外文翻译----电路交换网与vb调用数据库-vb程序(编辑修改稿)

外文翻译----电路交换网与vb调用数据库-vb程序(编辑修改稿)内容摘要：

nt. The sender and receiver can use any bit rate, format, or framing method they want to. The carrier does not know or care. With packet switching, the carrier determines the basic parameters. A rough analogy is a road versus a railroad. In the former, the user determines the size, speed, and nature of the vehicle。 in the latter, the carrier does. It is this transparency that allows voice, data, and fax to coexist within the phone system. A final difference between circuit and packet switching is the charging algorithm. With circuit switching, charging has historically been based on distance and time. For mobile phones, distance usually does not play a role, except for international calls, and time plays only a minor role(., a calling plan with 2020 free minutes costs more than one with 1000 free minutes and sometimes night or weekend calls are cheaper than normal).With packet switching, connect time is not an issue, but the volume of traffic sometimes is. For home users, ISPs usually charge a flat monthly rate because it is less work for them and their customers can understand this model easily, but backbone carriers charge regional works based on the volume of their traffic. The differences are summarized in . 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 11 Time of possible congestion At setup time On every packet Potentially wasted bandwidth Yes No Storeandforward transmission No Yes Transparency Yes No Charging Per minute Per packet Both circuit switching and packet switching are important enough that we will e back to them shortly and describe the various technologies used in detail. THE MOBILE TELEPHONE SYSTEM The traditional telephone system(even if it some day gets multigigabit endtoend 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 and surf the Web from all these locations and more consequently ,there is a tremendous amount o f interest in wireless telephony .In the following sections we will study this topic in some detail. Wireless telephones e 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 working ,so we will not examine them further .Instead we will concentrate on the mobile system ,which is used for wide area voice and data munication. Mobile phones have gone through three distinct generations ,with different technologies: 1. Analog voice 2. Digital voice 3. Digital voice and data(Inter , ,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 . by ATamp。 T and mandated for the 12 whole country by the FCC .As a result ,the entire . 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 governmentrun PTTs got together and standardized on a single system(GSM),so any European mobile phone will word anywhere in Europe .By then ,the . 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 . now has two major inpatible digital mobile phone systems in operation(plus one minor one). Despite an initial lead by the . , mobile phone ownership and usage in Europe is now far greater than in the . Having a single system for all of Europe is part of the reason ,but there is more .A second area where the . and Europe differed is in the humble matter of phone numbers .In the . mobile phones are mixed in with regular (fixed) telephones .Thus ,there is no way for a caller to see if ,say ,(212)23456。