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Glossary
output queue
This queue resides on the RP and is
used to hold the packet until the packet can be copied
to the buffers on the SP/SSP. From there, it is for-
warded to the specified interface processor.
outside-in troubleshooting
Troubleshooting
method that consists of choosing the opposite end of
the connection and working back toward you or the
user that reported the problem.
overrun
Phenomenon that occurs when the receiver
receives packets faster than it can transfer them to the
hardware buffer.
packet
In data communications, the basic logical
unit of information transferred. A packet consists of a
certain number of data bytes, wrapped or encapsu-
lated in headers and/or trailers that contain informa-
tion about where the packet came from, where it's
going, and so on. The various protocols involved in
sending a transmission add their own layers of header
information, which the corresponding protocols in
receiving devices then interpret.
packet switch
A physical device that makes it pos-
sible for a communication channel to share several
connections. Its functions include finding the most
efficient transmission path for packets.
packet switching
A networking technology based
on the transmission of data in packets. Dividing a con-
tinuous stream of data into small units--packets--
enables data from multiple devices on a network to
share the same communication channel simulta-
neously but also requires the use of precise routing
information.
PAP
Password Authentication Protocol: In Point-
to-Point Protocol (PPP) networks, a method of vali-
dating connection requests. The requesting (remote)
device must send an authentication request, contain-
ing a password and ID, to the local router when
attempting to connect. Unlike the more secure CHAP
(Challenge Handshake Authentication Protocol),
PAP sends the password unencrypted and does not
attempt to verify whether the user is authorized to
access the requested resource; it merely identifies the
remote end. See also: CHAP.
parity checking
A method of error-checking in
data transmissions. An extra bit (the parity bit) is
added to each character or data word so that the sum
of the bits will be either an odd number (in odd parity)
or an even number (even parity).
partial mesh
A type of network topology in which
some network nodes form a full mesh (where every
node has either a physical or a virtual circuit linking it
to every other network node), but others are attached
to only one or two nodes in the network. A typical use
of partial-mesh topology is in peripheral networks
linked to a fully meshed backbone. See also: full mesh.
path determination
Condition where the router is
aware of a route that leads to the desired destination
address.
PCR
Peak Cell Rate: As defined by the ATM Forum,
the parameter specifying, in cells per second, the max-
imum rate at which a source may transmit.
PDN
Public Data Network: Generally for a fee, a
PDN offers the public access to computer communica-
tion network operated by private concerns or govern-
ment agencies. Small organizations can take
advantage of PDNs, to aid them in creating WANs
without their having to invest in long-distance equip-
ment and circuitry.
PGP
Pretty Good Privacy: A popular public-key/pri-
vate-key encryption application offering protected
transfer of files and messages.
Physical layer
The lowest layer--Layer 1--in the
OSI reference model, it is responsible for converting
data packets from the Data-Link layer (Layer 2) into
electrical signals. Physical-layer protocols and stan-
dards define, for example, the type of cable and con-
nectors to be used, including their pin assignments and
the encoding scheme for signaling 0 and 1 values. See
also: Application layer, Data-Link layer, Network
layer, Presentation layer, Session layer,
and Transport
layer.
physical test equipment
A genre of testing equip-
ment including multi-meters, cable testers, TDRs, and
OTDRs. Used for testing cable integrity and end-to-
end physical connectivity.
ping
packet Internet groper: A Unix-based Internet
diagnostic tool, consisting of a message sent to test the
accessibility of a particular device on the IP network.
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