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H.323

A videoconferencing standard developed by the International Telecommunication Union (ITU).

Overview

H.323 is a comprehensive suite of protocols for voice, video, and data communications between computers, terminals, network devices, and network services. H.323 is designed to enable users to make point-to- point multimedia phone calls over connectionless packet-switching networks like private Internet Protocol (IP) internetworks and the Internet. H.323 was designed to replace the earlier H.320 standard that ran over circuit-switched services like Integrated Services Digital Network (ISDN) telephone systems.

H.323 is widely supported by manufacturers of videoconferencing equipment, Voice over IP (VoIP) equipment, and Internet telephony software and devices. Any hardware or software system that supports the H.323 standard can communicate easily with any other H.323 system. With ordinary telephone communications, you can purchase a telephone from any vendor, plug it into your phone system, and the equipment works and interoperates with other devices already on the system. The idea behind H.323 is to make multimedia communications over the Internet easy to set up and use.

Architecture

The H.323 protocol stack performs various functions related to establishing and maintaining real-time communications over voice, video, and data. H.323 does not represent a single standard, but rather an umbrella of many other ITU standards. The H.323 protocol stack includes more than two dozen G-level, H-level, and T-level protocols, including

H.323 also specifies standard codecs for audio (G.723) and for video (H.263) that enable H.323 products to send and receive voice and video images. Specifically, H.323 defines standards for data codecs for audio transmission rates of 14.4 kilobits per second (Kbps) or faster, and video transmission rates of 28.8 Kbps or faster. H.323 recognizes the T.120 protocol for data conferencing and provides a set of standards-based conferencing features for audio and video conferencing.

Implementation

An H.323 network consists of four different types of entities:

Marketplace

Many vendors provide H.323 videoconferencing systems and appliances. Examples include Polycom's ViewStation MP, a popular system in many enterprise environments. Other vendors include Sony Corporation with its Contact system (which also works over ISDN) and VCON's MC6000 that is multicast-capable.

H.323. Components of an H.323 network.

PC-based H.323 collaboration software products include PictureTel Corporation's 900 Series appliances, which support both H.323 and H.320 communications; VCOM's MC8000; Intel Corporation's TeamStation; and VTEL Corporation's SmartStation.

Prospects

H.323 was extolled by industry in the 1990s but is now beginning to be considered legacy technology, suitable mainly for migrating legacy telephony systems to VoIP and for interoperating with legacy Private Branch Exchanges (PBXs). The problem is that H.323 was designed with videoconferencing strictly in mind and is not flexible enough to support a full range of VoIP features.

A newer protocol proposed by the Internet Engineering Task Force (IETF) is Session Initiation Protocol (SIP), which is seen in some quarters as preferable for newer VoIP deployments. SIP is designed from the ground up to support IP and is slimmer and more flexible than H.323, a protocol known to have excessive overhead.

Notes

The OpenH323 Project is an initiative of Equivalence Pty Ltd. to develop an open-source version of the H.323 protocol suite licensed under Mozilla Public License (MPL).

See Also session initiation protocol (SIP) ,T.120 ,Voice over IP (VoIP)

H.323 gateway

A service that allows you to connect an H.323-based communication system on the Internet to a telephony system.

H.323 gateway. Making a phone call using an H.323 gateway.

Overview

You can use H.323 gateways to connect corporate networks to telephony systems for supporting integrated audio and video conferencing. For example, you can use an H.323 gateway to place a Microsoft NetMeeting call to someone's telephone. The Uniform Resource Locator (URL) format for doing so is "callto:<address>", in which <address> is the fully qualified domain name (FQDN), Internet Protocol (IP) address, or e-mail address of the person you are calling. You can use this URL as a link on a Web page or enter it directly in the address box of your Web browser.

NetMeeting does not include any H.323 gateway software, but third-party gateways are supported.

See Also gateway ,

hacking

The action of trying to compromise the security of a computer system or network.

Overview

In the 1980s the term hacker was used to describe curious programmers who enjoyed getting into the nuts and bolts of how computer systems worked, usually UNIX systems and Transmission Control Protocol/Internet Protocol (TCP/IP) networks. In the 1990s hacking emerged as a malicious pursuit with the goal of stealing information, crashing systems, bringing down networks, and interfering with the daily activities of businesses and governments worldwide.

Various kinds of activities constitute hacking, but their overall similarity is that they are performed with malicious intent and without the knowledge or permission of the systems being hacked. Some of the different types of activities that hackers perform include

Types

Hacker attacks fall under a number of different categories, including the following:

Notes

The Openhack Project was developed by eWeek Labs to help publicize the issues around hacking of business networks. The project challenged people to try deliberately attacking a test bed network, and the results of these attempts were published to help educate people on how hackers perform their attacks.

For More Information

Visit the Microsoft Security site at www.microsoft.com/security.

See Also denial of service (DoS) ,security

Hailstorm

An emerging platform for development of Internet- based services from Microsoft Corporation based on its new .NET platform.

Overview

In many ways, Microsoft Hailstorm represents the next evolutionary stage of the Internet. Hailstorm is a user- centric architecture based on Microsoft's .NET platform and on the open standards Extensible Markup Language (XML), Simple Object Access Protocol (SOAP), and Universal Description and Discovery Interface (UDDI).

Hailstorm provides users with a standard set of Web services that enables users to collaborate and participate in business transactions seamlessly and transparently. Hailstorm places control squarely in the user's court-data is not shared unless users explicitly grant their consent to do so. This makes Hailstorm a secure platform for 21st-century business communications in the Internet economy.

Initial support for Hailstorm is included in Microsoft's Windows XP, Office XP, and Xbox platforms. Hailstorm transforms MSN Messenger into an enterprise and consumer development platform for building advanced user experiences that leverage the power of Internet-based instant messaging.

Architecture

Hailstorm services are designed to allow users to access their data from a variety of platforms, including traditional PCs, Tablet PCs, handheld Personal Digital Assistants (PDAs), mobile phones, and other Web appliances. User data is stored in a distributed fashion and can be securely accessed from any device and any location with the user's consent.

Users and applications access Hailstorm services through end points, which may be applications, devices, or services. With a user's consent, Hailstorm automatically connects end points to widely distributed Web services to perform transactions desired by the user. Hailstorm services are XML-enabled Web services running on the .NET platform and are built upon the foundation service Microsoft Passport, which provides security and authentication to perform transactions. Other parts of a user's experience supported by Hailstorm's Web services include profile information, location information, calendar information, and so on. The table lists the initial set of core Web services to be included in Hailstorm.

Initial Set of Web Services Included in Microsoft Hailstorm

Service

Description

myAddress

User's address

myApplication Settings

User's application settings

myCalendar

User's time and task management information

myContacts

User's list of contacts

myDevices

Settings and capabilities of user's devices

myDocuments

Location for storing user's personal files

myFavorite WebSites

User's favorite Uniform Resource Locators (URLs)

myInbox

User's inbox for e-mail and voice mail

myLocation

User's geographical location

myNotifications

User's subscriptions and notifications

myProfile

User's personal profile information such as nickname, birth date, etc.

myServices

Services provided to user by Hailstorm

myUsage

User's usage of Hailstorm services

MyWallet

Receipts, coupons, and payment instruments for user's transactions

For More Information

Find out more about Hailstorm at www.microsoft.com/hailstorm.

See Also .NET platform

HAL

Stands for hardware abstraction layer, a thin layer of software at the base of the Microsoft Windows 2000, Windows XP, and Windows .NET Server operating systems that provides portability between different machine architectures and processor platforms.

See Also hardware abstraction layer (HAL)

half-duplex

A mode of communication where at any given moment data can be either transmitted or received but cannot be transmitted and received simultaneously.

Overview

The simplest example of half-duplex communications is using a walkie-talkie: you have to press a button to talk and release the button to listen. When two people use walkie-talkies to communicate, at any given moment only one of them can talk while the other listens. If both try to talk simultaneously, a collision occurs and neither hears what the other says.

Communication through traditional hub-based Ethernet networks is another example of half-duplex communications. When one station on an Ethernet network transmits, the other stations detect the carrier signal and listen instead of transmitting themselves. If two stations transmit signals simultaneously, a collision occurs and both stations stop transmitting and wait random intervals of time before retransmitting.

By contrast, full-duplex communication enables stations to transmit and receive signals simultaneously, with the advantage of providing twice the bandwidth of equivalent half-duplex technologies. However, full- duplex requires two communication channels to achieve these results-one to transmit and one to receive signals. Hub-based Ethernet networks can only be used for half-duplex communications-for full- duplex Ethernet, you need switches instead of hubs.

A third mode of communications is called simplex, which involves transmission in one direction only, with one station transmitting signals and the other receiving them. A television broadcast is a simple example of simplex communications.

See Also Ethernet ,full-duplex ,simplex

HAN

Stands for home area network, a network of several computers at a user's home.

See Also home area network (HAN)

handheld

Generally used to refer to Personal Digital Assistants (PDAs) and other computers that can be held in the palm of the hand.

Overview

In the late 1990s the handheld market became popular among consumers for managing contacts, appointments, and other personal information. At the end of the decade, the issue arose as to which of the two common platforms, Palm and Microsoft Windows CE, would dominate in the enterprise market. The outcome is still uncertain. Palm has the advantage of a large installed base, a wide range of third-party applications, and support by popular wireless services like AvantGo. The new PocketPC platform has made the Windows CE operating system competitive with the PalmOS by providing interoperability with Windows-based services, and pocket versions of Microsoft Office applications, Microsoft Internet Explorer. At the end of 2000, analysts reported that Palm held three-quarters of the handheld market share and Windows CE held only 18 percent, but the Windows CE has been gaining market share steadily.

A newcomer to the handheld market is the Linux PDA, represented by Agenda Computing's VR3. It is too soon to say what impact porting Linux to the handheld platform will have on market leaders Palm and PocketPC.

Handheld Device Markup Language (HDML)

A markup language loosely modeled on Hypertext Markup Language (HTML) and optimized for providing Internet access to cell phones and other handheld devices.

Overview

Handheld Device Markup Language (HDML) was developed by Unwired Planet (now Phone.com) to enable mobile communications devices like cell phones and Personal Digital Assistants (PDAs) to access content on the Internet. Such devices usually have limited size displays (typically four lines by 20 characters or smaller) and limited processing power that make them require that information they download from the Internet be specially formatted to meet these requirements.

HDML is not intended as a means of delivering standard HTML-formatted Web content to such devices-most standard Web pages simply cannot be reformatted to fit on devices with such small display areas. HDML is not a subset or scaled-down version of HTML but an entirely new markup language specifically designed from the ground up for these devices.

HDML can be used to deliver a broad range of time-sensitive information to handheld wireless devices, including appointments, weather information, stock quotes, telephone directory white pages, inventory, catalog pricing, and similar business and commercial information. Scripts can be developed to extract this kind of information from the databases in which it resides and format it into HDML cards in much the same way that Perl can be used to write scripts to access database information stored on UNIX servers or that Active Server Pages (ASP) can be used to build Web- based applications that connect to Structured Query Language (SQL) databases.

HDML is an open standard guided and licensed by Phone.com. The current version of this technology is HDML 2.

Implementation

HDML provides a way to format information for delivery to handheld wireless devices that support packet- switched Internet Protocol (IP) services instead of traditional circuit-switched cellular services. HDML uses the same Hypertext Transfer Protocol (HTTP) as its application layer protocol, just as HTML does. The difference is that an HDML/HTTP system uses three components (client, server, and gateway) instead of the traditional two (client and server) used by HTML/HTTP.

Handheld Device Markup Language (HDML). How an HDML communications system works.

