07Chapter07 - for36056_ch07.fm Page 145 Monday, January 31,...

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145 CHAPTER 7 Flow and Error Control In Chapters 3 and 4, we examined the transmission of signals across media links and the type of media that can be used in LANs. In Chapter 5, we discussed error detection at the physical layer. In Chapter 6, we showed how different systems can be connected to each other using different topologies. But unless accurately received by a second device (a client or server in a client/ server communication, a peer in a peer-to-peer communication, or a connecting device), a signal transmitted over a wire is just so much wasted electricity. With trans- mission alone we can put a signal onto a line, but we have no way of knowing if the intended receiver is ready and able to receive it. In the physical layer of the OSI model, we have transmission but we do not yet have communication. Communication requires at least two devices working together, one to send and one to receive. Even such a basic arrangement requires a great deal of coordination for an intelligible exchange to occur; we need flow and error control. Flow control defines how much data can be sent at any time. Error control defines how errors can be corrected. Communication networks control data flow and errors either at the link level (between two devices connected to the same link) or end to end (between the sender and the receiver) in an internetwork. End-to-end flow and error control is the responsi- bility of the transport layer; link-level flow and error control is the responsibility of the data link layer. In this chapter, we discuss these two concepts (flow and error control) at the data link layer. They are normally implemented as part of the LLC sublayer (at the data link layer). The implementation is necessary for smooth and reliable communication between two devices connected to the same link. 7.1 FLOW CONTROL In most protocols flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgment (ACK) from the receiver. The flow of data must not be allowed to overwhelm the receiver. Any receiving device has a
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146 CHAPTER 7 FLOW AND ERROR CONTROL limited speed at which it can process incoming data, and a limited amount of memory, in which to store incoming data. The receiving device must be able to inform the send- ing device before those limits are reached and to request that the transmitting device send fewer frames or stop temporarily. Incoming data must be checked and processed before they can be used. The rate of such processing is often slower than the rate of transmission. For this reason, each receiving device has a block of memory, called a buffer, reserved for storing incoming data until they are processed. If the buffer begins to fill up, the receiver must be able to tell the sender to halt transmission until it is once again able to receive. Two methods have been developed to control the flow of data across communica-
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07Chapter07 - for36056_ch07.fm Page 145 Monday, January 31,...

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