pacing
- See flow
control.
- Logical grouping of
information that includes a header containing control
information and (usually) user data. Packets are most
often used to refer to network layer units of data. The
terms datagram,
frame,
message,
and segment
are also used to describe logical information
groupings at various layers of the OSI reference model and
in various technology circles. See also PDU.
packet assembler/disassembler
See PAD.
packet buffer
- See buffer.
packet internet groper
- See ping.
packet level protocol
- See PLP.
packet line card
- See PLC.
packet switch
- WAN device that routes
packets along the most efficient path and allows a
communications channel to be shared by multiple
connections. Sometimes referred to as a packet switch
node (PSN), and formerly called an IMP. See
also IMP.
packet-switched data network
- See PSN.
packet-switched network
- See PSN.
- Networking method in which
nodes share bandwidth with each other by sending packets.
Compare with circuit
switching and message
switching. See also PSN
(packet-switched network).
packet switch exchange
- See PSE.
packet-switching node
- See PSN.
- Packet assembler/disassembler.
Device used to connect simple devices (like character-mode
terminals) that do not support the full functionality of a
particular protocol to a network. PADs buffer data and
assemble and disassemble packets sent to such end devices.
paddle card
- See access
card.
Palo Alto Research Center
- See PARC.
- Pulse amplitude
modulation. Modulation scheme where the
modulating wave is caused to modulate the amplitude of a
pulse stream. Compare with AM
and FM.
See also modulation.
- Password
Authentication Protocol. Authentication protocol
that allows PPP peers to authenticate one another. The
remote router attempting to connect to the local router is
required to send an authentication request. Unlike CHAP,
PAP passes the password and host name or username in the
clear (unencrypted). PAP does not itself prevent
unauthorized access, but merely identifies the remote end.
The router or access server then determines if that user
is allowed access. PAP is supported only on PPP lines.
Compare with CHAP.
- Channel that uses bus and
tag cables as a transmission medium. Compare with ESCON
channel. See also bus
and tag channel.
parallelism
- Indicates that multiple
paths exist between two points in a network. These paths
might be of equal or unequal cost. Parallelism is often a
network design goal: if one path fails, there is
redundancy in the network to ensure that an alternate path
to the same point exists.
- Method of data transmission
in which the bits of a data character are transmitted
simultaneously over a number of channels. Compare with serial
transmission.
- Palo Alto Research
Center. Research and development center operated
by XEROX. A number of widely-used technologies were
originally conceived at PARC, including the first personal
computers and LANs.
PARC Universal Protocol
- See PUP.
parity check
- Process for checking the
integrity of a character. A parity check involves
appending a bit that makes the total number of binary 1
digits in a character or word (excluding the parity bit)
either odd (for odd parity) or even (for even
parity).
- Term describing a network in
which devices are organized in a mesh topology, with some
network nodes organized in a full mesh, but with others
that are only connected to one or two other nodes in the
network. A partial mesh does not provide the level of
redundancy of a full mesh topology, but is less expensive
to implement. Partial mesh topologies are generally used
in the peripheral networks that connect to a fully meshed
backbone. See also full
mesh and mesh.
Password Authentication
Protocol
- See PAP.
patch panel
- An assembly of pin locations
and ports which can be mounted on a rack or wall bracket
in the wiring closet. Patch panels act like switchboards
that connect workstations cables to each other and to the
outside.
- Layer 3 in the SNA
architectural model. This layer performs sequencing
services related to proper data reassembly. The path
control layer is also responsible for routing. Corresponds
roughly with the network
layer of the OSI model. See also data
flow control layer, data
link control layer, physical
control layer, presentation
services layer, transaction
services layer, and transmission
control layer.
- SNA concept that consists of
lower-level components that control the routing and data
flow through an SNA network and handle physical data
transmission between SNA nodes. Compare with NAU.
path cost
- See cost.
path name
- Full name of a UNIX, DOS, or
LynxOS file or directory, including all directory and
subdirectory names. Consecutive names in a path name are
typically separated by a forward slash (/) or a backslash
(\), as in /usr/app/base/config.
payload
- Portion of a frame that
contains upper-layer information (data).
