A serial transmission standard that defines a physical interface for synchronous communication between data terminal equipment (DTE) and data communications equipment (DCE). V.35 was part of the V series standards developed by the International Telecommunication Union (ITU).
V.35 supports transmissions of up to 1.544 Mbps at distances of up to 1200 meters by using balanced lines for signaling and unbalanced lines for control. The V.35 interface is typically used to connect high-speed routers to Channel Service Unit/Data Service Units (CSU/DSUs) for communication over leased lines such as T1 lines. V.35 uses a block-shaped 34-pin block connector for DTE-to-DCE connections.
Graphic V-1.V.35. A V.35 connector.
TIP
You can sometimes use V.35-to-RS-232 gender changers to connect V.35 equipment using 25-pin (DB25) serial cables.
NOTE
Although the V.35 standard was replaced by the V10 and V11 standards in 1988, it remains one of the most popular DTE/DCE interfaces in use today for LAN/WAN connectivity.
V.90
A serial transmission standard for high-speed communication using modems. V.90, which is part of the V series standards developed by the International Telecommunication Union (ITU), evolved from the earlier x2 modem technology developed by U.S. Robotics (later bought by 3Com) and the K56flex modem technology developed by Rockwell. V.90 claims to support modem communication at a speed of 56 Kbps, but this is limited by several factors:
The top speed of 56 Kbps is achievable only for downstream traffic from the Internet service provider (ISP) via the telco central office (CO) to the customer premises. This downstream speed is achieved by using a pure digital downstream connection at the telco side, thus eliminating the quantization error that occurs during analog-to-digital conversion (ADC). The digital data path is actually 64 Kbps throughout the Public Switched Telephone Network (PSTN), but due to digital-to-analog conversion (DAC) at the CO where the digital PSTN backbone is switched to the analog local loop, information loss results, which reduces the possible speed to 56 Kbps.
The actual top downstream speed for V.90 is only 53 Kbps. This is because of Federal Communications Commission (FCC) regulations on communication over telephone lines, which mandate the maximum signal power that can be transmitted over the PSTN to prevent crosstalk from degrading signal quality.
The actual top downstream speed is less if line distances exceed 3.5 miles or if line conditions are poor. Typical downstream speeds are 40 to 50 Kbps. V.90 includes a mechanism whereby the client modem issues a request for a test tone during negotiation, which is used to gauge the line quality to determine the optimal downstream transmission speed.
Upstream speed for V.90 is limited to 33.6 Kbps and operates according to the older V.34b standard.
V.90 is possible only on telephone networks that have only one analog segment in the communication path (the local loop at the subscriber end) instead of two analog segments (at both the subscriber and ISP/CO ends).
value entry
A data entry in the Microsoft Windows registry. Value entries are contained in keys and are analogous to variables. They consist of three parts:
Name: The name of the value entry (for example, MaintainServerList)
Data type: The type of data stored in the value entry (for example, REG_SZ represents human-readable text)
Value: The actual data contained in the value entry (for example, the string “Auto”)
In Windows NT, five data types can be used in value entries. These are described in the table on the next page. Windows 2000 supports a few data types in addition to those shown in the table.
NOTE
The maximum size of a registry value in Windows NT is about 1 MB. Applications can define other data types, but the ones shown in the table are the only ones that you can display and modify using the registry editor (regedt32.exe).
Graphic V-2.90. Digital and analog segments with V.90 modems and earlier modems.
Value Entry Data Types in Windows NT
Data Type
Description
REG_BINARY
Raw binary data, usually displayed in the registry editor in hexadecimal format
REG_DWORD
Data represented by a number that is 4 bytes long and is displayed in the registry editor in binary, hexadecimal, or decimal format
REG_EXPAND_SZ
An expandable data string (text that contains a variable that is replaced when called by an application)
REG_MULTI_SZ
A multiple string, typically used for lists that are in human-readable text with entries separated by NULL characters
REG_SZ
A sequence of characters in human-readable text, typically used for descriptive information
See also key in Microsoft Windows registry, registry editor
VBScript
Stands for Visual Basic Scripting Edition, a scripting language developed by Microsoft and designed primarily for use in Web browsers such as Microsoft Internet Explorer. VBScript is a lightweight subset of the more powerful Visual Basic for Applications (VBA) programming language used in Microsoft Office and other Microsoft development platforms. VBScript omits features such as file I/O and direct access to the operating system to provide a secure scripting platform for developing Web-based applications using technologies such as Active Server Pages (ASP). Internet Explorer includes a scripting engine for interpreting and running scripts written in VBScript. VBScript can run on all versions of Microsoft Windows as well as on certain UNIX platforms.
