A type of customer premises equipment (CPE) that is used to terminate a T1 line and distribute its services across the organization. T1 channel banks allow T-carrier services to connect at the customer premises to
Data terminal equipment (DTE) such as routers or remote access servers that provide wide area network (WAN) data connections for corporate networks
Private Branch Exchange (PBX) units that provide integrated phone/fax services
How It Works
A typical T1 channel bank consists of a modular chassis unit to which you can add various expansion cards to provide digital communication services for CPEs. The modular chassis allows customers to add additional channels and upgrade fractional T1 services to full T1 or higher. It also allows customers to multiplex several channels together to provide higher bandwidth for high-speed data connections to routers, Web servers, and other DTEs. The chassis typically includes a built-in T1 Channel Service Unit (CSU) for terminating the T1 circuit at the customer premises, plus a variety of expansion cards for specific uses. Some chassis support up to four T1 lines, which can be configured for both active and backup purposes to provide redundant WAN connections.
Graphic T-1. T1 channel bank.
Each expansion card typically services one or two DS0 channels, which means that different channels can supply different services (such as voice, fax, or data connections). Typical types of expansion cards include the following:
Data service cards: Usually have a dual channel format that supports two DS0 channels and use a serial interface such as RS-232, RS-530, or V.35 for directly connecting to bridges and routers with integrated Channel Service Unit/Data Service Units (CSU/DSUs)
High-speed data cards: Support up to 1.544 Mbps in 64-Kbps or 56-Kbps increments by selecting the number of DS0 channels to multiplex together
Voice cards: Used to connect to a PBX or even directly to a telephone using standard 4-wire connections
Modem cards: Turn your channel bank into a modem pool for remote access functionality
NOTE
Individual DS0 channels might have bandwidths of 56 Kbps or 65 Kbps, depending on the carrier.
See T-carrier
An umbrella standard representing a suite of eight International Telecommunication Union (ITU) standards that define how real-time multipoint communication for tasks such as data conferencing and interactive game playing takes place over a network. The standards define such things as
Multipoint services for conferencing
Standard network services
Guidelines for defining data channels
Whiteboard methodologies
Application-sharing protocols
File transfer methodologies
T.120 is a multilevel standard that is tuned for reliable transmission within high-bandwidth enterprise environments running TCP/IP and other standard connection types. T.120 is the standard suite of protocols that makes possible collaboration and conferencing software such as Microsoft NetMeeting and is supported by many other telecommunications providers and application vendors.
How It Works
The architecture of the T.120 standard follows that defined by the Open Systems Interconnection (OSI) reference model for networking. The T.120 architecture can be divided into two parts:
Network-layer and transport-layer standards (T.122 through T.125): Allow data to be transmitted and received between conferencing nodes over a variety of supported network connections. These standards also provide platform independence and the capacity for simultaneously managing multiple participants running on different operating system platforms and conferencing software.
Application-layer standards (T.126 through T.128): Support multiuser conferencing functions such as whiteboarding, file transfer, and application sharing across different platforms and networks.
The following table shows the details of the various standards included under the T.120 umbrella.
T.120 Suite of Conferencing Standards
Standard | Description |
T.121 | A required standard for T.120 applications that defines how conference nodes register themselves with a T.120 node controller. Also defines the generic application template (GAT) for building T.120 application protocols and management facilities. |
T.122 | Defines multipoint communication services (MCS) over various topologies to enable multiple participants to send data as part of a conference. The MCS defined by T.122 are implemented by T.125. |
T.123 | Defines flow control, error control, and sequencing mechanisms for connect, disconnect, send, and receive functions over different network connections. |
T.124 | Defines how multipoint conferences are initiated and administered, and defines the generic conference control (GCC) that manages and monitors users, address lists, data flow, and MCS resources. |
T.125 | Defines how data is transmitted during a conference, specifying the private and broadcast channels that transport conference data. T.125 implements the MCS defined by T.122. |
T.126 | Defines mechanisms for transmitting and receiving whiteboard information between conference nodes and manages the multiuser whiteboard workspace. |
T.127 | Defines mechanisms for file transfer between conference nodes in either broadcast or directed mode. |
T.128 | Defines mechanisms for application sharing between conference nodes so that users can share their local programs with others for collaborative purposes. |
NOTE
T.120 also forms the basis of the Remote Desktop Protocol (RDP), which is used in Microsoft Windows NT Server, Terminal Server Edition, and in Terminal Services of Microsoft Windows 2000 Advanced Server.
Assuming ownership of an object—usually a file or a folder—on an NTFS volume and thereby gaining the right to share the object and assign permissions to it. The user who creates a file or folder on an NTFS volume is the owner. To take ownership of a file or a folder that you do not own, you must have one of the following:
Membership in the Administrators local group
NTFS full control permission on the object
NTFS special permission O (taking ownership) on the object
NTFS special permission P (change permission) on the object, so that you can give yourself O permission
NOTE
Ownership can be taken, but it cannot be assigned.
See also NTFS permissions (Windows 2000), NTFS permissions (Windows NT), NTFS special permissions (Windows 2000), NTFS special permissions (Windows NT)
A general term for a class of devices for backing up data to a magnetic tape for disaster recovery planning and archiving purposes. A number of technologies with incompatible tape formats have been implemented in these devices. The following list describes some of the formats. Note that the capacity and speed of tape drives is rapidly improving. This list gives you an idea of the range of possibilities. Note also that the capacities mentioned in this list are for uncompressed data. With data compression, most of these formats can approximately double the amount of data they hold.
Digital audio tape (DAT): 4-millimeter tape manufactured by a number of different vendors that has tape cartridge capacities of 4 GB and higher. DAT drives use the DDS-2 format for 4-GB tape cartridges, the DDS-3 format for 12-GB cartridges, and the DDS-4 format for 20-GB cartridges. Transfer speeds are typically 1.5 Mbps or higher. DAT cartridges are standardized with a 3.5-inch form factor.
Quarter-inch cartridge (QIC): Quarter-inch tape manufactured by Tandberg Data that can have tape cartridge capacities of 13 GB and higher. QIC generally uses 5.25-inch cartridges, but these are being replaced with 3.5-inch minicartridges. Transfer speeds are typically 1.5 Mbps or higher.
8 millimeter: Can have tape cartridge capacities of 20 GB and higher. Transfer speeds are typically 3 Mbps or higher. 8-millimeter tapes have a form factor of 5.25 inches. Sony and Exabyte are two manufacturers of 8-millimeter tapes.
Digital linear tape (DLT): Uses technology developed by Digital Equipment Limited (which was acquired by Quantum in 1994). DLT can have tape cartridge capacities of 25 GB and higher. Transfer speeds are typically 5 Mbps or higher. DLT cartridges have a form factor of 5.25 inches. Quantum and Tandburg are two manufacturers of DLT.
See also backup
See Telephony Application Programming Interface (TAPI)
A Microsoft Windows NT and Windows 2000 utility that you can invoke by clicking the Task Manager button in the Windows Security dialog box. You access the dialog box by pressing Ctrl+Alt+Del, the secure attention sequence (SAS) keystroke combination. You can use Task Manager to
Start, view, change the base priority of, and terminate processes
Display CPU and memory usage graphs and data
Terminate poorly behaving applications
Assign a process to a particular microprocessor on a multiprocessor system
Graphic T-2. Task Manager.
A Microsoft Windows 2000 utility that lets you schedule when to run or open a script, program, or document. Task Scheduler is a useful tool for regularly running system maintenance. Task Scheduler is also the name of the Windows 2000 service that underlies the operation of this utility.
How It Works
You schedule a new task by using the Scheduled Tasks wizard, which you can access through the Scheduled Tasks folder in Control Panel. This wizard prompts you for the following information:
The script, program, or other file you want to run or open.
The schedule for the task. For example, you can configure a task to run daily, weekly, monthly, when you log on, when the system starts, or only a single time.
The credentials under which the task will run. You must specify credentials with sufficient privileges to run the script or program, or to open the file.
Advanced options, including how different power management conditions will affect the execution of the task, stopping the task if it runs for too long, or configuring the task to execute when the computer is idle for a specific period of time.
The result of scheduling a task is a task file, which has the extension .job. You can send these task files to and receive them from other users as attachments to e-mail messages. Users can then drag these files into their local Scheduled Tasks folder. Administrators can also view and modify tasks displayed in the Scheduled Tasks folder called \Winnt\Tasks on remote computers by using My Network Places.
Once a task has been scheduled, you can modify, delete, disable, or stop its execution. The service creates a log file of past scheduled tasks that can be viewed using the Advanced menu of the Scheduled Tasks folder.
NOTE
Task Scheduler provides a friendlier, GUI-based interface for scheduling system tasks than the at command used at the command prompt for scheduling tasks in Microsoft Windows NT. The at command is still available in Windows 2000, and tasks scheduled using this command appear in the Scheduled Tasks folder. However, if you use the GUI-based Task Scheduler to modify a task that was scheduled using the at command, you no longer will be able to use the at command to modify the task.
TIP
You can also schedule a task by dragging the icon for a script, program, or document from My Computer or Windows Explorer into the Scheduled Tasks folder.