When a user with an HDML-enabled client device (a device running an HDML browser) wants to access HDML-formatted content on a Web server, the client device communicates over the provider's cellular network with a special gateway server called an UP.Link Gateway. This gateway server then "gates" the user's request over a Transmission Control Protocol/Internet Protocol (TCP/IP) wireline connection to the Web server. The Web server responds with the requested content that is then "gated" back to the client over the cellular network.

HDML uses a navigation model that is based on a filing card metaphor. Cards of HDML-formatted information are grouped into "activities," which facilitates the development of HDML-based applications. The user can enter information and share it among cards using variables, and the user can parameterize cards so that a family of cards differing only in the value of a variable can be stored more efficiently in the limited-size cache of these devices. You can use variables to create forms for entering information in HDML applications. HDML currently does not support scripting, branching, or conditional statements.

Marketplace

HDML is currently used by more than 80 cellular and mobile carriers in the United States, including AT&T, Ameritech, and Bell Atlantic. Some analysts estimate that more than 90 percent of Internet-enabled phones sold in the United States support HDML and include Phone.com's HDML browser. Outside the United States, the competing Wireless Markup Language/Wireless Application Protocol (WML/WAP) combination is more popular (WAP shares many of the features of HDML and was developed from HDML).

Prospects

WAP is currently gaining popularity at the expense of HDML and may eventually replace it. One indication of this is Nokia and Ericsson, which are two major players in the world cellular market and which provide WAP- enabled phones. Nokia is even giving away the source code for its WAP browser to promote this standard. Meanwhile, enterprises that want to deploy mobile clients with Internet access have to support two interoperable solutions, HDML and WAP.

See Also Hypertext Markup Language (HTML) ,Hypertext Transfer Protocol (HTTP) Wireless Application Protocol (WAP), Wireless Markup Language (WML)

handshaking

The process that establishes communication between two telecommunications devices.

Overview

When two computers first connect with each other through modems, the handshaking process determines which protocols, speeds, compression, and error- correction schemes will be used during the communication session. Handshaking is necessary at the start of each session because typically the modems differ in their vendor, model, or hardware/software configuration. The handshake ensures that communication is possible despite these differences.

The term handshaking arises from the analogy of two people meeting to conduct business. They first shake hands to greet each other, announcing their names, titles, and intentions. They might also "size each other up" to determine the other person's capacities and capabilities.

During the handshaking process, both modems send the other a series of control signals and respond to each other's signals. Handshaking is also referred to as "flow control" because the process establishes the ground rules for managing the flow of data between the two devices. Some of the parameters that the modems need to negotiate are

Modern modems transmit the control signals in full- duplex mode over the RS-232 interface that connects them to the computers.

See Also flow control ,modem

hardware abstraction layer (HAL)

A thin layer of software at the base of the Microsoft Windows 2000, Windows XP, and Windows .NET Server operating systems that provides portability between different machine architectures and processor platforms.

Overview

The hardware abstraction layer (HAL) hides hardware differences from the operating system so that uniform code can be used for all hardware. The HAL thus offers a uniform interface between the underlying hardware and the higher layers of the operating system. All underlying hardware looks the same to the Windows 2000, Windows XP, and Windows .NET Server operating systems because they "see" the hardware through the filtered glasses of the HAL.

The HAL is located at the base of the Executive Services, and it encapsulates most hardware-specific functions that are performed by the operating system. If another portion of the operating system wants to access a hardware device, it must refer its request to the HAL. The HAL handles all communication between the kernel of the operating system and the hardware, particularly those regarding processor commands, system interrupts, and input/output (I/O) interfaces.

Implementation

The HAL is implemented in Windows 2000, Windows XP, and Windows .NET Server as a loadable kernel mode module called hal.dll that is located in the System32 directory. If a hardware vendor needs to protect proprietary technology, the company can develop a custom implementation of the HAL. This means, for example, that different processor configurations such as multiprocessor machines might use different HAL drivers.

Hardware Abstraction Layer (HAL). How a Windows 2000 application communicates with hardware through the HAL.

Usually, what happens when Windows 2000, Windows XP, or Windows .NET Server want to access a hardware device is that the operating system issues a command to the appropriate device driver, which then talks to the HAL, which talks directly to the hardware.

Using the wrong HAL driver can result in degraded system performance since all hardware calls made by the operating system go through the HAL. Contact your vendor to make sure that you have the correct HAL for your system. To check which HAL is installed on your system, open the Setup.log file found in the \Winnt\Repair directory and examine the filename after the equal sign. The following table shows a list of HALs that can be installed from the Windows 2000, Windows XP, and Windows .NET Server compact disc.

HALs Found on the Windows 2000, Windows XP, and Windows .NET Server Compact Disc

File name

Description

hal.dll

Used for standard PCs

halacpi.dll

Used for PCs supporting Advanced Configuration and Power Interface (ACPI)

halapic.dll

Used for PCs supporting Advanced Programmable Interrupt Controller (APIC)

hapaacpi.dll

Used for PCs supporting both ACPI and APIC

halmps.dll

Used for multiprocessor PCs

halmacpi.dll

Used for multiprocessor PCs that support ACPI

halsp.dll

Used for Compaq SystemPro servers

For More Information

Find out about the HAL Development Kit at www.microsoft.com/DDK/halkit.

See Also Windows 2000 ,Windows .NET Server ,Windows XP

hardware address

Also called a MAC address, a unique 6-byte (48-bit) address that is usually permanently burned into a network interface card (NIC) or other physical-layer networking device and uniquely identifies the device on an Ethernet-based network.

See Also MAC address

Hardware Compatibility List (HCL)

A list of hardware that is compatible with a Microsoft Windows operating system product.

Overview

The Hardware Compatibility List (HCL) for a Microsoft Windows platform defines all supported hardware, including computer systems as well as individual hardware components such as video cards, motherboards, and sound cards. When in doubt, you should consult the HCL before installing an operating system on a nonstandard or customized machine or installing new hardware into a system with an existing operating system. Using components not included on the HCL can lead to installation failures or system instabilities. Microsoft has determined that drivers for hardware devices listed on the HCL are compatible with that version of Windows; Microsoft supports only these drivers.

If you use drivers for devices that are not on the HCL, you might not be entitled to Microsoft product support for your system configuration. If you must use non- HCL devices, contact the device's manufacturer to see whether a supported driver exists for the particular Windows operating system you are using. If you contact Microsoft Product Support Services (PSS) about a problem and the support engineer determines that a hardware device in your system is not on the HCL, you will likely incur a charge for the call even if the problem cannot be resolved.

Notes

You can usually find the HCL for a particular Windows operating system on its distribution CD. (The file might be called hcl.txt.) A more up-to-date version of the HCL for all Windows operating system platforms is available on the Web at the HCL site listed in the "For More Information" section of this entry.

For More Information

Visit Microsoft Product Support Services at www.microsoft.com/support. Look at the Microsoft HCL site at www.microsoft.com/hwtest/hcl.

hardware profile

Information about the configuration of devices and services that are used to boot an operating system into a certain configuration.

Overview

A hardware profile tells the operating system which devices are present when the computer boots.

You can create several different hardware profiles on Microsoft Windows operating systems and select the one you want to use at boot time. For example, if Windows 2000 is installed on a laptop computer, you can create two hardware profiles for that computer:

When the laptop is booted, you select the appropriate hardware profile from a menu generated by the operating system.

Notes

Hardware profiles are usually not necessary for plug-and- play laptops running Windows 2000, Windows XP, or Windows .NET Server. These computers can recognize when new hardware becomes available (for example, by docking) and automatically configure themselves.

hashing algorithm

A mathematical procedure for randomizing (hashing) information to make it more secure in transmission.

Overview

Hasting algorithms take information and scramble it repeatedly to create a fixed-length string of numbers and characters called a hash. A good hashing algorithm has the following characteristic: if you apply a hashing algorithm to some data and then change only a few bits in the data and apply the algorithm again, the two resulting hashes will differ in almost every bit.

Hashing algorithms are used extensively in cryptography for encrypting keys or messages. Examples of popular cryptographic hashing algorithms include MD2, MD4, MD5, and SHA-1. Message Digest 5 (MD5) uses a 128-bit hash, and Secure Hash Algorithm (SHA) uses a 60-bit hash. The more bits in a hash, the greater the security of the encryption process.

Hashing is also used in some database technology for creating indexes of items in a database. Hashes of database objects are generally smaller than the objects themselves, so they can be indexed and searched more quickly. You can generate unique hashes of fixed length for each database record, creating a hash table that you can use for quick searches for records.

See Also cryptography ,encryption

HBA

Stands for host bus adapter, a device that converts Peripheral Component Interconnect (PCI) bus signals into Small Computer System Interface (SCSI) or Fibre Channel format.

See Also host bus adapter (HBA)

H channel

A designation for groups of channels on Basic Rate Interface ISDN (BRI-ISDN) services.

Overview

H channel standards are defined by the International Telecommunication Union (ITU) and are composed of different combinations of Integrated Services Digital Network (ISDN) B channels. The most common configurations are as follows:

See Also B channel ,Integrated Services Digital Network (ISDN)

HCL

Stands for Hardware Compatibility List, a list of hardware that is compatible with a Microsoft Windows operating system product.

See Also Hardware Compatibility List (HCL)

HDLC

Stands for High-level Data Link Control, an encapsulation protocol for point-to-point serial wide area network (WAN) links.

See Also High-level Data Link Control (HDLC)

HDML

Stands for Handheld Device Markup Language, a markup language loosely modeled on Hypertext Markup Language (HTML) and optimized for providing Internet access to cell phones and other handheld devices.

See Also Handheld Device Markup Language (HDML)

HDSL

Stands for High-bit-rate Digital Subscriber Line, a form of Digital Subscriber Line (DSL) technology that provides T-1 speeds in both directions.

See Also High-bit-rate Digital Subscriber Line (HDSL)

header

The initial portion of a packet or a frame.

Overview

The header typically contains control information such as addressing, routing, and protocol version. The format of this information depends on the protocol being used. For example, an Internet Protocol (IP) header contains information about the version of the IP protocol, the length of the header, the type of service used, the packet's Time to Live (TTL), the source and destination address, and so on. Headers are used to control the flow of packets through the network or over the communication link.

The end of a frame sometimes has a smaller structure called a footer or trailer, but this usually contains only error-checking information. Control information is always placed in the header because this is the first portion of the packet or frame that is read by a networking device such as a switch or a router.

See Also frame ,packet

heartbeat

A polling feature of cluster servers.

Overview

An internal communications interface in Microsoft Cluster Server (MSCS) in Microsoft Windows NT Server, Enterprise Edition, and in the Cluster service in Windows 2000 Advanced Server and Windows .NET Enterprise Server, the heartbeat continuously provides interserver communication between cluster nodes in a cluster. One function of the heartbeat is to generate a message that the Cluster service running on one node regularly sends to the Cluster service on the other node to detect a failure within the cluster. Nodes in a cluster communicate status information with each other through the heartbeat using MSCS or the Cluster service. These messages appear as network traffic between the two nodes in the cluster on the dedicated network connection between the nodes, which is called the private network of the cluster. The primary heartbeat network interface is usually a crossover network cable directly attached between cluster nodes or in a private network. If the heartbeat is lost over the private connection, Cluster Server reverts to a public network or alternate connection for its heartbeat and other Cluster service traffic. You can configure the polling interval for the heartbeat using Cluster Administrator.

See Also clustering

Help

Any of several systems of online help for Microsoft Corporation products.

Overview

The type of help features available depends on the nature of the product, the date of release, and the context in which it is invoked. Examples of help functions include the following:

heterogeneous network

A network that uses multiple network architectures and operating systems.