How It Works
You cannot use VBScript to write stand-alone programs. Instead, you must embed script into standard Hypertext Markup Language (HTML) files. The script is executed when a Web browser opens the HTML file. Alternatively, ASP can use VBScript to generate client-side script on the fly.
Script within an HTML page is enclosed within <SCRIPT> … </SCRIPT> tags. For example, the following HTML segment displays a button that, when pressed, displays a message box that reads "Hello World!":
<INPUT TYPE=BUTTON VALUE="Click me" NAME="BtnHello"> <SCRIPT LANGUAGE="VBScript"> Sub BtnHello_OnClick MsgBox "Hello World!", 0, "An active document" End Sub </SCRIPT>
In this code, the <INPUT> tag creates the command button and the <SCRIPT> tag contains the script for the event handler that handles the button click.
On the Web
•
Microsoft Windows Script Technologies home page : http://msdn.microsoft.com/scripting/
See also JavaScript, JScript
vCalendar
An Internet protocol for interoperability between scheduling programs running on different platforms. vCalendar allows Personal Data Interchange (PDI) programs to exchange scheduling information over the Internet to book meetings, schedule events, and so on. The vCalendar standards define the format by which scheduling information can be exchanged over the Internet. The basic units of scheduling information are the event, which consists of a scheduled activity and its time, date, and duration, and the to-do, a work item or assignment that is delegated to an individual. vCalendar is defined in Requests for Comments (RFCs) 2445 to 2447 and is broadly supported in the e-mail and groupware industry. The current version is vCalendar 1.0.
NOTE
Microsoft Outlook 98 and later can import and export information in vCalendar format.
On the Web
•
Internet Mail Consortium’s PDI page : http://www.imc.org/pdi
vCard
An Internet protocol for exchanging business information. vCard enables users to exchange the kind of information found on business cards by using standard Internet applications such as e-mail clients and Web browsers. It can also be integrated into fax, cellular phone, pager, smart card, and other communication technologies. You can use vCard to electronically communicate information such as a user’s name, title, business, address, phone numbers, e-mail addresses, URLs (Uniform Resource Locators), company logo, photographic likeness, audio clip, and just about anything else. vCard specifies the format for encoding information for transmission over the Internet. vCard is defined in Requests for Comments (RFCs) 2425 and 2426. The current version is vCard 3.0.
NOTE
Microsoft Outlook 98 and later and Microsoft Outlook Express 5 support the vCard protocol.
On the Web
•
Internet Mail Consortium’s PDI page : http://www.imc.org/pdi
virtual circuit
A logical path between nodes in a network, typically a telecommunications network. The path is made up of discrete segments of the network that are connected using switches. The nodes on the circuit communicate as if they were directly connected using physical wires, but the switches actually establish and tear down the virtual communication path. You can change the logical paths within the network by simply reconfiguring the switches to establish new paths and tear down old ones—you need not manually reconnect wires to create new circuits.
There are two types of virtual circuits:
Permanent virtual circuits (PVCs): The switches are set up manually by the communication management station (the telco’s central office) and offer performance comparable to dedicated lines. These circuits are always on and are generally used for high-speed connectivity. PVCs are a costly solution for wide area networks (WANs) because they require telco resources (switches) to be dedicated to a particular communication circuit, whether or not that circuit is being used.
Switched virtual circuits (SVCs): The switches are set up automatically when a communication session is established. SVCs are freed up when the session is finished and can be used for establishing other communication paths. Ordinary telephone communication functions in this fashion. SVCs are generally used in WANs where backups to dedicated leased lines are required and are charged by the amount of time or traffic.
