11.61
Algorithm 11.7 Go-Back-N sender algorithm
(continued)
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Data Link Control Part 2-Data Communication and Computer Networks-Lecture Slides, Slides of Data Communication Systems and Computer Networks
These are a set of Lecture Slides on the subject of Data Communication and Computer Networks at Univeristy of Delhi by Dr. Sonam Zinta. It includes: Algorithm, Sender, ARQ, Size, Window, Selective, Repeat, Frame, Send, Receive
Typology: Slides
2011/2012
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Algorithm 11.7 Go-Back-N sender algorithm
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Algorithm 11.7 Go-Back-N sender algorithm (continued)
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Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note
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Figure 11.18 Send window for Selective Repeat ARQ
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Figure 11.20 Design of Selective Repeat ARQ
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This example is similar to Example 11.3 in which frame 1 is lost. We show how Selective Repeat behaves in this case. Figure 11.23 shows the situation. One main difference is the number of timers. Here, each frame sent or resent needs a timer, which means that the timers need to be numbered (0, 1, 2, and 3). The timer for frame 0 starts at the first request, but stops when the ACK for this frame arrives. The timer for frame 1 starts at the second request, restarts when a NAK arrives, and finally stops when the last ACK arrives. The other two timers start when the corresponding frames are sent and stop at the last arrival event.
Example 11.
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Another important point is that a NAK is sent after the second arrival, but not after the third, although both situations look the same. The reason is that the protocol does not want to crowd the network with unnecessary NAKs and unnecessary resent frames. The second NAK would still be NAK1 to inform the sender to resend frame 1 again; this has already been done. The first NAK sent is remembered (using the nakSent variable) and is not sent again until the frame slides. A NAK is sent once for each window position and defines the first slot in the window.
Example 11.8 (continued)
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The next point is about the ACKs. Notice that only two ACKs are sent here. The first one acknowledges only the first frame; the second one acknowledges three frames. In Selective Repeat, ACKs are sent when data are delivered to the network layer. If the data belonging to n frames are delivered in one shot, only one ACK is sent for all of them.
Example 11.8 (continued)
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In Selective Repeat ARQ, the size of the
sender and receiver window
must be at most one-half of 2m.
Note
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Figure 11.21 Selective Repeat ARQ, window size
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Algorithm 11.9 Sender-site Selective Repeat algorithm (continued)
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Algorithm 11.9 Sender-site Selective Repeat algorithm (continued)
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Algorithm 11.10 Receiver-site Selective Repeat algorithm
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