Communications Systems Glossary

Packet.  A packet is a group of data bits that include the information as well as overhead including source and destination addresses, identification information, and frame check sequence.

Packet assembly/disassembly (PAD).  The packet assembly/disassembly is the data terminal’s interface with a packet-switching network that breaks messages into packets for transmission and reassembles the packets into messages at receiver.

Packet switching.  The method of packet switching provides both the response of circuit switching and the efficient link utilisation of message switching by imposing a maximum length on the transmitted units by splitting all messages into packets.

Parity bit.  A parity bit is an additional check bit appended to a group (normally seven) of binary digits.

PBX (Private branch exchange).  A PBX (Private branch exchange) is an organisation’s internal telephone exchange.

Peak envelope power (PEP).  The output power of a transmitter that transmits an amplitude-modulated carrier is often specified in terms of its peak
envelope power (PEP).

Permanent virtual circuit (PVC).  A permanent virtual circuit (PVC) provides a permanent link between two nodes across a packet-switched network.

Phase modulation (PM).  Phase modulation is a form of modulation in which the carrier wave’s phase is caused to vary in accordance with the modulating signal.Phase modulation is a form of angle modulation.

Phase-shift keying (PSK).  Phase-shift keying is a form of modulation in which the phase of the carrier signal is shifted to represent digital data.

Physical layer (Layer 1).  The physical layer is Layer 1 of the International Standards Organization's OSI reference model.

Picocell.  The picocell is the smallest of the cells in a cellular personal communications systems (PCS)-normally covering an office area.

Plain text.  Plain text is the unencrypted (or decrypted) version of a message.

Polarisation.  Polarisation is the orientation of the electric field in an electromagnetic wave. Commonly used types of polarisation include horizontal, vertical and circular.

Polling.  Polling is a process in which data terminals are asked whether they have any data to transmit.

Power (P).  The dissipation of energy over time is called power (P).

Pre-emphasis / de-emphasis.  Pre-emphasis networks artificially amplify the high frequencies in the transmitter before modulation to prepare them better for the noise introduced in demodulation in the receiver. After demodulation, de-emphasis networks returns the high frequencies to their original amplitudes before passing to the AF amplifier.

Preamble.  A preamble is a bit pattern at the beginning of a data transmission, usually used to synchronise the transmitter and receiver.

Presentation layer (Layer 6).  The presentation layer is Layer 6 of the OSI model.

Print server.  A print server is a computer that allows a number of users to direct printed output to the same printer.

Private access branch exchange (PABX).  The term private access branch exchange (PABX) is normally used to refer to an organisation’s internal telephone exchange.

Promiscuous mode.  Promiscuous mode is a mode in which a network device listens to all addresses, not just its own.

Propagation delay.  Propagation delay is the amount of time it takes for a signal to travel from its source to its destination.

Protocol.  A protocol is a set of rules or conventions that govern one or more of the many aspects of communication.

Public key.  A public key is one of the two keys used in an asymmetric encryption system.

Public key authority (PKA).  A public key authority issues public keys for use in exchanging data using public key encryption.

Public-key encryption.  Public-key encryptions is a form of asymmetric encryption.

Public switched telephone network (PSTN).  Public switched telephone network (PSTN) is the formal name for telephone networks operated by telecommunications carriers.

Pulse amplitude modulation (PAM).  In pulse-amplitude modulation, the signal is sampled at regular intervals to obtain a pulse whose amplitude is proportional to the amplitude of the signal at the instant of sampling.

Pulse code modulation (PCM).  Pulse-code modulation (PCM) is similar to pulse-position modulation (PPM) in that the received information is not determined by the shape of the pulse, but has the additional advantage that the precise location of the pulse is not important either. The analogue signals are first sampled using pulse amplitude modulation. The pulse amplitude modulation pulses are then encoded into a binary code that is transmitted as a digital stream. At the receiver these pulse code modulation (PCM) codes are decoded into pulse amplitude modulation (PAM) pulses that are then used to reconstruct the analogue waveform.

Pulse duration modulation (PDM).  Pulse duration modulation (PDM), also known as pulse-length modulation (PLM) or pulse-width modulation (PWM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in pulse-amplitude modulation (PAM)), but by its duration.

Pulse length modulation (PLM).  Pulse length modulation (PLM), also known as pulse-width modulation (PWM) or pulse-duration modulation (PDM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in PAM), but by its duration.

Pulse position modulation (PPM).  Pulse position modulation (PPM) is a pulse modulation technique that uses pulses that are of uniform height and width but displaced in time from some base position according to the amplitude of the signal at the instant of sampling.

Pulse width modulation (PWM).  Pulse width modulation (PWM), also known as pulse-length modulation (PLM) or pulse-duration modulation (PDM), is a pulse modulation technique that transmits amplitude information of an analogue signal, not by the height of the pulse (as in PAM), but by its duration.

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