One example of WAN technology that uses virtual circuits is frame relay, which enables PVCs to be established between nodes over a public or private carrier network. Another example is X.25 networking technology.
virtual directory
A directory that appears to Web browser users as a subdirectory of a Web site’s home directory but in fact might be located in a different folder, drive, or server. Virtual directories are supported by Internet Information Server (IIS) in Microsoft Windows NT and Internet Information Services (IIS) in Microsoft Windows 2000.
For example, consider a user accessing a Web site by using the following Uniform Resource Locator (URL):
http://www.microsoft.com/otherstuff/file.htm
The directory otherstuff appears to the user to be a real subdirectory of the home directory www.microsoft.com, while in fact it could be a virtual directory that is mapped to a share on a different server on the Web provider’s network.
The advantage of using virtual directories is that content does not need to be stored only on the Web server—it can be distributed on other servers throughout the Web provider’s network. These servers can be located at secure, strategic locations for easy access by Web content developers. The disadvantage is a slight performance hit due to data being transmitted over the network.
virtual hosting
See reverse hosting
virtual LAN (VLAN)
A networking technology that allows networks to be segmented logically without having to be physically rewired.
How It Works
Traditional Ethernet networking devices such as hubs, bridges, and routers allow the creation of large internetworks that consist of physically separate “islands” of hosts. For example, each department in a building might have its own separate local area network (LAN) that was created using hubs, and these hubs might be interconnected to a main Ethernet switch in the main wiring room of the building. Using a switch instead of a hub as the main concentrator makes for a more efficient network because the network can be partitioned into several smaller collision domains. However, broadcasts sent by any host are still received by all hosts on the network, even if all of the hosts do not need to receive them. Also, if the organization of the departments changes, the hubs must be rewired to reflect the new topology of the network.
To overcome these problems, many Ethernet switches support virtual LAN (VLAN) technologies. By replacing all hubs with VLAN switches, the network administrator can create virtual network segments whose logical topology is independent of the physical topology of the wiring. Each station can be assigned a VLAN identification number (ID), and stations with the same VLAN ID (no matter what physical switch they are connected to) can act and function as though they are all on the same physical network segment. Broadcasts sent by one host are received only by hosts with the same VLAN ID. The assignment of VLAN IDs is done at the port level on the switches themselves and can be managed remotely using network management software. Moving a host to another department only requires the assignment of a different VLAN ID to the port on the switch to which the host is connected—no rewiring of patch cables is needed.
Graphic V-3.Example of a VLAN.
VLAN switches can function in different ways. They can be switched at the data-link layer (layer 2 of the Open Systems Interconnection reference model) or the network layer (layer 3), depending on the type of switching technology used. The main advantage of using VLAN technologies is that users can be grouped together according to their need for network communication, regardless of their actual physical locations. Membership in a VLAN segment (called a VLAN group) is controlled by the network management software, which allows users to be grouped according to their need for resource access and security privileges. The only disadvantage is that additional configuration is required to set up and establish the VLANs when implementing these switches.
NOTE
Many high-end Cisco Systems switches and routers support VLAN capability. A protocol called Inter-Switch Link (ISL) is used to carry traffic between switches and encapsulates Ethernet frames by adding a header of 30 bytes, which includes a 2-byte VLAN ID.
VLAN techniques are also typically used in Asynchronous Transfer Mode (ATM) networks to partition the network into smaller segments for LAN emulation (LANE).
virtual memory
A mechanism by which applications function as though the system has more RAM than it actually does. Virtual memory operates by paging unneeded code to a file on the hard drive called the swap file or page file.
How It Works
In Microsoft Windows 2000 and Windows NT, the Virtual Memory Manager maps the virtual addresses belonging to the address space of a running process to physical pages of memory in the computer. This ensures that each process has sufficient virtual memory to run efficiently and does not trespass on the memory of other processes. The Virtual Memory Manager handles paging between RAM and the page file, swapping pages by using a process called demand paging. The result is that each application has access to up to 4 GB of memory. A similar process in Microsoft Windows 95 and Windows 98 uses a comparable structure called the swap file. In Microsoft Windows 2000, you can use the System Properties property page to change the maximum size of the page file.
