724
Glossary
priority 2
Production network performance seri-
ously degraded. This is the second highest priority
when opening a ticket with the Cisco TAC.
priority 3
Network performance degraded. This is
the third highest priority when opening a ticket with
the Cisco TAC.
priority 4
Information needed on Cisco products.
This priority is associated with cases that do not
require immediate troubleshooting support.
priority queueing
A routing function in which
frames temporarily placed in an interface output
queue are assigned priorities based on traits such as
packet size or type of interface.
problem definition
The step in the troubleshooting
model where details are used to define the most likely
cause of a problem. This should be a concise yet accu-
rate description of the problem at hand.kh 1
process switching
As a packet arrives on a router
to be forwarded, it's copied to the router's process
buffer, and the router performs a lookup on the Layer
3 address. Using the route table, an exit interface is
associated with the destination address. The processor
forwards the packet with the added new information
to the exit interface, while the router initializes the
fast-switching cache. Subsequent packets bound for
the same destination address follow the same path as
the first packet.
PROM
programmable read-only memory: ROM
that is programmable only once, using special equip-
ment. Compare with: EPROM.
propagation delay
The time it takes data to
traverse a network from its source to its destination.
protocol
In networking, the specification of a set of
rules for a particular type of communication. The term
is also used to refer to the software that implements a
protocol.
protocol analyzer
Also known as network analyz-
ers. A device that collects and analyzes data on a con-
nected broadcast domain. The information provided
is a packet decode of data transiting the network. This
also includes a protocol analysis of the traffic that is
captured from the network. This data is used to trou-
bleshoot network problems.
protocol parameters
The options that are passed
with connection setup within many connection-ori-
ented protocols. An example would be the TCP win-
dow size.
protocol stack
A collection of related protocols.
PSE
Packet Switch Exchange: The X.25 term for a
switch.
PSN
packet-switched network: Any network that
uses packet-switching technology. Also known as
packet-switched data network (PSDN). See also:
packet switching.
PSTN
Public Switched Telephone Network: Collo-
quially referred to as "plain old telephone service"
(POTS). A term that describes the assortment of tele-
phone networks and services available globally.
PVC
permanent virtual circuit: In a Frame-Relay
network, a logical connection, defined in software,
that is maintained permanently. Compare with: SVC.
See also: virtual circuit.
PVP
permanent virtual path: A virtual path made up
of PVCs. See also: PVC.
PVP tunneling
permanent virtual path tunneling: A
technique that links two private ATM networks across
a public network using a virtual path; wherein the
public network transparently trunks the complete col-
lection of virtual channels in the virtual path between
the two private networks.
q.921
The q.921 is the Layer 2 protocol used by
ISDN, on the D channel. It establishes a connection
between the central office switch and the router.
q.931
The third layer of ISDN is addressed in the
ITU-T I.451 specification, which is also called q.931.
This protocol includes several message commands,
which are viewed with the debug isdn q931 command.
These commands include call setup, connect, release,
cancel, status, disconnect, and user information.
QoS
Quality of Service: A set of metrics used to mea-
sure the quality of transmission and service availabil-
ity of any given transmission system.
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com