T1

  Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps through the telephone-switching network, using AMI or B8ZS coding. Compare with E1. See also AMI, B8ZS, and DS-1.

T3

  Digital WAN carrier facility. T3 transmits DS-3-formatted data at 44.736 Mbps through the telephone switching network. Compare with E3. See also DS-3.

TAC

  1.) Terminal Access Controller. Internet host that accepts terminal connections from dial-up lines.
  2.) Technical Assistance Center. Cisco TACs provide technical assistance to partners and end users, and form the hub of Cisco global support.

TACACS

  Terminal Access Controller Access Control System. Authentication protocol, developed by the DDN community, that provides remote access authentication and related services, such as event logging. User passwords are administered in a central database rather than in individual routers, providing an easily scalable network security solution. See also TACACS+.

TACACS+

  Proprietary Cisco enhancement to TACACS. Provides additional support for authentication, authorization, and accounting. See also TACACS.

tagged traffic

  ATM cells that have their CLP bit set to 1. If the network is congested, tagged traffic can be dropped to ensure delivery of higher-priority traffic. Sometimes called DE (discard eligible) traffic. See also CLP.

TAXI 4B/5B

  Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte. Encoding scheme used for FDDI LANs as well as for ATM. Supports speeds of up to 100 Mbps over multimode fiber. TAXI is the chipset that generates 4B/5B encoding on multimode fiber. See also 4B/5B local fiber.

T-carrier

  TDM transmission method usually referring to a line or cable carrying a DS-1 signal.

TCP

  Transmission Control Protocol. Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack. See also TCP/IP.

TCP/IP

  Transmission Control Protocol/Internet Protocol. Common name for the suite of protocols developed by the U.S. DoD in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite. See also IP and TCP.

TCS

  Test and control system. Independently-powered subsystem used to initialize, monitor, and troubleshoot the hardware on a LightStream 2020 ATM switch. The TCS consists of a hub residing on the switch card and slaves on NPs and line cards.

TCU

  Trunk coupling unit. In Token Ring networks, a physical device that enables a station to connect to the trunk cable.

TDM

  Time-division multiplexing. Technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit. Compare with ATDM, FDM, and statistical multiplexing.

TDR

  Time domain reflectometer. Device capable of sending signals through a network medium to check cable continuity, length, and other attributes. TDRs are used to find physical layer network problems.

Technical Assistance Center

  See TAC.

Technical Office Protocol

  See TOP.

telco

  Abbreviation for telephone company.

telecommunications

  Term referring to communications (usually involving computer systems) over the telephone network.

Telecommunications Industry Association

  See TIA.

telephony

  Science of converting sound to electrical signals and transmitting it between widely removed points.

telepole

  Telescoping pole with a hook at one end. It is used to get cable across a ceiling or attic quickly.

telex

  Teletypewriter service allowing subscribers to send messages over the PSTN.

Telnet

  Command used to verify the application layer software between source and destination stations. This is the most complete test mechanism available.

Tempest

  U.S. military standard. Electronic products adhering to the Tempest specification are designed to withstand EMP. See also EMP.

TERENA

  Trans-European Research and Education Networking Association. Organization that promotes information and telecommunications technologies development in Europe. Formed by the merging of EARN and RARE. See also EARN and RARE.

term ip netmask-format

  Command used to specify the format of network masks for the current session.

termid

  SNA cluster controller identification. Termid is meaningful only for switched lines. Also called Xid.

terminal

  Simple device at which data can be entered or retrieved from a network. Generally, terminals have a monitor and a keyboard, but no processor or local disk drive.

Terminal Access Controller

  See TAC.

Terminal Access Controller Access System

  See TACACS.

terminal adapter

  Device used to connect ISDN BRI connections to existing interfaces such as EIA/TIA-232. Essentially, an ISDN modem.

terminal emulation

  Network application in which a computer runs software that makes it appear to a remote host as a directly attached terminal.

terminal server

  Communications processor that connects asynchronous devices such as terminals, printers, hosts, and modems to any LAN or WAN that uses TCP/IP, X.25, or LAT protocols. Terminal servers provide the internetwork intelligence that is not available in the connected devices.

terminator

  Device that provides electrical resistance at the end of a transmission line to absorb signals on the line, thereby keeping them from bouncing back and being received again by network stations.

test and control system

  See TCS.

Texas Higher Education Network

  See THEnet.

TFTP

  Trivial File Transfer Protocol. Simplified version of FTP that allows files to be transferred from one computer to another over a network.

TH

  Transmission header. SNA header that is appended to the SNA basic information unit (BIU). The TH uses one of a number of available SNA header formats. See also FID0, FID1, FID2, FID3, and FID4.