See also working set
virtual private network (VPN)
Generally, a technology for connecting the components and resources of one network over another. In common usage, a virtual private network (VPN) is a private corporate network whose wide area network (WAN) connections are made over a shared public network, usually the Internet. A common telecommunications carrier provides connectivity that acts like dedicated lines, but the network backbone is actually shared between all users as in a public network. VPNs are usually more cost-effective for companies than providing traditional remote access services to remote employees by using modem pools, dedicated phones lines, and toll-free numbers.
How It Works
VPNs use tunneling technologies to allow users to access private network resources through the Internet or another public network. Users enjoy the same security and features formerly available only in their private networks. Tunneling solutions are typically based on Microsoft’s Point-to-Point Tunneling Protocol (PPTP) or Cisco Systems’ Layer 2 Tunneling Protocol (L2TP), depending on resources and requirements.
You can use Internet Connection Services for Microsoft Remote Access Service (RAS), which is included in the Microsoft Windows NT Option Pack, to build VPNs and provide employees with secure remote access to the corporate network over the Internet. Using Windows NT, with its built-in support for PPTP, network administrators must configure two computers on the corporate network:
A dedicated PPTP server for providing secure, encrypted tunneling of IP packets
An Internet Authentication Service (IAS) computer that automatically integrates with the company’s domain controller to remotely authenticate employees
The Internet service provider (ISP) that provides the far-end tunneling connection services for VPN customers can install a Remote Authentication Dial-In User Service (RADIUS) proxy server and configure it to recognize authentication requests from the customer’s remote employees and forward these requests to IAS on the customer’s private network. In this way, the VPN customer can keep control over remote access permissions for all of its employees.
The ISP can implement other tools from Internet Connection Services for RAS, including the following:
Connection Manager: By using the Connection Manager Administration Kit, ISPs can configure custom preconfigured dialers for their VPN customers’ employees. These dialers make the connection experience secure and simple for remote employees by supporting PPTP, including corporate and ISP support numbers, and including customized Help files. Employees only have to enter their username to use these dialers—the dialer does everything else.
Connection Point Services: This tool allows new corporate and ISP access numbers to be automatically transferred to each employee through the Connection Manager dialer, providing phone books that are always up to date.
Once everything is set up and configured on the corporate network and at the ISP, remote employees can establish secure, local connections to their private corporate networks from anywhere in the world by dialing local access numbers to their ISPs. The RADIUS proxy server at the ISP forwards their authentication requests to IAS on their corporate networks, which uses their corporate domain controllers to grant access to resources on the corporate network. With Microsoft Challenge Handshake Authentication Protocol (MS-CHAP), secure connections are established between remote employees and the PPTP server on the corporate network. The entire process is transparent to remote employees—as far as they are concerned, they appear to have a local area network (LAN) connection to the corporate network.
NOTE
Microsoft Windows 98 clients can also use PPTP to connect to VPNs. The Windows 98 client makes two connections to establish a VPN tunnel:
A physical connection to a network access server at an ISP using Dial-Up Networking and Point-to-Point Protocol (PPP). This type of connection is needed only if you use nondedicated dial-up connections.
A logical connection to the VPN tunnel server using PPTP control and data protocols. This is the only connection required if you have a dedicated connection to the ISP.
Microsoft Windows 2000 includes support for VPNs similar to that provided by Windows NT, along with the following enhancements:
L2TP, which, when used with Windows 2000 Internet Protocol Security (IPSec), provides an alternate method to PPTP for creating secure VPNs
Remote access policies to provide flexibility in configuring connection attributes and access permissions
The more secure version 2 of the MS-CHAP authentication protocol
Account lockout to help prevent against dictionary attacks
The new Extensible Authentication Protocol (EAP), which allows newer authentication methods such as smart cards to be integrated for VPN use
See also Layer 2 Tunneling Protocol (L2TP), Point-to-Point Tunneling Protocol (PPTP)
virtual server
A technology that allows multiple independent Web sites to be hosted on a single Web server. A virtual server is sometimes referred to as a Web site.
Internet Information Services (IIS) supports virtual servers in three ways:
By binding multiple IP addresses to the server’s network interface card (NIC). Each virtual server can be assigned its own unique IP address. DNS servers can then resolve domain names into their respective IP addresses, allowing multiple companies to host their Web sites on a single IIS server. This is the preferred method if a large pool of available IP addresses exists for the server.
