Glossary U

T

T reference point

Used with an S reference point to change a four-wire ISDN network to a two-wire ISDN network.

T1

Digital WAN that uses 24 DS0s at 64KB each to create a bandwidth of 1.536Mbps, minus clocking overhead, providing 1.544Mbps of usable bandwidth.

T3

Digital WAN that can provide bandwidth of 44.763Mbps.

TACACS+

Terminal Access Control Access Control System: An enhanced version of TACACS, this protocol is similar to RADIUS. See also: RADIUS.

tag switching

A high-performance technology used for forwarding packets. Based on the concept of label swapping, whereby packets or cells are designated to defined-length labels that control the manner in which data is to be sent. 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.

TCAM

Ternary Content Addressable Memory: this uses a third binary option-the 'don't care'-to make bridge table lookups faster. See also: CAM.

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 header compression

A compression process that compresses only the TCP header information, which is typically repetitive. This would not compress the user data. See also: compression.

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. Contrast with: ATDM, FDM, and statistical multiplexing.

TE

terminal equipment: Any peripheral device that is ISDN-compatible and attached to a network, such as a telephone or computer. TE1s are devices that are ISDN-ready and understand ISDN signaling techniques. TE2s are devices that are not ISDN-ready and do not understand ISDN signaling techniques. A terminal adapter must be used with a TE2.

TE1

A device with a four-wire, twisted-pair digital interface is referred to as terminal equipment type 1. Most modern ISDN devices are of this type.

TE2

A TE2 device is a non-ISDN device which must be connected to an ISDN interface. This could be a non-ISDN phone or fax machine, or even a refrigerator. TE2 devices do not understand ISDN signaling techniques, and a terminal adapter must be used to convert the signaling.

telco

A common abbreviation for the telephone company.

Telnet

The standard terminal emulation protocol within the TCP/IP protocol stack. A 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.

terminal adapter

A hardware interface between a computer without a native ISDN interface and an ISDN line. In effect, a device to connect a standard async interface to a non-native ISDN device, emulating a modem.

terminal emulation

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

TFTP

Trivial File Transfer Protocol: Conceptually a 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 provide the abundance of functions that FTP does. In particular, it has no directory-browsing abilities; it can do nothing but send and receive files. See also: FTP.

Thicknet

Also called '10Base5.' Bus network that uses a thick cable and runs Ethernet up to 500 meters.

Thinnet

Also called '10Base2.' Bus network that uses a thin coax cable and runs Ethernet media access up to 185 meters.

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. 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.

toll network

WAN network that uses the Public Switched Telephone Network (PSTN) to send packets.

trace

IP command used to trace the path a packet takes through an internetwork.

traffic shaping

Used on Frame Relay networks to provide priorities of data.

transparent bridging

The bridging scheme used in Ethernet and IEEE 802.3 networks, it passes frames along one hop at a time, using bridging information stored in tables that associate end-node MAC addresses within bridge ports. This type of bridging is considered transparent because the source node does not know it has been bridged, because the destination frames are sent directly to the end node. Contrast with: SRB.

transparent Ethernet

A variety of techniques used to make a customer's wide or metropolitan network look like an end-to-end Ethernet.

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: Application layer, Data Link layer, Network layer, Physical layer, Presentation layer, and Session layer.

trie

A pointer used with a data structure, where the data structure does not actually contain the data. This allows for data to be stored in the way that best suits the type of data being stored, and the lookup process to be decoupled from the data.

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. Ports may be set to speeds of either 4Mbps or 16Mbps independently of each other.

trunk link

Link used between switches and from some servers to the switches. Trunk links carry information about many VLANs. Access links are used to connect host devices to a switch and carry only VLAN information that the device is a member of.

TTL

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

TUD

Trunk Up-Down: A protocol used in ATM networks for the monitoring of trunks. If a trunk misses 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.