ToggIT Technopedia
 

Toggit's Encyclopedia of Networking Terms and Computer Dictionary.

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T See tera-.

Search Alphabetical Listings for the letter 'T' Technology Speed Distance Limitation Conventional modem 56Kbps downstream, up to 33.6Kbps upstream None ISDN Up to 128Kbps symmetric 18,000 ft Cable modem Up to 30Mbps downstream, to 10Mbps upstream 30 mi ADSL 1.5 to 8Mbps downstream, to 1.544Mbps upstream 18,000 ft SDSL 1.544 to 2.048Mbps symmetric 10,000 ft HDSL 1.544 to 2.048Mbps symmetric (over 3 phone lines) 15,000 ft VDSL 13 to 52Mbps downstream, 1.5 to 2.3Mbps upstream 4,500 ft   container administrator device; they will not operate from a workstation. In some systems, the console is a virtual device that can be invoked from any workstation by a network administrator with the appropriate rights and privileges. THE MAC SUBLAYER medium attachment unit Abbreviated MAU. A transceiver that attaches to the AUI (Attachment Unit Interface) port on an Ethernet adapter and provides electrical and mechanical attachments to fiber-optic cable, twisted-pair cable (TP), or other media types. THE OSI REFERENCE MODEL   out-of-band signaling T See tera-. T1 A long-distance, point-to-point circuit, providing 24 channels of 64Kbps, giving a total bandwidth of 1.544Mbps. The standard T1 frame is 193 bits long, made up of twenty-four 8-bit voice samples and one synchronization bit. It transmits 8000 frames per second. When a T1 service is made available in single 64Kbps increments, it is known as fractional T1. In Europe, the comparable circuit is known as E-1, and it has a speed of 2.054Mbps. T1 has been superseded by the CCITT DS-1 designation. See also fractional T1; T-carrier. T1 multiplexer A statistical multiplexer that divides the 1.544MbpsT1 bandwidth into 24 separate 64Kbps channels of digitized data or voice. See also fractional T1; T1. T1 small aperture terminal Abbreviated TSAT. A small satellite terminal used for digital communications that can handle T1 data rates of up to 1.544Mbps. See also fractional T1; T1. T.120 A group of communications and applications protocols that support realtime, multipoint data communications over local area networks and ISDN, dial-up, and Internet connections. T.120 became well known after Microsoft incorporated it into the NetMeeting package. See also H.323; Microsoft NetMeeting. T2 A long-distance, point-to-point communications service, providing up to 4 T1 channels. T2 offers 96 channels of 64Kbps, for a total bandwidth of 6.3Mbps. T2 is not available commercially, although it is used within telephone company networks. See also T1; T-carrier. T3 A long-distance, point-to-point communications service, providing up to 28 T1 channels. T3 can carry 672 channels of 64Kbps, for a total bandwidth of 44.736Mbps, and is usually available over fiber-optic cable. T3 is used almost exclusively by AT&T and the regional telephone operating companies, although certain large private corporations are using T3 with digital microwave or fiberoptic networks. In Europe, T3 has been superseded by the CCITT DS-3 designation. See also T1; T-carrier.   T4 T4 A long-distance, point-to-point communications service, providing up to 168 T1 channels. T4 can carry 4032 channels of 64Kbps, for a total bandwidth of 274.176Mbps. T4 can be used for both digitized voice and data transmission. See also T1; T-carrier. table In a relational database system, a table is comparable to a database file, but is more highly structured. The organization of a table is logical, not physical. Each row (or record) in a table contains a unique key, or primary key, so that any item of data in the table can be retrieved by referring only to that key. Through the process known as normalization, all data items in a row are made to depend only on this primary key. View and data dictionaries in a relational database take the form of two-dimensional tables. tag An element in HTML used to annotate a document. A tag is text enclosed by angle brackets that tells the client Web browser how to display each part of the document. For example, the tag <H1> indicates the start of a level one heading, and the tag </H1> indicates the end of a level one heading. See also element; HyperText Markup Language. tag switching A technology from Cisco Systems, Inc., that integrates data-link layer switching with network layer routing for large-scale networks, allowing the integration of ATM switches into the Internet and implementing routing on top of those switches. See also Asynchronous Transfer Mode; IP over ATM; IP switching. tap A connector that attaches to a cable without blocking the passage of information along that cable; a connection onto the main transmission medium of the network. tape cartridge A self-contained tape storage module, containing tape much like that in a video cassette. Tape cartridges are primarily used to back up hard-disk systems. See also DC-2000; quarter-inch cartridge; Zip drive. tape drive A computer