PABX (PBX) A Private
Automatic Branch eXchange/telephone exchange linked to the PSTN.
It handles calls automatically i.e., unattended. Normally a manual,
user-owned exchange.
Packet A collection
of bits, including the address, data and control, that are switched
and transmitted together. The terms frame and packet are often
used synonymously.
Packet Buffer Memory
space reserved for storing a packet awaiting transmission or for
storing a received packet.
Packet Switching A
method of switching data in a network. Individual packets of a
set size and format are accepted by the network and delivered
to their destination. The sequence of packets is maintained, and
destination established, by the exchange of control information
(also contained in the packets) between the sending terminal and
the network before the transmission starts. The network is open
to all users, all the time, with packets from the various nodes
being interleaved throughout the network. The packets can be sent
in any order, as the control information sent at the beginning
of the transmission ensures they are interpreted in the correct
order at the receiving end. Because each packet carries its own
control instructions, it can use any route to reach its destination.
The link lasts only as long as the trarsmission. An ITU-TS standard
for packet switched networks information.
Predictive dialing Automated
dialing feature in which the systems predicts from pre-set algorithms
when an agent/operator will become free and makes the call in
advance.
Preview dialing Automated
dialing feature in which the system determines the next call to
be made but allows the agent/operator as a supervisor to check
and activate the call.
Presentation Layer The
sixth of seven layers in the OSI model. Its job is to ensure that
two computers wanting to communicate establish the ground rules
for how they can interoperate.
PRA (Primary Rate Access) An
ISDN access method that uses maximum data rates of 2.048 Mbit/s
in Europe, or 1.544 Mbit/s in the US and Japan.
PRI (Primary Rate Interface) ISDN's
carrier or large user interface comprising 30BD channels in Europe,
supporting around 1,920Mbit/s.
Primary domain controller The
server at which the master copy of a domain's user accounts database
is maintained. The primary domain controller also validates logon
requests. A LAN Manager term.
Print Server A computer
and/or software to provide users or a network with access to a
central printer. The printer server acts as a buffer, holding
the information to be printed out in memory until the printer
is free. It is possible to program the printer server to print
jobs in the order they arrive or to give priority to particular
users who, in effect, jump the queue. The advantages of a printer
server include maximum exploitation of expensive resources like
laser printers and not having to retry if the printer is initially
busy.
Printer driver A program
that controls printing and sets options such as print quality
and paper size for a particular printer. For example, in LAN Manager,
each printer queue has a single printer driver associated with
it.
Private (dedicated) line Term
for a dedicated voice or data circuit, usually leased from a Common
Carrier.
Privilege level For
user-level security, one of three settings -user, admin or guest
is assigned for each user account. The privilege level defines
interfaces was drawn up in 1980, known as Recommendation X.25.
Hence packet switched networks are often referred to as X.25.
In some cases X.25 is being superseded by frame relay and eventually
by ATM.
PAD (Packet Assembler/Disassembler) A
device used in a packet switched network to convert data to an
appropriate format so that a non X.25 (non-packet switching) device
can send information over an X.25 (packet switched) network, The
pad is responsible for call set up and addressing routines.
Paging A one-way alert
or alphanumeric message service. Messages are normally left via
a bureau with a human or automatic operator.
Parallel interface An
interface that can handle a group of bits transmitted at the same
time, by sending each bit over a separate wire. Probably the best
known parallel interface of all is the one developed by Centronics,
a printer manufacturer, that has 36 pins and can handle a byte
of data at a time. The transmission of the data is controlled
by a computer generated strobe pulse.
Parity Bit An extra
bit added to a group of bits, usually to a seven-bit byte. The
parity bit can be of 0 or 1 value so that every byte will then
add up to an odd or even number, depending on whether odd or even
parity is chosen. When the transmitting device frames a character,
it counts the numbers of 0s and 1s in the frame and attaches the
appropriate parity bit. The recipient counts all the 0s and 1s
and compares the total to the odd or even message contained in
the parity bit. If there's a discrepancy the recipient asks for
the data to be resent.
PCM (Pulse Code Modulation) One
of the most common ways of converting an analog signal to a digital
signal. This is done by sampling the signal and coding the sample.
It usually samples at 8,000 times per second; each sampling representing
8bits. This produces a transmission rate of 641Kbit/s, with the
traffic on the line in interleaved data format. At the receiving
end, the interleaved signals are separated and converted back
to analog.
Phase Modulation One
of the three ways of modulating or altering a signal so that it
is able to carry information. The other two are amplitude and
frequency modulation. Phase modulation is used in high speed modems.
Polling Method of
controlling terminals on a multi-point line by which a computer
acting as master calls or polls each of the terminals in turn
to find out if they have any data to send. This access method
is used in star topology networks but is generally being superseded
by the preferred interrupt method.
Port A computer interface
capable of attachment to another device, such as a modem for communicating
with a remote terminal, or, if the port is within a hub, to a
workstation.
PPP (Point to Point Protocol) An
addition to the Internet protocol suite to help connect devices
where dissimilar transport protocols exist, for example for the
transfer of routing the range of actions a user can perform on
the network.
Profile A set of parameters
defining the way in which a device acts. In the LAN world, a profile
is often used by one or more workstations to determine the connection
they will have with other devices. Profiles work like batch files,
executing a number of commands to save a user time and effort.
Protocol A set of
rules governing the information flow within a communications infrastructure,
often known as "data link control". Protocols control
format, timing, error essential correction and running order.
They are essential for a device to be able to interpret incoming
information. Suites of protocols are often used in networks, with
each protocol responsible for one part of a communications function.
Protocol converter A
device that translates between two protocols to facilitate communications
between different computers or different systems.
Protocol Independent Routing A
proprietary routing method designed for IBM networks developed
by General DataComm, improving on Source Routing. The router checks
all LAN packets for their source address contained in each frame.
Adopting the proprietary DSPF (Discover Shortest Path First) router-to-router
protocol, a PIR router finds the best path between pairs of LANs,
rather than simply between pairs of routers as OSPF (Open Shortest
Path First). Because a router can have several LANs connected
to it, OSPF does not necessarily produce the best result, whereas
PIR load-balances across each available LAN link for improved
throughput and performance. From a Token Ring LAN, PIR looks like
a Source Routing bridge; in an Ethernet environment, PIR appears
as a Transparent bridge.
Proxy agent A network
management agent that sits in front of a device attached to a
network that needs to be managed, but has no intrinsic network
management capabilities. The proxy agent provides network management
"by proxy" for that device.
PSN (Packet Switching Node) The
name of an Arpanet packet switch; PSNs were formerly called IMPS.
PDN (Public Data Network) A
network established by a PTT, PTO or TA to provide data transmission
services for public use.
PUP (Parc Universal Packet) In
the internet system developed by Xerox Corporation, a PUP is the
fundamental unit of transfer, just as the IP datagram in a TCP/IP
internet. The name was derived from the name of the laboratory
at which the Xerox internet was developed, the Palo Alto Research
Center (PARC).
Punch-down block Telephony
term describing the connector arrangements for distributing and
connecting UTP/STP wiring inside a building. Typically found in
phone wiring closets.
PVC (Permanent Virtual Circuit) A
fixed virtual circuit between two users: the public data network
equivalent of a leased line. No call setup or clearing procedures
are needed.