By using a single IP address but assigning a unique port number to each virtual server. Clients must know the exact port number to connect to the server instead of using the default Hypertext Transfer Protocol (HTTP) port number 80.
By enabling host headers on the IIS server. The server has only one IP address and uses the default HTTP port number 80. The client Web browser attempting to access a Web site on the IIS server must be HTTP 1.1–compliant to seamlessly access the site.
NOTE
The term “virtual server” is also used in Microsoft Cluster Server (MSCS), where it is a collection of services that appears to clients as a physical Microsoft Windows NT Server. The same terminology is also used in the Windows Clustering component of Microsoft Windows 2000 Server. A virtual server is usually a resource group that contains all the resources necessary for running an application, including the network name and IP address resources.
virus
Any piece of code that is deliberately written to cause damage or annoyance to computer users on a network. Tens of thousands of different “strains” of viruses have been detected over the years. The effects of these viruses range from harmless messages announcing the presence of an “infection” to malicious deletion of crucial system and data files. The first recorded PC virus was the Pakistani Brain virus detected in 1987.
Common categories of viruses include the following:
Boot-sector viruses: Viruses that infect the boot sector of a floppy or hard disk and execute when a system is booted, causing various kinds of damage, including totally disabling systems. Notorious boot viruses have included the Michelangelo virus and the Stoned virus.
File viruses: Viruses that reproduce by attaching themselves to executable (.exe) files. When the executable file is run, the virus code is executed, causing the virus to reproduce itself and (typically) damage user files in the process.
Polymorphic viruses: Viruses that evolve as they reproduce, causing their signature to change and making them difficult to detect unless the specific evolution algorithm is known.
Macro viruses: Increasingly popular viruses written in the form of macros for word processing and spreadsheet applications. When the macro is executed, the virus infects the system and typically results in loss of files.
NOTE
Related types of malicious programs that are not strictly viruses because they do not reproduce can have similar effects. These include the following:
Trojan horses: Programs that masquerade as other programs and are typically used to steal credentials or other information from a user
Worms: Programs that invade memory to destroy files but are not disk resident
Logic bombs: Programs that are triggered when a certain event or sequence of events occurs
Many malicious programs combine the properties of viruses with one or more of these programs and are not easy to categorize.
TIP
To keep viruses from proliferating on your network, you can take the following measures:
Install top-quality virus-scanning software on each computer in your network, and keep their virus information files up to date.
Be sure that your anti-virus software scans for viruses in Hypertext Markup Language (HTML) files and attachments to Simple Mail Transfer Protocol (SMTP) e-mail messages if you have Internet connectivity.
Regularly perform backups of all important servers and include periodic archives in your backup schedule, since many viruses do not activate for weeks or months after infection.
Scan new computers for infection before bringing them onto the network.
Issue a company policy prohibiting users from installing any personal programs on their desktop computers. You might even disable their floppy drives, because infection via floppy is a common route to virus infection.
On the Web
•
McAfee Virus Information Center : http://www.mcafee.com/centers/anti-virus/
VLAN
See virtual LAN (VLAN)
Voice over IP (VoIP)
An umbrella term for a set of technologies that allow voice traffic to be carried over Internet Protocol (IP) internetworks such as the Internet. The term “Voice over IP” was coined by the VoIP Forum, a consortium of companies dedicated to the development and extension of IP telephony technologies. Voice over IP (VoIP) is the prime example of “convergence” in the networking and telecommunications industry because it enables telephony and computer networking traffic to be combined into a single data stream of IP packets.
Various vendors have offered proprietary VoIP solutions, but most have recently moved toward standards-based solutions that implement the H.323 and T.120 communication protocols developed by the International Telecommunication Union (ITU). These include voice-encoding industry standards such as G.723 and G.729. One goal of these standards is to enable integration between IP telephony and global cellular telephony standards such as the Global System for Mobile Communications (GSM). However, the move toward standards has been slow, and only a few vendors offer end-to-end carrier-class VoIP services with acceptable levels of communication latency.
