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:

data transmitted in one direction only
both transmitting and receiving network layers are always ready
processing time can be ignored
finite buffer space is available
error free data link

The procedure works as follows:

A source entity transmits a PDU.
The destination entity indicates its willingness to accept another PDU by sending back an aknowledgment to the PDU just received.
The source must wait until it receives an ack before sending the next PDU

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 buffer size on the reciever may be limited
The longer the transmission, the more likely that there will be an error, requiring a retransmission
On a shared medium, it is usually desirable not to permit one station occupy the mdeium for an extended period

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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