Overview

An example of a heterogeneous network at the hardware level is a combination of Ethernet and Token Ring local area networks (LANs) connected with a Fiber Distributed Data Interface (FDDI) backbone. An example at the network operating system (NOS) level is a server room in which some machines are running Microsoft Windows NT, others are running UNIX, and still others are running Novell NetWare. An example at the application level is a messaging system that includes Microsoft Exchange Server, Novell GroupWise, UNIX Sendmail, and IBM PROFS mail systems.

Heterogeneous networks are generally more complex to administer than homogeneous networks. Most networks are heterogeneous because they evolve over time. Most startup companies cannot afford to buy a completely homogeneous, state-of-the-art network platform, and even if they could, it would soon become out of date. One advantage of deliberately maintaining a heterogeneous network is that customers can use any product they choose instead of being locked into a single vendor's system. Networking and operating system choices can be made on a "best-served" basis instead of an "only buy from us" approach.

See Also homogeneous network

hidden share

A share not visible when users browse the network for resources.

Overview

In Microsoft Windows operating systems, a share with a dollar sign ($) appended to the share name. Hidden shares are accessible on the network using Universal Naming Convention (UNC) paths or mapped network drives, but only if the user knows that they are present and knows their exact name. These shares do not show up in My Network Places or Windows Explorer or when you use the net view command. Users are unaware of their presence unless they are specifically informed.

It is a good idea not to acquaint ordinary users with hidden shares, because they might create such shares on their workstations if appropriate file and print services are installed. This can lead to secret, uncontrolled publishing of information within the company, which is usually against company policy.

Windows 2000, Windows XP, and Windows .NET Server also create certain hidden shares during the installation process that the operating system uses for specific purposes. These hidden shares are called administrative shares; permissions on them should not be modified.

See Also share

Hierarchical Storage Management (HSM)

A data storage system for archiving infrequently needed data while still making it easily available.

Overview

A Hierarchical Storage Management (HSM) system is a way of providing users with seemingly endless amounts of storage space. This is accomplished by moving much of the data from hard disks to an archival storage system, such as an optical drive or tape library. Pointers are created on the hard disks indicating where the archived data is located. Users who need access to data need only request it from the disk, and if the requested data has been archived, the pointers allow the data to be found and returned to the user. The whole process is transparent from the user's viewpoint-all the data appears to be stored on the hard disks.

Hierarchical Storage Management (HSM). How an HSM system works.

Implementation

HSM defines two types of storage:

Note that nearline storage differs from offline storage, which uses removable archive media (usually tape) that needs to be manually inserted in order to access information stored on the media. For HSM to work, the archival storage system employed must be automated or nearline in operation.

In HSM, when an online storage system approaches full capacity, HSM automatically starts to archive files, directories, or whole volumes to nearline storage. The particular data archived depends on the policies configured for HSM's operation, which typically initiate file archiving based on how frequently files have been accessed or when they have last been accessed. The archiving process happens automatically, and files moved from online storage are replaced with pointers to their nearline location. When a user tries to access a file stored nearline, the file is moved to online storage where the user can read it (the user cannot read nearline storage directly).

Marketplace

HSM systems have been around for some time in the UNIX world as third-party add-ons developed by different vendors for enterprise environments. Microsoft now includes an integrated HSM system in its Microsoft Windows 2000 Server platform called Remote Storage Service (RSS), which can archive data to Small Computer System Interface (SCSI) tape libraries using 4mm, 8mm, and digital linear tape (DLT) tape formats (quarter-inch cartridge [QIC] tapes and optical disk libraries are not supported). RSS works by using a new feature of NTFS version 5 called reparse points, which are similar to pointers used by UNIX file systems.

Issues

Traditional HSM loses its usefulness when used to archive large files like video feeds and database files. This is because users sometimes need to access only a portion of such a file, but in order for them to access it, the entire file must be fetched from nearline storage and moved to online storage, a process that can take excessive time (for example, retrieving a 2 gigabyte (GB) file from a DLT tape library could take five minutes or more). A recent solution to this problem is the implementation of file segmentation to HSM systems. Using file segmentation, large files are broken up into smaller segments for archiving and retrieval, and when a part of a file is modified, only the segment modified needs to be rearchived. File segmentation can be implemented on UNIX-based HSM systems by employing a segmentation attribute-setting this attribute causes a file, directory, or volume to be broken into segments when moved to nearline storage, with pointers to each segment created on the online storage where the object previously resided. The whole process is again transparent to the user and typically reduces seek/return time for portions of large files to seconds instead of minutes.

Notes

Note that HSM systems like Microsoft's RSS are not a substitute for regular tape backups. HSM will not archive critical operating system and application files, so regular backups are still necessary when using such systems.

See Also storage ,tape format ,tape library

High-bit-rate Digital Subscriber Line (HDSL)

A form of Digital Subscriber Line (DSL) technology that supports high-speed symmetric data transmission.

Overview

High-bit-rate Digital Subscriber Line (HDSL) can be used to transmit data over existing copper local loop connections at T1 or E1 speeds. It is used to transport data only (HDSL signals overlap the voice portion of the Plain Old Telephone Service [POTS] spectrum and therefore cannot carry voice) and is generally used in wide area networking (WAN) scenarios. The maximum distance for HDSL transmission is typically 15,000 feet (4500 meters) when running over unconditioned copper twisted-pair telephone wiring. Some providers, however, claim that their devices support twice this distance. This maximum distance from the telco's central office (CO) to an HDSL customer's premises is sometimes called the Carrier Service Area (CSA).

HDSL was the earliest symmetric version of DSL to be widely implemented and was designed in the early 1990s as an alternative to traditional T-1 services. T1 lines originally required intermediate repeaters to be installed every 6000 feet (1830 meters) between termination points in order to ensure the signal strength necessary to transport data at such high speeds. HDSL was developed by Bellcore as a repeaterless form of T1 that would save the cost of installing repeaters and speed deployment of T1 lines for customers.

HDSL comes in various formats, including the following:

Implementation

The main different between HDSL and the original T1 specification is the line coding scheme employed. T1 lines originally used bipolar alternate mark inversion (AMI) for signal encoding, which has a spectral efficiency of only 1 bit/baud (in other words, to transmit one bit of data requires one symbol or character). By contrast, HDSL employs 2B1Q (2 Binary 1 Quaternary) line coding, which transmits information at 2 bits/baud (2B1Q is also the encoding scheme for Integrated Services Digital Network [ISDN]). Each pair of copper phone wires in an HDSL implementation carry data at 784 Kbps over unconditioned lines without the need of repeaters. Two pairs of copper wires thus provide a throughput of 1.536 Mbps, which is the same as regular T1.

In a typical scenario, a line driver called an HTU-R terminates the lines at the customer premises end through a serial connection to a router that provides connectivity for the customer's local area network (LAN). At the telco end, a similar line driver called an HTU-C terminates the other end and provides a connection to the digital cross connect (DCC) that routes signals to trunk lines between central offices (COs). A group of HTU-Cs are typically combined in a rack to service more customers.

High-bit-rate Digital Subscriber Line (HDSL). Some possible deployment scenarios for HDSL.

Uses

Because it is a high-speed symmetrical service, HDSL is typically used to provide dedicated WAN links for enterprises. Many telcos now offer HDSL instead of traditional T1 lines because of reduced deployment costs associated with this repeaterless technology. An enterprise customer wanting to run a public Web server at her company site might use HDSL to provide a high-bandwidth connection between the server and the Internet.

HDSL can also be used for connecting bridges, routers, and telephone equipment such as Private Branch Exchanges (PBXs) over a campus using HDSL line drivers with built-in Channel Service Unit/Data Service Unit (CSU/DSU) functionality. It can also be used to build private data telecommunications networks, connections between points of presence (POPs), and other telecommunications services. You can use HDSL to connect campus networks and phone equipment at T1 speeds without the need for costly fiber-optic cabling. HDSL line drivers or line terminals generally support a variety of data interfaces, including V.35, G.703, and 10BaseT connections. They are configurable for Nx64 Kbps transmission speeds and sometimes include bridge or router functionality for framing of High-level Data Link Control (HDLC), Point-to-Point Protocol (PPP), Internet Protocol (IP), and other protocols. They can be used for LAN-LAN connections and for connecting LANs to frame relay networks or the Internet.

HDSL can also be used for other purposes such as videoconferencing, telecommuting, and shared Internet access for connecting business networks to the Internet. HDSL is not used for residential Internet access-Asymmetric Digital Subscriber Line (ADSL) is commonly used there instead. ADSL is able to provide line sharing (combined voice/data services over a single pair of copper lines), but HDSL carries only data and requires two pairs of lines.

Prospects

A newer form of HDSL called HDSL2 works similarly to HDSL but requires only one pair of copper lines in order to operate. HDSL2 is standardized by the International Telecommunication Union (ITU) as G.991.1 and by the American National Standards Institute (ANSI) as T1E1.4.

See Also Digital Subscriber Line (DSL) ,T-carrier

High-level Data Link Control (HDLC)

An encapsulation protocol for point-to-point wide area network (WAN) links.

Overview

High-level Data Link Control (HDLC) is a data-link layer protocol for synchronous communication over serial links. HDLC was developed in the 1970s by IBM as an offshoot of the Synchronous Data Link Control (SDLC) protocol for their Systems Network Architecture (SNA) mainframe computing environment. It was later standardized by the International Organization for Standardization (ISO) as a standard Open Systems Interconnection (OSI) Layer-2 protocol.

Architecture

HDLC is called an encapsulation protocol because it encapsulates bit stream data into fixed-length frames for transmission over synchronous serial lines. HDLC is a bit-stream protocol (bit streams are streams of data not broken into individual characters) that uses a 32-bit checksum for error correction and supports full-duplex communication. HDLC frames consist of a special flag byte (01111110) followed by address and control information, data bits, and a cyclic redundancy check (CRC) byte. A control field at the start of a frame is used for establishing and terminating data link connections. As a data-link protocol, HDLC is responsible for ensuring that data is received intact, without errors, and in the proper sequence. HDLC also supports flow-control mechanisms for managing synchronous data streams. HDLC also supports multiple protocols but does not include any mechanisms for authentication (since authentication is not really needed in dedicated point-to-point communications).

An HDLC link typically consists of a primary station and a secondary station, with the primary station issuing the commands and the secondary station issuing the responses. HDLC is used mainly for point-to-point communication, in contrast to other WAN transports such as Asynchronous Transfer Mode (ATM), frame relay, and X.25, which can be used for both point-to- point and point-to-multipoint communication. Because HDLC is used mainly in point-to-point communication, it does not need to have addressing implemented at the data-link layer because the local and remote stations are connected directly. In this configuration, either one station is the primary and the other the secondary (unbalanced point-to-point link) or both stations function in a primary/secondary capacity (balanced point-to-point link). You can also use HDLC in a more complex configuration in which one primary links to several secondaries (unbalanced multipoint configuration), but this is not common.

High-level Data Link Control (HDLC). Using HDLC for WAN communications over a leased line.

Implementation

HDLC is the default encapsulation protocol employed by Cisco routers for synchronous transmission over serial links like Integrated Services Digital Network (ISDN) and T-1 lines. For point-to-point communications over a leased line using Cisco routers at both ends, HDLC is a good choice. A typical leased-line configuration uses Channel Service Unit/Data Service Units (CSU/DSUs) to terminate the ends of the leased line, with Cisco routers connecting the CSU/DSUs to the two networks. The CSU/DSUs are nowadays often integrated into the routers for simplicity.

