P

Parabolic antenna

See "Satellite Dish".

 

Polarisation

The way the electrical field of a signal is organised. The SIRIUS Satellite System broadcast signals with the same frequencies but with two different polarisation (the Horizontal and the Vertical) to make an efficient use of the available spectrum.


Proxy Server

A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfil the requests itself. If not, it forwards the request to the real server. Proxy servers have two main purposes: Improve Performance: Proxy servers can dramatically improve performance for groups of users. This is because it saves the results of all requests for a certain amount of time. Real proxy servers support hundreds or thousands of users. The major online services such as Compuserve and America Online, for example, employ an array of proxy servers.


Pull

A system where data is sent only in response to a request, eg, video on demand. The opposite system is push technology, where data is sent without being requested. Push and pull are frequently used to describe data sent over the internet. The web is based on pull technologies, where a page isn't delivered until a browser requests it.

 

Push

Broadcast media is a push technology because information is transmitted regardless of whether anyone is watching. Client/server applications use push technology when data is sent to the client without it being requested.