T

T1

Digital WAN that uses 24 DS0s to create a bandwidth of 1.544Mbps.

T3

Digital WAN that can provide bandwidth of 44.763Mbps.

Terminal Access Controller Access Control System Plus

(TACACS+) Allows network administrators to centrally manage terminal access to Cisco network devices.

tag switching

Based on the concept of label swapping, where packets or cells are designated to defined-length labels that control the manner in which data is to be sent, tag switching is a high-performance technology used for forwarding packets. It incorporates Data Link layer (Layer 2) switching and Network layer (Layer 3) routing and supplies scalable, high-speed switching in the network core.

tagged traffic

ATM cells with their cell loss priority (CLP) bit set to 1. Also referred to as discard-eligible (DE) traffic. Tagged traffic can be eliminated in order to ensure trouble-free delivery of higher priority traffic, if the network is congested. See also: CLP.

TCP

Transmission Control Protocol: A connection-oriented protocol that is defined at the Transport layer of the OSI reference model. Provides reliable delivery of data.

TCP/IP

Transmission Control Protocol/Internet Protocol. The suite of protocols underlying the Internet. TCP and IP are the most widely known protocols in that suite. See also: IP and TCP.

TDM

Time-Division Multiplexing: A technique for assigning bandwidth on a single wire, based on preassigned time slots, to data from several channels. Bandwidth is allotted to each channel regardless of a station’s ability to send data. See also: ATDM, FDM, and multiplexing.

TE1

A device with a four-wire, twisted-pair digital interface is referred to as terminal equipment type one. Most modern ISDN devices are of this type.

TE

Terminal Equipment: Any peripheral device that is ISDN-compatible and attached to a network, such as a telephone or computer.

telco

A common abbreviation for the telephone company.

Telnet

The standard terminal emulation protocol within the TCP/IP protocol stack. Method of remote terminal connection, enabling users to log in on remote networks and use those resources as if they were locally connected. Telnet is defined in RFC 854.

10BaseT

Part of the IEEE 802.3 standard, 10BaseT is the Ethernet specification of 10Mbps baseband that uses two pairs of twisted-pair, Category 3, 4, or 5 cabling—one pair to send data and the other to receive. 10BaseT has a distance limit of about 100 meters per segment. See also: Ethernet and IEEE 802.3.

terminal adapter

A hardware interface between a computer and an ISDN line. In effect, an ISDN modem.

terminal emulation

The use of software, installed on a PC or LAN server, that allows the PC to function as if it were a “dumb” terminal directly attached to a particular type of mainframe.

TFTP

The stripped-down version of FTP, it’s the protocol of choice if you know exactly what you want and where it’s to be found. TFTP doesn’t give you the abundance of functions that FTP does. In particular, it has no directory browsing abilities; it can do nothing but send and receive files.

three-part firewall

The classic firewall system, called a “three-part firewall,” has three specialized layers:

An isolation LAN that is a buffer between the corporate internetwork and the outside world. (The isolation LAN is called the DMZ.)

A router that acts as an inside packet filter between the corporate internetwork and the isolation LAN.

Another router that acts as an outside packet filter between the isolation LAN and the outside internetwork.

thrashing

If you have multiple links between switches and are not running the STP protocol, the MAC address filter table of the switch will be totally confused about a source device’s location because the switch can receive the same frame from more than one link. The switch can get so caught up in constantly updating the MAC filter table with source hardware address locations that it fails to forward a frame. This is called thrashing the MAC table.

Tie-line

A part of traditional voice architecture, tie-lines are the connections between PBX switches.

token

A frame containing only control information. Possessing this control information gives a network device permission to transmit data onto the network. See also: token passing.

token bus

LAN architecture that is the basis for the IEEE 802.4 LAN specification and employs token passing access over a bus topology. See also: IEEE.

token passing

A method used by network devices to access the physical medium in a systematic way based on possession of a small frame called a token. Contrast with: circuit switching. See also: token.

Token Ring

IBM’s token-passing LAN technology. It runs at 4Mbps or 16Mbps over a ring topology. Defined formally by IEEE 802.5. See also: ring topology and token passing.

ToS

(Type of Service) A field in the IP header that allows traffic to be tagged as voice and payload size to test effectiveness of QoS policies.

totally stubby area

OSPF defines a totally stubby area as one that blocks external routes and summary routes (inter-area routes) from going into the area. This way, intra-area routes and the default of 0.0.0.0 are the only routes injected into that area.

traffic shaping

Often used with Frame Relay, traffic shaping applies to both PVCs and SVCs. This allows network designers and engineers to control the amount of data traffic sent onto a Frame Relay circuit.

transparent bridging

The bridging scheme used in Ethernet and IEEE 802.3 networks, it passes frames along one hop at a time, using routing information stored in tables that associate end nodes within bridge ports. This type of bridging is considered transparent because the source node doesn’t need to know the entire route, as it does with source-route bridging. Contrast with: SRB.

Transport layer

Layer 4 of the OSI reference model, used for reliable communication between end nodes over the network. The Transport layer provides mechanisms used for establishing, maintaining, and terminating virtual circuits, transport fault detection and recovery, and controlling the flow of information. See also: Data Link layer, Application layer, Physical layer, Network layer, Presentation layer, and Session layer.

Trap Message

A function of a SNMP trap, it is used to notify a network management station that an extraordinary event has occurred at an agent. When a trap condition occurs, the SNMP agent sends an SNMP agent trap message to each of the network management stations as specified in the trap receiver table.

TRIP

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

Trunk

A part of traditional voice architecture, trunks are the lines that feed tie-lines.

TTL

Time To Live: A field in an IP header, indicating the length of time a packet is valid.

TUD

Trunk Up-Down: A protocol used in ATM networks for the monitoring of trunks. Should a trunk miss a given number of test messages being sent by ATM switches to ensure trunk line quality, TUD declares the trunk down. When a trunk reverses direction and comes back up, TUD recognizes that the trunk is up and returns the trunk to service.

tunneling

A method of avoiding protocol restrictions by wrapping packets from one protocol in another protocol’s packet and transmitting this encapsulated packet over a network that supports the wrapper protocol. See also: encapsulation.