TAC: Terminal Access Controller
Terminal Access Controller(TAG) is an Internet host that accepts terminal connections from dial-up lines.

Tag
A Tag, in internetworking, refers to the identification information included in a packet, which includes a number plus other information. A tag is used for traffic classification and prioritization.

Tag Switching
Tag switching, also called Label Switching, was orginally introduced by Cisco Systems, which uses tags (labels) containing forwarding information. Tag switching uses routers that sit on the periphery of the network and make forwarding decisions for all the routers in the backbone. They append this information to each packet in fixed positions in the header that can be quickly examined by interior backbone routers, saving the time involved in decoding the packet and its associated table lookups. Tag switching then was developed into the industrial standard technology called Multiprotocol Label Switching (MPLS).

Tagged Frame
Tagged Frame, a concept in the IEEE 802.1Q Virtual LAN, refers to the frames with an 802.1Q VLAN identifier.

Tagged Traffic
Tagged Traffic, sometimes called DE traffic, refers to the ATM cells that have their Cell Loss Priority (CLP) bit set to 1 for low priority. If the network is congested, tagged traffic can be dropped to ensure the delivery of higher-priority traffic (CLP=0).

Tail-End
Tail-End, also known as tunnel tailend, refers to the downstream, receive end of a tunnel.

TAPI: Telephony Application Programming Interface
The Telephony Application Programming Interface (TAPI) is an API developed by Microsoft and Intel, which enables PCs running Microsoft Windows to use telephone services.

TAXI 4B/5B: Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte
Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte(TAXI 4B/5B) is an encoding scheme used for FDDI LANs as well as for ATM. TAXI 4B/5B supports speeds of up to 100 Mbps over multimode fiber. TAXI is the chipset that generates 4B/5B encoding on multimode fiber.

TB: Transparent Bridging
A transparent bridge is a computer network device that is used to interconnect several computers in a network enabling the exchange of data among them. The bridge is called transparent because its activity is transparent to the network hosts. A transparent bridge directs the outgoing data traffic using a forwarding table that associates addresses to ports. The table can be static or built by learning the network topology from the analysis of the incoming traffic.

TC: Telephony Controller
Telephony Controller, also known as Signaling Controller (SC) or Virtual Switch Controller (VSC) or call manager, is a componnt in the VOIP network to manage and control calls.

TC: Transmission Convergence
Transmission Convergence(TC) is a sublayer of the ATM physical layer that transforms the flow of cells into a steady flow of bits for transmission over the physical medium. When transmitting, the TC sublayer maps the cells into the frame format, generates the HEC, and sends idle cells when there is nothing to send. When receiving, the TC sublayer delineates individual cells in the received bit stream and uses HEC to detect and correct errors.

TCL/TK: Toolkit Command Language windowing toolkit
Toolkit Command Language windowing toolkit(TCL/TK) is a combination of a scripting language (TCI) with a windowing toolkit (TK). TCL/TK is open source (based on a BSD-style license), used for rapid prototyping and application development, such as web and desktop applications, network programming, embedded development, testing, general purpose programming, system administration, database work, etc.

TCL: Toolkit Command Language
Toolkit Command Language(TCL) is a scripting language used for gateway products both internally and externally to Cisco IOS software code.

TCL: Transmission Control Layer
Transmission Control Layer (TCL) is the 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. TCL is similar to the transport layer of the OSI model.

TCP three-way handshake
TCP three-way handshake refers to the procedure used by Transmission Control Protocol (TCP) for establishing a session.

Tcpdump
Tcpdump is a popular computer network debugging and security tool which allows the user to intercept and display TCP/IP packets being transmitted or received over a network to which the computer is attached. Tcpdump allows us to precisely see all the traffic and enables us to create statistical monitoring scripts.

TCR: Tagged Cell Rate
Tagged Cell Rate (TCR) is an ATM ABR service parameter. TCR limits the rate at which a source may send out-of-rate forward RM-cells. TCR is a constant fixed at 10 cells/second.

TCU: Trunk Coupling Unit
Trunk Coupling Unit(TCU), a component in Token Ring networks, is a physical device that enables a station to connect to the trunk cable.

TDF: Timeout Decrease Factor
Timeout Decrease Factor (TDF) is an ATM ABR service parameter. TDF controls the decrease in ACR associated with TOF. TDF is in units of 1/seconds. For ABR traffic, TDF effects the time rate of change of the ACR. This value effects TCP round trip time calculations, which in turn effects TCP throughput.