If scheduled tasks do not run when expected, check the system date and time on your computer to see whether they are accurate.
If you have trouble using the at command to schedule a task, you might have accidentally changed the security context (credentials) under which the command runs. Check this using the Advanced menu of the Scheduled Tasks folder.
A series of digital communication services provided by telcos for high-speed permanent voice and data connections. T-carrier services were first developed by Bell in the 1960s for digital transmission of analog voice communication. Telcos typically use T1 lines to connect telephone exchange switching equipment within the telco’s central office (CO).
Common uses for T1 lines from a networking perspective include the following:
Building dedicated, high-speed wide area networks (WANs)
Providing corporate networks with high-speed access to the Internet
Connecting corporate intranet/extranet web servers to the Internet
Providing high-speed remote access solutions for companies with mobile users
Providing integrated voice/fax/data services to businesses
Graphic T-3. Uses for T1 routers.
How It Works
The T-carrier system is based on the DS1 signaling standard defined by AT&T. A DS1 channel is formed from a combination of 24 DS0 (Digital Signal Zero) channels with 64 Kbps of bandwidth each, for a total bandwidth of 1.544 Mbps. This configuration is called a T1 circuit and is the base circuit from which other T-carrier circuits are derived. The 24 DS0 channels can either be used separately for voice and data or be combined by using a technique called time-division multiplexing (TDM), in which voice or data information from each channel is interleaved into a single bit stream. A DS1 frame is thus composed of 1 byte (8 bits) from each DS0 channel plus 1 bit of framing control. The transmission rate of frames is set at 8000 frames per second, which means that the total bandwidth of a T1 circuit or DS1 communication channel can be calculated using this formula:
T1 = 8000 frames/sec x ((24 x 8) + 1) bits/frame = 1544000 bits/sec = 1.544 Mbps
Because of the framing bit, the actual bandwidth of a T1 line for data transmission is slightly less than this, at 1.536 Mbps. Synchronization is maintained between T1 equipment at the customer premises and the telco CO by varying the framing bit using a predetermined algorithm. The most flexible T1 solution for customer premises is to use a T1 channel bank to interface local area networks (LANs), Private Branch Exchanges (PBXs), and telephone and fax equipment with a T1 circuit leased from a local telco.
Other special-purpose T1 equipment that you can buy or lease includes
T1 CSU/DSU, a Channel Service Unit/Data Service Unit for connecting bridges or routers to T1 circuits
T1 MUX, a multiplexer for aggregating T1 circuits for high-speed communication
T1 bridges and routers, with or without integrated T1 CSU/DSUs, for dedicated point-to-point or multipoint WAN connections for enterprise-level internetworks
T1 access routers for providing remote access services and allowing multiple, simultaneous remote access connections to be channeled through a single T1 line at the customer premises
Other T-carrier services provided by telecommunications carriers include the following:
Fractional T1 circuits, consisting of 4, 8, 12, or more DS0 channels multiplexed together
Multiples of T1, such as T2, T3, and T4, as shown in the following table
T-Carrier Services
T-Carrier Service | Number of DS0 Channels | Bandwidth |
T1 | 24 | 1.544 Mbps |
T2 | 4 x 24 = 96 | 6.312 Mbps |
T3 | 18 x 24 = 432 | 44.736 Mbps |
T4 | 168 x 24 = 4032 | 274.176 Mbps |
NOTE
T-carrier services such as T1 lines are often used to provide networks with high-bandwidth, permanent WAN connections between sites. T1 is the preferred technology for combining voice, fax, and data transmissions over an enterprise-level internetwork. T1 lines can be expensive because, whether or not they are being used, they are always “on.” A cheaper solution is to lease a fractional T1 service such as 4 x DS0 = 256 Kbps and then upgrade it to higher speeds as necessary. Fractional T1 is usually cheaper than using individual DS0 circuits at the customer premises and multiplexing them together.
T1 cannot be run over Plain Old Telephone Service (POTS) lines. They must use specially conditioned two-pair copper lines. Two wires are used for transmission (TX interface) and two for receiving (RX interface) for full-duplex communication. A repeater is typically required every 915 meters or 3000 feet to regenerate the signal. T1 lines typically terminate at the customer premises using an RJ-48 jack, which looks like an RJ-45 jack but is pinned differently. T1 lines can use unshielded twisted-pair (UTP) cabling, coaxial cabling, or fiber-optic cabling.
T1 circuits use either AMI (Alternate Mark Inversion) or B8ZS line coding. AMI encodes zeros as 01 and ones alternately as 00 and 11. B8ZS substitutes a special byte if eight consecutive zero bits are detected in order to maintain a specific ones density to help maintain synchronization. (Ones density is a scheme that allows a CSU/DSU to recover the data clock reliably. The CSU/DSU derives the data clock from the data that passes through it. To recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it. Ones density is also called pulse density.)
TIP
To test T1 customer premises equipment (CPE) such as channel banks and CSU/DSUs, you can use a cable simulator, a passive device that simulates a standard 22-gauge twisted-pair T1 line that is 400 meters (1310 feet) long. Connect two cable simulators between your CPE and your T1 test equipment using the TX and RX interfaces to analyze the performance of your device. (Or use 400 meters of 22-gauge twisted-pair cabling instead!)
A “wet” T1 line carries a small DC current of about 140 mA (milliamperes) at several hundred volts for powering the CSU/DSU at the customer premises. “Dry” lines carry no current, so CSU/DSUs must be powered from the customer premises. Don’t touch a T1 line—a wet line can give you a serious shock!
See also RJ connectors, T1 channel bank
See Transmission Control Protocol (TCP)
An abbreviation for Transmission Control Protocol/Internet Protocol, an industry-standard protocol suite for wide area networks (WANs) developed in the 1970s and 1980s by the U.S. Department of Defense (DoD). TCP/IP is a routable protocol that is suitable for connecting dissimilar systems (such as Microsoft Windows and UNIX) in heterogeneous networks, and it is the protocol of the worldwide network known as the Internet. Microsoft’s implementation of TCP/IP supports industry standards, and TCP/IP is implemented for all Windows operating systems.
Graphic T-4. TCP/IP.
How It Works
The architecture of the TCP/IP protocol suite has four layers that map loosely to the seven-layer Open Systems Interconnection (OSI) reference model (as shown in the diagram). The TCP/IP model is sometimes called the DoD model because TCP/IP was developed in connection with the ARPANET project of the U.S. Department of Defense. Each layer of the TCP/IP protocol suite has its associated component protocols, the most important of which are listed here:
Application layer protocols: Responsible for application-level access to TCP/IP networking services. These include Dynamic Host Configuration Protocol (DHCP), Domain Name System (DNS), Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Telnet, Simple Mail Transfer Protocol (SMTP), and Simple Network Management Protocol (SNMP). In the Microsoft implementation of TCP/IP, application layer protocols interact with transport layer protocols by using either Windows Sockets or NetBIOS over TCP/IP (NetBT).
Transport layer protocols: Establish communication through connection-oriented sessions and connectionless broadcasts. Protocols at this layer include Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Internet layer protocols: Responsible for routing and encapsulation into IP packets. Protocols at this layer include Internet Protocol (IP), Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP), and Internet Group Management Protocol (IGMP).
Network layer protocols: Place frames on the network. These protocols include the various local area network (LAN) architectures (such as Ethernet and Token Ring) and WAN telecommunication service technologies—such as Plain Old Telephone Service (POTS), Integrated Services Digital Network (ISDN), and Asynchronous Transfer Mode (ATM).
TCP/IP uses two naming schemes to identify hosts and networks on an internetwork:
IP addresses: Logical 32-bit (4-byte) numeric addresses of the form w.x.y.z. They are partitioned (using a subnet mask) into two segments, a network ID and a host ID. For example, the IP address 205.116.8.44 is partitioned using the subnet mask 255.255.255.0 into the network ID 25.116.8.0 and the host ID 44. IP addresses are the basic or primary way of identifying hosts and networks on an internetwork; they can be assigned to computers manually or by using DHCP.
Fully qualified domain names (FQDNs): Alphanumeric names of the form host_name.domain_name in which domain_name is part of DNS, a hierarchical worldwide naming system. For example, the FQDN server12.microsoft.com represents a host named server12 that belongs to a network whose domain name is microsoft, which belongs to the top-level domain named .com, which belongs to the root DNS domain named dot. FQDNs are friendly names. They are easier to remember than IP addresses and are resolved into IP addresses by using a DNS server or a local hosts file.
NOTE
TCP/IP is a constantly evolving protocol suite whose development is steered by such bodies as the Internet Society (ISOC), the Internet Architecture Board (IAB), and the Internet Engineering Task Force (IETF). The current version of TCP/IP is called IPv4 (Internet Protocol version 4); a new version called IPv6 is under development.
An add-on for Microsoft Windows for Workgroups that provides a 32-bit implementation of the TCP/IP protocol. This add-on includes features such as the following:
An industry-standard implementation of the TCP/IP protocol suite
Windows Sockets and NetBIOS programming interfaces
The TCP/IP client utilities File Transfer Protocol (FTP) and Telnet
The TCP/IP diagnostic utilities arp, ipconfig, nbtstat, netstat, ping, route, and tracert
Support for Dynamic Host Configuration Protocol (DHCP) and Windows Internet Naming Service (WINS)
Support for the User Datagram Protocol (UDP), Address Resolution Protocol (ARP), and Internet Control Message Protocol (ICMP)
NOTE
You can create installation disks for this software by using the Network Client Administrator tool in Microsoft Windows NT.