THC over X.25

  Feature providing TCP/IP header compression over X.25 links, for purposes of link efficiency.

THEnet

  Texas Higher Education Network. Regional network comprising over 60 academic and research institutions in the Texas (United States), area.

Thinnet

  Term used to define a thinner, less expensive version of the cable specified in the IEEE 802.3 10BASE2 standard. Compare with Cheapernet. See also 10BASE2, Ethernet, and IEEE 802.3.

throughput

  Rate of information arriving at, and possibly passing through, a particular point in a network system.

TIA

  Telecommunications Industry Association. Organization that develops standards relating to telecommunications technologies. Together, the TIA and the EIA have formalized standards, such as EIA/TIA-232, for the electrical characteristics of data transmission. See also EIA.

TIC

  Token Ring interface coupler. Controller through which an FEP connects to a Token Ring.

tie-wraps

  Plastic ties used for holding cables together or for holding cables in place.

time-division multiplexing

  See TDM.

time domain reflectometer

  See TDR.

time domain reflectometry

  Technique of sending an electrical signal down a cable and then timing the signal's reflection back from the end of the cable.

Time Notify

  See TNotify.

time-out

  Event that occurs when one network device expects to hear from another network device within a specified period of time, but does not. The resulting time-out usually results in a retransmission of information or the dissolving of the session between the two devices.

Time To Live

  See TTL.

TN3270

  Terminal emulation software that allows a terminal to appear to an IBM host as a 3278 Model 2 terminal. The Cisco TN3270 implementation allows users to access an IBM host without using a special IBM server or a UNIX host acting as a server.

TNotify

  Time Notify. Specifies how often SMT initiates neighbor notification broadcasts. See also SMT.

token

  Frame that contains control information. Possession of the token allows a network device to transmit data onto the network. See also token passing.

token bus

  LAN architecture using token passing access over a bus topology. This LAN architecture is the basis for the IEEE 802.4 LAN specification. See also IEEE 802.4.

token passing

  Access method by which network devices access the physical medium in an orderly fashion based on possession of a small frame called a token. Contrast with circuit switching and contention. See also token.

Token Ring

  Token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to IEEE 802.5. See also IEEE 802.5, ring topology, and token passing.

Token Ring interface coupler

  See TIC.

Token Ring Interface Processor

  See TRIP.

TOP

  Technical Office Protocol. OSI-based architecture developed for office communications.

topology

  Physical arrangement of network nodes and media within an enterprise networking structure.

topology map

  Tool for managing a LightStream 2020 ATM switch that examines a network and displays the status of its nodes and trunks. The topology map is an HP OpenView-based application that runs on an NMS.

TOS

  Type of service. Field within an IP datagram that indicates how the datagram should be handled. See COS (class of service).

to switch unit

  See TSU.

total length

  Field within an IP datagram that indicates total length of the header + the data.

TP0

  Transport Protocol Class 0. OSI connectionless transport protocol for use over reliable subnetworks. Defined by ISO 8073.

TP4

  Transport Protocol Class 4. OSI connection-based transport protocol. Defined by ISO 8073.

trace

  Command that uses Time-To-Live (TTL) values to generate messages from each router used along the path. This is very powerful in its ability to locate failures in the path from the source to the destination.

traffic management

  See ControlStream traffic management.

traffic policing

  Process used to measure the actual traffic flow across a given connection and compare it to the total admissable traffic flow for that connection. Traffic outside of the agreed upon flow can be tagged (where the CLP bit is set to 1) and can be discarded en route if congestion develops. Traffic policing is used in ATM, Frame Relay, and other types of networks. Also know as admission control, permit processing, rate enforcement, and UPC (usage parameter control). See also tagged traffic.

traffic profile

  Set of COS attribute values assigned to a given port on a LightStream 2020 ATM switch. The profile affects numerous parameters for data transmitted from the port including rate, cell drop eligibility, transmit priority, and inactivity timer. See also COS.

traffic shaping

  Use of queues to limit surges that can congest a network. Data is buffered and then sent into the network in regulated amounts to ensure that the traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is used in ATM, Frame Relay, and other types of networks. Also known as metering, shaping, and smoothing.

trailer

  Control information appended to data when encapsulating the data for network transmission. Compare with header.

transaction

  Result-oriented unit of communication processing.

transaction services layer

  Layer 7 in the SNA architectural model. Represents user application functions, such as spreadsheets, word-processing, or electronic mail, by which users interact with the network. Corresponds roughly with the application layer of the OSI reference model. See also data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, and transmission control layer.

transceiver

  See MAU.

transceiver cable

  See AUI.

