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ProxyServer User Guide
Transport Layer: Layer 4 of the Open Systems Interconnection (OSI) model; provides reliable, end-to-end delivery of data,
and detects transmission sequential errors.
Transport Protocol Data Unit (TPDU): A transport header, which is added to every message, contains destination and
source addressing information that allows the end-to-end routing of messages in multilayer NAC networks of high
complexity. They are automatically added to messages as they enter the network and can be stripped off before being
passed to the host or another device that does not support TPDU's.
Trunk: Transmission links that interconnect switching offices.
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.
Tunneling: Encapsulation data in an IP packet for transport across the Internet.
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
UART (Universal Asynchronous Receiver/Transmitter) (pronounced "you art"): A chip that transmits and receives data
on the serial port. It converts bytes into serial bits for transmission, and vice versa, and generates and strips the start and
stop bits appended to each character.
UNIX: An operating system developed by Bell Laboratories that features multiprogramming in a multi-user environment.
Unshielded Twisted Pair (UTP): Telephone-type wiring. Transmission media for 10Base-T.
User Datagram Protocol (UDP): A TCP/IP protocol describing how messages reach application programs within a
destination computer. This protocol is usually bundled with IP-layer software. UDP is a transport layer, connectionless mode
protocol, providing a (potentially unreliable, unsequenced, and/or duplicated) datagram mode of communication for delivery
of packets to a remote or local user.
V
V.25bis: An ITU-T standard for synchronous communications between a mainframe or host and a modem using HDLC or
other character-oriented protocol.
V.54: The ITU-T standard for local and remote loopback tests in modems, DCEs and DTEs. The four basic tests are:
• local digital loopback (tests DTE send and receive circuits),
• local analog loopback (tests local modem operation),
• remote analog loopback (tests comm link to the remote modem), and
• remote digital loopback (tests remote modem operation).
Virtual Circuit: A logical connection. Used in packet switching wherein 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).
W
Wide Area Network (WAN): 1. A network that provides communication services to a geographic area larger than that
served by a local area network or a metropolitan area network, and that may use or provide public communication facilities.
2. A data communications network designed to serve an area of hundreds or thousands of miles; for example, public and
private packet-switching networks, and national telephone networks. Contrast with local area network (LAN).
Wide Area Telecommunications Service (WATS): A low-cost toll service offered by most long distance and local phone
companies. Incoming (800 call service, or IN-WATS) and outgoing WATS are subscribed to separately, but over the same
line.
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.
Y
Yellow Alarm: An error indication sent by the T1 device when it has not gotten a receive signal, or cannot synchronize on
the receive signal received. Contrast "Red Alarm" and "Blue Alarm".
Z
Zero Byte Time Slot Interchange (ZBTSI): A method for allowing 64 Kbps unrestricted user data (allowing all 0s in the
user data). An alternative to (but not as popular as) B8ZS.
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MTPSR1-202ST
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