User guide

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Glossary
Topology: Physical layout of network components (cables,
stations, gateways, and hubs). Three basic interconnection
topologies are star, ring, and bus networks.
Transmission Control Protocol (TCP): A communications
protocol used in Internet and in any network that follows the US
Department of Defense standards for internetwork protocol. TCP
provides a reliable host-to-host protocol between hosts in packet-
switched communications networks and in interconnected
systems of such networks. It assumes that the Internet protocol
is the underlying protocol.
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 multi-layer 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 10BaseT
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 64K bps unrestricted user data (allowing all 0s in the
user data). An alternative to (but not as popular as) B8ZS.