Issues

When HDLC was first developed, vendors made their own modifications to the protocol, with the result that versions of HDLC from different vendors cannot interoperate with each other. This was the driving force behind the development by the Internet Engineering Task Force (IETF) of Point-to-Point Protocol (PPP), a newer encapsulation protocol that has largely replaced HDLC in WAN environments. PPP supports both synchronous and asynchronous communications and has a standard implementation by every vendor to ensure interoperability between networking equipment from different vendors. Both PPP and Link Access Protocol, Balanced (LAPB), the data-link protocol for X.25, evolved from HDLC and were built upon its foundation.

Notes

A version of HDLC known as Normal Response Mode (NRM) HDLC is basically the same thing as Synchronous Data Link Control (SDLC), a protocol developed by IBM as a replacement for its BiSync protocol.

See Also Channel Service Unit/Data Service Unit (CSU/DSU) ,leased line ,Open Systems Interconnection (OSI) reference model ,Point-to-Point Protocol (PPP) ,Synchronous Data Link Control (SDLC) ,wide area network (WAN) ,X.25

High Performance File System (HPFS)

A file system designed for version 1.2 of the Microsoft/IBM OS/2 operating system as a successor to the file allocation table (FAT) file system used by MS-DOS.

Overview

High-Performance File System (HPFS) improves on the performance of the FAT file system for MS-DOS in the following ways:

Other features of HPFS that were considered advanced at the time of its development included the following:

Notes

HPFS is supported by Microsoft Windows NT version 3.51 but not by the Windows NT 4, Windows 2000, Windows XP, or Windows .NET Server operating systems, which use the more advanced NTFS file system (NTFS). HPFS was designed to function as an Installable File System (IFS), which is implemented as a dynamic-link library (DLL) that hooks into the file system component of the operating system kernel. In Windows NT 3.51, you configured HPFS through an IFS= line in the config.sys file. The CD file system (CDFS) was also designed to support IFS.

See Also file allocation table (FAT) ,file system ,NTFS file system (NTFS)

High-Performance Parallel Interface (HIPPI)

A gigabit networking technology for point-to-point communications.

Overview

High-Performance Parallel Interface (HIPPI) is an American National Standards Institute (ANSI) standard for point-to-point networking at gigabit speeds. HIPPI was developed in the 1980s as a technology for interconnecting supercomputers, mainframes, and their storage devices. Fibre Channel is the closest networking technology to HIPPI in terms of its use and capabilities and is more widely used than HIPPI.

Uses

While HIPPI is comparable in speed to Gigabit Ethernet (GbE), it is not widely used in general networking applications since it is basically a connection-oriented technology suitable for point-to-point communications due to latency involved in negotiating connections. HIPPI also lacks the multicasting capability provided by Internet Protocol (IP)-based GbE. HIPPI is mostly deployed in a few high-speed research projects by universities and for high-speed data collection in specialized scientific experiments, one of the most notable being at the European Center for Particle Physics (CERN), the world's largest particle physics facility.

Implementation

HIPPI operates at the physical and data-link layers to provide connection-oriented communications between two points. HIPPI can operate at either 800 megabits per second (Mbps) or 1.6 gigabits per second (Gbps), and it has a simple flow control command set that can establish or tear down a connection in under a microsecond using a HIPPI switch. There is also a new standard called HIPPI-6400 that supports speeds up to 6.4 Gbps for distances up to 164 feet (50 meters) over twisted pair copper and up to 0.62 miles (1 kilometer) over fiber-optic cabling.

In its original format, HIPPI was a parallel transmission technology that used 50-pair shielded twisted-pair (STP) copper cabling for transmission of 800 Mbps over a distance of up to 82 feet (25 meters). Because longer parallel cables are impractical, a newer version called Serial HIPPI was developed in 1997 that encoded HIPPI's parallel bit stream into a single serial bit stream over fiber-optic cabling. For 800 Mbps transmission, the signaling rate is 1.2 Gbaud and two fibers are used, one for simplex transmission in each direction, giving full-duplex communications. For 1.6 Gbps transmission, four fibers are used. The maximum distances over which Serial HIPPI can operate are:

Marketplace

Vendors of HIPPI equipment include Ascend, Cray Research, GigaLabs, Hewlett-Packard Company, IBM, and Silicon Graphics, Inc. (SGI). Products offered include adapter boards and interfaces, frame buffers, storage devices, tape servers, switches and routers, cables, and test equipment.

For More Information

Find out more about HIPPI standards activities at www.hippi.org

See Also Fibre Channel ,Gigabit Ethernet (GbE)

High-Speed Circuit Switched Data (HSCSD)

An interim upgrade for Global System for Mobile Communications (GSM) cellular communications systems.

Overview

High-Speed Circuit Switched Data (HSCSD) is the first upgrade available for GSM that boosts GSM's data- carrying capacity above its current maximum of 14.4 kilobits per second (Kbps). HSCSD is a pre-2.5G upgrade for GSM that has been deployed by a few carriers in lieu of General Packet Radio Service (GPRS), a 2.5G upgrade for GSM and other Time Division Multiple Access (TDMA) cellular systems that is just beginning to be widely deployed. Unlike GPRS, which requires that existing base station hardware be upgraded, HSCSD is a software-only upgrade and is easily implemented.

Implementation

HSCSD works by aggregating together groups of GSM time slots. Each GSM frame consists of eight time slots, each of which can provide up to 14.4 Kbps throughput for data transmission. HSCSD uses one slot for upstream communications and aggregates four slots for downstream communications. HSCSD is thus an asymmetrical data transmission technology that supports speeds of 14.4 Kbps upstream and 57.6 Kbps downstream. However, the overhead required by slot aggregation usually results in downstream bandwidth of only 28.8 Kbps instead of the theoretical 57.6 Kbps.

HSCSD supports two different transmission modes:

Prospects

HSCSD test deployments began in 1999. Some GSM operators that have upgraded their networks to HSCSD include Sonera in Finland and SingTel in Singapore. The majority of operators are upgrading directly to GPRS, however, and HSCSD is likely to be eclipsed quickly. This may be unfortunate, because HSCSD's circuit-switched transmission method has advantages over packet-switched GPRS in some applications like digital video, where low latency and guaranteed quality of service (QoS) are important.

See Also General Packet Radio Service (GPRS) ,Global System for Mobile Communications (GSM) ,Time Division Multiple Access (TDMA)

High-Speed Serial Interface (HSSI)

A serial interface that supports speeds up to 52 megabits per second (Mbps).

Overview

High-Speed Serial Interface (HSSI) is defined by the standard EIA 612/613. HSSI supports serial transmission at speeds much higher than those supported by traditional serial interfaces like RS-232 and V.35. At a top speed of 52 Mbps, HSSI is capable of supporting

Uses

HSSI is typically used to connect access servers and routers to T3 circuits for enterprise wide area network (WAN) connectivity. Top-end Cisco routers support HSSI through installable HSSI Interface Processor (HIP) cards that provide one HSSI port per router. HSSI is necessary for companies that want to deploy T3 WAN services such as DS-3 frame relay links because slower V.35 and RS-232 serial interfaces cannot operate at such speeds.

Notes

HSSI uses a 50-pin Centronics connector similar to a Small Computer System Interface (SCSI) connector, but you cannot use a SCSI cable to connect a HSSI router to a Data Service Unit (DSU)-you have to use a special HSSI cable instead.

See Also router ,RS-232 ,serial transmission ,T-carrier ,V.35

High-Speed Token Ring (HSTR)

A high-speed version of Token Ring that never got off the ground.

Overview

High-Speed Token Ring (HSTR) was a development project of the High-Speed Token Ring Alliance, which consisted of Token Ring hardware vendors IBM, Olicom, and Madge Networks. HSTR was initially intended as a 100 megabits per second (Mbps) version of 802.5 Token Ring networking architecture that would provide a logical upgrade for existing 4 and 16 Mbps Token Ring networks. Speeds up to 1 gigabit per second (Gbps) were envisioned as being possible down the line.

Unfortunately, customer interest in HSTR waned with the rapid development of Fast Ethernet and Gigabit Ethernet (GbE) technologies, and in 1998 IBM withdrew from the alliance and the effort collapsed. The effective result is that Ethernet has finally won the local area network (LAN) wars with Token Ring and Fiber Distributed Data Interface (FDDI) networking architectures, and these two architectures are now considered legacy technologies that have no real future.

See Also 802.5 ,Ethernet ,Fast Ethernet ,Fiber Distributed Data Interface (FDDI) ,Gigabit Ethernet (GbE) ,Token Ring

HiperLAN/2

An emerging high-speed wireless networking standard.

Overview

HiperLAN/2 is a global standard for broadband wireless networking developed by the Broadband Radio Access Networks (BRAN) group of the European Telecommunications Standards Institute (ETSI). HiperLAN/2 is designed to be an extension of Asynchronous Transfer Mode (ATM) technology to the wireless networking arena and is expected to be ratified as a standard sometime in 2002. HiperLAN/2 is an alternative to the existing 802.11a high-speed wireless networking standard developed by the Institute of Electrical and Electronics Engineers (IEEE) and is intended as a worldwide wireless networking platform that is interoperable with ATM, third-generation (3G) cellular systems, and 1394 (Firewire) systems.

Adoption of HiperLAN/2 standards is being driven by the HiperLAN2 Global Forum (H2GF), an open industry forum launched in 1999 and whose founding members were Bosch, Dell Computer Corporation, Ericsson, Nokia, Telia, and Texas Instruments. Two other HiperLAN standards being promoted include

Architecture

HiperLAN/2 provides raw data transfer speeds of up to 54 megabits per second (Mbps), which corresponds to roughly 20 Mbps actual sustained throughput. Instead of operating in the slower, crowded 2.5 gigahertz (GHz) Industrial, Scientific, and Medical (ISM) frequency band, HiperLAN/2 uses the higher 5 GHz frequency band for greater throughput. Actual frequencies used depend on the country in which the technology is deployed, for example:

HiperLAN/2 shares some similarities with the IEEE's 802.11n and 802.11a wireless networking standards, especially at the physical (PHY) layer. Specifically, both technologies use channels 20 megahertz (MHz) wide and employ orthogonal frequency division multiplexing (OFDM) at the PHY layer to support high data transfer rates. In HiperLAN/2, however, these 20 MHz channels are shared using Time Division Multiple Access (TDMA) at the media access control (MAC) layer, instead of the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) MAC employed by 802.11a. This makes HiperLAN/2 more like ATM than Ethernet in its operation, and, in fact, HiperLAN/2 supports Quality of Service (QoS) features similar to those found in ATM, something 802.11 does not.

Prospects

HiperLAN/2's QoS features make it an attractive solution for wireless transport of multimedia and video. The main issue with the technology, however, is its use of the 5 GHz frequency spectrum, which conflicts with the Federal Communication Commission's (FCC) spectrum allocation in the United States. This means that HiperLAN/2 is likely to be much more popular in Europe, where it was developed, than in the United States (HiperLAN/2 is accepted everywhere except Japan). Furthermore, 802.11a has the advantage of being first to market and is already beginning to be widely deployed, which may make it prevail over HiperLAN/2. Large enterprises may find it easier to deploy 802.11a globally throughout their offices than to try to support two different technologies.

A company called Atheros Communications has proposed a standard called 5-UP (5 GHz Unified Protocol) that would enable HiperLAn/2 and 802.11a technologies to interoperate. The 5-UP standard has been submitted to the IEEE for consideration.

See Also 802.11a ,Asynchronous Transfer Mode (ATM) ,Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) ,Orthogonal Frequency Division Multiplexing (OFDM) ,quality of service (QoS) ,Time Division Multiple Access (TDMA) ,wireless networking

HIPPI

Stands for High-Performance Parallel Interface, a gigabit networking technology for point-to-point communications.