peripheral device that reads from and writes to magnetic tape. The drive may use tape on an open reel or from an enclosed tape cartridge. Because tape-management software must search from the beginning of the tape every time it wants to find a file (a process called sequential access), tape is too slow to use as a primary storage system; however, tapes are frequently used to back up hard disks. See also streaming tape. TAPI See Telephony API. tar In Unix, a utility program that can create, list, add to, and retrieve from an archive file, which is usually stored on tape. The archive file has the filename extension .tar. The archive created by tar is not compressed and can be further processed by the Unix   T-carrier compress (extension .Z) or gzip (extension .gz) utilities to give compound extensions such as filename.tar.Z or filename.tar.gz. task Any independent running program and the set of system resources that it uses. A task may be an operating system process or part of an application. See also context switching; multitasking; task switching. Task Manager In Microsoft Windows NT, an application that allows a user to manually inspect the tasks running on the computer and select individual tasks for shutdown. Task Manager is opened by pressing Ctrl+Alt+Del. task switching To change from one running program to another quickly, either at the direction of the operating system or at the request of the user. TB See terabyte. T-carrier A digital communications service from a common carrier for voice or data transmission. The four-level, time-division multiplexing specification for the U.S. telephone system allows the bit stream of the smaller carriers to be multiplexed into the larger ones. The following are the four service levels: _ T1 Provides 24 channels of 64Kbps, giving a total bandwidth of 1.544Mbps. When a T1 service is made available in single 64Kbps increments, it is known as fractional T1. _ T2 The equivalent of 4 T1 services, T2 offers 96 channels of 64Kbps, for a total bandwidth of 6.3Mbps. _ T3 The equivalent of 28 T1 circuits, T3 offers 672 channels of 64Kbps, for a total bandwidth of 44.736Mbps. T3 is available commercially, but is not often used for LANs. _ T4 The equivalent of 168 T1 circuits, T4 provides 4,032 channels of 64Kbps, for a total bandwidth of 274.176Mbps. See also T1; T2; T3; T4.   Tcl T-CARRIER Tcl See Tool Command Language. TCM See trellis-coded modulation. TCO See total cost of ownership. T-connector A T-shaped device, used with coaxial cable, that connects two thin Ethernet cables and also provides a third connector for the network interface card.   TCP ports T-CONNECTOR TCP See Transmission Control Protocol. TCP/IP See Transmission Control Protocol/Internet Protocol. TCP ports In a TCP/IP network when a computer connects with another computer to access a specific service, and end-to-end connection is established and a socket is set up at each end of the connection. This socket is created at a particular port number, depending on the application in use. Table T.1 lists the port numbers used for some of the common protocols. TABLE T.1 PORT NUMBERS FOR COMMON PROTOCOLS TDM See time-division multiplexing. TDMA See Time Division Multiple Access. TDR See time-domain reflectometer. Technical and Office Protocol Abbreviated TOP. An Ethernet implementation for use in an engineering environment, developed by Boeing Corporation. See also Manufacturing Automation Protocol. telecommunications A general term for the electronic transmission of all forms of information, including digital data, voice, fax, sound, and video, from one location to another over some form of communications link. Telecommunications Industry Association Abbreviated TIA. An important trade group, active in the areas of standards development, trade shows, international marketing, and legislative efforts. The TIA often works in close association with the Electronic Industries Association (EIA). For more information on TIA, see www .tiaonline.org . See also Electronic Industries Association. telecommuting Working at home on a computer connected to the office by modems and telephone lines instead of commuting to the office. Telecommuting saves time, cuts down on automobile use and pollution, and decreases stress. Some local and state governments actively encourage telecommuting to keep the number of commuters as low as possible. Most studies indicate that home workers are happier and more productive. However, some jobs do not lend themselves to telecommuting; welding and brain surgery are very difficult to do over the phone. See also small office/home office. teleconferencing The use of audio, video, animation, and application sharing, linked by a communications channel, to allow widely separated individuals to take part in a discussion or meeting. Desktop video and chalkboard programs such as Microsoft NetMeeting are becoming more and more common, and groupware applications such as Lotus Notes are helping people work together. Telephony API Abbreviated TAPI. A standard telephone interface for Microsoft Windows, developed by Intel and Microsoft, designed to allow applications to set up and control calls. TAPI does not define the method of data transmission used once a call is in progress. It is completely independent of the telephone network itself and can be used on public-switched telephone networks, ISDN, and IP networks. See also Telephony Services API. Telephony Services API Abbreviated TSAPI. A standard telephone interface developed by AT&T and Novell, designed to allow applications to set up and control calls. TSAPI is available as a NetWare Loadable Module for NetWare 4.01 and later.   