A method of initializing a Transmission Control Protocol (TCP) session between two hosts on a TCP/IP network. The handshake establishes a logical connection between the hosts by synchronizing the sending and receiving of packets and communicating TCP parameters between the hosts.
How It Works
All TCP communication is connection oriented. A TCP session must be established before the hosts in the connection exchange data. Packets that are transferred between hosts are accounted for by assigning a sequence number to each packet. An ACK, or acknowledgment, is sent after every packet is received. If no ACK is received for a packet, the packet is re-sent. The three-way handshake ensures that the initial request is acknowledged, that the data is sent, and that the data is acknowledged.
These are the three stages of a TCP three-way handshake:
The initiating host sends a TCP packet requesting a new session. This packet contains the initiating host’s sequence number for the connection. The packet includes information such as a set SYN (synchronization) flag and data about the size of the window buffer on the initiating host.
Graphic T-5. TCP three-way handshake.
The target host sends a TCP packet with its own sequence number and an ACK of the initiating host’s sequence number.
The initiating host sends an ACK containing the target sequence number that it received.
NOTE
A similar three-way process is used to terminate a TCP session between two hosts. Using the same type of handshake to end the connection ensures that the hosts have completed their transactions and that all data is accounted for.
See Transport Driver Interface (TDI)
See time-division multiplexing (TDM)
See Time Division Multiple Access (TDMA)
See time domain reflectometry (TDR)
See Microsoft TechNet
Stands for telephone company and can be one of the following:
A local provider of a Plain Old Telephone Service (POTS) connection through the local loop connection from its central office (CO)
Any telecommunications service provider (including long-distance carriers)
Telcos offer a variety of high-speed data transmission services, including Integrated Services Digital Network (ISDN), frame relay, T1 lines, and Asymmetric Digital Subscriber Line (ADSL). Networks connect their customer premises equipment (CPE) to similar equipment at telco COs for such purposes as
Point-to-point or multipoint connections for creating a wide area network (WAN) with other company locations
High-speed dedicated access to the Internet for corporate users
Dial-up remote access solutions using modems or ISDN
A national trade organization representing all aspects of the telecommunications industry in the United States. Working in conjunction with its subsidiary, the MultiMedia Telecommunications Association (MMTA), and its industry peer organization, the Electronic Industries Alliance (EIA), the Telecommunications Industry Association (TIA) represents its members in activities such as establishing public policies and government regulatory issues, developing standards for communication and networking, and organizing trade shows and other events. The goal of the TIA is to provide member companies, which are drawn mostly from service providers and hardware vendors in the communication industry, with a forum for discussing industry issues and a voice for representing members’ interests on the national and international level.
Active in telecommunications standards development, the TIA is endorsed and accredited by the American National Standards Institute (ANSI). The Standards and Technology Department consists of five divisions expressed in over 70 groups responsible for formulating new standards. These five divisions are as follows:
Fiber Optics
User Premises Equipment
Network Equipment
Wireless Communications
Satellite Communications
On the Web
•
TIA Online : http://www.tiaonline.org
Internetworking services provided by telcos and long-distance carriers to businesses. To link geographically separated local area networks (LANs) into a metropolitan area network (MAN) or a wide area network (WAN), businesses must lease or purchase services from their local telco and long-distance carriers because these companies own the wires that make internetworking possible. In addition to the regular Public Switched Telephone Network (PSTN) service, most telecommunications companies offer additional services, including the following:
Leased lines, such as T1 lines for permanent, dedicated point-to-point WAN connections
Circuit-switched services, such as Integrated Services Digital Network (ISDN) for temporary or dial-up point-to-point WAN connections
Packet-switched services, such as X.25 services for permanent multipoint WAN connections
Various data communication technologies are used to provide these services, including the following:
Asynchronous Transfer Mode (ATM)
Digital data service (DDS)
Frame relay
ISDN
Switched 56
T1 and fractional T1
A set of standard application programming interfaces (APIs) for accessing telephony services developed by Microsoft and Intel and implemented in Microsoft Windows.
Telephony Application Programming Interface (TAPI) receives requests from applications and forwards them to telephony devices such as modems, telephones, Integrated Services Digital Network (ISDN) equipment, or Private Branch Exchanges (PBXs). TAPI manages such functions as
Signal
Hold and transfer
Conference and call park
Other PBX functions
TIP
If you travel frequently and use a laptop, create a TAPI location for each geographical site that you commonly visit. A TAPI location is a set of information used by dial-up networking that specifies a country, area code, dial-out information, and calling card information. TAPI-aware applications such as dial-up networking use TAPI locations to correctly dial from a given location.
A standard TCP/IP protocol for running programs on remote hosts. The term “telnet” also refers to the software (client or server component) that implements this protocol on a particular platform or system. Telnet is defined in Request for Comments (RFC) 854.
How It Works
Telnet is a terminal emulation program, which is a command-line interface for issuing commands on a remote computer. A user running telnet client software can interactively run command-line applications on a remote host that is running the telnet service or daemon. The user enters information at the telnet client; this information is processed on the telnet server and its output is returned to the user. For example, if you use telnet to connect to a UNIX server, you can issue UNIX commands to remotely perform operations on that server.
NOTE
Microsoft Windows NT includes a telnet client implemented as a Microsoft Windows application, but does not include telnet server software. Windows 2000 includes both a telnet client implemented as a command-line utility and telnet server software that supports up to 63 simultaneous client connections but is licensed to only provide up to two simultaneous client connections. If you require support for additional client connections, you should obtain the Windows Services for UNIX add-on pack for Windows 2000 Server.
TIP
You can use a telnet client to connect to a Web server on port 80 or a Simple Mail Transfer Protocol (SMTP) mail server on port 25 and issue Hypertext Transfer Protocol (HTTP) or SMTP commands directly to the server for troubleshooting purposes.
Generally, any device that terminates a communication channel. In computer terminology, a terminal is an input/output (I/O) device, usually consisting of a keyboard and monitor, that acts as a front end for a mainframe, terminal server, or other back-end processing device. The earliest terminals were called teletypes (abbreviated TTY), which were essentially electric typewriters in which users would enter commands and data for a mainframe, and on which the mainframe would type the output returned to the user. A terminal that supports only text output is sometimes called an ASCII terminal.
How It Works
Terminals generally have little or no inherent data-processing power and rely entirely on the back-end system to do the processing. The terminal is responsible only for processing and queuing input from the keyboard (or other input device such as the mouse), transmitting this in a recognized format to the back-end host (mainframe or terminal server), and receiving output from the host and presenting it on the screen in suitable format for the user (ASCII text in older systems, graphical desktop environment in newer systems). This explains the origin of the term “dumb terminal,” which means that a terminal by itself is generally useless without connecting to the back-end system. However, there are also “smart” or “intelligent” terminals that have various degrees of inherent processing capability. The information the user enters on the keyboard is typically transmitted to a mainframe over an RS-232 or RS-423 asynchronous serial connection, but it is sometimes transmitted over an Ethernet or a Token Ring local area network (LAN) connection. The mainframe processes the input and returns the output to the terminal, which displays the output on the monitor. In other words, the application runs in one location (the mainframe), while the user interface is in a different location (the terminal).
Terminals originated in the mainframe environment, and a number of standards (terminal protocols) have evolved that govern their use. The VT-100 terminal originated by Digital Equipment Corporation was a popular ASCII-text-based terminal standard that is still used in places such as library online catalog systems, which remote users typically access by running a telnet client over a dial-up connection. IBM’s 3270 terminal protocol is still widely used in IBM mainframe environments, while 5250 is popular in AS/400 mid-range computing environments. Other terminal standards include ANSI, VT52, and VT220.
Terminals can be one of the following:
Local terminals that are directly connected to the back-end host via a serial or LAN connection
Remote terminals that are typically connected via a phone line with a modem at both ends
The popularity of terminals declined in the late 1980s and early 1990s with the advent of distributed client/server environments and the eclipse of mainframe computing environments. In a client/server environment, data processing is shared between the front-end client computer, usually a full-featured PC with a graphical user interface (GUI) such as Microsoft Windows, and the back-end server, which can be a Windows NT–based server, a Novell NetWare server, an AS/400, or some other system. In the late 1990s, the pendulum started to swing back toward terminals with the rising popularity of PC-based terminal emulators and terminal servers. A terminal emulator is hardware and/or software that runs on a stripped-down PC with no operating system and causes the PC to function as a terminal, while a terminal server is a back-end server that generates and delivers the user desktop environment to the terminals and performs all the processing. This arrangement allows for low-cost “thin clients” to be used and centralizes system administration at the back end, reducing deployment and management costs associated with a distributed client/server systems environment.