See Also High-Performance Parallel Interface (HIPPI)

History

A folder in Microsoft Internet Explorer that contains shortcuts to Web pages that you have recently browsed.

Overview

The History folder makes it simple to return to sites that you have recently visited but have not been designated as Favorites. By clicking the History button on the Internet Explorer toolbar, you can view the contents of the History folder and revisit any of the links. You can configure how many days items should be kept in the folder, and you can delete the files manually if desired. Using the offline browsing feature of Internet Explorer, you can browse the History folder while you are disconnected from the Internet.

hive

A physical file containing part of the registry in Microsoft Windows.

Overview

Hives are opposed to subtrees, which are logical sections of the registry. The term hive is loosely connected with the idea of the cellular structure of a beehive.

Hives consist of a discrete collection of keys and subkeys that have a root at the top of the registry. Five of these hives are located in the folder %SystemRoot%\ system32\config; the sixth hive (Ntuser.dat), which contains user profile information, is stored on machines running Windows NT in the folder %SystemRoot%\ Profiles\username. On machines running Windows 2000, Windows XP, or Windows .NET Server, it is stored in the folder

Each hive has an associated transactional .log file that logs all modifications made to the registry and provides fault tolerance. Each hive file also has a .sav file, which is a backup copy of the hive file. The functions of the hives and the logical key they map to are indicated in the following table.

Windows NT Hives

Hive

Key

Function

Default

HKEY_USERS\ DEFAULT

Contains the default system profile used when the logon screen is displayed.

SAM

HKEY_ LOCAL_ MACHINE \SAM

Contains information for the Security Account Manager (SAM). This hive cannot be viewed with the registry editor and must be accessed using specific application programming interfaces (APIs).

Security

HKEY_ LOCAL_ MACHINE \SECURITY

Contains the computer's security policy information. This hive also cannot be viewed with the registry editor and must be accessed using specific APIs.

Software

HKEY_ LOCAL_ MACHINE \SOFTWARE

Contains global configuration information for installed software.

System

HKEY_ LOCAL_ MACHINE \SYSTEM and HKEY_CURRENT_CONFIG

Contains configuration information for installed hardware devices and services.

Ntuser.dat

HKEY_ CURRENT_ USER

Contains user-specific configuration settings for the user who is currently logged on interactively.

See Also registry

H-node

A NetBIOS name resolution method used for name registration and resolution.

Overview

H-node is a type of NetBIOS over Transmission Control Protocol/Internet Protocol (TCP/IP) node defined in RFCs 1001 and 1002. It's supported by Microsoft Windows NT, Windows 2000, Windows XP, and Windows .NET Server. H-node is also one of four basic methods for resolving NetBIOS host names (computer names) into Internet Protocol (IP) addresses. Name resolution is the process of converting the name of a host on the network into a network address, such as an IP address. Name resolution must be performed to establish communication over a network.

Implementation

When a computer running Windows NT, Windows 2000, Windows XP, or Windows .NET Server is configured as an H-node machine, it first tries to use a NetBIOS name server to resolve names of other hosts on the network; this is the way a P-node machine functions. A server running Windows NT, Windows 2000, Windows XP, or Windows .NET Server with the Windows Internet Name Service (WINS) configured on it is the typical example of a NetBIOS name server. If name resolution fails this way (for example, if all WINS servers are down), an H-node machine tries to use broadcasts to resolve the names of the hosts; this is the way a B-node machine functions. H-node is thus a combination of P-node and B-node (in that order), which explains the origin of the term H-node : H stands for hybrid.

If no WINS server is on a network, machines running Windows NT, Windows 2000, Windows XP, and Windows .NET Server automatically configure themselves as B-nodes. If at least one WINS server is on the network, however, the systems default to H-node.

See Also B-node ,M-node ,NetBIOS name resolution ,P-node

home area network (HAN)

A network of several computers at a user's home.

Overview

With the rapid growth in the PC market, an estimated 20 percent of homes in the United States that have one PC have at least one additional PC. As a result, ordinary PC users are becoming interested in networking their home PCs to use them for purposes such as

Implementation

A variety of different technologies can be used for deploying home area networks (HANs). Examples include the following:

Marketplace

The number of vendors offering Ethernet kits for home networking is legion. Vendors for phone line networking kits include 3Com Corporation and Intel Corporation. In the wireless networking market, Proxim offers a product called Symphony that uses PCI or PCM cards with antennae attached and no central access point. Intel also offers a popular wireless solution called AnyPoint that features plug-and-play USB connectivity.

See Also 10BaseT ,802.11b ,Ethernet

home folder

Also called home directory, a user's private folder for storing personal files.

Overview

Home folders for users are usually centrally located on a network server for the following reasons:

In Microsoft Windows 2000 and Windows XP, home folders are not part of the user profile of roaming users, so they must be specified. Use the Active Directory Users and Computers console to assign home folder locations. Open the property sheet for the user and specify the location of the user's home folder on the Profile tab.

HomeRF

A wireless networking specification targeted toward consumers.

Overview

HomeRF is a wireless networking technology that supports both voice and data transmission. HomeRF is designed for consumer applications such as wireless home networks, communication between Personal Digital Assistants (PDAs) and laptops, and similar uses.

HomeRF is based on a specification called RangeLan2 developed by Proxim. It operates in the same 2.4 gigahertz (GHz) unlicensed Industrial, Scientific, and Medical (ISM) band of the electromagnetic spectrum used by 802.11b and Bluetooth wireless networking technologies and by devices like microwave ovens. HomeRF is less costly than these competing technologies due to the simplicity of its design and operation.

Implementation

The original HomeRF specification supports networking of PCs and other devices at data transfer rates up to 1.6 megabits per second (Mbps). In April 2001 the HomeRF Working Group released the HomeRF 2 specification, which boosts speeds to 10 Mbps. The Working Group aims at releasing a HomeRF 3 standard in 2002 that will push data rates up to 22 Mbps.

In contrast to the competing 802.11 wireless networking technologies that use direct sequence spread spectrum (DSSS) radio transmission, HomeRF instead employs frequency hopping spread spectrum (FHSS) transmission. FHSS employs a signal that "hops" over a number of different frequencies to find one that is clear for transmission and provides better resistance to electromagnetic interference (EMI) than DSSS. Voice transmission is supported using time division multiple access (TDMA), but data uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

HomeRF is deployed by using a central device called a home gateway, which enables communications between various types of devices including PCs, display pads, Web appliances, PDAs, stereos, home automation devices, and cordless phones. HomeRF uses a new protocol called Shared Wireless Access Protocol (SWAP) to enable its home networking features. Using SWAP, a user can do things like

Marketplace

HomeRF products are available from a number of vendors, including Cayman Systems, Compaq Computer Corporation, Intel Corporation, Motorola, and Proxim. Intel's AnyPoint wireless networking platform supports HomeRF networking and is a popular choice.

Issues

Proponents of 802.11 have criticized HomeRF because it can potentially interfere with 802.11-based wireless communications. To a DSSS 802.11 receiver, FHSS signals produced by a HomeRF transmitter appear as random noise that can disrupt communications channels. Proponents of HomeRF often respond by saying that their product is targeted to a different market, the home networking market, than is 802.11, which is aimed mainly at the enterprise. They also point out that HomeRF fills a niche for combined data-voice wireless networking that 802.11, which is strictly data-oriented, has not filled. The Federal Communications Commission (FCC) has tried to resolve these concerns by limiting HomeRF hardware to power transmission under 125 milliwatts. This limits the range of HomeRF transmission to about 150 feet (45 meters) and makes it less likely to interfere with a neighboring 802.11 network.

Market prospects for HomeRF look good, but it has to play catch up to 802.11, which already has a large installed base worldwide. Intel is one company that already has home network products available in the consumer market that are based on the HomeRF standard.

For More Information

Visit the HomeRF Working Group at www.homerf.org

See Also 802.11 ,Bluetooth ,wireless networking

homogeneous network

A network that uses a single network architecture and operating system.

Overview

Homogeneous networks are the simplest networks to administer, but most corporate networks are heterogeneous networks for the following reasons:

A simple example of a homogenous network would be an Ethernet local area network (LAN) in which all machines are running Microsoft Windows 2000. Another example would be an enterprise-level messaging system based solely on Microsoft Exchange Server with no other mail systems used.

See Also heterogeneous network

honey pot

A dummy server used to distract hackers from real targets.

Honey pot. Two ways of setting up honey pots to attract and catch hackers.

Overview

A honey pot emulates a real server, with the goal of drawing hackers away from your actual production servers. For example, you could set up a dummy Web server, mail server, or authentication server as a honey pot. Honey pots should include

Implementation

There are several ways to deploy honey pots on the perimeter of your corporate network:

Honey pots require resources to be dedicated to them and are essentially useless unless you have the time and personnel to analyze their intrusion logs and respond to their alerts. In other words, a honey pot that is set up and then ignored is worse than no honey pot at all. Honey pots are designed to provide administrators with information not just for detecting attacks, but also for tracking down those performing the attacks, but this requires skill and time to accomplish.

See Also firewall ,router ,security

hood

Sometimes called a boot, the protective enclosure at the ends of cabling that houses the pins.

Overview

The hood protects the contacts between the cable's wires and the pins in the enclosed connector. The term hood is usually applied to serial cables for serial transmission interfaces such as RS-232 and V.35. The RJ-45 termination of unshielded twisted-pair (UTP) cabling and the SC termination of fiber-optic cabling are simply called connectors or jacks. Hoods are generally made of metal or plastic. Metal hoods are used on shielded cabling to provide shielding against electromagnetic interference (EMI) at the cable ends. Removable metal hoods are also used for running cable through tight spaces, such as conduits, or for repinning connections. Plastic hoods, which are less expensive, are used primarily on unshielded cabling. Molded plastic hoods are also used to provide durable, tamper-proof housings for pins.

Hood. An example of a hood.

hop count

The logical distance between two networks based on the number of routers that must be traversed by packets sent between them.

Overview

In Transmission Control Protocol/Internet Protocol (TCP/IP) internetworking, the number of hops between two hosts would be the number of routers that an Internet Protocol (IP) packet would have to pass through in order to reach its destination.

Hop count. Two hosts that are three hops apart on an internetwork.

The illustration shows a network path that is three hops long. As the packet travels from source to destination, the packet's header maintains information about the "hop count" (the number of hops traversed). This information is stored as a Time to Live (TTL) parameter within each packet that typically starts with a value of 128 and is decremented by 1 at each router (that is, after each hop). If router congestion delays the packet at a router, the TTL might be decremented by more than 1 to indicate this. If the TTL is decremented to 0 before the packet reaches its ultimate destination, the next router drops the packet and retransmission is required from the source host.

Uses

Hop counts are used by dynamic routers to determine the best route for forwarding data across a large internetwork. The route that has the smallest total number of hops is generally the best route for sending the data. Hop counts are also used to prevent packets from endlessly circulating around an internetwork by having packets dropped (discarded) once they reach a certain maximum hop count (that is, once the TTL decrements to zero).

Notes

You can use the tracert command in Microsoft Windows 2000 to watch the hop count decrease as an IP packet traverses an internetwork toward its destination.

See Also routing

horizontal cabling

In premise cabling, any cabling that is used to connect a floor's wiring closet to wall plates in the work areas to provide local area network (LAN) drops for connecting users' computers to the network.