terminator TSAPI requires that the PBX (private branch exchange) be linked to a server and so has greater control over the call than does Telephony API. See also Telephony API. Telnet A terminal emulation protocol, part of the TCP/IP suite of protocols and common in the Unix world, that provides remote terminal-connection services. The most common terminal emulations are for Digital Equipment Corporation (DEC) VT-52, VT-100, and VT-220 terminals, although many companies offer additional add-in emulations. See also tn3270. temporary swap file A swap space that is created every time it is needed. A temporary swap file can consist of several discontinuous pieces of hard-disk space. A temporary swap file does not occupy harddisk space if the application that created it is not running. See also swap space; permanent swap file; virtual memory. ter A term describing a tertiary CCITT recommendation, an alternative or extension to the primary or secondary recommendation. See also bis. tera- Abbreviated T. A prefix that means 10 12 in the metric system, 1,000,000,000,000; commonly referred to as one trillion in the American numbering system, and one million million in the British numbering system. terabyte Abbreviated TB. In computing, usually 2 40 , or 1,099,511,627,776 bytes. Terabytes are used to represent extremely large hard-disk capacities. terminal emulation A method of operation or software that makes a personal computer or a workstation act like a terminal attached to a mainframe, usually for the purpose of telecommunications. Communications programs often include popular emulations, such as ANSI, VT-52, VT-100, VT-200, and TTY. See also Telnet; tn3270. Terminal services In Microsoft Windows 2000 Server, an optional component that provides multiple user sessions from the same machine; also known as thinclient computing, multiuser Windows, or server-based computing. With Terminal services installed, the major portion of all application processing takes place on the server, rather than on the client, allowing you to use cheaper client hardware, install application software on the server rather than on every client, and administer the server from a remote console. terminate-and-stay-resident program Abbreviated TSR. A small accessory or utility program that stays loaded in memory, even when it is not actually running. A TSR can be invoked quickly to perform a specific task. TSR’s are often used with single-tasking operating systems such as MS-DOS. terminator A device attached to the last peripheral device in a series or the last node on a network.   TEX For example, the last device on a SCSI bus must terminate the bus; otherwise, the bus will not perform properly. A 50-ohm resistor is placed at both ends of an Ethernet cable to prevent signals reflecting and interfering with the transmission. See also active termination; forced perfect termination; passive termination. TEX A typesetting language developed by Donald E. Knuth of Stanford University, capable of professional-quality typeset text, particularly of mathematical equations and scientific, Japanese, Chinese, Cyrillic, and Arabic text. TEX is not easy for the casual user to master, but several packages are available containing macros designed to solve specific typesetting problems. text file See ASCII file. text mode A mode in which the computer displays characters on the screen using the built-in character set, but does not show any graphics characters or a mouse pointer. Also known as character mode. TFTP See Trivial File Transfer Protocol. thick Ethernet Connecting coaxial cable used on an Ethernet network; also known as thicknet. The cable is 1 centimeter (0.4 inch) thick, almost as thick as your thumb, and can be used to connect network nodes up to a distance of approximately 1006 meters (3300 feet). Thick Ethernet is primarily used for facility-wide installations. See also thin Ethernet. thicknet See thick Ethernet. thin client In a client/server network, a client that requires relatively small amounts of local memory and hard-disk space and leaves most of the processing to the server. Sometimes called a Windows terminal. In some cases, the client operating system, as well as the applications the client runs and the data it manipulates, all reside on the server. See also NetPC; network computer; total cost of ownership; Zero Administration for