- Private branch
exchange. Digital or analog telephone switchboard
located on the subscriber premises and used to connect
private and public telephone networks.
- Protocol control
information. Control information added to user
data to comprise an OSI packet. The OSI equivalent of the
term header. See also header.
- Pulse code
modulation. Transmission of analog information in
digital form through sampling and encoding the samples
with a fixed number of bits.
- Peak cell rate.
Parameter defined by the ATM Forum for ATM traffic
management. In CBR transmissions, PCR determines how often
data samples are sent. In ABR transmissions, PCR
determines the maximum value of the ACR. See also ABR
(available bit rate), ACR,
and CBR.
- Public data network.
Network operated either by a government (as in Europe) or
by a private concern to provide computer communications to
the public, usually for a fee. PDNs enable small
organizations to create a WAN without all the equipment
costs of long-distance circuits.
- Protocol data unit. In the OSI model, the generic name for the structure of data (messages) sent between peer layers of communicating network devices. Packet = IP datagram carrying TCP or UDP; datagram = generic layer 3 PDU.
peak cell rate
- See PCR.
peak rate
- Maximum rate, in kilobits
per second, at which a virtual circuit can transmit.
- Peer-to-peer computing calls
for each network device to run both client and server
portions of an application. Also describes communication
between implementations of the same OSI reference model
layer in two different network devices. Compare with client-server
computing.
- One of five categories of
network management defined by ISO for management of OSI
networks. Performance management subsystems are
responsible for analyzing and controlling network
performance including network throughput and error rates.
See also accounting
management, configuration
management, fault
management, and security
management.
peripheral node
- In SNA, a node that uses
local addresses and is therefore not affected by changes
to network addresses. Peripheral nodes require boundary
function assistance from an adjacent subarea node.
permanent virtual circuit
- See PVC.
permanent virtual connection
- See PVC.
permanent virtual path
- See PVP.
permit processing
- See traffic
policing.
- Poll/final bit.
Bit in bit-synchronous data link layer protocols that
indicates the function of a frame. If the frame is a
command, a 1 in this bit indicates a poll. If the frame is
a response, a 1 in this bit indicates that the current
frame is the last frame in the response.
- Pretty Good Privacy.
Public-key encryption application that allows secure file
and message exchanges. There is some controversy over the
development and use of this application, in part due to
U.S. national security concerns.
phase
- Location of a position on an
alternating wave form.
- Situation in which the
relative position in time between the clock and data
signals of a transmission becomes unsynchronized. In
systems using long cables at higher transmission speeds,
slight variances in cable construction, temperature, and
other factors can cause a phase shift, resulting in high
error rates.
- Physical sublayer.
One of two sublayers of the FDDI physical layer. See also PMD.
physical address
- See MAC
address.
- Layer 1 in the SNA
architectural model. This layer is responsible for
the physical specifications for the physical links between
end systems. Corresponds to the physical
layer of the OSI model. See also data
flow control layer, data
link control layer, path
control layer, presentation
services layer, transaction
services layer, and transmission
control layer.
- Layer 1 of the OSI
reference model. The physical layer defines the
electrical, mechanical, procedural and functional
specifications for activating, maintaining, and
deactivating the physical link between end systems.
Corresponds with the physical
control layer in the SNA model. See also application
layer, data
link layer, network
layer, presentation
layer, session
layer, and transport
layer.
physical layer convergence
procedure
- See PLCP.
physical media
- See media.
physical medium
- See media.
physical medium dependent
- See PMD.
physical sublayer
- See PHY.
physical unit
- See PU.
Physical Unit 2
- See PU
2.
Physical Unit 2.1
- See PU
2.1.
Physical Unit 4
- See PU
4.
Physical Unit 5
- See PU
5.
Physics Network
- See PHYSNET.
- Physics Network.
Group of many DECnet-based physics research networks,
including HEPnet. See also HEPnet.
piggybacking
- Process of carrying
acknowledgments within a data packet to save network
bandwidth.
- Protocol Independent
Multicast. Multicast routing architecture that
allows the addition of IP multicast routing on existing IP
networks. PIM is unicast routing protocol independent and
can be operated in two modes: dense mode and sparse mode.