Overview

Horizontal cabling is usually installed in a star topology that connects each work area to the wiring closet, as shown in the illustration. Four-pair 100-ohm unshielded twisted-pair (UTP) cabling (Category 5 [Cat5] cabling or enhanced Category 5 [Cat5e] cabling) is usually recommended for new installations because it supports both voice and high-speed data transmission. To comply with Electronic Industries Association/Telecommunications Industries Association (EIA/TIA) wiring standards, individual cables should be limited to 295 feet (90 meters) in length between the wall plate in the work area and the patch panels in the wiring closet. Patch cords for connecting the patch panel to hubs and switches in the wiring closet should be no longer than 23 feet (7 meters) total, with a maximum of two patch cords per line, neither of which can exceed 19.7 feet (6 meters) in length. Cables connecting users' computers to wall plates should be limited to 9.8 feet (3 meters) in length.

Horizontal cabling. Deploying horizontal cabling to connect a work area to a wiring closet on the same floor.

Horizontal cabling is most easily installed during construction or renovation of the building because proper installation might require opening false ceilings or walls. If this is not feasible, installing external cable trays and conduits might be the best solution because loose cables on the floor pose a hazard and should be avoided at all costs.

Notes

Avoid installing cables near motors, generators, transformers, or power lines in order to minimize electromagnetic interference (EMI). Keep cables away from photocopying machines and elevators because these machines generate a lot of EMI. If you anticipate increased bandwidth needs in the near future, use multimode fiber-optic cabling instead of unshielded twisted pair (UTP) cabling for horizontal cabling. You probably want to install two four-pair UTP cables in each work area, one for voice and the other for data transmission. Be sure to use the right kind of wall plates (RJ-11 for voice and RJ-45 for data).

See Also cabling ,premise cabling ,structured wiring

host

Any device on a Transmission Control Protocol/Internet Protocol (TCP/IP) network that has an Internet Protocol (IP) address.

Overview

Examples of hosts include servers, workstations, network-interface print devices, and routers. The terms node and host are often used interchangeably in this regard. Sometimes the term host specifically means a device on a TCP/IP network that can both receive data and initiate contact with other devices. For example, a computer configured as a Simple Mail Transfer Protocol (SMTP) host receives e-mail messages and forwards them to their destination.

Notes

In mainframe computing environments, a host is a mainframe computer that is accessed by users through remote terminals.

See Also IP address ,Transmission Control Protocol/Internet Protocol (TCP/IP)

host bus adapter (HBA)

A device that converts Peripheral Component Interconnect (PCI) bus signals into Small Computer System Interface (SCSI) or Fibre Channel format.

Overview

Host bus adapters (HBAs) have traditionally been SCSI-based devices used in Storage Area Networks (SANs). They are also used in Fibre Channel SANs to provide connectivity with Gigabit Ethernet (GbE) networks. The Storage Network Industry Association is developing a proposed set of standard application programming interfaces (APIs) for HBAs to ensure interoperability between products from different vendors. With the emergence of the new Infiniband bus standard, things are likely to change and host bus adapters may decline in importance.

Marketplace

A number of vendors produce host bus adapters for storage networking, including Emulex Corporation, whose LightPulse and Giganet Fibre Channel PCI host bus adapter is a popular choice for enterprise SANs. One new development in this market is the recent marriage of high-speed switching technology with host bus adapters, represented by companies such as Qlogic Corporation.

See Also Fibre Channel ,Infiniband (IB) ,storage area network (SAN)

Host Data Replicator

A Microsoft Corporation database replication tool that copies data from legacy DB2 database tables on an IBM mainframe to Microsoft SQL Server database tables.

Overview

The Host Data Replicator consists of a data replicator service implemented as a service in Microsoft Windows NT and Windows 2000 plus an administrative tool called the Data Replication Manager. The Host Data Replicator performs the replication by taking a snapshot of the entire source table. It then either copies the entire snapshot to the target database table, overwriting any records in the existing target table, or appends the information to the target table, depending on how it is configured. The Host Data Replicator can also replicate SQL Server tables back to a DB2 database. The Host Data Replicator can copy data either on demand or according to a predefined schedule.

The Host Data Replicator is supported by Microsoft SNA Server version 3 and later and Microsoft SQL Server version 6.5 and later.

host header names

A feature of Hypertext Transfer Protocol version 1 (HTTP/1.1) that enables multiple Web sites to be hosted on the same Web server.

Overview

Host headers are supported by Microsoft Internet Information Services (IIS) and allow multiple Web sites having the same Internet Protocol (IP) address and HTTP port number to be hosted on the same Web server. Web browsers such as Microsoft Internet Explorer also support host header names and can access such IIS Web sites seamlessly. Web browsers that support host header names include Internet Explorer 3 and later or Netscape Navigator 3 and later.

Implementation

To configure multiple Web sites on an IIS server to share one IP address and use host header names for client access to them, perform the following steps:

  1. Open the property sheet for each virtual server in Internet Services Manager.

  2. Select the Web Site tab and specify the IP address and port number (usually port 80), or select All Unassigned if your server has only one IP address.

  3. Click the Advanced button, select the identity you want to use from the Multiple Identities For This Web Site list, and add a host header name. Note that each identity can have only one host header name, but each virtual server can have multiple identities.

  4. Register the host header name with your Domain Name System (DNS) server or Internet service provider (ISP).

Once the IIS server and DNS name resolution environments are configured properly, a Web browser that supports host header names, such as Internet Explorer 4 and later, can use these names to access different virtual servers that have the same IP address and port number and are located on the same machine running IIS. The browser makes an HTTP 1.1-compliant GET request, which contains in its header the host header name being requested.

Notes

IIS 4 and later also support host header names for older noncompliant browsers through the mechanism of cookies. For very old browsers that do not support cookies, a tool in the IIS Resource Kit called the Cookie Munger enables these browsers to access such sites on IIS 4 as well.

Do not create host header names for the Default Web Site because this might affect other services installed on the server that expect the Default Web Site to have no host header names.

See Also Hypertext Transfer Protocol (HTTP) ,IP address ,Web server

host ID

The portion of an Internet Protocol (IP) address that uniquely identifies a host on a given Transmission Control Protocol/Internet Protocol (TCP/IP) network.

Overview

You can determine the host ID from a host's IP address by logically NANDing the binary form of the IP address with the binary form of the subnet mask for the network. The remaining part of an IP address is the network ID, which specifies the network to which the host belongs.

For example, if a host has an IP address of 172.16.8.55 on a network with a subnet mask of 255.255.0.0 (the default subnet mask), the host ID is 0.0.8.55 or simply 8.55. The host ID uniquely identifies the host but only within the boundary of the network ID for the network on which the host is installed.

See Also IP address ,network ID

Host Integration Server

Microsoft Corporation's gateway and application integration platform.

Overview

Microsoft Host Integration Server 2000 is the successor to Microsoft's previous BackOffice gateway platform, SNA Server. Host Integration Server is part of the new .NET Enterprise Server platform and provides access to applications and data stores on IBM mainframe and AS/400 platforms. Host Integration Server supports network interoperability between Microsoft Windows 2000 client/server networks and Systems Network Architecture (SNA)-based mainframe computing platforms. Host Integration Server helps enterprises maximize their existing investments in legacy mainframe systems while leveraging the power of the Windows 2000 Server platform.

Features of Host Integration Server include

For More Information

Find out more about Host Integration Server at www.microsoft.com/hiserver.

See Also .NET Enterprise Servers

host name

An alias given to a computer on a Transmission Control Protocol/Internet Protocol (TCP/IP) network to identify it on the network.

Overview

Host names are a friendlier way of identifying TCP/IP hosts than Internet Protocol (IP) addresses. Host names can be resolved into IP addresses by host name resolution using either a Domain Name System (DNS) server or the hosts file. Host names can include the characters a-z, A-Z, 0-9, period, and dash (-). To ensure full compatibility with DNS, do not use any other special characters in host names.

Notes

To find out the host name of a computer running any version of Microsoft Windows, type Hostname at the command prompt.

See Also Domain Name System (DNS) ,

host name resolution

The process by which a host determines the Internet Protocol (IP) address of another host on a Transmission Control Protocol/Internet Protocol (TCP/IP) network given its host name or fully qualified domain name (FQDN).

Overview

Suppose you go to the command prompt of a machine running Microsoft Windows 2000 and type ping followed by a host name or FQDN of another host on the network. The host name or FQDN of the target host must first be resolved into its IP address before the TCP/IP utility ping can occur. This process is called host name resolution.

A number of different methods can be used to perform host name resolution. The following table shows the order in which these are attempted on a Microsoft network. The methods are tried in succession until the host name is resolved into its IP address or until name resolution finally fails. Some methods will not be available-for example, if there is no Domain Name System (DNS) server or NetBIOS Name Server (NBNS) on the network.

Host Name Resolution Methods in the Order Applied

Host Name Resolution Methods

Comments

Check whether the target host is the local host.

The local host knows its own host name!

Check local hosts file.

This check is performed only if a hosts file has been configured.

Contact DNS server.

This check is performed only if the DNS tab of the TCP/IP property sheet has a DNS server specified on it. The local host tries again at intervals of 5, 10, 20, and 40 seconds.

Check local NetBIOS name cache. (Unique to Microsoft networks.)

The cache contains recently resolved NetBIOS names. (On Microsoft networks, NetBIOS names and host names are usually the same.)

Contact NBNS. (Unique to Microsoft networks.)

This check is performed if NBNS has been configured by creating a Windows Internet Name Service (WINS) record within the DNS database. On a Microsoft network, this is usually a WINS server. The local host tries three times to contact the WINS server and then tries the secondary WINS server three times.

Perform local broadcast. (Unique to Microsoft networks.)

Local host broadcasts a NetBIOS name query request packet three times.

Check local lmhosts file. (Unique to Microsoft networks.)

This check is performed if a lmhosts file has been configured.

If all methods fail, an error message states that the computer could not be found on the network.

Notes

There is a separate series of steps for attempting to resolve NetBIOS names on a network that uses WINS. For more information, see the entry "NetBIOS name resolution" elsewhere in this book.

See Also Domain Name System (DNS) ,

host routing

The routing process that occurs when a host (computer) on a network forwards a packet to a destination host on the network.

Overview

Host routing is different from router routing, which is what happens when a router receives a packet that needs to be forwarded to a destination host. Host routing essentially involves a simple decision: should the packet be forwarded directly to its destination host, or should it be forwarded to a router? The host makes this decision by comparing the packet's destination address with entries in its internal routing table.

To perform host routing, the host must first obtain the internetwork address of the destination host using some form of name resolution. For example, in order for a host to forward a packet to a remote host called northwind.microsoft.com, it could first use the Domain Name System (DNS) to obtain the Internet Protocol (IP) address of the remote host. The host then compares this address with entries in its internal routing table to determine whether the destination has a local or remote network address. If the address is a local network address, the host forwards the packet directly to its destination using the physical layer or physical address of the remote host. This process is known as direct delivery. On Transmission Control Protocol/Internet Protocol (TCP/IP) networks, the physical address of a host is its MAC address, which is obtained by using the Address Resolution Protocol (ARP).

If, however, the host determines that the destination host has a remote network address (that is, the destination host is on a different network than the sending host), the host forwards the packet to a nearby router after first obtaining the physical address of the near-side interface of the router. This process is known as indirect delivery. It is the router's responsibility to ensure that the packet is forwarded toward its destination, although in a typical internetwork the destination might be several hops away, in which case the router's responsibility extends only to the next hop on the path.

See Also Address Resolution Protocol (ARP) , Domain Name System (DNS) , IP address, router, routing

hosts file

A text file that provides a local method for name resolution of Internet Protocol (IP) hosts.

Overview

A hosts file can be used to resolve a host name or fully qualified domain name (FQDN) into its associated IP address. Hosts files are a local alternative to using distributed Domain Name System (DNS) name servers for performing name resolution on Transmission Control Protocol/Internet Protocol (TCP/IP) networks. Hosts files are used mainly on small networks where maintaining a DNS server is impractical or as a backup in case no name servers are available to perform name lookups.