Windows. thin-client computing See Terminal services. thin Ethernet Connecting coaxial cable used on an Ethernet network; also known as thinnet. The cable is 5 millimeters (0.2 inch) thick, about as thick as your little finger, and can be used to connect network nodes up to a distance of approximately 165 meters (500 feet). Thin Ethernet is primarily used for office installations. See also thick Ethernet. thinnet See thin Ethernet. thrashing An excessive amount of disk activity that causes a virtual memory system to spend all its time swapping pages in and out of memory, and no time executing the application. Thrashing can be caused when poor system configuration creates a swap file that is too small or when insufficient memory is installed in the computer. Increasing the size of the swap file or adding memory are often the best ways to reduce thrashing.   TIA/EIA structured cabling standards thread 1. A concurrent process that is part of a larger process or program. In a multitasking operating system, a single program may contain several threads, all running at the same time. For example, one part of a program can be making a calculation while another part is drawing a graph or a chart. 2. A connected set of postings to a Usenet newsgroup. Many newsreaders present postings as threads rather than in strict chronological order. See also multiprocessing; newsgroup; newsreader; Usenet. threaded newsreader An application used to read the articles posted to Usenet newsgroups. A threaded newsreader groups the newsgroup posts into threads of related articles, whereas unthreaded newsreaders present them in their original order of posting. Of the two types, threaded newsreaders are much more convenient to use. See also newsgroup; newsreader; Usenet. throughput A measure of the data-transfer rate through a complex communications or networking scheme. Throughput is considered an indication of the overall performance of the system. For example, the throughput of a server depends on the processor type, operating system in use, hard disk capacity, network interface card in use, and the size of the data transfer buffer. In communications, throughput is usually measured as the number of bits or packets processed each second. See also bandwidth. TIA See Telecommunications Industry Association. TIA/EIA structured cabling standards Standards specified by the Electronics Industry Association/ Telecommunications Industries Association (EIA/TIA), including: _ ANSI/EIA/TIA-568-1991 Commercial Building Telecommunications Wiring. _ EIA/TIA TSB-36 Additional Cable Specifications for UTP Cables. 1991. _ EIA/TIA TSB-40 Telecommunications Systems Bulletin—Additional Transmission Specifications for UTP Connecting Hardware. 1992. _ ANSI/EIA/TIA-568A 1995 revises the original 568 document and adds material from TSB-36 and TSB-40. _ ANSI/EIA/TIA-569-1990 Commercial Building Standard for Telecommunications Pathways and Spaces. _ ANSI/EIA/TIA-570-1991 Residential and Light Commercial Telecommunications Wiring Standard. _ ANSI/EIA/TIA-606-1993 Administration Standard for the Telecommunications Infrastructure of Commercial Buildings. _ ANSI/EIA/TIA-607-1994 Commercial Building Grounding and Bonding Requirements for Telecommunications. Local codes and standards may impose additional requirements. See also cabling standards.   ticket ticket A token within the Kerberos authentication system that contains the user’s name and address, as well as the service the user requested, security information, a time deadline, and other authentication information. See also authentication; Kerberos. tie line A private circuit, leased from a communications carrier, connecting two or more points in a single organization. Time Division Multiple Access Abbreviated TDMA. A technique used to allocate multiple channels on the same frequency on a cellular phone or satellite communications system. See also Code Division Multiple Access. time-division multiplexing Abbreviated TDM. A method of sharing a transmission channel by dividing the available time equally between competing stations. At the receiving end, the different signals are merged back into their individual streams. See also frequency-division multiplexing; inverse multiplexing; statistical multiplexing. time-domain reflectometer Abbreviated TDR. A diagnostic tool used to detect cabling faults. A TDR calculates the length of a cable by measuring the time it takes for a reflected pulse to return to the TDR and then multiplying that by the nominal velocity of propagation. time to live Abbreviated TTL. A mechanism used to ensure that misdirected information doesn’t end up traveling a TCP/IP network or the Internet for all eternity. Each IP datagram contains a TTL value; once this value reaches zero, the datagram is simply assumed to be undeliverable and is discarded. timeout Many procedures require a device to respond or reply to an inquiry within a certain period of time; if the device does not respond, a timeout condition occurs, thus preventing the