DELPHI办公管理系统 第4页
Item Circuit Switched Packet Switched
Call setup Required Not needed
Dedicated physical path Yes No
Each packet follows the same route Yes No
Packets arrive in order Yes No
Is a switch crash fatal Yes No
Bandwidth available Fixed Dynamic
Time of possible congestion At setup time On every packet
Potentially wasted bandwidth Yes No
Store-and-forward transmission No Yes
Transparency Yes No
Charging Per minute Per packet

Both circuit switching and packet switching are important enough that we will come back to them shortly and describe the various technologies used in detail.

2.6 THE MOBILE TELEPHONE SYSTEM
The traditional telephone system(even if it some day gets multi-gigabit end-to-end fiber) will still not be able to satisfy a growing group of users :people on the go .People now expect to make phone calls from airplanes ,cars ,swimming pools ,and while jogging in the park .Within a few years they will also expect to send e-mail and surf the Web from all these locations and more consequently ,there is a tremendous amount of interest in wireless telephony .In the following sections we will study this topic in some detail.
Wireless telephones come in two basic varieties :cordless phones and mobile phones(sometimes called cell phones),Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. These are never used for networking ,so we will not examine them further .Instead we will concentrate on the mobile system ,which is used for wide area voice and data communication.
Mobile phones have gone through three distinct generations ,with different technologies:
1. Analog voice
2. Digital voice
3. Digital voice and data(Internet ,e-mail ,etc.).
Although most of our discussion will be about the technology of these systems ,it is interesting to note how political and tiny marketing decisions can have a huge impact .The first mobile system was devised in the U.S. by AT&T and mandated for the whole country by the FCC .As a result ,the entire U.S. had a single(analog) system and a mobile phone purchased in California also worked in New York .In contrast ,when mobile came to Europe ,every country devised its own system ,which resulted in a fiasco.
Europe learned from its mistake and when digital came around, the government-run PTTs got together and standardized on a single system(GSM),so any European mobile phone will word anywhere in Europe .By then ,the U.S. had decided that government should not be in the standardization business ,so it left digital to the marketplace .This decision resulted in different equipment manufacturers producing different kinds of mobile phones .As a consequence ,the U.S. now has two major incompatible digital mobile phone systems in operation(plus one minor one).
Despite an initial lead by the U.S. , mobile phone ownership and usage in Europe is now far greater than in the U.S. Having a single system for all of Europe is part of the reason ,but there is more .A second area where

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