See also terminal emulator, terminal server
Hardware and/or software that allows a PC to operate as a terminal and connect to a back-end mainframe or terminal server. Terminal emulators can be designed to emulate specific terminal modes such as ANSI, VT52, VT100, VT220, TN3270, or TN5250. Microsoft HyperTerminal, included with 32-bit Microsoft Windows operating systems, supports a variety of different terminal emulation modes.
TIP
The emulation mode on the clients must match the terminal mode running on the back-end system in order for communication to work. If you are trying to connect to an unknown mainframe or other back-end system and your emulator cannot automatically detect the terminal mode needed, try using ANSI mode first. If that fails, try VT100 and other popular terminal modes.
See also terminal
Generally, a server that provides the back-end support needed for terminals to function. This can be a mainframe system, a UNIX host running X Windows, or a PC-based server running software such as Microsoft Windows NT Server, Terminal Server Edition, or Microsoft Windows 2000 Server. The terminal server generates the desktop environment presented to the user on the terminal and performs all processing of data submitted by the user. The main advantages of such a system are as follows:
Lower hardware costs: “Thin clients” (special devices or stripped-down PCs) can be used instead of full-featured desktop PCs. For example, Windows NT Server, Terminal Server Edition, can present a 32-bit Windows user environment on older PCs that lack the hardware requirements for running a local copy of the latest versions of Microsoft Windows operating systems.
Lower management costs: Operating systems and applications are installed and run only on the back-end terminal servers, which simplifies deployment and troubleshooting and makes administration more centralized. Windows NT Server, Terminal Server Edition, and Windows 2000 Server, for example, support running applications such as Microsoft Office from central servers instead of installing them on every desktop client in the enterprise.
Multiplatform support: Allows the same applications and desktop environments to be presented on a variety of client platforms, including Windows-based PCs, Macintosh computers, UNIX workstations, and other devices.
Some vendors produce rack-mountable terminal server devices with 8 or 16 RJ-45 ports that can be used to connect asynchronous terminals to an Ethernet local area network (LAN) running TCP/IP or some other network protocol. Such devices can be used to provide terminals (or PCs running terminal emulation software) with access to network file servers or dial-up access to the Internet. Windows-based management software allows these devices to be remotely managed from a PC for viewing and configuring port information. Built-in support for Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and Remote Authentication Dial-In User Service (RADIUS) are often included to control user access. Users can dial in to the device, be authenticated, and select a desired host on the LAN they want to communicate with.
Single-port terminal servers are sometimes used in mainframe environments to allow users connected to different controllers to communicate over the corporate LAN without needing a dedicated point-to-point communication link. In a typical configuration, the controller is connected to a terminal server via an RS-232 serial connection, while the terminal server is linked to the LAN via an Ethernet interface.
See also terminal, terminal emulator
An optional component of Microsoft Windows 2000 Server that enables users to access the Windows 2000 desktop and run Microsoft Windows applications on remote computers and terminal devices. Terminal Services enables Windows 2000 Server to function as a terminal server and provide terminal emulation for a wide range of client computers. By moving all processing to the server, Terminal Services reduces total cost of ownership by
Simplifying system administration by centralizing the installation and management of all applications on the server and supporting full remote administration from a single desktop
Extending the life of legacy hardware by enabling client computers with minimal processing power and memory to run standard Windows applications
Extending the life of legacy operating systems by allowing applications designed for Windows 2000 to run on legacy versions of Windows
Increasing security by using encrypted sessions between clients and servers, by enabling administrators to fully monitor and control user operations by shadowing client sessions from another client computer, and by enabling administrators to input keyboard and mouse actions during client sessions for remote control purposes
How It Works
In order for Terminal Services to work on a network, you must implement three components:
Terminal server: A Windows 2000–based server that provides each client computer with its own Windows desktop, receives and processes all keystroke and mouse actions performed by clients, and sends the display output for the operating system and applications to the appropriate client.
Remote Desktop Protocol (RDP): A protocol suite based upon the T.120 standard from the International Telecommunication Union (ITU), which provides the basis for communication between the client and the terminal server.
Graphic T-6. Terminal Services.
Terminal Services client: A “thin client” that displays the Windows 2000 desktop and running applications within a window on the client computer. Terminal Services clients are provided for all versions of Windows, including 32-bit clients that can run on computers running Windows 2000, Windows NT 4.0, Windows NT 3.51, Windows 98, or Windows 95 on either Intel or Alpha platforms, and a 16-bit client for Windows for Workgroups 3.11. Special client software can also be embedded in devices such as Windows-based terminals and handheld PCs.
You can install Terminal Services during Setup or afterward using Add/Remove Programs in Control Panel. To use Terminal Services, you must install both Terminal Services and Terminal Services Licensing, and you need to specify the directory location of the licensing server database. Once the services are installed, you can configure the security of the terminal server to allow users to remotely run multiuser applications, configure user accounts to allow them to log on to the terminal server, create user profiles and home directories if desired, and install Terminal Services client software on client computers. You can install client software either by downloading it across the network or by creating client installation disks for manual installation.
NOTE
The implementation of RDP on Windows 2000 requires that TCP/IP be implemented as the underlying network transport.
TIP
By installing the Citrix MetaFrame add-on, non-Windows clients such as UNIX, Macintosh, and OS/2 Warp can also access a Windows 2000–based server running Terminal Services in order to run Windows 2000 applications.
A good rule of thumb is that a terminal server needs an additional 4 to 8 MB of RAM for each additional client it supports. Also, do not run legacy MS-DOS or 16-bit Windows on the terminal server, as this can significantly reduce the number of concurrent users that the server can support and increase the memory requirements for each connected client.
Install Terminal Services on a member server instead of a domain controller. Installation on a domain controller can affect the domain controller’s performance as a result of the additional load that Terminal Services places on server processor, memory, and network interface.
A device connected to one end of a bus or cable that absorbs signals. Terminators prevent signal reflection, which can produce interference that causes signal loss. Most communication systems such as networks and computer buses require some form of termination at the ends of the data path, although this is often provided internally by the devices at the ends of the data path.
How It Works
In a bus-based system, a single wire or series of wire segments connects network components in a chain formation. If the ends of the cable are not terminated, a signal placed on the wire by one component will bounce back and forth between the ends of the cable, hogging the cable and preventing other components from signaling. Terminators eliminate this signal bounce by absorbing the signal after each component has seen it once, allowing other components to place their signals on the cable.
Terminators can be passive (simple resistors) or active (more complex electronics) depending on the type of bus being terminated. By supplying a load equal to the impedance of the cable, the terminator prevents reflections or standing waves from developing on the cable. Passive terminators use resistors to provide this impedance matching, while active terminators generally use voltage regulators.
Terminator types include the following:
Coaxial cabling terminators: Passive terminators that come in various sizes and use BNC threading to terminate
RG-58 thinnet cabling for 10Base2 Ethernet networks with termination resistance of 50 ohms
RG-59 cable television terminators with resistance of 75 ohms
RG-62 ARCNET cabling terminators with resistance of 93 ohms
Small Computer System Interface (SCSI) terminators: The ends of a SCSI cable must always be terminated in a chain of SCSI devices. The internal termination is usually supplied by the SCSI adapter card, and the external termination is supplied by the last device in the chain. SCSI terminators can be passive, active, differential, or forced-perfect. Forced-perfect terminators compensate for the differences in impedance along the length of a SCSI bus. Diagnostic terminators analyze and display the condition of the data paths within a SCSI bus and are useful for high-availability uses such as clustering.
Free connectors: Connectors on the hubs at both ends of a series of stackable hubs. These terminators are specific to the type of hub sold by a vendor.
TIP
You can test the termination of a long 10Base2 network without having to hunt for the ends of the cable. Simply use an ohmmeter and test the resistance between the central conductor and the shield of any BNC T-connectors (after removing the cable from the network card it is attached to). If the reading is around 25 ohms, the cable is properly terminated; if the reading is around 50 ohms, one of the terminators is loose or missing. If the cable appears to be properly terminated but network problems persist, remove one of the terminators and use the ohmmeter to test the connection to the T-connector that you just exposed. If the result is less than 50 ohms, you probably have a short in the cable; if it is more than 56 ohms, there is probably a loose T-connector somewhere on the network.
Graphic T-7. A terminator can be used to test thinnet cabling.
An initiative from the European Telecommunications Standards Institute (ETSI) for a single standard for digital mobile radio services. Terrestrial Trunked Radio (Tetra) is defined in a memorandum of understanding between a number of different equipment vendors, service providers, testing bodies, and regulatory agencies that was laid out in 1994. Tetra consists of two complementary standards:
A standard Time Division Multiple Access (TDMA) cellular communication system for voice and data communication on 25-kHz channels
A Packet Data Optimized (PDO) protocol for packet-switched data-only transmission at 36 Kbps on 25-kHz channels
Tetra includes support for security features such as multilevel authentication and encryption, allows voice and data communication to be combined using the same equipment, and supports multiplexing of up to four channels to provide data rates of up to 144 Kbps. Tetra is complementary to the Global System for Mobile Communications (GSM) cellular communication standard: GSM can be considered an extension of the Integrated Services Digital Network (ISDN) to the wireless domain, while Tetra is an extension of ISDN Private Branch Exchange (PBX) systems to the same domain. Tetra thus provides additional communication functionality not built into GSM, such as direct mobile-to-mobile communication that bypasses the communication infrastructure, support for broadcast and group call features, fast call setup, priority call, and so on.
