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Node
NRZ
Out of Frame State
(OOF)
PABX
Packet
Packet Switching
Parity Bit
Parity Check
PBX
Phase Modulation
Phase Shift Keying
(PSK)
Point to Point
Port
Private Line
GLOSSARY
A point where one or more functional units interconnect transmission lines
(ISO); a physical device that allows for the transmission of data within a network;
an end-point of a link or a junction common to two or more links in a network
(IBM SNA); typically includes host processors, communications controllers, clus-
ter controllers, and terminals.
Non-return to zero; a binary encoding and transmission scheme where "ones"
and "zeros" are represented by opposite, and alternating, high and low voltages;
where there is no return to a reference (zero) voltage between encoded bits.
An OOF state begins when any two out of four or two out of five consecutive fram-
ing bits received from either side are incorrect. In response to receiving an OOF
from one side, the CSU returns a Yellow alarm, and transmits an AIS to the other
side. A Red alarm signal occurs when on OOF state exists for more than 2.5 sec-
onds. An OOF state ends when reframe occurs.
Private automatic branch exchange.
A sequence of data, with associated control elements, that is switched and trans-
mitted as a whole; refers mainly to the field structure and format defined within
the CCITT X.25 recommendation; multiple packets may be required to carry one
complete document or a lengthy block of information.
A data transmission technique wherein user information is segmented
routed in discrete data envelopes called packets, each with its own appended con-
trol information for routing, sequencing, and error checking; a
technique that allows a communications channel to be shared by many users,
each using the circuit only for the time required to transmit a single packet; a
network that operates in this manner.
An additional non-information bit appended to a group of bits, typically to a 7- or
8-bit byte, which indicates whether the number of ones in the group of bits is an
odd or even number; a basic and elementary mechanism for error checking.
Process of error checking using the parity bit; varied methods include longitudi-
nal parity check and transverse parity check.
Private branch exchange; telephone switch located on a customer's premises that
primarily establishes voice-grade circuits, over
users and the switched telephone network.
A data transmission encoding method wherein the phase angle of the carrier
wave is varied, usually by 90 or 180 degrees, to represent a different bit value to
the receiver; the encoding technique used in phase shift keying. The process of
changing the phase of a carrier waveform to reflect digital (binary) information
for the purpose of transmission over analog facilities.
The phase-modulation encoding technique employed by many modems.
A circuit that connects two points directly, with generally no intermediate pro-
cessing nodes or computers, although there could be switching facilities; a type of
connection, such as a phone-line circuit, that links two, and only two, logical enti-
ties.
A point of access into a computer, a network, or other electronic device; the physi-
cal or electrical interface through which one gains access; the interface between a
process and a communications or transmission facility.
A leased line, an unswitched circuit.
B-7
transmission
tie-lines,
between
individual
and
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