History

Hosts files predated the DNS as a name resolution method. During the 1970s, when the ARPANET, the Internet's precursor, consisted of only a few hundred different hosts, using hosts files was a relatively painless way of enabling name resolution to be performed. When someone added a new host to the network, they would e-mail the Network Information Center (NIC), which would update the information in their master hosts file stored on a server called SRI-NIC. Users around the ARPANET typically checked this server a few times each week to see if an updated hosts file was updated and if so would download it to their own hosts using File Transfer Protocol (FTP).

When the ARPANET migrated to TCP/IP in the early 1980s, the number of hosts exploded and a new method of name resolution had to be found. This new method had to overcome the problems with hosts: it scaled poorly, had no automated process for registering new hosts, and had no way of pushing out updates from the master hosts file server. The result was the development of DNS, a distributed name resolution service that is still in use and forms the backbone of communications on the Internet.

Implementation

Hosts files are text files that consist of a series of FQDN-to-IP address mappings, one per line. Each line in the hosts file contains the IP address of a host followed by the FQDN of the host, followed by an optional comment prefixed with a pound sign (#). Hosts files should contain mappings for hosts on both local and remote networks. Mappings can consist of an IP address and one or more host names (aliases). If you are using hosts files to resolve host names on a network, each computer on the network should have a hosts file.

On a UNIX system, the hosts table is found in /etc/hosts, and on Microsoft Windows 2000, Windows XP, and Windows .NET Server systems, it is located in the %SystemRoot%\system32\ drivers\etc\Hosts directory.

Notes

Although DNS has largely eliminated the need for maintaining hosts files, they are still useful in two scenarios:

Notes

Place the host names that need to be most frequently resolved near the top of the hosts file because the file is parsed linearly from the beginning.

See Also lmhosts file ,Networks file ,protocol file ,services file

hot

An adjective referring to actions that can be performed on systems or hardware devices while the power for the device is still turned on or while the device still has a live network connection or telecommunications link.

Overview

"Hot" networking and system technologies are used in

Examples of disk storage systems that use "hot" technologies include the following:

For example, when you buy a hardware RAID unit, you might want to be sure that it has hot-rebuild, hot-spare, and hot-swap capabilities.

hotfix

A fix for a bug in an application or operating system.

Overview

Hotfix is a term used by Microsoft Corporation to describe a software patch issued to correct a problem with a version of Microsoft Windows or a Microsoft application. For the BackOffice platform, hotfixes are now called Quick Fix Engineering (QFE).

Generally, you should apply a hotfix (QFE) only if you are experiencing a problem related to an issue the hotfix is designed to correct. In other words, if you are not experiencing the problem described in the documentation for a hotfix, it is generally best to wait for the next release of a service pack (a group of hotfixes applied in one shot) instead of applying the hotfix individually. This is especially true of hotfixes that have not been "regression tested," that is, whose interaction with other software is currently uncertain.

Some hotfixes are made available publicly through Microsoft's File Transfer Protocol (FTP) site. Others are only available from Microsoft Product Support Services (PSS) and require a support agreement to be in place before downloading them.

For More Information

You can find current hotfixes for Microsoft products at ftp.microsoft.com.

See Also service pack

Hot Standby Router Protocol (HSRP)

A proprietary Cisco technology for implementing fault-tolerant routing.

Overview

Hot Standby Router Protocol (HSRP) was developed by Cisco Systems as a way of providing fault tolerance for routed internetworks. Normally, when a router goes down, routing protocols communicate this fact to all the routers on the network, which then reconfigure their routing tables to select alternate routes to avoid the downed router. The problem is that the process of convergence, the updating of all routing tables on the network, is slow and can take some time to complete. HSRP was designed to work around this problem of slow convergence by allowing a standby router take over from a router that has gone down and fill its role in a manner completely transparent to hosts on the network.

HSRP is outlined in RFC 2281 but is a proprietary standard and not an Internet Engineering Task Force (IETF) standard. The IETF has proposed its own vendor-independent fault-tolerant routing technology called Virtual Router Redundancy Protocol (VRRP), which is documented in RFC 2338.

Implementation

HSRP works by creating virtual groups that consist of two routers, an active router and a standby router. HSRP creates one virtual router in place of each pair of actual routers, and hosts need only to have their gateway address pointed to the virtual router. When a host forwards an Internet Protocol (IP) packet to its default gateway (the virtual router), it is actually handled by the active router. Should the active router go down, the standby router steps in and continues to forward packets it receives. To IP hosts on the network, the virtual router acts like a real router and everything works transparently whether the active or standby router actually does the forwarding.

Hot Standby Router Protocol (HSRP). How HSRP works.

HSRP allows for complex arrangements of virtual routers. For example, actual routers can belong to more than one virtual group, and there can be up to 255 virtual groups on the network. HSRP can also be used to provide backup default gateways for hosts capable of addressing only a single default gateway.

See Also default gateway ,router ,routing

HPFS

Stands for High Performance File System, a file system designed for version 1.2 of the Microsoft/IBM OS/2 operating system as a successor to the file allocation table (FAT) file system used by MS-DOS.

See Also High Performance File System (HPFS)

HP-UX

Hewlett-Packard's version of the UNIX operating system platform.

Overview

HP-UX is the only version of UNIX that supports application development on UNIX, Linux, and Microsoft Windows platforms. The most recent version HP-UX 11i includes features like

For More Information

Visit Hewlett-Packard online at www.hp.com

See Also Linux ,UNIX

HSCSD

Stands for High-Speed Circuit Switched Data, an interim upgrade for Global System for Mobile Communications (GSM) cellular communications systems.

See Also High-Speed Circuit Switched Data (HSCSD)

HSM

Stands for Hierarchical Storage Management, a data storage system for archiving infrequently needed data while still making it easily available.

See Also Hierarchical Storage Management (HSM)

HSRP

Stands for Hot Standby Router Protocol, a proprietary Cisco protocol for implementing fault-tolerant routing.

See Also Hot Standby Router Protocol (HSRP)

HSSI

Stands for High-Speed Serial Interface, a serial interface that supports speeds up to 52 megabits per second (Mbps).

See Also High-Speed Serial Interface (HSSI)

HSTR

Stands for High-Speed Token Ring, a 100 megabits per second (Mbps) version of Token Ring that never got off the ground.

See Also High-Speed Token Ring (HSTR)

HTML

Stands for Hypertext Markup Language, a formatting or markup language used to create documents for the World Wide Web (WWW).

See Also Hypertext Markup Language (HTML)

HTMLA

Stands for HTML-based administration, a method for remotely administering Microsoft Internet Information Services (IIS) Web servers.

See Also HTML-based administration (HTMLA)

HTML-based administration (HTMLA)

A method for remotely administering Microsoft Internet Information Services (IIS) Web servers.

Overview

HTML-based administration (HTMLA) allows administrators to manage IIS Web servers using a standard Web browser such as Microsoft Internet Explorer as the client-side interface. IIS supports administration using HTMLA from any Web browser that supports the use of frames and JScript.

HTMLA is based on Microsoft Corporation's Component Object Model (COM) and Distributed Component Object Model (DCOM) programming architectures. The purpose of HTMLA is to simplify remote management of network resources and services by requiring that client machines used for remote administration of Web servers need only be running a standard Web browser. Most of today's networking products are moving toward one form or another of Web-based administration, and Microsoft's HTMLA feature of IIS was a pioneer in this area.

See Also Component Object Model (COM) ,Distributed Component Object Model (DCOM) ,Web server

HTML Extension (HTX)

A text file with the extension .htx that is used by Indexing Service in Microsoft Windows 2000, Windows XP, and Windows .NET Server.

Overview

HTML Extension (HTX) files are used to format the result set issued by Index Server in response to a catalog search query. These catalog search queries are issued using an Internet Data Query (IDQ) file. HTX files are also used by the Internet Database Connector (IDC) to format the result set of a database query.

The .htx file formats the result set of a query into a Hypertext Markup Language (HTML) page that the user who issued the query can read and understand. The .htx file acts as a template for formatting the result set and consists of HTML statements with additional tags and specific read-only variables such as

HTTP

Stands for Hypertext Transfer Protocol, a protocol that defines how Web browsers communicate and download information from Web servers.

See Also Hypertext Transfer Protocol (HTTP)

HTTP Keep-Alives

An enhanced version of Hypertext Transfer Protocol (HTTP)-persistent connections supported by Microsoft Internet Information Services (IIS).

Overview

HTTP Keep-Alives allow a client Web browser to keep connections open with the Web server instead of closing them after the request has been answered and reopening them for each new Hypertext Transfer Protocol (HTTP) request, which consumes system resources. For this feature to work, however, both the client Web browser and the Web server must support HTTP Keep-Alives. Web browsers that support this feature include Microsoft Internet Explorer version 2 and later and Netscape Navigator 2 and later.

Notes

HTTP Keep-Alives are enabled by default in IIS. They are not the same as Transmission Control Protocol (TCP) Keep-Alives, which are periodic packets sent between machines to determine whether an idle connection is still active.

HTTPS

A secure version of Hypertext Transfer Protocol (HTTP).

Overview

Hypertext Transfer Protocol Secure (HTTPS) is essentially a combination of HTTP and the Secure Sockets Layer (SSL) protocol, and the designation HTTPS comes from the combination of the words HTTP and secure . HTTPS was originally developed by Netscape to enable the secure transmission of Web content over the Internet.

HTTPS is based on a public key cryptography system and allows information transmitted over the Internet to be encrypted for greater security. To run HTTPS on a Web server, you must first install a digital certificate on the server. Web browsers then connect to the server by providing a Uniform Resource Locator (URL) that begins with the prefix https:// rather than http://, which is usually used. HTTPS uses the TCP well-known port number 443 instead of port 80, which is used by standard HTTP.

Notes

Note that HTTPS is not the same as Secure Hypertext Transfer Protocol (S-HTTP), another protocol for secure HTTP transmission.

See Also Hypertext Transfer Protocol (HTTP) ,Secure Sockets Layer (SSL)

HTTP status codes

Codes that Hypertext Transfer Protocol (HTTP) servers return in response to requests from HTTP clients.

Overview

HTTP status codes are three-digit codes that Web servers return in response to HTTP requests from clients. They are also known as HTTP error codes because most of them signify that some sort of error has occurred. An HTTP status code is one of the first pieces of information returned by a Web server in response to a request from a Web browser, and it informs the browser of the status of the request-whether the request was successful or not, and if not, why. When the server cannot satisfy the client's request for some reason, a status code is returned to the client, which displays it for purposes of troubleshooting the problem. The client typically displays a message along with the status code to help the user understand the nature of the problem.

HTTP status codes are grouped into different categories by their first digit, as shown in the following table.

HTTP Status Codes by Category

Status Code

Category

Description

1xx

Informational

An acknowledgement message from the server.

2xx

Success

The requested action was understood and completed.

3xx

Redirection

Further action must be taken to fulfill the request.

4xx

Client error

The request could not be performed because of client error.

5xx

Server error

The request could not be performed because of server error.

Examples

Some common status codes that are fairly self- explanatory include the following:

Notes

Web servers running Microsoft Internet Information Services (IIS) allow administrators to create customized HTTP status code pages that can be considerably more informative to users than traditional status codes such as "403 Forbidden."

See Also Hypertext Transfer Protocol (HTTP) ,Web server

HTX

Stands for HTML Extension, a text file with the extension .htx that is used by Indexing Service in Windows 2000, Windows XP, and Windows .NET Server.

See Also HTML Extension (HTX)

hub

A device used to connect shared Ethernet segments into a single local area network (LAN).