NOTE
Because of the recent growth of the Internet and wide demand for high-speed wireless mobile data services, a new high-speed wireless mobile packet-switching system called the Digital Advanced Wireless System (DAWS) is currently being developed by the ETSI to supersede the Tetra PDO standard.
On the Web
•
Tetra home page : http://tetramou.com
See also Digital Advanced Wireless System (DAWS)
A general name for equipment used to configure, diagnose, and troubleshoot networking and telecommunications systems. Test equipment is invaluable to busy network administrators for troubleshooting local area network (LAN) or wide area network (WAN) connections, to system integrators who install networks and communication services at customer premises, and to wiring and cabling installation service people. You can buy test equipment for dedicated, single-use testing purposes, but multifunction test equipment is more cost effective.
Test equipment comes in all shapes and sizes, from rack-mounted equipment for enterprise troubleshooting, to hand-held scanners and packet sniffers, to laptops that run special software and use special PCMCIA-attached probes. Here are some examples:
Copper cable testers: Typically hand-held devices that can test installed copper cabling for compliance with EIA/TIA standards for cabling system performance. These are usually multifunction devices that support both coaxial cabling and twisted-pair cabling. Two-way testers enable you to test a cable from both ends.
Fiber-optic cable testers: Usually a separate category from coax/twisted-pair cable testers. These devices might support testing of single-mode fiber-optic cabling, multimode fiber-optic cabling, or both, and provide detailed measurements in decibels for optical link budget (OLB) calculations to ensure that a fiber installation will support the intended equipment layout. A typical fiber tester consists of two modules: a light source for injecting signals into the system at 850 or 1300 nanometers (depending on the type of fiber) and a power meter to measure what comes out the other end. Some devices include both functions and can be used to test fiber that is still on the spool.
Token Ring testers: Test for shorts, opens, and grounds on shielded twisted-pair (STP) cabling in Token Ring installations.
LAN analyzers (sniffers): For troubleshooting problems with LAN protocols at all levels of the Open Systems Interconnection (OSI) reference model protocol stack, from lower-level protocols such as Data Link Control (DLC), IPX/SPX, NetBEUI, and TCP/IP to higher-level protocols such as File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), NetBIOS, Server Message Block (SMB), and Simple Mail Transfer Protocol (SMTP). These devices basically capture LAN traffic and allow you to analyze and filter packets that use specific protocols, that are transmitted and received from specific computers, that are portions of a specific communication session between two computers, and so on. Microsoft Network Monitor, which is included with Microsoft Systems Management Server (SMS), Microsoft Windows 2000, and Microsoft Windows NT, is a software-based sniffer that runs on any PC with a network card and can capture and analyze most forms of LAN traffic.
SCSI testers: Test Small Computer Systems Interface (SCSI) buses for shorts, opens, or improper termination. These are usually dedicated to a specific type of SCSI interface.
ISDN and T1 test equipment: Includes continuity testers, channel testers, and line-quality analyzers for testing Integrated Services Digital Network (ISDN) and T-carrier circuits. They can sample frames to check for jitter and lack of synchronization.
WAN analyzers: Test serial transmission protocols such as RS-232 and V.35, which are used to connect WAN devices such as routers and bridges to Channel Service Unit/Data Service Units (CSU/DSUs). They are typically used to troubleshoot frame relay, High-level Data Link Control (HDLC), Point-to-Point Protocol (PPP), Synchronous Data Link Control (SDLC), Serial Line Internet Protocol (SLIP), Systems Network Architecture (SNA), and X.25 connections. You can connect a WAN analyzer to a serial connection by using a Y-shaped connector called a data tap, which lets you monitor communication without interfering with the data being transmitted.
TIP
Use a cable tester on a new enhanced category 5 cabling installation before you install and configure your Fast Ethernet network equipment. Good-quality cable testers typically test all four pairs of wires in unshielded twisted-pair (UTP) cabling over frequencies of up to 100 MHz or higher, checking cable integrity for shorts and opens, measuring cable segment lengths using time domain reflectometry (TDR), and allowing measurement of attenuation, near-end crosstalk (NEXT), and PowerSum NEXT to an accuracy of 0.1 decibels or better.
Cable testers can trace cables through walls, ceilings, and floors by measuring the length of a cable and telling you whether the cable is terminated, has an open end, is connected to a port on a hub, and so on. You can plug two-way cable testers into a wall plate and test the patch panel to find out which cable connects to the wall plate.
See Terrestrial Trunked Radio (Tetra)
See ASCII file
An abbreviation for Trivial File Transfer Protocol, a TCP/IP file transfer protocol. TFTP differs from the popular File Transfer Protocol (FTP) in that it does not support any form of authentication. TFTP is defined in Request for Comments (RFC) 1350.
How It Works
TFTP copies files to and from remote hosts by using the User Datagram Protocol (UDP). The remote host must be running the TFTP service or daemon for the TFTP client to be able to communicate with it. In UNIX networks that use diskless workstations and the bootstrap protocol (BOOTP), TFTP is usually used to download the boot disk image from the BOOTP server to the workstation.
NOTE
Microsoft’s implementation of TCP/IP on Microsoft Windows NT does not include TFTP service but does include a command-prompt TFTP client. On the Microsoft Windows 2000 platform, in addition to the command-prompt TFTP client there is an optional TFTP service called the Trivial File Transfer Protocol Daemon (TFTPD), which is installed when the Remote Installation Services component of Windows 2000 Server is enabled.
The coaxial cabling used in standard Ethernet or 10Base5 networking. Thicknet coaxial cabling is usually 3/8 inch in diameter. It is fairly rigid, has an impedance of 50 ohms, and can carry signals up to 500 meters (1640 feet)—hence the designation 10Base5 for “10-Mbps baseband transmission over 500 meters.”
How It Works
To connect a computer to a thicknet cable, you attach a vampire tap to the cable. The tap pierces the cable’s insulation layers and makes contact with the signal-carrying copper core. The tap is connected to a transceiver, and a drop cable connects the transceiver to an AUI connector on the computer’s network interface card (NIC).
NOTE
Thicknet was commonly used in the 1980s, primarily for Ethernet cabling. It has largely been superseded by twisted-pair and fiber-optic cabling.
See also 10Base5, coaxial cabling
See thinnet
The thin coaxial cabling used for 10Base2 installations of Ethernet networking. Thinnet cabling is RG-58 coaxial cabling that is 3/16 inch in diameter and has an impedance of 50 ohms. Thinnet uses BNC connectors to connect cable segments, computers, and concentrators (hubs). Many older hubs, bridges, routers, and other networking devices have at least one thinnet port for connecting to 10Base2 networks. Thinnet was often used in the 1980s for workgroup or departmental local area networks (LANs); it has largely been replaced by unshielded twisted-pair (UTP) cabling.
TIP
Thinnet cables must be terminated at both ends. If communication is down, check the termination points, and then check for loose BNC T-connectors attached to the computers on the network. Thinnet cabling can become damaged if it is sharply bent or twisted, so handle it carefully. (It is not nearly as fragile as fiber-optic cabling, however.)
One place where thinnet is still useful is in electrically noisy environments such as shop floors in factories, where electromagnetic interference (EMI) caused by motors, generators, and other heavy equipment can disrupt communication over UTP. Coaxial cabling, with its internal shielding, can easily withstand the noise.
See also 10Base2, coaxial cabling
See Telecommunications Industry Association (TIA)
A certificate issued by the Kerberos service running on a Microsoft Windows 2000–based network that approves an authenticated session. A ticket contains a session key, the name of the user to whom the session key was provided, the expiration time for the ticket, and additional information. The ticket’s expiration time is configured so that the session length doesn’t exceed the period specified by the domain security policy. If a ticket expires while a client and server have an active session open, the Kerberos service informs both the client and the server to refresh the ticket and generates a new session key for them.
A cellular phone technology based on time-division multiplexing (TDM) techniques.
How It Works
Time Division Multiple Access (TDMA) is an analog cellular phone technology that evolved from the Advanced Mobile Phone Service (AMPS), which was developed in 1979. TDMA takes a cellular communication channel (frequency band) and slices it into a series of time segments, as in this example:
123123123...
Each cellular user is assigned the time slices with a given number and transmits information only for the duration of his or her time segments using the TDMA scheme. This means that voice communication must be buffered and transmitted as short bursts. The time segments are so small and the slicing frequency is so high that the user perceives a continuous communication channel. TDMA allows more communication sessions to be crammed onto a single cellular channel.
TDMA is used by both the 800-MHz frequency band of Digital Advanced Mobile Phone Service (D-AMPS) and the 1900-MHz frequency band of Personal Communications Services (PCS) technologies.
The first version of TDMA was developed in 1991 and was known as the IS-54 standard (developed by the EIA/TIA). It divided each 30-KHz channel into three multiplexed subchannels. A revised version using digital control channels was developed in 1994 and is known as the IS-136 standard or, more popularly, D-AMPS. Another cellular phone technology that is based on TDMA is the Global System for Mobile Communications (GSM), which multiplexes eight subchannels into a single 200-KHz channel.