procedure from hanging up the computer. Timeouts are also used in communications to detect transmission failures. Some timeouts are fixed, such as the amount of time during which an operating system will attempt to access a modem or printer; others can be specified by the user. time-slice multitasking A form of multitasking in which the operating system assigns the same small time period to each process in turn. See also cooperative multitasking; preemptive multitasking. time stamp An identification code that includes the time that an event took place. Most operating systems add a time stamp to indicate a file’s create time. Automatic error logging or security auditing processes often add a time stamp to critical events such as changes to passwords or accounts. time synchronization A method of synchronizing time across all servers on the network so that all servers report the same time.   token-ring network tn3270 A special version of the Telnet program specifically designed for use with large IBM computers using 3270 and 327 x series terminals. Most of the computers on the Internet use Unix, but if you ever encounter an IBM mainframe, you will definitely need tn3270. So how do you know when to use tn3270 rather than Telnet? It’s time to load up tn3270 if you try to connect to an Internet host with Telnet and one of the following happens: _ The on-screen messages are all in uppercase letters rather than the usual Unix mix of uppercase and lowercase letters. _ You see VM or MVS anywhere in the login message. These are both names of IBM operating systems. _ Your session is aborted before it really gets started. See also Telnet. token passing A network access method that uses a circulating electronic token to prevent multiple nodes from transmitting on the network simultaneously. Before a node can transmit, it must be in possession of the token. Fiber Distributed Data Interface (FDDI), Token Ring, and Token Bus networks all use token passing to avoid packet collisions. See also Carrier Sense Multiple Access/ Collision Detection; token-ring network. Token Ring network IBM’s implementation of the token-ring network architecture, which uses a token-passing protocol transmitting at 4 or 16Mbps. Using standard telephone wiring, a Token Ring network can connect a maximum of 72 devices; with shielded twisted-pair (STP) wiring, each ring can support a maximum of 256 nodes. Although it is based on a closed-loop ring structure, a Token Ring network uses a star-shaped cluster of as many as eight nodes, all attached to the same wiring concentrator or Multistation Access Unit (MAU). The MAUs are then connected to the main ring circuit. A Token Ring network can include personal computers, minicomputers, and mainframes. The IEEE 802.5 standard defines token-ring networks. token-ring network A LAN with a ring structure that uses token passing to regulate traffic on the network and avoid collisions. On a token-ring network, the controlling network interface card generates a token that controls the right to transmit. This token is continuously passed from one node to the next around the network. When a node has information to transmit, it captures the token, sets its status to busy, and adds the message and the destination address. All other nodes continuously read the token to determine if they are the recipient of a message. If they are, they collect the token, extract the message, and return the token to the sender. The sender then removes the message and sets the token status to free, indicating that it can be used by the next node in sequence. See also 802.5; Carrier Sense Multiple Access/Collision Detection; Token Ring network.   Tool Command Language TOKEN RING NETWORK Tool Command Language Abbreviated Tcl, pronounced “tickle.” Developed by John Ousterhout while at the University of California at Berkeley, Tcl is a general-purpose extensible scripting language supplied as a C library on Unix systems. Tcl is also available in versions for MS-DOS, Windows, and the Macintosh. See also Perl; REXX; scripting. TOP See Technical and Office Protocol. top-level domains On the Internet, the highest category of host name, which either signifies the type of institution or the country of its origin. In the United States, the most common toplevel domains include: _ com A commercial organization. Most companies end up in this category. _ edu An educational establishment such as a university. _ gov A branch of the U.S. government. _ int An international organization such as NATO or the United Nations. _ mil A branch of the U.S. military.   