| PAL |
Phased Alternate Line, analog standard for television transmission (mainly Europe), frame 4:3, 625 lines. |
| Parabolic antenna |
See "Satellite Dish”. |
| Pay TV |
Services or group of services which require to pay a subscription fee to watch them. |
| Pay-per-View |
Programming services which are paid for by subscribers on the basis of the number of hours or programmes watched rather than through a straight subscription fee. |
| 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 |
To request data from another program or computer. The opposite of pull is push, where data is sent without a request being made. The terms push and pull are used frequently to describe data sent over the Internet. The World Wide Web is based on pull technologies, where a page isn't delivered until a browser requests it. Increasingly, however, Information services are harnessing the Internet to broadcast information using push technologies. |
| Push |
In client/server applications, to send data to a client without the client requesting it. The World Wide Web is based on a pull technology where the client browser must request a Web page before it is sent. Broadcast media, on the other hand, are push technologies because they send information out regardless of whether anyone is tuned in. Increasingly, companies are using the Internet to deliver information push-style. One of the most successful examples of this is PointCast, which delivers customised news to users' desktops. Probably the oldest and most widely used push technology is e-mail. This is a push technology because you receive mail whether you ask for it or not -- that is, the sender pushes the message to the receiver. |