TDFF: TDF Factor
TDF Factor (TDFF) is a relative measure of Timeout Decrease Factor (TDF), with the relationship: TDF=TDFF/RDF times the smallest power of 2 greater or equal to PCR. TDFF is either zero or a power of two in the range 1/64 to 1 in units of 1 /cells. (RDF: Rate Decrease Factor and PCR: Peak Cell Rate)

TDR: Time-Domain Reflectometer
Time-Domain Reflectometer(TDR) is a device capable of sending signals through a network medium to check cable continuity and other attributes. TDRs are used to find physical layer network problems.

TE: Terminal Equipment
Terminal Equipment(TE) refers to any ISDN-compatible device that can be attached to the network, such as a telephone, a fax, or a computer.

Technology prefix
Technology prefix is a discriminator used to distinguish between gateways having specific capabilities within a given zone. In the exchange between the gateway and the gatekeeper, the technology prefix is used to select a gateway after the zone has been selected. Technology prefixes can be used to tell the gatekeeper that a certain technology is associated with a particular call (for example, 15# could mean a fax transmission), or it can be used like an area code for more generic routing. No standard defines what the numbers in a technology prefix mean; by convention, technology prefixes are designated by a pound (#) symbol as the last character.

TEI: Terminal Endpoint Identifier
Terminal Endpoint Identifier(TEI) is a field in the Link Access Protocol - D Channel (LAPD) address that identifies a device on an ISDN interface.

Telco-Return CM
Telco-Return cable modem(CM) is a device which uses the cable network only for subscriber downstream traffic, and uses the PSTN for subscriber upstream traffic (which is necessary in older cable plants).

Telecommunication
Telecommunication refers to the transmission of signals over a distance for the purpose of communication. Telecommunication is widespread and devices that assist the process such as the television, radio and telephone are common in many parts of the world. There is also a vast array of networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet, such as e-mail and instant messaging, is just one of many examples of telecommunication.

Telemetry
Telemetry is the capability of transmitting or retrieving data over long distance communication links, such as satellite or telephone links. With the Telemetry technology, one can conduct the remote measurement and reporting of information of interest to the system designer or operator.

Telephone Number Mapping
Telephone Number Mapping (ENUM), also referred to "E164 NUmber Mapping", is a proposed technology for mapping telephone numbers to the Domain Name System (DNS). ENUM includes a suite of protocols to unify the telephone system with the Internet by using E.164 addresses with DDDS and DNS. ENUM is a DNS-based protocol that is best suited to offer services that not only help facilitate, but expand on ways to complete calls over VoIP networks. It provides a user with a domain name on an E.164 server in order to associate a common international telephone number with a URI and provide other DNS-related services. The server is maintained by the Service Provider and is expected to become standard since it can successfully address locating URIs with nothing more than a common international telephone number.

Telephony
Telephony is a general term for the technologies behind voice communications, including traditional circuit switched telephone networks and the newer IP telephony technologies (VOIP) over the packet switched networks.

TERENA: Trans-European Research and Education Networking Association
Trans-European Research and Education Networking Association(TERENA) is an organization that promotes information and telecommunications technologies development in Europe. TERENA was formed by the merger of EARN and RARE.

Termid
Termid, also called XID, is an SNA cluster controller identification for switched lines only.

Terminal
Terminal, also called computer terminal, is a device at which data can be entered or retrieved from a network. Generally, terminals have a monitor and a keyboard and mouse, but no processor or local disk drive, which called dumb terminal. Today, PCs are wided used as a terminal.

Terminal Adapter
Terminal Adapter, is a device used to connect ISDN BRI connections to existing interfaces, such as EIA/TIA-232. Essentially, Terminal Adapter is an ISDN modem.

Terminal Emulation
Terminal Emulation is a network application in which a computer runs software that makes it appear to a remote host as a directly attached terminal.

Terminal Server
Terminal Server is an equipment that allows serial line devices to connect to a network. Simple terminal servers only allow dumb terminals (perhaps via modems and telephone lines) something like telnet access. Sophisticated terminal servers allow (semi-)intelligent terminals (e.g. PCs) to connect to the network using higher-level protocols (e.g. PPP). If connections are only via modems, then this equipment may also be referred to as a modem server.

Terminal Server Session
Terminal Server Session refers to an end-to-end virtual connection between a terminal and a terminal server. Usually, the terminal server session begins when the a session such as a call goes on line and ends when the call disconnects. A terminal server session can be either local or remote.