traffic _ net A network organization. _ org A nonprofit organization. Most countries also have unique domains named after their international abbreviations; for example, ca represents Canada, uk represents Great Britain, and jp represents Japan. See also domain; domain name: Domain Name Service. topology The map of a network. Physical topology describes where the cables are run and where the workstations, nodes, routers, and gateways are located. Networks are usually configured in bus, ring, star, or mesh topologies. Logical topology refers to the paths that messages take to get from one user on the network to another. TOPS A local-area network (LAN) from TOPS Corporation that uses the LocalTalk protocol to connect Apple computers, PCs, and Sun workstations. Toshiba Corporation One of the largest manufacturers of electrical equipment in the world, a major supplier of notebook computers, and a co-developer of the digital video disc. For more information on Toshiba Corporation, see www.toshiba.com . total cost of ownership Abbreviated TCO. A term first used by the Gardener Group in an attempt to quantify the real cost of a particular computer solution. TCO encompasses the direct costs of the hardware and the software required and then adds in costs for maintenance and support, costs for the users performing their own technical support rather than their official job, and system productivity costs. See also NetPC; network computer; thin client; Zero Administration for Windows. TP See twisted-pair cable. trace log In Microsoft Windows 2000, a System Monitor log used to monitor events such as disk input/output or page faults; when the event occurs, it is logged into the trace log. See also alert log; counter log; page fault; System Monitor. tracert A utility used on TCP/IP networks to trace the route that datagrams take between the server and another system. As tracert also tells you how long each hop takes, it can be a useful tool in identifying system trouble spots. trackball A device used for pointing, designed as a space-saving alternative to the mouse. A trackball contains a movable ball that you rotate with your fingers to move the cursor on the screen. Because it does not need the area of flat space that a mouse needs, trackballs are popular with users of portable computers. The Apple PowerBook includes a trackball as part of the keyboard case, Microsoft has released a small trackball that clips onto the side of a laptop computer, and IBM has developed a dualbutton, touch-sensitive pointing stick called the TrackPoint. traffic The flow of messages and data carried by a communications channel or link. Traffic on a data network is normally measured in bits transferred in a given time period.   transaction transaction A single activity within a computer system, such as an entry into an airline reservation database, that is executed in real time rather than as a batch process. transaction processing A processing method in which transactions are executed immediately when they are received by the system, rather than at some later time as in batch-processing systems. Airline reservation databases and automatic teller machines are examples of transactionprocessing systems. See also on-line transaction processing; roll back; roll forward; two-phase commit. Transaction Tracking System Abbreviated TTS. A fault-tolerant feature of Novell NetWare that maintains the integrity of databases by backing out or rolling back incomplete transactions that result from a failure in a network component. transceiver A contraction of transmitter/ receiver. A device capable of both transmitting and receiving data. The data may be located on the network interface card that connects a workstation to a network, or it may be on a separate device. A transceiver can convert between an AUI (Attachment Unit Interface) Ethernet connection and another type of cabling, such as fiber-optic, coaxial, or unshielded twisted pair (UTP). transfer rate See data-transfer rate. transient See surge. transitive trust See two-way transitive trust. Transmission Control Protocol Abbreviated TCP. The transport-level protocol used in the TCP/IP suite of protocols. It works above IP in the protocol stack and provides reliable data delivery over connection-oriented links. TCP adds a header to the datagram that contains the information needed to get the datagram to its destination. The source port number and the destination port number allow data to be sent back and forth to the correct processes running on each computer. A sequence number allows the datagrams to be rebuilt in the correct order in the receiving computer, and a checksum verifies that the data received is the same as the data sent. In addition to these fields, the TCP header contains the following information: _ Acknowledgment number Indicates that the data was received successfully. If the datagram is damaged in transit, the receiver discards the data and does not send an acknowledgment to the sender. After a specified timeout expires, the sender retransmits data for which no acknowledgment has been received. _ Offset Specifies the length of the header. _ Reserved Variables set aside for future use. _ Flags Indicate that this packet is the end of the data or that the data is urgent.   