IBM SC30-3865-04 462

packet loss ratio. The probability that a packet will

not reach its destination or not reach it within a

speciﬁed time.

packet mode operation. Synonym for

packet

switching

packet switching. (1) The process of routing and

transferring data by means of addressed packets so

that a channel is occupied only during transmission of a

packet. On completion of the transmission, the channel

is made available for transfer of other packets. (I) (2)

Synonymous with

packet mode operation

. See also

circuit switching

parallel bridges. Apair of bridges connected to the

same LAN segment, creating redundant paths to the

segment.

parallel transmission groups. Multiple transmission

groups between adjacent nodes, with each group

having a distinct transmission group number.

path. (1) In a network, any route between any two

nodes.A path may include more than one

branch. (T) (2) The series of transport network

components (path control and data link control) that are

traversed by the information exchanged between two

network accessible units. See also

explicit route (ER)

route extension

, and

virtual route (VR)

path control (PC). The function that routes message

units between network accessible units in the network

and provides the paths between them. It converts the

basic information units (BIUs) from transmission control

(possibly segmenting them) into path information units

(PIUs) and exchanges basic transmission units

containing one or more PIUs with data link control. Path

control differs by node type: some nodes (APPN nodes,

for example) use locally generated session identiﬁers for

routing, and others (subarea nodes) use network

addresses for routing.

path cost. In link-state routing protocols, the sum of

the link costs along the path between two nodes or

networks.

path information unit (PIU). Amessage unit

consisting of a transmission header (TH) alone, or a TH

followed by a basic information unit (BIU) or a BIU

segment.

pattern-matching character. Aspecial character such

as an asterisk (*) or a question mark (?) that can be

used to represent one or more characters.Any

character or set of characters can replace a

pattern-matching character.Synonymous with

global

character

and

wildcard character

permanent virtual circuit (PVC). In X.25 and

frame-relay communications, a virtual circuit that has a

logical channel permanently assigned to it at each data

terminal equipment (DTE). Call-establishment protocols

are not required. Contrast with

switched virtual circuit

(SVC)

physical circuit. Acircuit established without

multiplexing. See also

data circuit

. Contrast with

virtual

circuit

physical layer. Inthe Open Systems Interconnection

reference model, the layer that provides the mechanical,

electrical, functional, and procedural means to establish,

maintain, and release physical connections over the

transmission medium. (T)

physical unit (PU). (1) The component that manages

and monitors the resources (such as attached links and

adjacent link stations) associated with a node, as

requested by an SSCP via an SSCP-PU session.An

SSCP activates a session with the physical unit in order

to indirectly manage, through the PU, resources of the

node such as attached links. This term applies to type

2.0, type 4, and type 5 nodes only.(2) See also

peripheral PU

and

subarea PU

ping command. The command that sends an Internet

Control Message Protocol (ICMP) echo-request packet

to a gateway,router, or host with the expectation of

receiving a reply.

Point-to-Point Protocol (PPP). Aprotocol that

provides a method for encapsulating and transmitting

packets over serial point-to-point links.

polling. (1) On a multipoint connection or a

point-to-point connection, the process whereby data

stations are invited, one at a time, to transmit. (I) (2)

Interrogation of devices for such purposes as to avoid

contention, to determine operational status, or to

determine readiness to send or receive data. (A)

port. (1)An access point for data entry or exit. (2) A

connector on a device to which cables for other devices

such as display stations and printers are attached. (3)

The representation of a physical connection to the link

hardware.A port is sometimes referred to as an

adapter; however,there can be more than one port on

an adapter.There may be one or more ports controlled

by a single DLC process. (4) In the Internet suite of

protocols, a 16-bit number used to communicate

between TCP or the User Datagram Protocol (UDP) and

a higher-level protocol or application. Some protocols,

such as File TransferProtocol (FTP) and Simple Mail

Transfer Protocol (SMTP), use the same well-known

port number in all TCP/IP implementations. (5)An

abstraction used by transport protocols to distinguish

among multiple destinations within a host machine. (6)

Synonymous with

socket

port number. InInternet communications, the

identiﬁcation of an application entity to the transport

service.

462 MRS V3.2 Protocol Conﬁg Ref Vol2