1、實驗一Ci rcuit Switching and Packet Switching一、實驗目的1、擴大通信技術的英語詞匯量。2、熟悉電路交換、分組交換等專業術語。3、了解科技文章的表達特點和掌握科技英語的翻譯技巧。二、實驗內容文章翻譯Circuit Switching and Packet SwitchingThere are two basic types of switching techniques: circuit switching and message switching. In circuit switching, a total path of connected line

2、s is set up from the origin to the destination at the time the call is made, and the path remains allocated to the source-destination pair (whether used or not) until it is released by the communicating parties. The switches, cal led circui t swi tches (or office exchange in telephone jargon), have

3、no capabi1ity of storing or manipulating users data on their way to the destination. The circuit is set up by a special signaling message that finds its way through the network, seizing channels in the path as it proceeds. Once the path is established, a return signal informs the source to begin tra

4、nsmission. Direct transmission of data from source to destination can then take place without any intervention on the part of the subnet.In message switching, the transmission unit is a well-defined block of data called a message. In addition to the text to be transmitted, a message comprises a head

5、er and a checksum. The header contains information regarding the source and destination addresses as well as other control information; the checksum is used for error control purposes. The switching element is a computer referred to as a message processor, with processing and storage capabilities. M

6、essages travel independently and asynchronously, finding their own way from source to destination. First the message is transmitted from the host to the message processor to which it is attached. Once the message is entirely received, the message processor examines its header, and accordingly decide

7、s on the next outgoing channel on which to transmit it. If this selected channel is busy, the message waits in a queue until the channel becomes free, at which time transmission begins. At the next message processor, the message is again received, stored, examined, and transmitted on some outgoing c

8、hannel, and the same process continues until the message is delivered to its destination. This transmission technique is also referred to as the store-and-forward transmission technique.A variation of message switching is packet switching. Here the message is broken up into several p i eces of a g i

9、 ven max i mum 1 ength, called packets. As with message switching, each packet contains a header and a checksum. Packets are transmitted independently in a store-and-forward manner.With circuit switching, there is always an initial connection cost incurred in setting up the circuit. It is cost-effec

10、tive only in those situations where once the circuit is set up there is a guaranteed steady flow of information transfer to amortize the initial cost. This is certainly the case with voice communication in the traditional way, and indeed circuit switching is the technique used in the telephone syste

11、m. Communication among computers, however, is characterized as bursty. Burstiness is a result of the high degree of randomness encountered in the message-generation process and the message size, and of the low delay constraint required by the user. The users and devices require the communication res

12、ources relatively infrequently; but when they do, they require a relatively rapid response. If a fixed dedicated end-to-end circuit were to be set up connecting the end users, then one must assign enough transmission bandwidth to the circuit in order to meet the delay constraint with the consequence

13、 that the resulting channel utilization is low. If the circuit of high bandwidth were set up and released at each message transmission request, then the set-up time would be large compared to the transmission time of the message, resulting again in low channel utilization. Therefore, for bursty user

14、s (which can also be characterized by high peak-to-average data ate requirements), store-and-forward transmission techniques offer a more cost-effective solution, since a message occupies a particular communications link only for the duration of its transmission on that link; the rest of the time it

15、 is stored at some intermediate message switch and the link is available for other transmissions. Thus the main advantage of store-and-forward transmission over circuit switching is that the communication bandwidth is dynamically allocated, and the allocation is done on the fine basis of a particula

16、r link in the network and a particular message (for a particularsource-destination pair).Packet switching achieves the benefits discussed so far and offers added features. It provides the full advantage of the dynamic allocation of the bandwidth, even when messages are long. Indeed, with packet swit

17、ching, many packets of the same message may be in transmission simultaneously over consecutive links of a path from source to destination, thus achieving a “pipelining effect and reducing considerably the overall transmission delay of the message as compared to message switching. It tends to require

18、 smaller storage allocation at the intermediate switches. It also has better error characteristics and leads to more efficient error recovery procedures, as it deals with smaller entities. Needless to say, packet switching presents design problems of its own, such as the need to reorder packets of a given message that may arrive at the destination