TH: Transmission Header
Transmission Header(TH) is an SNA header that is appended to the SNA basic information unit (BIU). The TH uses one of a number of available SNA header formats.

THC over X.25
TCP/IP Header Compression (THC) over X.25 is a feature providing TCP/IP header compression over X.25 links, for purposes of link efficiency.

THEnet: Texas Higher Education Network
Texas Higher Education Network(THEnet) is a regional network comprising over 60 academic and research institutions in the Texas (United States) area.

Three-way Handshake
Three-way handshake is a process in the Transmission Control Protocol (TCP) that occurs between a client and server when initiating or terminating a TCP connection. TCP is a connection-oriented protocol, means that, before any data can be transmitted, a reliable connection must be obtained and acknowledged. TCP level data transmissions, connection establishment, and connection termination maintain specific control parameters that govern the entire process. The control bits are listed as follows:
URG: Urgent Pointer field significant
ACK: Acknowledgement field significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender

Throughput
Throughput refers to the rate of information arriving at, and possibly passing through, a particular point in a network system.

TIC: Token Ring Interface Coupler
Token Ring Interface Coupler(TIC) is a controller through which an Front End Processor (FEP) in an SNA network connects to a Token Ring network.

TID: Terminal Identifier
Terminal Identifier(TID) is a code identifying a specific terminal.

TID: Tunnel Identifier
Tunnel Identifier (TID), as a concept in the Layer 2 Tunnel Protocol (L2TP), is a two-octet value that denotes a tunnel between an L2TP access concentrator (LAC) and an L2TP Network Server (LNS).

Timeout
Timeout is an event that occurs when one network device expects to hear from another network device within a specified period of time, but does not. Timeout usually results in a retransmission of information or the dissolving of the session between the two devices.

TM: Traffic Management
Traffic management includes subjects such as network bandwidth control and allocation, communication delay reduction and congestion minimization. The objectives of the traffic management and engineering is to manage network resources efficiently and provide users the bandwidth and service level as desired. Many advanced statistical techniques such as queuing theory are used to predict and engineer the behaviour of large telecommunications networks such as telephone networks or the Internet.

TN3270
TN3270 is a terminal emulation software that allows a terminal to appear to an IBM host as a 3278 Model 2 terminal.

Tnotify: Time Notify
Time Notify (Tnotify) is a parameter in an FDDI network which specifies how often Station Management (SMT) initiates neighbor notification broadcasts.

TOE: TCP Offload Engine
TCP Offload Engine(TOE) is a technology for the acceleration of TCP/IP, specifically by moving TCP/IP processing to a separate dedicated sub-system from the main host CPU, the overall system TCP/IP performance is improved. Originally TCP was designed for unreliable low speed networks (such as early dial-up modems) but with the growth of the internet in terms of internet backbone transmission speeds (OC-48, OC-192, GigE and 10GigE links) and faster and more reliable access mechanisms (such as Digital Subscriber Line and Cable Modem) it is now used commonly in datacenter and desktop PC environments at speeds of up to 1 Gigabit per second.

TOF: Time Out Factor
Time Out Factor (TOF) is an ATM ABR service parameter. TOF controls the maximum time permitted between sending forward RM-cells before a rate decrease is required. For ABR traffic, TOF effects the time rate of change of the Allowed Cell Rate (ACR). This value effects TCP round trip time calculations, which in turn effects TCP throughput.

TOFF: TOF Factor
Time Out Factor (TOF) Factor (TOFF) is a relative adjustment of TOF with the relationship as: TOF=TOFF+1. TOFF is a power of 2 in the range: 1/8 to 4,096.

Token
Token, a concenpt in a Token Ring network, is a frame that contains control information. Possession of the token for a node allows the network device to transmit data onto the network.

Token Bucket
Token bucket is an algorithm for network traffic shaping or rate limiting, which is used to control the amount of data that is injected into a network, allowing for "bursts" of data to be sent. A token bucket has three components: a burst size, a mean rate, and a time interval (Tc). A token bucket algorithm is implemented in a device that regulates the rate of a flow.

Token Bus
Token Bus is a LAN architecture using token passing access over a bus topology. Token bus was standarized by the IEEE 802.4 Working Group.

Token Passing
Token Passing is an access method by which network devices access the physical medium in an orderly fashion based on possession of a small frame called a token. This technique is used in networks with Token Ring architecture.

