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Glossary
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trunk link
Link used between switches and from
some servers to the switches. Trunk links carry infor-
mation about many VLANs. Access links are used to
connect host devices to a switch and carry only VLAN
information that the device is a member of.
TTL
Time To Live: A field in an IP header, indicating
the length of time a packet is valid.
TUD
Trunk Up-Down: A protocol used in ATM net-
works for the monitoring of trunks. Should a trunk
miss a given number of test messages being sent by
ATM switches to ensure trunk line quality, TUD
declares the trunk down. When a trunk reverses direc-
tion and comes back up, TUD recognizes that the
trunk is up and returns the trunk to service.
tunneling
A method of avoiding protocol restric-
tions by wrapping packets from one protocol in
another protocol's packet and transmitting this encap-
sulated packet over a network that supports the wrap-
per protocol. See also: encapsulation.
U reference point
Reference point between a TE1
and an ISDN network. The U reference point under-
stands ISDN signaling techniques and uses a 2-wire
connection.
UDP
User Datagram Protocol: A connectionless
Transport layer protocol in the TCP/IP protocol stack
that simply allows datagrams to be exchanged without
acknowledgments or delivery guarantees, requiring
other protocols to handle error processing and
retransmission. UDP is defined in RFC 768.
underrun
Occurs when the transmitter runs at a
higher rate than the packets sent from the hardware
buffer.
unnumbered frames
HDLC frames used for con-
trol-management purposes, such as link start-up and
shutdown or mode specification.
UTP
unshielded twisted-pair: Copper wiring used in
small-to-large networks to connect host devices to
hubs and switches. Also used to connect switch to
switch or hub to hub.
VBR
Variable Bit Rate: A QoS class, as defined by
the ATM Forum, for use in ATM networks that is sub-
divided into real time (RT) class and non-real time
(NRT) class. RT is employed when connections have a
fixed-time relationship between samples. Conversely,
NRT is employed when connections do not have a
fixed-time relationship between samples, but still need
an assured QoS.
VCC
Virtual Channel Connection: A logical circuit
that is created by VCLs. VCCs carry data between two
endpoints in an ATM network. Sometimes called a
virtual circuit connection.
VIP
(1) Versatile Interface Processor: An interface
card for Cisco 7000 and 7500 series routers, providing
multilayer switching and running the Cisco IOS soft-
ware. The most recent version of VIP is VIP2. (2) Vir-
tual IP: A function making it possible for logically
separated switched IP workgroups to run Virtual Net-
working Services across the switch ports of a Catalyst
5000.
virtual circuit
Abbreviated VC, a logical circuit
devised to assure reliable communication between two
devices on a network. Defined by a virtual path con-
nection (VPC)/virtual path identifier (VCI) pair, a vir-
tual circuit can be permanent (PVC) or switched
(SVC). Virtual circuits are used in Frame Relay and
X.25. Known as virtual channel in ATM. See also:
PVC
and SVC.
virtual ring
In an SRB network, a logical connection
between physical rings, either local or remote.
VLAN
Virtual LAN: A group of devices on one or
more logically segmented LANs (configured by use of
management software), enabling devices to communi-
cate as if attached to the same physical medium, when
they are actually located on numerous different LAN
segments. VLANs are based on logical instead of phys-
ical connections and thus are tremendously flexible.
VLSM
variable-length subnet mask: Helps opti-
mize available address space and specify a different
subnet mask for the same network number on vari-
ous subnets. Also commonly referred to as "subnet-
ting a subnet."
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