Page 116 - DCAP406_DCAP_207_Computer Networks

Unit 8: Data Link Protocols

At the destination node, the receiving machine waits for reception of the frame from the physical Notes
layer. Once, the frame is received, the sequence number is checked. If it is found correct, the
packet is passed to the next layer i.e. network layer. The destination machine sends an
acknowledgement for the frame to the source machine for the sequence number just received. If
an incorrect or out of sequence frame arrives, the destination machines requests for retransmission
to the source node.
PAR involves only one outstanding frame at a time, a sequence number must be used to determine
if any frames are lost or damaged. This scheme requires only two sequence numbers because at
a time there will be only one outstanding frame and sequence number will not change until a
positive acknowledgement is received. This protocol simply uses ‘1’ and ‘0’ as sequence numbers.
The PAR protocol is simple to implement. Sequence numbers and transmitting an
acknowledgement for frame received in sequence keeps the source and destination hosts in
synchronization.

The protocol is able to handle congestion, lost frames and damaged frames because frames are
retransmitted until a positive acknowledgement is received. However, this characteristic makes
the PAR protocol inefficient because the number of frames sent to transfer the entire message
reliably becomes large frames on loss of acknowledgements.

8.2 Sliding Window Protocols

In data communications systems, it may be defined as the control of the rate at which data are
transmitted from a terminal so that the data can be received by another terminal. By maintaining
a compatible data transfer rate between sending and receiving ends, it prevents network
congestion. A high-speed computer, for example, may generate traffic faster than the network
can transfer it or faster than the destination device can receive and process it. In order to ensure
effective transmission, source device requires an acknowledgment from the destination after a
certain number of packets have been transmitted. This is a Windowing flow-control scheme.
If the destination does not receive one or more of the packets for some reason, such as overflowing
buffers, it does not receive enough packets to send an acknowledgment. The source then
retransmits the packets at a reduced transmission rate.
As we have seen in the foregoing topic that flow control is a technique whose primary purpose
is to properly match the transmission rate of sender to that of the receiver and the network.
However, it is not the same as congestion control. Congestion control is primarily concerned
with a sustained overload of network intermediate devices such as IP routers.
In order to maintain a proper flow of data a window field is created to adjust the rate of flow of
the byte stream between communicating devices. Figure 8.1 illustrates the concept of the sliding
window.
In this simple example, there is a 4-byte sliding window. Moving from left to right, the window
“slides” as bytes in the stream are sent and acknowledged. The size of the window and how fast
to increase or decrease the window size is an area of great research.

LOVELY PROFESSIONAL UNIVERSITY 109