Transmission Control Protocol/Internet Protocol _ Window size Provides a way to increase packet size, which can improve efficiency in data transfers. _ Urgent pointer Gives the location of urgent data. _ Options Reserved for future use or for special options as defined by the protocol. _ Padding Ensures that the header ends on a 32-bit boundary. The data immediately follow this header information. See also Internet Protocol; Transmission Control Protocol/Internet Protocol; User Datagram Protocol. Transmission Control Protocol/ Internet Protocol Abbreviated TCP/IP. A set of communications protocols first developed by the Defense Advanced Research Projects Agency (DARPA) in the late 1970s. The set of TCP/IP protocols encompasses media access, packet transport, session communications, file transfer, e-mail, and terminal emulation. TCP/IP is a widely published open standard, and while completely independent of any specific hardware or software company, it is supported by a huge number of vendors and is available on many different computers, from PCs to mainframes, running many different operating systems. Many corporations, universities, and government agencies use TCP/IP, and it is also the basis of the Internet. TCP/IP is separated from the network hardware and will run over Ethernet, token-ring, X.25 networks, and dial-up connections. It is a routable protocol, so datagrams can be sent over specific routes, and it has reliable and efficient data-delivery mechanisms. TCP/IP uses a common expandable addressing scheme, so any system can address any other system, even in a network as large as the Internet, and new networks can be added without service disruptions. The popularity that the TCP/IP family of protocols enjoys today did not arise just because the protocols were available or even because the U.S. government mandated their use. They are popular because they are robust, solid protocols that solve many of the most difficult networking problems and do so in an elegant and efficient way. See also Address Resolution Protocol; File Transfer Protocol; Hypertext Transfer Protocol; Internet Control Message Protocol; Internet Mail Access Protocol; Internet Protocol; Post Office Protocol; Simple Mail Transfer Protocol; Simple Network Management Protocol; Telnet; tn3270; User Datagram Protocol.   transmission medium TCP HEADER TRANSMISSION CONTROL PROTOCOL/INTERNET PROTOCOL transmission medium The physical cabling used to carry network information, such as fiber-optic, coaxial, shielded twisted-pair (STP), and unshielded twistedpair (UTP) cabling.   trusted domain transmission mode The manner in which communications can take place between a sender and a receiver. Several modes are defined, as follows: _ Simplex Communications can go in only one direction, so the sender can use the whole of the available bandwidth. _ Half-duplex Communications can go in two directions, but only in one direction at a time. _ Full-duplex Communications can go in two directions simultaneously. _ Echo-plex A rare mode in which characters are retransmitted to the sender for error-checking purposes. Transmit Data Abbreviated TXD. A hardware signal defined by the RS-232-C standard that carries information from one device to another. See also Receive Data. transport layer The fourth of seven layers of the OSI Reference Model for computer- to-computer communications. The transport layer defines protocols for message structure and supervises the validity of the transmission by performing some error checking. See also OSI Reference Model. trap See software interrupt. trap door An entry point in a computer network, through which an intruder can gain access without authentication. tree In Microsoft Active Directory, a hierarchy of domains linked via trust relationships that share the same namespace, Directory Schema, and Global Catalog. See also Active Directory; Directory Schema; Global Catalog. Tree object In Novell Directory Services (NDS), the hierarchical organization of all the objects on the network into a single structure. See also container object; Novell Directory Services; Organization object. trellis-coded modulation Abbreviated TCM. A form of quadrature amplitude modulation used in modems that operate at 9600 bits per second or higher. TCM encodes data as a set of bits associated with both phase and amplitude changes. Trivial File Transfer Protocol Abbreviated TFTP. A little-used and simplified version of the TCP/IP file transfer protocol that does not include password protection. Because it has no security associated with it, most system administrators do not support its use and recommend File Transfer Protocol instead. See also File Transfer Protocol. Trojan Horse A type of computer virus that pretends to be a useful program, such as a game or a utility, to entice you to use it, when in reality it contains special code that will intentionally damage any system onto which it is loaded. See also logic bomb. trusted domain In Microsoft Windows NT Server, a domain that a trusting domain will allow to authenticate logons.   