data to it at 56 Kbps. It is a relatively low cost service, widely used in North America for telecommuting, videoconferencing and high speed data transfers. Many phone companies are phasing out switched 56 in favor of ISDN service.
switched line—In communications, a physical channel established by dynamically connecting one or more discreet segments. This connection lasts for the duration of the call after which each segment may be used as part of a different channel. Contrast with leased line.
switched network—A network in which a temporary connection is established from one point via one or more segments.
synchronous transmission—The transmission of data which involves sending a group of characters in a packet. This is a common method of transmission between computers on a network or between modems. One or more synchronous characters are transmitted to confirm clocking before each packet of data is transmitted. Compare to Asynchronous Transmission.
T
T—Provides the connection between the NT1 and NT2. Can be internal to or external from the PABX. Provides the connection between the NT1 equipment and the TA or TE-1. Can provide both Primary Rate and Basic Rate services.
TA—Seeterminal adapter.
TCP/IP (Transmission Control Protocol / Internet Protocol)—A multi-layer set of protocols developed by the US Department of Defense to link dissimilar computers across dissimilar and unreliable LANs.
TEI (terminal endpoint identifier)—A unique number that is assigned to an ISDN terminal adapter at subscription time. The TEI is used by the central office to identify the various TAs connected to the ISDN network. The TEI number can be fixed (in the range 0f 0- 63, or dynamic, automatically assigned at the CO each time the TA is connected to the ISDN interface and powered up.
TE1 (Terminal Equipment Type 1)—A terminal device with a standard ISDN interface (an ISDN-compatible terminal).
TE2 (Terminal Equipment Type 2)—A terminal device with a non- ISDN interface, such as RS232, RS449, V.35 or X.21. Can be a telephone, data terminal, or voice/data worrkstation, etc.
telecommunication service—A function, such as bearer service and teleservice, offered by an RPOA to its customer in order to satisfy a specific telecommunications requirement. See also ISDN, RPOA, and bearer service.
telematics—User-oriented ISDN information transfer services (e.g., teletex, videotex, facsimile).
Teleservices—A telecommunications service that provides the complete capability for communication between subscribers according to protocols agreed to by RPOAs. A set of ISDN features using communications links, including E-Mail and fax services.
Teletext—A telecommunications service that provides a one-way information retrieval service that uses unused portions of a TV channel bandwidth. A fixed number of text pages are repetitively broadcast for decoding by a decoder at the TV set that can select and display pages. See also ISDN.
terminal—The screen and keyboard device used in a mainframe environment for interactive data entry. Terminals have no box, which is to say they have no file storage or processing capabilities.
terminal adapter (TA)—An ISDN DTE device for connecting a non-ISDN terminal device to the ISDN network. Similar to a protocol converter or an interface converter, a TA connects a non- ISDN device between the R and S interfaces.
tone dialing—One of two methods of dialing a telephone, usually
associated with Touch-Tone® (push button) phones. Compare with pulse dialing.
TSR (terminate and stay resident)—A software program that remains active and in memory after its user interface is closed. Similar to a daemon in UNIX environments.
twisted pair wiring—A type of cabling with one or more pairs of insulated wires wrapped around each other. An inexpensive wiring method used for LAN and telephone applications, also called UTP wiring.
U
Ureference point—A twisted-pair subscriber loop that connects the LT to the NT1. Normally a 2-wire BRI or PRI line is used, but a 4- wire BRI can also be used. In the US, the U Reference Point marks the separation point between CPE and the network.
UART (universal asynchronous receiver/transmitter)—An integrated circuit that transmits and receives data on the serial port. It converts parallel bytes into serial bytes for transmission, and vice versa, and generates and strips the start and stop bits appended to each character.
UTP (unshielded twisted pair) —Telephone-type wiring.
V
V.110—One of the terminal rate adaptation protocols for the ISDN B-channel. V.110 is more hardware-intensive than the V.120 B- channel standard.
V.120—One of the terminal rate adaptation protocols for the ISDN B-channel. V.120 is more software-intensive than the V.110 B- channel standard.
Videotex—A two-way information-retrieval service that can be accessed by terminals and by a TV set (with installed decoder). Allows interactive retrieval of information pages from a central resource. See also ISDN.
virtual circuit—A logical connection. Used in packet switching wherin a logical connection is established between two devices at the start of transmission. All information packets follow the same route and arrive in sequence (but do not necessarily carry a complete address).
X
X.25—ITU-T’s definition of a three-level packet-switching protocol to be used between packet-mode DTEs and network DCEs. X.25 corresponds with layer 3 of the 7-layer OSI model.
X.75—An ITU-T standard for linking X.25 packet-switched net- works. X.75 defines the connection between public networks, i.e., for a gateway between X.25 networks.