See also cellular phone technology
A multiplexing method for transmitting multiple data streams in a single communication path.
How It Works
In time-division multiplexing (TDM), the data from different input channels is divided into fixed-length segments and then combined in round-robin fashion into a single output data stream, which can then be transmitted over a single channel transmission system and demultiplexed at the destination location. The segments can be created by the multiplexer itself or can be inherent in the input channel signals, such as fixed-length frames. For example, if input streams A, B, and C are divided into segments as shown here
A A1, A2, A3,... B B1, B2, B3,... C C1, C2, C3,...
the output stream will look like this:
MUX(ABC) A1, B1, C1, A2, B2, C2, A3, B3, C3,...
One weakness in this mechanism is that if an input channel does not have anything important to carry for a time, empty segments are inserted into the output stream anyway. For example, if channel A is not transmitting data, one-third of the output channel is not being used. You can overcome this weakness by using a more sophisticated multiplexing technique called statistical multiplexing.
NOTE
TDM is used in T1 lines to enable them to simultaneously carry 24 data channels by interleaving data into portions of a single 193-bit frame. For example, bits 1 through 8 represent channel 1, bits 9 through 16 represent channel 2, and so on to bits 185 through 192 for channel 24, plus bit 193 for synchronization. This framing process occurs 8000 times per second, producing a total throughput of 1.544 Mbps.
See also multiplexing
A cable testing technique for finding breaks or shorts in a cable.
How It Works
A time domain reflectometer is a device that sends a pulse onto a cable and measures the time that it takes for the reflection to return from a short or break in the cable. (This is analogous to the use of sonar to determine the depth of a sea.) The time interval between transmission and reception of the signal is called the signal delay; this delay can be used to determine the location of the short or break, typically within a few centimeters, even though the break might be hidden within the cable’s jacket and not visible. You can also use the reflectometer to determine the length of an undamaged cable and identify cables running through walls and false ceilings in a cabling installation.
Most high-quality cable testers can perform time domain reflectometry (TDR) tests in addition to their other functions. Time domain reflectometers are available for testing both copper cabling and fiber-optic cabling.
A form of Telnet service that enables access to mainframe hosts over a TCP/IP network. By using Microsoft SNA Server, users running a TN3270 client can connect to mainframe computers using the TN3270 service included with SNA Server.
How It Works
TN3270 (Telnet 3270) was developed as an alternate to the regular Telnet service for accessing mainframe computers. TN3270 provides a better look and feel than standard Telnet, but its numeric field handling and keyboard interface are somewhat clumsy. TN3270 provides keyboard emulation and block-mode service at the client level, thus freeing the mainframe from translation functions. TN3270 provides workstation emulation only and does not include file-transfer or printer-emulation services.
You can also use TN3270 to connect to AS/400 systems, but the AS/400 systems must translate the 3270 data stream into 5250 format and provide keyboard mapping between the 3270 and 5250 key sequences, a process that consumes additional CPU resources on the AS/400.
A form of Telnet service for letting users access AS/400 systems over a TCP/IP network using a TN5250 client terminal emulator.
How It Works
TN5250 (Telnet 5250) is to the AS/400 what TN3270 is to the mainframe. A TN5250 service included with Microsoft SNA Server lets TN5250 clients connect to AS/400 systems using SNA Server without installing TCP/IP on the AS/400. TN5250 offers full 5250 terminal emulation, including hot backup and security features similar to those included with the TN3270 service.
TN5250 provides 5250 workstation emulation that supports almost all the field attributes and keyboard sequences of a “real” SNA 5250 except text assist. TN5250 has a natural and intuitive look and feel; no conversion is required inside the AS/400. TN5250 provides workstation emulation only and does not include file-transfer or printer-emulation services.
A popular local area network (LAN) technology developed by IBM that still has a large installed base in many shops but has been greatly outpaced in recent years by different forms of Ethernet. Token Ring was standardized in the IEEE 802.5 specifications, which describe the implementation of a token-passing ring network configured as a physical star topology.
How It Works
In a Token Ring network, stations (computers) are wired in a star formation to a central wiring concentrating unit called a Multistation Access Unit (MAU). This unit concentrates wiring in a star topology but internally forms a logical ring topology over which network traffic can travel. Lobes connect the individual stations to the MAU. The maximum cable length for a lobe is 22.5 meters or 100 meters, depending on the cable type, but you can extend this distance up to 2.4 kilometers using repeaters designed for Token Ring networks. MAUs typically support 8 or 16 connections for attaching lobes. You can extend a Token Ring network by connecting MAUs to ring-out and ring-in ports to form larger rings that can support larger numbers of stations. Stackable MAUs simplify this process. You can connect up to 33 MAUs to form a network. Many MAUs support being connected by fiber-optic cabling to create networks that span a building or campus. Most MAUs also support in-band management by using Simple Network Management Protocol (SNMP) plus out-of-band management by using a serial interface.
Graphic T-8. Token Ring.
Token Ring networks typically operate at speeds of 4 or 16 Mbps, although speeds of up to 100 Mbps are possible with equipment from some vendors. Token Ring networks come in two types, both of which can operate at 4 or 16 Mbps:
Type 1: Generally uses shielded twisted-pair (STP) cabling with a special data connector developed by IBM for Token Ring installations. However, 16-Mbps MAUs generally have ports for RJ-45 or DB9 connectors.
Type 3: This type uses standard unshielded twisted-pair (UTP) cabling with RJ-45 connectors.
Type 1 is often considered more reliable than Type 3, but the larger installed base of UTP cabling makes Type 3 a viable option for new Token Ring installations. Type 1 configurations support up to 260 stations per ring, while Type 3 can support up to 72 stations per ring.
Token Ring stations pass a single data packet called a token from one computer to the next rather than let each node transmit independently, as in a contention-based network such as Ethernet. Only one token can be on the network at a time, so collisions do not occur in Token Ring networks as they do in Ethernet networks. This process is analogous to sending messages to a group of people by passing a hat.
In order to pass a token in a Token Ring network, each station must know who its neighbors are and must perform a check to make sure that the circuit is unbroken. Messages containing this information are continually sent around the ring. The token circulates so long as this message is received. To generate the required information, the first station online in the ring assumes the role of Active Monitor Station. It creates the token and is responsible for taking action if the token is lost or damaged. The Active Monitor Station sends out an Active Monitor Present frame every seven seconds to the next node down the line. Each node in turn informs its downstream neighbor that it is its Nearest Active Upstream Neighbor. An error-detection process called beaconing occurs if the ring breaks and the token fails to circulate. If the Active Monitor Station fails, another station assumes its role of monitoring the status of the network and generating a new token if the existing one is lost.
If a station wants to transmit data over the network, it waits until the token comes by; if the token has not been claimed by another station, it claims the token and inverts the monitor setting bit to mark it “busy” so that no other station can claim the token for a predefined but variable amount of time. The originating station then removes the last byte from the token (called the delimiter byte), appends data to the token, and appends the delimiter byte to the end to form a frame of variable length (up to 8000 bytes). The token with data circulates around the ring in one direction from station to station. (Each station acts as a repeater to regenerate and forward the token.) When it returns to the originating station, the token and the data are removed and a new token is generated and placed onto the network.
NOTE
The term “Multistation Access Unit” is sometimes abbreviated as MSAU instead of MAU to distinguish it from “media attachment unit,” a term used in older Ethernet networking technologies.
Distances between MAUs and attached stations are specified as lobe lengths, which refer to round-trip signal paths. Thus, a station with a lobe length of 200 meters actually uses a cable 100 meters long.
TIP
STP cabling for Type 1 Token Ring comes in nine types, two of which are common now:
Type 1 cable: Uses two-pair 22-gauge shielded, grounded solid copper wire. Use this type for longer cable runs such as those between wiring closets and work areas. The maximum lobe length is 200 meters.
Type 6 cable: Uses two-pair 26-gauge stranded, shielded copper wire and is more flexible (and looks nicer!) than Type 1 cable. Use this type for work areas in which cables will be visible or where equipment will be moved around frequently, and especially for patch cables. The maximum lobe length is 45 meters.
You can get both types of cable in an adapter cable version (terminated at one end with an IBM data connector and at the other end with a DB9 male connector) or a patch panel version (terminated at both ends with data connectors). Use patch panel cables to connect MAUs, and use adapter cables to connect stations to MAUs.
You can also get baluns, which can convert Type 1 IBM cabling to UTP cabling to connect different Token Ring types, and you can get special adapters that allow data connectors to be connected to RJ-45 ports so that you can use installed UTP cabling with Type 1 MAUs.
TIP
Some network interface cards (NICs) for Token Ring networking support software-configurable physical layer addressing. All your NICs must have unique MAC addresses.
Most MAUs and NICs are dual-speed and can run at either 4 or 16 Mbps, but not both. However, you can use bridges or routers to connect 4-Mbps Token Ring networks to 16-Mbps Token Ring networks.
The following table provides suggestions for troubleshooting Token Ring network problems.