Token Ring
Token-Ring is a local area network (LAN) technology originally developed and promoted by IBM in the early 1980s and standardised as IEEE 802.5. Initially very successful, it went into steep decline after the introduction of 10BASE-T for Ethernet and the EIA/TIA 568 cabling standard in the early 1990s. IBM no longer uses or promotes Token-Ring.

TokenTalk
TokenTalk is the Apple Computer's data-link product that allows an AppleTalk network to be connected by Token Ring cables.

Topology
Topology in networking refers to the physical arrangement of network nodes and media within an organization's networking structure.

Torrent
A Torrent in the Internet world is a site that uses BitTorrent technologies to host file for P2P file download and sharing. A torrent contains the location of data files that can be download from the BitTorrent peer-to-peer network. The torrent also contains some identifying information about P2P files. Torrent files can be found on numerous Web sites. BitTorrents can be loaded into the BitTorrent P2P client to initiate the actual download. When saved on a computer, they also serve as bookmarks to available files for future reference.

ToS: Type of Service
Type of Service (TOS) is a one byte field in the IPv4 header that has had various purposes over the years. The modern definition of the TOS byte is a six-bit Differentiated Services Code Point and a two-bit Explicit Congestion Notification field.

TP: Termination Point
Termination Point(TP) is a transmission line or path that terminates or originates on a Network Element (NE), such as the Line Card unit.

TPD: Trail Packet Discard
Trail Packet Discard(TPD) is a mechanism used by some ATM switches that allows the remaining cells supporting an AAL5 frame to be discarded when one or more cells of that AAL5 frame are dropped. This avoids sending partial AAL5 frames through the ATM network when they have to be retransmitted by the sender.

TR VLAN: Token Ring Virtual LAN
Token Ring Virtual LAN(TR VLAN) refers to the virtual LAN configured on the Token Ring network. In a Token Ring VLAN, logical ring domains are formed by defining groups of ports that have the same ring number. The IEEE calls such a port group a Concentrator Relay Function (CRF).

Traceroute
Traceroute, originally written by Van Jacobson in 1988, is a TCP/IP utility which allows the user to determine the route packets taken to reach a particular host. The term traceroute now refers to both a utility and the output of the utility. Traceroute shows you the address and how long it takes to get to each hop in the path. It can be very useful for locating and detecting network congestion, failures, and other various troubleshooting issues.

Tracert
Tracert is the Microsoft Windows version of Traceroute, a UNIX tool for displaying the path taken by packets on a network.

Traffic Analysis
Traffic Analysis refers to capturing network traffic for analysis using certain tools. The characterisics include the identities and locations of the source(s) and destination(s), and the presence, amount, frequency, and duration of occurrence.

Traffic contract & usage parameter control
Traffic contract & usage parameter control refers to ATM connections-specified unidirectional Quality Of Service (QOS) parameters. These parameters are specified at connection set-up and are guaranteed by the network. To guarantee the QOS, the network must be able to obtain enough information from the user about the connection and be able to ensure that no other connections that share the resources degrade the QOS. A user must enter into a contract with the network about the parameters of the connection. Then the network will implement traffic control to avoid problems with degraded QOS before they occur. This includes Network Resource Management (NRM), Connection Admission Control (CAC) and Usage Parameter Control (UPC). The network will also control the case where congestion does occur by implementing Explicit Forward Congestion Indication (EFCI), selective cell discard and reaction to UPC failure.

Traffic Engineering
Traffic Engineering refers to the technique and processe that cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods were used.

Traffic Engineering Tunnel
Traffic Engineering Tunnel is a label-switched tunnel that is used for traffic engineering. Such a tunnel is set up through means other than normal Layer 3 routing; it is used to direct traffic over a path different from the one that Layer 3 routing could cause the tunnel to take.

Traffic Path
Traffic Path is a route of a bearer channel that carries voice traffic.

Traffic Policing
Traffic Policing, used in ATM, Frame Relay, and other types of networks, is a process used to measure the actual traffic flow across a given connection and compare it to the total admissible traffic flow for that connection. Traffic outside of the agreed-upon flow can be tagged (where the CLP bit is set to 1 for low priority traffic in ATM network) and can be discarded en route if congestion develops.

Traffic Profile
Traffic Profile is a set of Class of Service (CoS) attribute values assigned to a given port on an ATM switch. The profile affects numerous parameters for data transmitted from the port, including rate, cell drop eligibility, transmit priority, and inactivity timer.

Traffic Shaping
Traffic Shaping, used in ATM, Frame Relay, and other types of networks, is a network traffic management method that uses 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 fits within the promised traffic envelope for the particular connection.