Vendors tout the following advantages of using VoIP technologies:
Lower long-distance costs: VoIP enables users to avoid long-distance charges by placing local calls to their Internet service provider (ISP) and having the encapsulated voice traffic move free of charge over the Internet. The problem with this approach is that transmission over the Internet is unpredictable and often has unacceptable levels of latency. One solution is for vendors to construct their own private IP internetworks for VoIP services, but this is an expensive prospect even for the largest carriers and has contributed to the slow progress in voice/data convergence. Some companies offer their own VoIP services to employees using the internal company network, but this approach is still costly and requires expertise to maintain.
New solutions for remote access by mobile users: VoIP solutions allow users to communicate with remote stations using both voice and data integrated into a single IP data stream. For example, a user can browse a Web site and talk to customer support at the site simultaneously by using a single modem connection.
voice profile for Internet mail (VPIM)
An Internet Engineering Task Force (IETF) specification that defines a unified way of transmitting voice mail and fax messages over the Internet. Voice profile for Internet mail (VPIM) uses e-mail systems that support Multipurpose Internet Mail Extensions (MIME) and Simple Mail Transfer Protocol Service Extensions (ESMTP) standards for Internet messaging. VPIM can also be deployed over corporate intranets for integrated business messaging solutions. VPIM includes a proposed directory service that enables lookup of routable addresses and includes a mapping specification to support interoperability with other voice messaging systems.
How It Works
VPIM defines the mechanisms by which voice mail and fax messages can be exchanged between Simple Mail Transfer Protocol (SMTP) mail servers on a TCP/IP internetwork. However, VPIM leaves open the way in which specific mail clients interface with these SMTP servers to send and receive voice and fax messages, which will initially be accomplished by implementing VPIM helper applications. VPIM is implemented as a MIME profile, which allows voice and fax information to be encoded using any SMTP mail server that supports MIME. You can implement VPIM by running additional VPIM software on existing SMTP mail servers or by installing VPIM-SMTP gateways on the TCP/IP internetwork. VPIM gateways support messaging between telephones, cell phones, fax machines, pagers, and computers. VPIM will be able to use Lightweight Directory Access Protocol (LDAP) or X.500-based directories for white pages lookup to address messages to users.
NOTE
VPIM is supported by the popular UNIX mail forwarder software called Sendmail; other vendors are also implementing VPIM. The current standard, VPIM v2, can be found in Request for Comments (RFC) 2421, and VPIM v3 is under development.
On the Web
•
VPIM information : http://www.ema.org/vpimdir
VoIP
See Voice over IP (VoIP)
VoIP gateway
A device that allows telephone calls to be transmitted over Internet Protocol (IP) backbone networks by converting voice signals into IP packets and transmitting them over the network. The reverse process takes place at the other end of the call.
How It Works
Most VoIP gateways are chassis-based units that have either digital or analog built-in Private Branch Exchange (PBX) interfaces. A gatekeeper keeps track of IP address to phone number mappings for routing calls between gateways. Built-in local area network (LAN) and/or wide area network (WAN) interfaces are included for connecting the gateway to the IP backbone. Some gateways also have built-in routing capabilities. The LAN interface is usually Ethernet, but some gateways support Token Ring. The WAN interface is typically T1 or E1, but smaller gateways designed for small office/home office (SOHO) environments support Integrated Services Digital Network (ISDN) interfaces. The number of voice interfaces per chassis typically ranges from 72 to 960, depending on the vendor. Voice interfaces are typically the digital signal cross-connect level (DSX-1) type, but some gateways also support the foreign exchange station (FXS) type interfaces for direct attachment of analog telephones.
TIP
When you shop for a VoIP gateway, you should consider the following:
Audio quality should be your primary consideration. The audio quality with low traffic congestion should be close to that of a digital PBX. With heavy traffic congestion, latency and jitter should remain low enough that voice quality is acceptable to average users. A packet loss of 15 percent or more results in transmission with borderline intelligibility; delays of over 700 milliseconds are unacceptable to most users.
Find out what extra features are supported by the gateway, such as dialed number identification service (DNIS) for call routing, automatic number identification (ANI) and caller ID for identifying the incoming caller, and interactive voice response (IVR) for creating telephone menus.