See also PIM
dense mode and PIM
sparse mode.
- One of the two PIM
operational modes. PIM dense mode is data-driven and
resembles typical multicast routing protocols. Packets are
forwarded on all outgoing interfaces until pruning and
truncation occurs. In dense mode, receivers are densely
populated, and it is assumed that the downstream networks
want to receive and will probably use the datagrams that
are forwarded to them. The cost of using dense mode is its
default flooding behavior. Sometimes called dense mode
PIM or PIM DM. Contrast with PIM
sparse mode. See also PIM.
PIM DM
- See PIM
dense mode.
PIM SM
- See PIM
sparse mode.
- One of the two PIM
operational modes. PIM sparse mode tries to constrain data
distribution so that a minimal number of routers in the
network receive it. Packets are sent only if they are
explicitly requested at the RP (rendezvous point). In
sparse mode, receivers are widely distributed, and the
assumption is that downstream networks will not
necessarily use the datagrams that are sent to them. The
cost of using sparse mode is its reliance on the periodic
refreshing of explicit join messages and its need for RPs.
Sometimes called sparse mode PIM or PIM SM.
Contrast with PIM
dense mode. See also PIM
and RP
(rendezvous point).
pin location
- A color-coded slot on a
patch panel. Cable wires are punched down using a punch
tool to make an electrical connection that allows the
network to function.
- Short for Packet Internet Groper or Packet Inter-network Groper, a utility to determine whether a specific IP address is accessible. It works by sending a packet to the specified address and waiting for a reply. PING is used primarily to troubleshoot Internet connections.
ping-ponging
- Phrase used to describe the
actions of a packet in a two-node routing loop.
pixel
- The smallest element of a
display image, corresponding to a single displayed spot or
color triad on a display, or to a single input spot from a
camera. (A word coined from the phrase "picture
element.")
plain old telephone service
- See PSTN.
- Packet line card.
Card on the LightStream 2020 ATM switch that can be
configured only as an edge card. A PLC, in conjunction
with an access card, supports up to eight Ethernet or two
FDDI edge ports.
- Physical layer
convergence procedure. Specification that maps
ATM cells into physical media, such as T3 or E3, and
defines certain management information.
- Term describing digital
signals that are sourced from different clocks of
comparable accuracy and stability. Compare with asynchronous
transmission, isochronous
transmission, and synchronous
transmission.
- Packet level
protocol. Network layer protocol in the X.25
protocol stack. Sometimes called X.25 Level 3 or X.25
Protocol. See also X.25.
- Primary LU.
The LU that is initiating a session with another LU. See
also LU.
- Physical medium
dependent. Sublayer of the FDDI physical layer
that interfaces directly with the physical medium and
performs the most basic bit transmission functions of the
network. See also PHY.
- Private
Network-Network Interface. ATM Forum
specification that describes an ATM virtual circuit
routing protocol, as well as a signaling protocol between
ATM switches. Used to allow ATM switches within a private
network to interconnect. Sometimes called Private
Network Node Interface.
point of presence
- See POP.
Point-to-Point Protocol
- See PPP.
poison reverse updates
- Routing updates that
explicitly indicate that a network or subnet is
unreachable, rather than implying that a network is
unreachable by not including it in updates. Poison reverse
updates are sent to defeat large routing loops. The Cisco
IGRP implementation uses poison reverse updates.
policy-based routing
- See policy
routing.
- Routing scheme that forwards
packets to specific interfaces based on user-configured
policies. Such policies might specify that traffic sent
from a particular network should be forwarded out one
interface, while all other traffic should be forwarded out
another interface.
poll/final bit
- See P/F.
polling
- Access method in which a
primary network device inquires, in an orderly fashion,
whether secondaries have data to transmit. The inquiry
occurs in the form of a message to each secondary that
gives the secondary the right to transmit.
- Point of presence.
Point of presence is the point of interconnection between
the communication facilities provided by the telephone
company and the building's main distribution facility.
port
- 1.) Interface on an
internetworking device (such as a router).