Trans-European Research and Education Networking Association

  See TERENA.

transfer priority

  See transmit priority.

transit bridging

  Bridging that uses encapsulation to send a frame between two similar networks over a dissimilar network.

translational bridging

  Bridging between networks with dissimilar MAC sublayer protocols. MAC information is translated into the format of the destination network at the bridge. Contrast with encapsulation bridging.

transmission control layer

  Layer 4 in the SNA architectural model. This layer is responsible for establishing, maintaining, and terminating SNA sessions, sequencing data messages, and controlling session level flow. Corresponds to the transport layer of the OSI model. See also data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, and transaction services layer.

Transmission Control Protocol

  See TCP.

Transmission Control Protocol/Internet Protocol

  See TCP/IP.

transmission group

  In SNA routing, one or more parallel communications links treated as one communications facility.

transmission header

  See TH.

transmission link

  See link.

transmit priority

  Queuing scheme in which each internal TOS of a LightStream 2020 ATM switch correlates to a relative priority in queues in the ATM network. This priority determines which traffic is serviced first in the case of contention for a network resource. Also known as forwarding priority and transfer priority.

TRANSPAC

  Major packet data network run by France Telecom.

Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte

  See TAXI 4B/5B.

transparent bridging

  Bridging scheme often used in Ethernet and IEEE 802.3 networks in which bridges pass frames along one hop at a time based on tables associating end nodes with bridge ports. Transparent bridging is so named because the presence of bridges is transparent to network end nodes. Contrast with SRB.

transport layer

  Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control. Corresponds to the transmission control layer of the SNA model. See also application layer, data link layer, network layer, physical layer, presentation layer, and session layer.

Transport Protocol Class 0

  See TP0.

Transport Protocol Class 4

  See TP4.

trap

  Message sent by an SNMP agent to an NMS, console, or terminal to indicate the occurrence of a significant event, such as a specifically defined condition or a threshold that has been reached. See also alarm and event.

tree topology

  LAN topology similar to a bus topology, except that tree networks can contain branches with multiple nodes. Transmissions from a station propagate the length of the medium and are received by all other stations. Compare with bus topology, ring topology, and star topology.

TRIP

  Token Ring Interface Processor. High-speed interface processor on the Cisco 7000 series routers. The TRIP provides two or four Token Ring ports for interconnection with IEEE 802.5 and IBM Token Ring media with ports independently set to speeds of either 4 or 16 Mbps.

Trivial File Transfer Protocol

  See TFTP.

trunk

  Physical and logical connection between two ATM switches across which traffic in an ATM network travels. An ATM backbone is composed of a number of trunks.

trunk card

  Line card on a LightStream 2020 ATM switch that is configured to communicate with other ATM switches. LightStream 2020 trunk cards offer a variety of interface types. CLCs, LSCs, and MSCs can operate as trunk cards. See also edge card.

trunk coupling unit

  See TCU.

trunk up-down

  See TUD.

TSU

  To switch unit. Subsystem on each LightStream 2020 ATM switch line card that appends ATM routing information to outgoing cells and sends the cells to the switch card.

TTL

  Time To Live. Field in an IP header that indicates how long a packet is considered valid.

tunneling

  Architecture that is designed to provide the services necessary to implement any standard point-to-point encapsulation scheme. See also encapsulation.

TUD

  Trunk up-down. Protocol used in ATM networks that monitors trunks and detects when one goes down or comes up. ATM switches send regular test messages from each trunk port to test trunk line quality. If a trunk misses a given number of these messages, TUD declares the trunk down. When a trunk comes back up, TUD recognizes that the trunk is up, declares the trunk up, and returns it to service. See also trunk.

TUV

  German test agency that certifies products to European safety standards.

twisted pair

  Relatively low-speed transmission medium consisting of two insulated wires arranged in a regular spiral pattern. The wires can be shielded or unshielded. Twisted pair is common in telephony applications and is increasingly common in data networks. See also STP and UTP.

two-way simultaneous

  See TWS.

TWS

  Two-way simultaneous. Mode that allows a router configured as a primary SDLC station to achieve better utilization of a full-duplex serial line. When TWS is enabled in a multidrop environment, the router can poll a secondary station and receive data from that station while it sends data to or receives data from a different secondary station on the same serial line.

TYMNET

  See XStream.

Type 1 operation

  IEEE 802.2 (LLC) connectionless operation.

Type 2 operation

  IEEE 802.2 (LLC) connection-oriented operation.

type of service

  See TOS.