P

Packet
A collection of data with address and supervisory information. This is the basic unit of data that passed through a packet switched network.

PAD
Packet Assembler / Dis-assembler. A device of taking a continuous stream of data and assembling it into a packet. Usually referring to asynchronous or SNA data.

PAL
Phas Alternate Line, the UK analogue terrestrial TV standard

PAMA
Pre-Assigned Multiple Access

Parabolic Antenna
The most common satellite antenna, with a reflector formed from a section of a parabola

PBX
Private Branch (telephone) Exchange

PCM
Pulse Code Modulation, a method for encoding speech for transmission over digital networks

PVC
Permanent Virtual Connection / Circuit: a route between one application and another across a network that does not need to be established or torn down following use. It is commonly used in fast packet switch networks such as ATM and Frame Relay.

Point-to-Multipoint link
A system that connects one place to several others in a star network. Usually implemented as a number of independent two-way connections.

Point-to-Point link
An independent link that connects one place with another

Polarisation
All Electromagnetic signals comprise two components, one Electric and one Magnetic. These components are at right angles to each other and to the direction of travel. The angle the Electric component makes with the horizon is the polarisation. This can be a fixed angle, horizontal or vertical, in which case the polarisation is said to be linear. Alternatively, the polarisation angle can change as the signal propagates such that it twists as it moves through space. At the receiver, signals with opposite polarisations (horizontal vs. vertical or right hand twist vs. left-hand twist) can be distinguished from one another, even if they are the same frequency. In this way the frequency band from the satellite can be used twice, once on each polarisation.

POR
Pacific Ocean Region, a group of orbital positions for satellite serving Asia and the Americas

Pre-emptible
Satellite capacity can be booked on the understanding that if a higher priority user requires the capacity the lower priority customer must relinquish the capacity. This is the system used to generate revenue from satellites that have been provided to be used for disaster recovery.

Private network
A network that is restricted to carrying traffic originating within the company that owns it. This is a pre-requisite for licences to be granted in some instances.

Protocol
A set of rules and standards that define how two applications will communicate

PSTN
The Public Switched Telephone Network

PTT
Postal Telephone and Telegraph, a term referring mainly to principle national and international telecommunications companies, often publicly owned.