- 2.) In IP terminology, an upper-layer process that is receiving information from lower layers.
- 3.) To rewrite software or microcode so that it will run on a different hardware platform or in a different software environment than that for which it was originally designed. 4. A female plug on a patch panel which accepts the same size plug as an RJ-45 jack. Patch cords are used in these ports to cross connect computers wired to the patch panel. It is this cross connection which allows the LAN to function.
4.) A female plug on a patch panel which accepts the same size plug as an RJ-45 jack. Patch cords are used in these ports to cross connect computers wired to the patch panel. It is this cross connection which allows the LAN to function.
- 2.) In IP terminology, an upper-layer process that is receiving information from lower layers.
- Power-on self test.
Set of hardware diagnostics that runs on a hardware device
when that device is powered up. On a LightStream 2020 ATM
switch, for example, the NP, switch card, and line card
all perform the POST.
Post, Telephone, and
Telegraph
- See PTT.
POTS
- Plain old telephone
service. See PSTN.
power-on self test
- See POST.
- Feature on the LightStream
2020 ATM switch that allows faulty components to be
diagnosed, removed, and replaced while the rest of the
switch continues to operate normally. Sometimes
abbreviated POS. Sometimes called hot swapping.
See also OIR.
power tray
- Power supply for a
LightStream 2020 ATM switch. A LightStream 2020 switch can
have one or two bulk power trays. In a redundant system,
the two power trays load share, but each can power the
entire system in the event that the other fails. The power
tray can provide either AC or DC power to the switch.
- Point-to-Point
Protocol. A successor to SLIP, PPP provides
router-to-router and host-to-network connections over
synchronous and asynchronous circuits. See also SLIP.
- Layer 6 of the OSI
reference model. This layer ensures that information
sent by the application layer of one system will be
readable by the application layer of another. The
presentation layer is also concerned with the data
structures used by programs and therefore negotiates data
transfer syntax for the application layer. Corresponds
roughly with the presentation
services layer of the SNA model. See also application
layer, data
link layer, network
layer, physical
layer, session
layer, and transport
layer.
- Layer 6 of the SNA
architectural model. This layer provides network resource
management, session presentation services, and some
application management. Corresponds roughly with the presentation
layer of the OSI model. See also data
flow control layer, data
link control layer, path
control layer, physical
control layer, transaction
services layer, and transmission
control layer.
Pretty Good Privacy
- See PGP.
- Primary Rate
Interface. ISDN interface to primary rate access.
Primary rate access consists of a single 64-Kbps D channel
plus 23 (T1) or 30 (E1) B channels for voice or data.
Compare to BRI.
See also BISDN,
ISDN,
and N-ISDN.
primary
- See primary
station.
Primary LU
- See PLU.
Primary Rate Interface
- See PRI.
- One of the two rings that
make up an FDDI or CDDI ring. The primary ring is the
default path for data transmissions. Compare with secondary
ring.
- In bit-synchronous data link
layer protocols such as HDLC and SDLC, a station that
controls the transmission activity of secondary stations
and performs other management functions such as error
control through polling or other means. Primary stations
send commands to secondary stations and receive responses.
Also called, simply, a primary. See also secondary
station.
print server
- Networked computer system
that fields, manages, and executes (or sends for
execution) print requests from other network devices.
priority queuing
- Routing feature in which
frames in an interface output queue are prioritized based
on various characteristics such as packet size and
interface type.
private branch exchange
- See PBX.
Private Network-Network
Interface
- See PNNI.
Private Network Node
Interface
- See PNNI.
process switching
- Operation that provides full
route evaluation and per-packet load balancing across
parallel WAN links. Involves the transmission of entire
frames to the router CPU where they are repackaged for
delivery to or from a WAN interface, with the router
making a route selection for each packet. Process
switching is the most resource-intensive switching
operation that the CPU can perform.
programmable read-only
memory
- See PROM.
