11.1 Flow and Error Control Flow Control Error Control.

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Presentation transcript:

11.1 Flow and Error Control Flow Control Error Control

Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Note:

Error control in the data link layer is based on automatic repeat request, which is the retransmission of data. Note:

11.2 Stop-and-Wait ARQ Operation Bidirectional Transmission

11.1 Normal operation

11.2 Stop-and-Wait ARQ, lost frame

11.3 Stop-and-Wait ARQ, lost ACK frame

In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames. Note:

11.4 Stop-and-Wait ARQ, delayed ACK

Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost. Note:

11.5 Piggybacking

11.3 Go-Back-N ARQ Sequence Number Sender and Receiver Sliding Window Control Variables and Timers Acknowledgment Resending Frames Operation

11.6 Sender sliding window

11.7 Receiver sliding window

11.8 Control variables

11.9 Go-Back-N ARQ, normal operation

11.10 Go-Back-N ARQ, lost frame

11.11 Go-Back-N ARQ: sender window size

In Go-Back-N ARQ, the size of the sender window must be less than 2m; the size of the receiver window is always 1. Note:

11.4 Selective-Repeat ARQ Sender and Receiver Windows Operation Sender Window Size Bidirectional Transmission Pipelining

11.12 Selective Repeat ARQ, sender and receiver windows

11.13 Selective Repeat ARQ, lost frame

In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2 m. Note:

11.14 Selective Repeat ARQ, sender window size

Example 1 In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link? Solution The bandwidth-delay product is 1  10 6  20  = 20,000 bits The system can send 20,000 bits during the time it takes for the data to go from the sender to the receiver and then back again. However, the system sends only 1000 bits. We can say that the link utilization is only 1000/20,000, or 5%. For this reason, for a link with high bandwidth or long delay, use of Stop-and-Wait ARQ wastes the capacity of the link.

Example 2 What is the utilization percentage of the link in Example 1 if the link uses Go- Back-N ARQ with a 15-frame sequence? Solution The bandwidth-delay product is still 20,000. The system can send up to 15 frames or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged frames, the utilization percentage is much less because frames have to be resent.