Trail
In the context of wavelength routing, a trail is the physical connection of two network ports. A single trail is equal to either an OC-48 or OC-192 wavelength between two switches.

Trailer
Trailer is the control information appended to data when encapsulating the data for network transmission. In many protocols, trailer is a reguired field in the protocol.

Transfer Syntax
Transfer Syntax is the actual format or representation of data as it is transmitted over a network, as opposed to abstract syntax which is independent of machine-oriented structures. Transfer Syntax is expressed as a string of bits.

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

Transit Node
A transit node interfaces with other nodes and transfers packet data. Transit node does not directly interface with customer equipment.

Translational Bridging
Translational Bridging refers to the bridging between networks with dissimilar data link layer protocols. The protocol information is translated into the format of the destination network at the bridge. For example, a translationonal bridge can forward data link layer frames to and from LANs utilizing different protocols, such as Ethernet to FDDI, or Ethernet to Token Ring.

Transmission Group
A transmission group, a term in APPN and SNA networking, is the set of lines connecting two nodes. The difference between a transmission group in SNA and in APPN is that the current APPN architecture limits a transmission group to a single link, although multilink transmission groups are expected to be implemented in the future. The topology database contains NNs and transmission groups that connect NNs.

TRANSPAC
TRANSPAC is a major packet data network run by France Telecom.

Transport Layer
Transport Layer is the layer 4 of the OSI 7 layers reference model, which 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. TCP and UDP are two well-known protocols in the transport layer.

Trap
Trap, also known as SNMP trap, is a message sent by an SNMP agent to a manager, a console, or a terminal to indicate the occurrence of a significant event, such as a specifically defined condition or a threshold that was reached.

TRBRF: Token Ring Bridge Relay Function
Token Ring Bridge Relay Function(TRBRF) is an internal multiport bridge function used to interconnect token rings to form a domain.

TRCRF: Token Ring Concentrator Relay Function
Token Ring Concentrator Relay Function(TRCRF) is a logical ring domain formed by defining groups of ports that have the same ring number.

Tree Topology
Tree Topology is one of the local area network (LAN) topologies, In which, the network nodes are arranged as a tree, which resembles an interconnection of star networks in that individual peripheral nodes are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the function of the central node may be distributed.

Triple Play
The Triple Play service refers to providing the three services: high-speed Internet, television (Video on Demand or regular broadcasts) and telephone service over a single broadband connection and network infrastructure. Triple Play services can be offered by cable television operators as well as by telecommunication operators. For telecom providers, Triple Play is delivered using a combination of optical fiber and digital subscriber line (DSL) technologies to its residential base. Cable television operators use a similar architecture called hybrid fibre coaxial (HFC) to provide subscriber homes with broadband, but use the available coaxial cable rather than a twisted pair for the last mile transmission standard. Subscriber homes can be in a residential environment, multi-dwelling units, or even in business offices.

TRM
TRM is an Available Bit Rate (ABR) service parameter that provides an upper bound on the time between forward RM-cells for an active source. It is 100 times a power of two with a range of 100*2-7 to 100*20. For ABR traffic, TRM effects the time rate of change of the ACR. This value effects TCP round trip time calculations, which in turn effects TCP throughput.

Trunk
Trunk refers to physical and logical connections between two switches across which network traffic travels. A switched backbone is composed of a number of trunks.

TSAPI: Telephony Services Application Programming Interface
Telephony Services Application Programming Interface(TSAPI) is a call control model developed by Lucent and Novell for NetWare servers.

TSI: Transport Session Identifier
Transport Session Identifier is the unique identifier used by both the Pragmatic General Multicast (PGM) Host and PGM Router Assist features to identify each individual session

TWS: Two-way Simultaneous
Two-way Simultaneous(TWS) is a 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.

Type 1 Operation
Type 1 Operation refers to the IEEE 802.2 (Logic Link Control - LLC) connectionless service.

Type 2 Operation
Type 2 Operation refers to the IEEE 802.2 (Logic Link Control - LLC) connection-oriented service.

Type 3 Operation
Type 2 Operation refers to the IEEE 802.2 (Logic Link Control - LLC) connectionless service with acknowledgement.

Type A Traffic
Type A Traffic in networking refers to the transactional traffic or host to host traffic. For example, the conversational traffic exchanged between a client and a server for queries and responses.

Type B Traffic
Type B Traffic refers to the messaging Traffic. For example, the e-mail application traffic is the tybe B traffic.