Overview

Hubs are the foundation of traditional 10/100BaseT shared Ethernet networks. They can be used to connect several dozen computers together to form a workgroup LAN, or they can be cascaded to create larger LANs of up to a hundred computers or more. Hubs typically have 4, 8, 12, or 24 ports for connecting stations using unshielded twisted pair (UTP) cabling terminated with RJ-45 connectors.

When a hub receives a signal from a station connected to a port, it repeats the signals to all other stations connected to the hub's ports (hence the alternative name "multiport repeater" is sometimes used to describe a hub). From a logical cabling point of view, stations wired into a hub form a star topology with the hub at the center and the stations at the points of the star. The physical topology of a hub, however, is bus topology, and all stations connected to a hub share the same bus connection.

Hubs were originally passive devices that simply repeated signals, but today's hubs are active hubs in which the signal received from one port is regenerated (amplified) and retransmitted to the other ports on the hub. Hubs typically have one or more uplink ports that are used for connecting the hub to other hubs in a cascaded star topology. You connect an uplink port on one hub to a regular port on another hub using a standard UTP cable. Alternatively, you can connect two hubs using the uplink ports by using a crossover cable.

Hubs generally have various light-emitting diode (LED) indicator lights to indicate the status of each port, link status, collisions, and so on. Hubs with several different types of LAN connectors such as RJ-45, BNC, and AUI are commonly called combo hubs.

Types

There are numerous types of hubs for various specialized uses. These include the following:

Prospects

Hubs have largely been supplanted by switches in today's enterprise-level Ethernet networks. Although a shared hub receives frames and blindly forwards them to all other attached stations, an Ethernet switch forms a temporary internal pathway to transmit the frame directly to the port belonging to the destination station. Shared hubs were still popular in the mid-1990s when price per port for switches was high, but with switch prices now comparable to those of hubs, any new enterprise deployment should consist mainly of switches instead of hubs. About the only remaining places where hubs can sensibly be deployed are in small workgroup scenarios and for home networks. In most other cases shared hubs usually represent a bad investment.

Notes

A device called a wiring concentrator or Multistation Access Unit (MAU) is similar to a hub but is used in Token Ring networks. Another device called a patch panel is used to organize UTP cabling in structured wiring implementations. Do not confuse either of these devices with a hub.

See Also 10BaseT ,100BaseT ,Ethernet ,Ethernet switch

HybridAuth

An authentication protocol proposed by the Internet Engineering Task Force (IETF) for Virtual Private Networking (VPN).

Overview

To deploy a secure VPN, companies must implement a public key infrastructure (PKI) for issuing and managing digital certificates. But deploying a PKI is complex, and the IETF has proposed a new protocol called HybridAuth to simplify the process.

HybridAuth is an extension to the Internet Key Exchange (IKE) protocol that defines how a user's credentials are exchanged over secure IPsec tunnels. IKE supports tunneling of two forms of authentication: digital certificates and preshared keys. HybridAuth enables digital certificates to be used asymmetrically between users and authentication servers and is simpler to deploy than a full-scale PKI solution.

See Also digital certificate ,public key infrastructure (PKI) ,virtual private network (VPN)

HyperTerminal

A communication utility included with Microsoft Windows that provides terminal access to remote computers using a modem.

Overview

HyperTerminal can be used to send and receive files between a local and remote computer over a modem and to connect to remote computer bulletin board systems. Network administrators can also use HyperTerminal to remotely connect to routers, switches, and other devices that support VT100 terminal emulation, and to enter text commands for configuring the device.

If you want to access files and printers on a remote computer running Windows 2000, Windows XP, or Windows .NET Server using a modem, use Network and Dial-Up Connections instead of HyperTerminal.

See Also router

Hypertext Markup Language (HTML)

A formatting or markup language used to create documents for the World Wide Web (WWW).

Overview

Hypertext Markup Language (HTML) is the standard formatting language used to publish information on Web servers for delivery to Web browsers over the Internet. HTML allows linked sets of documents (that is, hypertext) to be created, stored, and accessed from Web servers using Uniform Resource Locators (URLs). HTML is simple to learn and was a key enabler in the explosion of the Internet in the 1990s.

History

HTML was created together with Hypertext Transfer Protocol (HTTP) in 1991 by Tim Berners-Lee as a means of disseminating information more easily among physicists and other scientists at CERN, the European Laboratory for Particle Physics. HTML was designed as a simplified version of Standardized Generalized Markup Language (SGML), a much more complicated formatting language that was developed previously but never widely implemented. HTML was created by combining the concept of hypertext, a concept created in the 1940s for linking documents together, with URLs, a method of uniquely naming hypertext documents so they could be easily accessed regardless of which servers they reside on. HTTP provides the additional protocol for communications between clients (called HTTP clients or Web browsers) and the servers (called Web servers or HTTP servers) on which HTML documents are stored.

In 1993, Marc Andreesen developed Mosaic at the National Center for Supercomputing Applications (NCSA). Mosaic was the first Web browser that supported documents containing both text and graphics (previous Web browsers supported only text documents). Andreesen left the NCSA to cofound Netscape Communications (now just Netscape), which produced the popular Netscape Navigator browser. Meanwhile, Microsoft Corporation licensed code from Mosaic through the commercial outlet Spyglass and developed its own popular browser, Microsoft Internet Explorer. Both Netscape and Microsoft pushed the evolution of HTML by introducing their own proprietary extension tags, the first being the <font> tag from Netscape. The result was that HTML rapidly developed through several versions, including

To steer the evolution of HTML and prevent different vendor-specific versions from emerging, Berners-Lee formed the World Wide Web Consortium (W3C) in 1994. HTML 3.2 was the first version of HTML standardized by the W3C. Current work on HTML by the W3C involves development of the XHTML standard, a version of HTML written as an Extensible Markup Language (XML) application instead of an SGML one. The complementary HTTP protocol is standardized by a different group from the W3C, namely the Internet Engineering Task Force (IETF).

HTML has gone through several versions since it was created. At the time of this writing, the current version is HTML 4. The original HTML did not provide much control over how documents were formatted-that is, how objects such as text and graphics were laid out on a page. Its original set of tags was quite limited and was intended primarily for linking documents using hyperlinks to form hypertext. As the Web grew in popularity, however, first Netscape and then Microsoft introduced their own proprietary HTML tags to provide Web developers with more control over document formatting, thus increasing the pace at which the W3C developed the HTML standard. HTML 4 includes standards for creating cascading style sheets, which provides powerful formatting capabilities for precise placement of objects on a Web page.

Implementation

HTML in its simplest form uses tags to format ASCII text documents. These tags then indicate text that should be displayed as boldface, italic, bulleted, hyperlinked, centered, and so on. HTML tags usually come in pairs, can be nested, can contain additional attributes, and are used to "mark up" the text. For example, the text "Save 50%" can be displayed on Web browsers in boldface by marking it up using the <STRONG> tag (which means "turn on the bold style") and the </STRONG> tag (which means "turn off the bold style"). The resulting HTML would look like this:

<STRONG>Save 50%</STRONG>

HTML pages can be created using a broad range of tools from simple text editors like Notepad to advanced publishing tools like Microsoft FrontPage. HTML pages are saved as files with the extension .htm or .html to indicate they contain HTML text. Once created, HTML pages can be stored on Web servers like Microsoft Windows 2000 servers running Microsoft Internet Information Services (IIS). When a Web browser such as Internet Explorer downloads an HTML page from a Web server, it interprets the tags and displays the document with the appropriate formatting.

Advantages and Disadvantages

The main advantage of HTML is its simplicity-a child can learn enough HTML to create a simple Web page. Because HTML was originally intended for exchanging information between scientists, support for formatting documents was initially limited. As the Web became more popular, new tags were introduced to make snazzy-looking documents possible. Together with style sheets, HTML documents can now be as complex in their formatting as documents produced for desktop publishing.

The main disadvantages of HTML are that it cannot easily create data hierarchies or structures, has a fixed set of tags, and cannot communicate information about the document's content, only its format. As a result, a richer language called XML has been created to allow self-describing structured documents to be created. While HTML is designed for document presentation, XML is primarily designed to facilitate the exchange of hierarchically structured information, such as between businesses in a supply chain.

Prospects

Due to its simplicity, HTML will probably live on and not be replaced by the more powerful and complex XML. Several offshoots of HTML have been developed for mobile communications platforms that use devices whose display is too small to show traditional HTML pages. Some examples of these new platforms include

For More Information

See the W3C's HTML 4.0 Specification at www.w3c.org/TR/REC-html40.

See Also Handheld Device Markup Language (HDML) ,Hypertext Transfer Protocol (HTTP) Internet Engineering Task Force (IETF), Uniform Resource Locator (URL), Wireless Markup Language (WML), World Wide Web (WWW), World Wide Web Consortium (W3C), XML

Hypertext Transfer Protocol (HTTP)

A protocol that defines how Web browsers download information from Web servers.

Overview

Hypertext Transfer Protocol (HTTP) is an application- level Transmission Control Protocol/Internet Protocol (TCP/IP) protocol first developed in 1990 as HTTP/0.9 and later standardized in 1996 by RFC 1945 as HTTP/1.0. The current version number for the protocol is HTTP/1.1 as defined by RFC 2616.

HTTP specifies the syntax for communication between HTTP clients and servers, which are defined as follows:

Implementation

HTTP is implemented as a stateless request/response protocol. That is, clients make requests to servers, which then respond to the client. The HTTP protocol is called "stateless" because each request/response session is independent.

Hypertext Transfer Protocol (HTTP). How a typical HTTP session works.

An HTTP request consists of a message containing a request method, the file requested, and information about the client's capabilities. The request message consists of a series of headers encoded in plain text. The request method identifies the nature of the request and can be any of the following (not all methods may be supported by a particular HTTP server):

An HTTP response may be either

or

The simplest example of an HTTP session is a client requesting a Web page from the server. The client sends an HTTP GET request over port 80 to the server. Other ports may be used, but port 80 is the default TCP port for HTTP requests, and Web servers typically listen to port 80 for such requests in order to fill them. When the server receives the request, it parses the headers and determines which file to return to the client. The file is then encoded using the appropriate Multipurpose Internet Mail Extensions (MIME) format and is returned as a message body with a series of headers attached. The client receives the message, parses the headers, extracts the file from the message body, and displays the downloaded Web page.

In the original HTTP/1.0 protocol, each request requires a separate TCP connection to be initiated and torn down afterward. This is slow-a Web page that has five images in it would need to be accessed using six requests, one for the text and one for each embedded image. As a result, a newer version, HTTP/1.1, was developed that allows a persistent TCP connection to be maintained so that many HTTP requests can be issued using it. This new feature is called HTTP Keep-Alives. Other features of HTTP/1.1 include

Notes

IIS supports the HTTP/1.1 version of HTTP.

HTTP's stateless nature makes it difficult to build Web applications that remember client requests across sessions. A common way of working around this issue is to use cookies, small files placed on the client by the server to store information across multiple sessions.

Do not confuse HTTP with HTML! HTTP is the protocol through which Web servers communicate with Web browsers. It is a control language for passing commands between clients and servers. HTML is Hypertext Markup Language, the language for constructing Web pages (the actual content passed from Web servers to Web clients in an HTTP request).

For More Information

Find out more about HTTP at the World Wide Web Consortium (W3C) Web site (www.w3.org).

See Also browser (Web browser) , cookie , Web server



Microsoft Encyclopedia of Networking
Microsoft Encyclopedia of Networking
ISBN: 0735613788
EAN: 2147483647
Year: 2002
Pages: 36
Authors: Mitch Tulloch, Ingrid Tulloch
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