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CS 635 Lecture 3, Part 2 Logical Link Control

Flow Control Mechanisms

Technique for assuring that a transmitting entity does not overwhelm a receiving entity.

Stop-and-Wait Flow Control

Supports the acknowledged connectionless LLC service. The assumptions are: The procedure works as follows: The procedure works best the a message is sent in a few large PDUs. However, the source will often break up the messager into smaller blocks and transmit the data in many PDUs. This is done for the following reasons: The pro blem with this protocol is that it is very inefficient. See page 155, figure 5.3

Sliding-Window Flow Control

This protocol requires full-duplex communication. Instead of waisting the bandwidth on sending ack PDUs separately, the reciever waits until it has data to send and attaches acknowledgement to the outgoing data frames (piggybacking). The receiving station must allocate space for n PDUs. Thus the receiver can accept n PDUs, and the sender can send n PDUs without waiting for any acknowledgements. To keep track of which PDUs have been acknowledged, each is labeled with a sequence number. The sender acknowledges a PDU by sending an acknowledgement that includes the sequence number of the next PDU expected. This ack also implicitly announces that the receiver is prepared to receive the next n PDUs. See page 157, figure 5.4

Advantage of the Sliding-Window protocol is that it saves bandwitdth better than stop-and-wait,  more efficient, serves as a flow control.

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