trustee See also trusting domain; trust relationship. trustee In Novell NetWare, a user or group object that has been granted rights to work with a directory, an object, or a file. See also Public trustee; trustee assignment. trustee assignment In Novell Net- Ware, a mechanism that determines how a user can access an object, a directory, or a file. Also known as trustee rights. For example, trustee rights regulate whether a user can read a file, change it, change its name, delete it, or control other users’ trustee rights to it. Trustee rights are assigned to individual users, and one user’s rights can be different from another user’s rights to the same directory See also Public trustee; trustee. trustee rights See trustee assignment. trusting domain In Microsoft Windows NT Server, a domain that lets users and groups in the trusted domain use its resources. See also trusted domain; trust relationship. trust relationship In Microsoft Windows NT Server, a link between domains that allows pass-through authentication, in which a trusting domain allows the logon authentication of a trusted domain. With the right trust relationships, a user with one user account in one domain has the potential to access the whole network. Global groups and user accounts defined in the trusted domain can be assigned rights and permissions in a trusting domain, even though those accounts do not exist in the trusting domain’s directory database. See also authentication; pass-through authentication; trusted domain; trusting domain; user account. TSAPI See Telephony Services API. TSAT See T1 small aperture terminal. TSR See terminate-and-stay-resident program. TTL See time to live. TTS See Transaction Tracking System. tunneling The encapsulation of one protocol within another, often used to transport a local-area network protocol across a backbone that does not support that particular protocol. Tunneling is also used to create pseudoconnections across connectionless networks such as the Internet and may be referred to as protocol encapsulation or synchronous pass-through. See also encapsulation; Point-to-Point Tunneling Protocol. twinaxial cable A cable with two coaxial cables inside a single insulating shield. Twinaxial cable is used with IBM AS/400 minicomputers.   Type 1–9 cable TWINAXIAL CABLE twisted-pair cable Abbreviated TP. Cable that comprises two or more pairs of insulated wires twisted together, at six twists per inch. In twisted-pair cable, one wire carries the signal and the other is grounded. The cable may be shielded or unshielded. Telephone wire installed in modern buildings is often twisted-pair wiring. two-phase commit A method used in transaction processing to ensure data is posted to shared databases correctly by dividing the writing of data into two steps. Each of the steps must receive a verification of completeness from the shared databases; otherwise, the transaction-processing system rolls back the transaction and tries again. See also on-line transaction processing; roll back; roll forward; transaction processing. two-way transitive trust In Microsoft Windows 2000 Server, an automatic trust relationship created between two Windows 2000 domains. If domain A trusts domain B, and domain B trusts domain C, then domain A trusts domain C, and domain C also trusts domain A. In Windows NT, all trust relationships were created and managed manually. See also downlevel trust; explicit trust; shortcut trust; TXD See Transmit Data. Type 1–9 cable IBM Cabling System specifications, as follows: _ Type 1 Shielded, twisted, dual-pair cable with 22-gauge solid conductors   type-ahead buffer and a braided shield. Used with Token Ring networks. _ Type 2 Two-pair, shielded cable with solid conductors and a braided shield. Type 2 also includes four pairs of unshielded voice-grade lines, giving a total of six pairs in the same sheath. _ Type 3 Four unshielded, solid, twisted pairs, used for voice or data. IBM’s variant of twisted-pair telephone wire. _ Type 4 No published specification. _ Type 5 Dual 100/140 micron fiber-optic cable; IBM now recommends 125-micron fiber-optic cable, which is the current industry standard for fiber-optic cable. _ Type 6 Shielded, two-pair, braided cable used for patch cables. Type 6 is more flexible than Type 1 cable. _ Type 7 No published specification. _ Type 8 Shielded, dual-pair cable with no twists, housed in a flat jacket; commonly used under carpets. _ Type 9 Shielded, dual-pair, plenum cable with solid or braided conductors and a fire-resistant outer coating, for use between floors in a building. See also American Wire Gauge; cabling standards; plenum cable; riser cable; structured wiring. type-ahead buffer See keyboard buffer. typefull naming In Novell Directory Services (NDS), a formal method used to name objects that includes name types for each part of the name. For example, .CN=Gary.OU=Marketing.O=Sybex . See also typeless naming. typeless naming In Novell Directory Services (NDS), a common method used to name objects that does not include name types for each part of the name. For example, .Gary.Marketing.Sybex . See also typefull naming.   unicast U