《计算机网络和 与因特网》课件第八章包、帧和 与差错检测.pptVIP

8.9 Detecting Errors With Checksums Many computer network systems send a checksum(校验和) along with each packet to help the receiver detect errors. To compute a checksum, the sender treats each pair of characters as a 16-bits integer and computes the sum. If the sum grows larger than 16 bits, the carry bits(进位) are added into the final sum. The small size of the checksum means the cost of transmitting the checksum is usually much smaller than the cost of transmitting the data. Checksums have the disadvantage of not detecting all common errors. For example: Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉 PART II Packet Transmission Chapter 8 Packets, Frames, and Error Detection 包、帧与差错检测 8.1 Introduction 电路交换 分组交换 8.2 The Concept of Packets The network system divides data into small blocks called packets,which it sends individually. Computer networks are often called packet networks or packet switching networks because they use packet technology. Two facts motivate the use of packets: First, a sender and receiver need to coordinate transmission to ensure that data arrives correctly. When transmission errors occur, data can be lost. Dividing the data into small blocks helps a sender and receiver determine which blocks arrive intact and which do not. Second, because multiple computer often share underlying connections and hardware, a network system cannot allow one computer to deny access to others. Using small packets helps ensure fairness. How long does a file transfer require? If a file contains 5 megabytes and the communication system can transfer 56,000 bits per second. 8.3 Packets and Time-Division Multiplexing A network that permits many sources to take turns accessing a shared communication resource is providing a form of time-division multiplexing. 8.4 Packets and Hardware Frames The term packet refers to the general concept of a small block of data, there is no universal agreement on t