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Unformatted text preview: Computer Networks Error correction with ARQ Saad Mneimneh Computer Science Hunter College of CUNY New York- I am Ok.- I am not Ok. Ack and Nak 1 Introduction We started our discussion of the DLC layer by saying that it is responsible for reliable transmission of packets over a single link , which means that every packet is deliv- ered once, only once, without errors, and in order. To achieve this goal, the DLC had to detect errors using framing and parity check codes. Now assuming that the DLC can detect errors, the question becomes how to correct these errors and how to make sure that each packet is delivered once, only once, and in order? The delivery once and ordering properties can be solved by including a sequence number in the packet header (in addition to possibly other framing information). The DLC can then determine whether a duplicate frame or an out of order frame is received, and ignores any such frame. So the essential problem appears to be error correction. The last statement may give the wrong impression that the problem of reliable communication is one sided. Most of the times, errors cannot be corrected at the receiving side; so if the receiver declares an error upon receiving a frame, the sender must resend that frame sometime in the future. The receiver may have to request such retransmission. Moreover, if the receiver does not receive any frames, then noth- ing needs to be corrected. But for reliable communication to exist, the sender must guarantee the delivery of frames. While on a single link, lost generally means received with errors, if frames can be lost (for whatever reason), the receiver may have to acknowledge the receipt of frames in some way. Therefore, the problem of reliable communication is a coordination problem among both the sender and the receiver DLCs. At any point in time, both DLCs must have a clear view of what has happened so far. But it appears from above that by simply acknowledging the receipt of frames (those that are not duplicates, are in order, and 1 contain no errors), the receiver can instruct the sender of the next frame to be sent. The solution however is not that simple. To appreciate the difficulty of the problem, let us consider a classical problem known as the coordinated attack problem. 2 The coordinated attack problem There are three armies, two colored blue, and one colored red. The red army separates the two blue armies. If the blue armies attack simultaneously, they win, but if they attack separately, the red army wins. The only communication between the blue armies is by sending messengers through the red army lines, but there is a possibility that any such messenger will be captured, causing the message to go undelivered. How should the blue armies coordinate their attack?...
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This note was uploaded on 03/27/2010 for the course CSCI 415 taught by Professor Saadmneimneh during the Spring '08 term at CUNY Hunter.
- Spring '08
- Computer Networks