VoIP gateway technologies are still evolving, so equipment from different vendors might not interoperate, even if the vendors claim to support the H.323 standards of the International Telecommunication Union (ITU). You should therefore buy VoIP from a single vendor, especially if you have an enterprise implementation with many gateways.
If you are concerned about eavesdropping on voice conversations on IP networks such as the Internet, be sure that your gateways support the H.245 encryption standard. The alternative practice of using a virtual private network (VPN) gateway to encrypt VoIP traffic usually results in additional incompatibility problems. Also be aware that it is difficult to configure a VoIP gateway to operate across a firewall that hides network IP addresses using network address translation (NAT), especially if the traffic is encrypted.
Graphic V-4.VoIP gateway.
See also Voice over IP (VoIP)
volume
In MS-DOS and earlier Microsoft Windows platforms, a portion of a hard disk that can be formatted with a file system and can have a unique drive letter assigned to it. In Windows 2000, a volume is a logical storage entity composed of portions of one or more physical disks. Windows 2000 volumes can be formatted using the NTFS file system or file allocation table (FAT) and can be assigned a drive letter.
Windows 2000 supports two different types of disk storage:
Basic storage: Supported by Windows NT version 4 or earlier and can include primary partitions, extended partitions, logical drives, volume sets, mirror sets, stripe sets, or stripe sets with parity
Dynamic storage: Volumes created using the Windows 2000 Computer Management snap-in for the Microsoft Management Console (MMC) that can include simple volumes, spanned volumes, striped volumes, mirrored volumes, or RAID-5 volumes
volume set
A single volume created using discontiguous free areas on hard disks. By using the Microsoft Windows NT administrative tool Disk Administrator, you can create volume sets by combining between 2 and 32 free areas on your disk drives.
NOTE
Volume sets are called spanned volumes in Microsoft Windows 2000, and they are created using the Computer Management snap-in for the Microsoft Management Console (MMC).
TIP
The Windows NT system partition and boot partition cannot be volume sets.
You can extend a volume set formatted with the NTFS file system without having to reformat the entire volume.
See also stripe set
VPIM
See voice profile for Internet mail (VPIM)
VPN
See virtual private network (VPN)
V series
A series of communication standards developed by the International Telecommunication Union (ITU). V series protocols define methodologies for exchanging data over the public telephone system by using telecommunications devices such as modems and multiplexers. Various V series protocols define interfaces between data terminal equipment (DTE) such as computers and data communications equipment (DCE) such as modems, or signaling methods for use over the public telephone system. Standards V.100 and above deal with internetworking the telephone system with other types of networks such as packet-switching networks, while standards below V.100 deal primarily with general DTE/DCE serial communication interfaces, analog modem technologies, flow control, and error control.
The following table describes some of the popular serial transmission standards relating to modem and other serial communication technologies.
Popular Serial Transmission Standards
V Standard
Description
V.22
Early standard for full-duplex serial transmission over one pair of wires at 1200 bps (or 2400 bps for the V.22bis version).
V.24
The ITU equivalent of the RS-232 serial interface.
V.32
Industry standard for 9600-bps serial transmission (or 14.4 Kbps with V.32bis).
V.33
Protocol for full-duplex synchronous serial communication over leased lines with two pairs of wires; supports speeds of up to 14.4 Kbps and is used primarily in IBM mainframe environments.
V.34
Supports 28.8-Kbps serial transmission (or 33.6 Kbps with V.34bis) over dial-up (one wire pair) or leased (one or two wire pairs) lines.
V.35
Protocol for synchronous serial communication at speeds of up to 48 Kbps; typically used for DTE/DCE communication between Channel Service Unit/Data Service Units (CSU/DSUs) and bridges/routers. Replaced by V.10/11 but still widely implemented.
V.42
Standard for data compression and error control in serial communication; uses both the LAPM (link access procedures for modems) and MNP (Microcom Networking Protocol) series protocols.
V.90
High-speed modem standard that supports asymmetric communication with a maximum downstream data rate of 56 Kbps and an upstream rate of 33.6 Kbps.
NOTE
Only two V series standards are covered in detail in this work: V.35 and V.90.