Troubleshooting Tips for Token Ring Networks
Problem | Suggestion |
Mismatched ring speed | Be sure that all connected stations use 4 Mbps or that all use 16 Mbps. Do not mix stations of different speeds. |
Stations cannot receive | Check cables and reset the MAU. |
Conflicting MAC addresses | Use NIC configuration software to change the MAC address on one of the conflicting computers. |
Traffic congestion on the network | Segment the network by using a bridge or a router. |
Any domain that is directly under the root domain in the hierarchical Domain Name System (DNS). Top-level domains are few in number and are used to identify broad classes of Internet services. Except for country domains, the various top-level domains currently in existence are listed in the following table. A number of additional top-level domains are yet to be finalized and implemented.
Top-Level Domains
Domain | Description |
.com | Commercial businesses and personal domains |
.edu | Mostly U.S. universities and colleges |
.org | Nonprofit organizations |
.net | Networking and telecommunications companies |
.gov | American government branches and agencies |
.mil | U.S. military |
In addition to the domains listed in the table, countries as well as states and provinces within countries are identified by two-letter country codes. For example, .uk is the top-level domain for the United Kingdom, .ca is the top-level domain for Canada, and mb.ca is the top-level domain for the province of Manitoba in Canada. Although the .com domain is by far the most popular one today due to the way it is marketed, many businesses are forced to use other domains such as .net or their country domain because of the shortage of commercial top-level domains.
NOTE
A special domain called in-addr.arpa is used for reverse DNS name lookups (resolving a host name given the host’s IP address).
See also country code
The physical layout of computers, cables, switches, routers, and other components of a network. This term can also refer to the underlying network architecture, such as Ethernet or Token Ring. The word “topology” comes from topos, which is Greek for “place.”
How It Works
When you design a network, your choice of topology will be determined by the size, architecture, cost, and management of the network. Basic network topologies include the following:
Bus topology: The stations are connected in a linear fashion. An example is the 10Base2 form of Ethernet.
Star topology: The stations are connected to a single concentrating device called a hub (Ethernet) or a Multistation Access Unit, or MAU (Token Ring physical topology).
Ring topology: The stations are connected in a ring. Examples are Fiber Distributed Data Interface, or FDDI (logical and physical ring), and Token Ring (logical ring and physical star).
Mesh topology: The stations are connected in a complex, redundant pattern. This topology is generally used only in wide area networks (WANs) in which different networks are connected using routers.
Variations of these basic topologies include the following:
Star bus topology: Consists of many star networks whose concentrators (hubs) are connected in a linear bus fashion
Star-wired topology or cascaded-star topology: Consists of star networks whose hubs are joined in star formation to other hubs, forming a kind of tree-shaped network with the main hub at the top
NOTE
The term “topology” can refer to either a network’s physical topology, which is the actual physical layout or pattern of the cabling, or its logical topology, which is the path that signals actually take around the network. This difference is most evident in Token Ring networks, whose cabling is physically arranged in a star but whose signal flows in a ring from one component to the next. The term “topology” without any further description is usually assumed to mean the physical layout.
A TCP/IP utility in Microsoft Windows for diagnosing and troubleshooting router connections in an internetwork such as the Internet. The term “tracert” stands for trace route. The tracert utility uses Internet Control Message Protocol (ICMP) echo packets that are similar to those used by the ping utility. These ICMP echo packets are assigned a steadily increasing Time to Live (TTL) to test network connectivity with routers and other hosts that are farther and farther along the network path until connectivity fails or the target host is finally contacted and successfully responds.
Example
If you run
tracert www.yahoo.com
from Winnipeg via a local Internet service provider (ISP), you might get a display similar to the following, depending on the route your packets take:
Tracing route to www.yahoo.com [204.71.177.75] over a maximum of 30 hops: 1 193 ms 188 ms 192 ms tnt01.escape.ca [204.112.225.50] 2 195 ms 189 ms 199 ms bb.escape.ca [204.112.225.4] 3 216 ms 575 ms 248 ms escape.mbnet.mb.ca [204.112.54.194] 4 227 ms 239 ms 531 ms e0.manitoba.mbnet.mb.ca [204.112.54.193] 5 211 ms 210 ms 358 ms psp.mb.canet.ca [192.68.64.5] 6 269 ms 251 ms 244 ms border1-atm1-0.quebec.canet.ca [205.207.238.45] 7 224 ms 240 ms 269 ms psp.ny.canet.ca [205.207.238.154] 8 249 ms 274 ms 251 ms borderx2-hssi2-0.Boston.mci.net [204.70.179.117] 9 238 ms 304 ms 258 ms core2-fddi1-0.Boston.mci.net [204.70.179.65] 10 315 ms 310 ms 365 ms bordercore2-loopback.Bloomington.mci.net [166.48.176.1] 11 701 ms 677 ms 360 ms internet-connection.Bloomington.mci.net [166.48.177.254] 12 389 ms 384 ms 357 ms www.yahoo.com [204.71.177.75] Trace complete.
The destination host was reached after a distance of 12 hops. Note the gradually increasing response times.
NOTE
The UNIX version of this utility is typically called traceroute.
A method of coordinating a series of changes to a set of resources distributed over the network. Transactions are units of work that must succeed or fail as a whole—a transaction can never partially succeed. If a transaction fails while only partially completed, the transaction is rolled back to the beginning. An example is a credit card purchase: The store requests the purchase amount from the credit card company, the company distributes the funds to the store, and the company bills the purchaser. If any part of the transaction fails, the entire transaction must fail in order to prevent money from being lost.
Component Services on Microsoft Windows 2000 (or Microsoft Transaction Server on Microsoft Windows NT), a tool that provides the underlying support, or “plumbing,” for creating scalable, distributed, transactional Web applications, provides failure isolations and mechanisms for recovering failed transactions and can run components of transactions as isolated processes for additional crash protection. Component Services uses the Distributed Component Object Model (DCOM) programming architecture for communication between components on Microsoft Windows networks.
A technology that provides fault tolerance and crash recovery for critical database files. Transaction logs are used in products such as the Microsoft Exchange Server directory services database and information store and Microsoft SQL Server.
How It Works
Using Exchange Server as an example, data is written to transaction log files before it is applied to the directory or information store databases. This improves the performance of write operations to the Exchange databases. Transaction logs also play an important role in providing fault tolerance and recoverability for databases. If a system crash corrupts the database files, you can use the transaction logs (if they are intact) to restore all changes to the database since the last backup. Transaction logs make online incremental and differential backups possible. Without transaction logs, you would be able to perform full backups only when backing up databases online.
NOTE
In Exchange, you might have several transaction logs in your database directory. When a database is backed up, the transaction logs are then purged.
TIP
Use the Microsoft Exchange Performance Optimizer tool to make sure that transaction logs are located on a stripe set for maximum performance.
An electronic device for connecting a computer to a baseband transmission network so that the computer can transmit and receive signals on the network. In the 1980s, transceivers were often separate devices attached to thicknet cabling using vampire taps, but today most network interface cards (NICs) have onboard transceivers.
NOTE
Some Fast Ethernet NICs have a media independent interface (MII) to which an external transceiver can be connected to provide different kinds of 100-Mbps networking. This allows you greater flexibility in your networking options. For example, 100BaseTX transceivers have an RJ-45 port for connecting unshielded twisted-pair (UTP) cabling, and 100BaseFX transceivers have an SC-type port for connecting fiber-optic cabling.
See drop cable
A transport layer protocol that enables reliable, connection-oriented network communication.
How It Works
Transmission Control Protocol (TCP) is a connection-oriented protocol that guarantees data will be delivered intact to its destination. TCP first establishes a session by using a TCP three-way handshake with TCP ports on each host. It then transmits the data in packets, each with a sequence number. When packets are received at their destination, TCP generates an acknowledgment to the sending host. If a packet in a sequence is not received, TCP on the sending host retransmits the packet after a certain interval of time.
Microsoft’s implementation of TCP on Windows platforms includes advanced features such as self-tuning to ensure that data is sent at a speed optimal for the receiving host, dead gateway detection, and checksums for ensuring error-free delivery.
A standard or specification for a common programming interface for developing Microsoft Windows NT and Windows 2000 file system drivers (server or redirector components) and for providing independence between transport layer protocols and file system drivers. The Transport Driver Interface (TDI) allows one file system driver to be bound to many protocols or one protocol to work with multiple file systems.
See also I/O Manager
Layer 4 of the Open Systems Interconnection (OSI) reference model. The transport layer is responsible for providing reliable transport services to the upper-layer protocols. These services include the following:
Flow control to ensure that the transmitting device does not send more data than the receiving device can handle
Packet sequencing for segmentation of data packets and remote reassembly
Error handling and acknowledgments to ensure that data is retransmitted when required
Multiplexing for combining data from several sources for transmission over one data path
Virtual circuits for establishing sessions between communicating stations
NOTE
The Transmission Control Protocol (TCP) of the TCP/IP protocol suite resides at the transport layer.
See domain tree
In switched Ethernet networking, any method of aggregating the physical network links into a single logical link. Trunking provides a way of overcoming the bandwidth limitations of a single physical link and is used in both switch-to-switch and switch-to-server connections to relieve traffic congestion. A number of vendors have implemented trunking hardware and/or software, and a standard called IEEE 802.3ad that ensures interoperability among the different vendor offerings should be approved soon.
