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StudyGuide5 - CH05_p75-90 4:42 PM Page 75 C HAPTER 5 Link...

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CHAPTER 5 Link Layer and Local Area Networks 75
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76 STUDY COMPANION FOR COMPUTER NETWORKING, THIRD EDITION Most Important Ideas and Concepts from Chapter 5 Link layer services. One of the key facets of a layered architecture is that a pro- tocol in one layer provides services to the protocols in the layer above. In partic- ular, a protocol in the link layer (layer 2) provides services to the protocols in the network layer (layer 3). The most basic service of any link-layer protocol is to move a datagram from one node to an adjacent node over a single communication link. As discussed on page 420 of the textbook, a useful analogy here is that of a tourist, a travel agent, transportation segments, and transportation modes (bus, plane, train, and so on). A transportation segment (say, between two cities) is anal- ogous to a link, a transportation mode is analogous to a link-layer protocol, a tourist is analogous to a packet, and a travel agent—who plans the trip from end-to-end— is analogous to a routing protocol. At the sender side, a link-layer protocol encapsulates the datagram in a link-layer frame, which is then passed to the physical layer for transmission over the link. At the receiving side, after receiving a frame, the link-layer protocol extracts the data- gram and passes it to the network layer. Other possible services a link-layer proto- col can provide include: medium access control, reliable delivery, flow control, error detection, and correction. Many of these services can also be provided in other lay- ers. For example, we learned in Chapter 3 that the TCP transport-layer protocol pro- vides end-to-end flow control, so that the sending side of a TCP connection does not overwhelm the receiving side of the connection. Flow-control in the link layer has a similar objective, but is no longer from end-to-end but instead from node to ad- jacent node. In this chapter we cover in detail many of the classic link-layer ser- vices. We also highlight many of the key services in the paragraphs below. Error detection and correction. Error detection is another example of a service that can be provided in different layers. The checksum in UDP and TCP are examples of an error detection service at the transport layer. Similarly, IP’s header checksum is an example of error detection at the network layer. Typically, the error detection in the network and transport layers is rather crude, only detecting single-bit errors and certain combinations of multiple-bit errors. Typically more sophisticated, a link- layer error detection scheme can detect (over a single link) single-bit and a wide-range of common multiple-bit errors. The main idea behind error detection is for the sender to create, as a function of the bits in the frame, a block of bits and include the block in a header field in the frame. When the receiving side of the link receives the frame, it runs the frame through the same function and compares the result with the block in the frame. If there is a match, the frame is considered error-free; if there is an in- consistency, the frame is considered corrupted. Section 5.2 discusses three error de-
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