【正文】 ng up a path in advance also opens up the possibility of reserving bandwidth in advance. If bandwidth is reserved, then when a packet arrives, it can be sent out immediately over the reserved bandwidth. With packet switching, no bandwidth is reserved, so packets may have to wait their turn to be forwarded. Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more packets show up than expected).On the other hand, when an attempt is made to establish a circuit, the attempt can fail due to congestion. Thus, congestion can occur at different times with circuit switching(at setup time) and packet switching(when packets are sent). If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit is wasted. It cannot be used for other traffic. Packet switching does not waste bandwidth and thus is more efficient form a systemwide perspective. Understanding this tradeoff is crucial for prehending the difference between circuit switching and packet switching. The tradeoff is between guaranteed service and wasting resources versus not guaranteeing service and not wasting 11 resources. Packet switching uses storeandforward transmission. A packet is accumulated. in a router’s memory, then sent on to the next router. With circuit switching, the bits just flow through the wire continuously. The storeandforward technique adds delay. Another difference is that circuit switching is pletely transparent. 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。T and mandated for the 13 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)2345678 is a fixed telephone (cheap or free call) or a mobile phone (expensive call).To keep people from getting nervous about using the telephone ,the telephone panies decided to make the mobile phone owner pay for ining calls .As a consequence ,many people hesitated to buy a mobile phone for fear of running up a big bill by just receiving calls .In Europe ,mobile phones have a special area code (analogous to 800 and 900 numbers) so they are instantly recognizable .Consequently , the usual rule of “caller pays” also applies to mobile phones in Europe (except for international calls where costs are split). A third issue that has had a large impact on adoption is the widespread use of prepaid mobile phones in Europe (up to 75% in some areas ).These can be purchased in many stores with no more formality than buying a radio .You pay and you go .They are preloaded with ,for example ,20 or 50 euro and can be recharged (using a secret PIN code ) when the balance drops to zero . As a consequence , practically every teenager and many small children in Europe have (usually prepaid ) mobile phones so their 14 parents can locate them ,without the danger of the child running up a huge bill .If the mobile phone is used only occasionally ,its use is essentially free since there is no monthly charge or charge for ining calls . FirstGeneration Mobile Phones :Analog Voice Enough about the politics and marketing