- Programmable
read-only memory. ROM that can be programmed
using special equipment. PROMs can be programmed only
once. Compare with EPROM.
propagation delay
- Time required for data to
travel over a network, from its source to its ultimate
destination.
protocol
- 1.) Formal description of a
set of rules and conventions that govern how devices on a
network exchange information. 2.) Field within an IP
datagram that indicates the upper layer (Layer 4) protocol
sending the datagram.
protocol address
- See network
address.
protocol analyzer
- See network
analyzer.
protocol control information
- See PCI.
protocol converter
- Enables equipment with
different data formats to communicate by translating the
data transmission code of one device to the data
transmission code of another device.
protocol data unit
- See PDU.
Protocol Independent
Multicast
- See PIM.
- Set of related
communications protocols that operate together and, as a
group, address communication at some or all of the seven
layers of the OSI reference model. Not every protocol
stack covers each layer of the model, and often a single
protocol in the stack will address a number of layers at
once. TCP/IP is a typical protocol stack.
protocol translator
- Network device or software
that converts one protocol into another, similar,
protocol.
proxy
- Entity that, in the interest
of efficiency, essentially stands in for another entity.
proxy Address Resolution
Protocol
- See proxy
ARP.
- Proxy Address
Resolution Protocol. Variation of the ARP
protocol in which an intermediate device (for example, a
router) sends an ARP response on behalf of an end node to
the requesting host. Proxy ARP can lessen bandwidth use on
slow-speed WAN links. See also ARP.
proxy explorer
- Technique that minimizes
exploding explorer packet traffic propagating through an
SRB network by creating an explorer packet reply cache,
the entries of which are reused when subsequent explorer
packets need to find the same host.
- Technique that alleviates
the load across an SDLC network by allowing routers to act
as proxies for primary and secondary nodes, thus keeping
polling traffic off of the shared links. Proxy polling has
been replaced by SDLC Transport. See SDLC
Transport.
- Packet-switched data
network. See PSN
(packet-switched network).
- Packet switch
exchange. Essentially, a switch. The term PSE is
generally used in reference to a switch in an X.25 PSN.
See also switch.
- 1. Packet-switched
network. Network that utilizes packet-switching
technology for data transfer. Sometimes called a packet-switched
data network (PSDN). See packet
switching.
2. Packet-switching node. Network node capable of performing packet switching functions. See also packet switching.
- Public Switched
Telephone Network. General term referring to the
variety of telephone networks and services in place
worldwide. Sometimes called plain old telephone service
(POTS).
- Post, Telephone, and
Telegraph. Government agency that provides
telephone services. PTTs exist in most areas outside North
America and provide both local and long-distance telephone
services.
- Physical unit.
SNA component that manages and monitors the resources of a
node, as requested by an SSCP. There is one PU per node.
- Physical Unit 2.
SNA peripheral node that can support only DLUs that
require services from a VTAM host and that are only
capable of performing the secondary LU role in SNA
sessions.
- Physical Unit type
2.1. SNA network node used for connecting peer
nodes in a peer-oriented network. PU 2.1 sessions do not
require that one node reside on VTAM. APPN is based upon
PU 2.1 nodes, which can also be connected to a traditional
hierarchical SNA network.
- Physical Unit 4.
Component of an IBM FEP capable of full-duplex data
transfer. Each such SNA device employs a separate data and
control path into the transmit and receive buffers of the
control program.
- Physical Unit 5.
Component of an IBM mainframe or host computer that
manages an SNA network. PU 5 nodes are involved in routing
within the SNA path control layer.
public data network
- See PDN.
Public Switched Telephone
Network
- See PSTN.
pull string
- Strong, heavy string used to
pull cable in multiple runs.
pulse amplitude modulation
- See PAM.
pulse code modulation
- See PCM.
- See ones
density.
Punch Tool
- Spring-loaded tool used for
cutting and connecting wire in a jack or on a patch panel.
- PARC Universal
Protocol. Protocol similar to IP developed at
PARC.
- Permanent virtual
circuit. Virtual circuit that is permanently
established. PVCs save bandwidth associated with circuit
establishment and tear down in situations where certain
virtual circuits must exist all the time. Called a permanent
virtual connection in ATM terminology. Compare with SVC.
- Permanent virtual path. Virtual path that consists of PVCs. See also PVC and virtual path.