Published online by Cambridge University Press: 17 May 2010
Foundation concept: Networks.
Overview
The standard model for a voice telephone system is a circuit switching network. When two users or end-stations wish to communicate, a direct connection must be made between them, and dedicated to that call for its duration. The connection may be through copper wires, fiber-optic cables, or even radio or microwave transmissions. It may pass though any number of switching stations, staffed by human operators with plug-boards, or electronically operated. It is even possible, using frequency shifting, for many conversations to be carried by the same wire at the same time. The key concept is that a complete connection must be made, then the conversation can happen, and then the connection may be dismantled.
Circuit switching allows very simple telephony equipment built on nineteenth-century technology to operate effectively (the first telephone exchange was working in 1877), but it does not allow very efficient use of the high-cost infrastructure (long-distance cables and transmitters).
The alternative is a Packet-switching network, and requires that all communications are in digital form. In a packetswitched network, every end-station has a permanent connection to at least one switching station; each switching station has a number of permanent connections to other switching stations. The connectivity does not change during use: connections are not made before data is transmitted or broken thereafter; there is a path (perhaps a very long path, through dozens of switching stations) from any end-station to any other.
To save this book to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.