How It Works
Trunking is essentially a form of inverse multiplexing and is often used to aggregate multiple wide area network (WAN) connections into a single connection. In the switched local area network (LAN) environment, trunking was originally used to reduce congestion in switch-to-switch connections. By aggregating several 100-Mbps links between Fast Ethernet switches, you can achieve data rates of 300 or 400 Mbps between the switches to accommodate network backbone traffic. In a full-duplex configuration, this means rates of 600 or 800 Mbps, which rivals the more expensive Gigabit Ethernet technology and gives new life to old switches.
You can also implement trunking in switch-to-server connections so that multiple connections to a single server can be aggregated. This form of trunking can be purely software based or can be implemented as a combination of both hardware and software. For example, trunking software installed on multiple network interface cards (NICs) in the server automatically handles load balancing across the various server interfaces and can remove an interface from the trunking group if the interface goes down. This provides increased bandwidth between the server and the switch and ensures fault-tolerant operation.
Graphic T-9. Trunking.
Trunking comes in two varieties:
Symmetrical trunking: Allows any port in a trunking group to transmit packets to any other port. Full-duplex connections are thus supported over all links in the group. For example, a server can both transmit and receive data at 400 Mbps in a trunked group of four interfaces and one switch.
Asymmetrical trunking: Allows any port in a trunking group to transmit packets but allows only one port (the port on the switch) to receive packets. The server can transmit data at 400 Mbps but can receive data at only 100 Mbps.
NOTE
Trunking by itself is limited to point-to-point connections between two switches or between a switch and a server. However, you can use the Multipoint Link Aggregation (MPLA) technology developed by 3Com to aggregate physical links connected to different switches into a single logical link. MPLA thus supports multipath trunking between multiple switches and servers, giving network administrators flexibility in configuring their hardware for optimal traffic servicing. Other vendors are working on similar multipath trunking technologies, but no standards have emerged yet.
Although the theoretical speed for quadruple-trunked full-duplex Fast Ethernet connections is 800 Mbps, in practice the maximum achievable rate is about 560 Mbps because of traffic overhead.
TIP
Software-based trunking adds overhead of up to 5 percent to the server’s CPU, depending on the software and the NIC used. Look for special NICs from trunking software vendors with on-board processors that can run the trunking software and thus reduce the load on the CPU. Also, don’t mix and match trunking software or hardware from different vendors in a single trunking group.
Switches must be intelligent if they are to properly support trunked connections. Check your switch documentation before you attempt to implement trunking on your network.
Not only is it often more economical to trunk Fast Ethernet lines than to upgrade to Gigabit Ethernet, but trunked Fast Ethernet cable runs can go farther than Gigabit Ethernet cable runs can. However, in certain situations trunking does not improve things. For example, trunking cannot speed up server-to-server backups.
See trust relationship
A secure communication channel between two domains in Microsoft Windows NT or Windows 2000. Trust relationships allow users in one domain to access resources in another domain. Trusts work by having one domain trust the authority of the other domain to authenticate its user accounts.
How It Works
In Windows NT, trusts are one-way—the trusting domain (or resource domain) trusts the trusted domain (or accounts domain). This means that global users in the trusted domain can be authenticated for accessing resources in the trusting domain. Global users from the trusted domain can log on to any computer in either domain and can access resources in either domain if they have the appropriate permissions.
If you want to establish a two-way trust between two domains, you must create two trusts, one in each direction. Administrators can set up trust relationships between domains by using the Policies menu in User Manager for Domains. The administrator on the accounts domain should permit the trust first, and then the administrator on the resource domain should complete the trust. Only global accounts (global users and global groups) can cross trusts.
Windows NT trusts are nontransitive. In other words, if domain A trusts domain B and domain B trusts domain C, it is not true that domain A trusts domain C.
By using trusts, you can join Windows NT domains into a variety of domain models, including the complete trust model, the master domain model, and the multiple master domain model. You can join domains to support 100,000 or more users for enterprise-level networks.
Windows NT trusts, which are based on the Windows NT Challenge/Response Authentication, are managed by the Windows NT Directory Services (NTDS).
Graphic T-10. Trust relationship.
In Windows 2000, trusts are always two-way. If domain A trusts domain B, users in either domain can access resources in the other domain if they have the appropriate permissions. Windows 2000 trusts are also transitive. In other words, if domain A trusts domain B and domain B trusts domain C, domain A also trusts domain C.
Windows 2000 trusts are much easier to manage than Windows NT trusts, primarily because there are far fewer trusts to manage. Windows 2000 domains are combined into hierarchical structures called domain trees. All users in a domain tree can access resources in any domain of the tree if they have suitable permissions. In Windows 2000, you can also use another type of trust called an explicit trust, which is a one-way trust similar to that implemented in Windows NT, to form a trust relationship between two domain forests.
Windows 2000 trusts are managed by Active Directory and are based on the Kerberos v5 security protocol.
TIP
If you are unable to establish a trust relationship between two domains, make sure that no sessions are open between the two primary domain controllers (PDCs) and that they are using common transport protocols.
See Remote Desktop Protocol (RDP)
A technology for sending frames from one network to another. In tunneling, frames from the source network are encapsulated in the frame format of a different protocol and then sent over the link, called a tunnel. Frames are unencapsulated at the destination network and forwarded to their destination node.
Tunneling technologies include the following:
Internetwork Packet Exchange (IPX) tunneling over Internet Protocol (IP) internetworks, which allows IPX packets to be encapsulated in an IP packet and routed over the TCP/IP internetwork until they reach the destination local area network (LAN), where they are unwrapped into IPX packets. This process permits NetWare clients and servers using IPX to communicate over a TCP/IP internetwork.
Systems Network Architecture (SNA) tunneling over IP internetworks.
Point-to-Point Tunneling Protocol (PPTP).
Layer 2 Tunneling Protocol (L2TP).
A form of coaxial cabling with twin central conducting cores. Twinax cabling typically uses 20 AWG stranded copper conductors, has an outside diameter of 1/3 inch, and comes with either a polyvinyl chloride or plenum jacket. Twinax cabling typically has an impedance of 80 to 100 ohms. Twinax cabling is used primarily for connecting IBM System 3X or AS/400 systems to 5250 terminals.
TIP
To extend a twinax connection over long distances, use a repeater. Twinax repeaters can typically transmit signals up to 1 mile over unshielded twisted-pair (UTP) cabling and over longer distances using duplex fiber-optic cabling. One repeater is required at both ends of the connection.
Use a multiport repeater (hub) to connect several terminals over a single connection to an AS/400 or System 3X host. You can use twinax-to-RJ-45 baluns to connect the terminals and mainframe host to the hub by using UTP cabling. Some repeaters have RJ-11 ports for extending twinax connections over standard telephone cabling. Twinax cabling is traditionally used in a daisy-chained topology, but if you use a multiport repeater, you can also use a star topology configuration.
A form of copper cabling that consists of one to four pairs of color-coded insulated stranded copper wires that are twisted together in pairs and enclosed in a protective outer sheath. Twisted-pair cabling is terminated with RJ-11 connectors and was originally developed for the telephone system. It is now also the cabling of choice for networking workgroups and departmental local area networks (LANs). Twisted-pair cabling for networking purposes has RJ-45 connectors at each end.
How It Works
In computer networking environments that use twisted-pair cabling, one pair of wires is typically used for transmitting data while another pair receives data. The twists in the cabling reduce the effects of crosstalk and make the cabling more resistant to electromagnetic interference (EMI), which helps maintain a high signal-to-noise ratio for reliable network communication. Twisted-pair cabling used in Ethernet networking is usually unshielded twisted-pair (UTP) cabling, while shielded twisted-pair (STP) cabling is typically used in Token Ring networks. UTP cabling comes in different grades for different purposes, the most common of which is category 5 cabling.
NOTE
In a telephone environment, one pair is sufficient for phone communication to take place. Most customer premises wiring established by telcos uses two-pair wiring in case a second phone line is later needed for fax or modem use.
A trust relationship between two domains in Microsoft Windows 2000. By default, a Windows 2000 trust is two-way, meaning that each domain trusts the authority of the other domain for authentication. A Windows 2000 trust is also transitive—if domain A trusts domain B and domain B trusts domain C, domain A trusts domain C. Windows 2000 two-way transitive trusts are based on the Kerberos v5 security protocol.
Because of the two-way transitive nature of Windows 2000 trusts, all domains in a domain tree implicitly trust each other. This means that resources of one domain are available to users in all other domains in the domain tree if they have suitable permissions.
NOTE
You can also create one-way nontransitive trusts for Windows 2000–based networks. These one-way trusts are similar to the trust relationships formed by Microsoft Windows NT domain controllers. A one-way trust between a domain and a domain tree provides users of the domain with access only to the domain in the tree to which it is joined. One-way trusts can be useful when domains require a less permanent relationship—for example, when two companies take part in a joint venture. Only the resources needed by the other company are made available to the trusted domain; the entire domain tree is not exposed.
See also Active Directory