There are two conditions for the delivery of frames

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layer. There are two conditions for the delivery of frames to the network layer: First, a set of consecutive frames must have arrived. Second, the set starts from the beginning of the window. Example 11.8 (continued)
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1 9 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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1 10 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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1 11 Figure 11.23 Flow diagram for Example 11.8
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1 12 Figure 11.24 Design of piggybacking in Go-Back-N ARQ
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1 13 11-6 HDLC High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It implements the ARQ mechanisms we discussed in this chapter. Configurations and Transfer Modes Frames Control Field Topics discussed in this section:
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1 14 Figure 11.25 Normal response mode
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1 15 Figure 11.26 Asynchronous balanced mode
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1 16 Figure 11.27 HDLC frames
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1 17 Figure 11.28 Control field format for the different frame types
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1 18 Table 11.1 U-frame control command and response
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