Tandem Network :
An arrangement of voice switches that enables calls to be routed through two or more switching centres in tandem fashion, such that each end office switch does not need to be directly connected to each other.

Tandem Switch :
A voice switch that is designed primarily with trunk interfaces rather than subscriber interfaces.

TCAP (Transaction Capabilities Application Part) :
The portion of the SS7 protocol that is used to make database queries to SCPs. It is used to support services such as 800 and LNP number translation, as well as other functions.

TCP (Transmission Control Protocol) :
A communications protocol used along with the Internet Protocol (the two together are called TCP/IP) to send data over the Internet in the form of packets between computers. TCP divides the data into packets and takes care of keeping track of them.

TDM (Time Division Multiplexing) :
A multiplexing scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is broken up into many segments, each having a very short duration and specific time slots within the channel. The slots are assigned whether or not any signals are available for transmission.

Tele/Data Communications Network :
A system of wires, optic fibres and satellite links, where hardware (such as hubs, routers and switches) directs information between one or several senders/receivers.

Telephony-Grade :
99.9994% uptime or the higher reliability standard circuit switches require (translating into 3 minutes of downtime a year), as opposed to the "carrier-class" reliability standard of 99.999% (called "five-nines").

TOS (Type Of Service) :
A field in the IP header assigned to a data packet that enables differentiation between packets of different priorities.

Twisted Pair :
Two insulated copper wires twisted together with the "twists" or "lays" varied in length to reduce potential signal interference between the pairs. Where cables comprise more than 25 pairs, they are usually bundled together and wrapped in a cable sheath. Twisted pair is the most commonly used medium for connecting telephones, computers and terminals to PABXs, supporting speeds up to 64kbits/sec.

Unicast :
A type of data packet addressed to a single destination. Most traffic is unicast.

VCC (Virtual Circuit Channel) :
Generic name for a central office located multiplexer that aggregates multiple customers via lower speed line to a higher speed trunk connection.

VLAN (Virtual LAN) :
A method of grouping together computers so that they can communicate with each other without interference from computers outside the VLAN. They do not have to be on the same physical network (hence "virtual" LAN). VLANs can also separate computers that may be on the same physical network. This is a very powerful tool for network managers.

Voice Over ATM (Voice over Asynchronous Transfer Mode) :
The process of transmitting voice traffic across an ATM-based packet network.

Voice Over DSL (Voice over Digital Subscriber Line) :
The process of transmitting voice phone calls over DSL.

Voice Over IP (Voice over Internet Protocol) :
Routing of encoded voice traffic over the network as IP data packets. Currently, traffic priority (and hence audio quality) can only be guaranteed over private networks.

WAN (Wide Area Network) :
A geographically dispersed telecommunications network (nationally or globally). The term usually connotes the inclusion of public (shared user) networks.

WCDMA (Wideband Code Division Multiple Access) :
A part of the new world-encompassing standard for mobile telephony - the Universal Mobile Telephone Standard (UMTS), which enables wireless transmission of data traffic at speeds up to 2 Mbps.

Wire Speed Router/Switch :
A switch or router that can handle the switching of data traffic at the highest speeds within the connected networks.

W-LAN (Wireless-Local Area Network) :
A wireless version of the LAN. It provides access to the LAN even when the user is not in the office.

Work Group Switch :
The simplest switch that is positioned closest to the user's computer in the network.