Data Link Control

Data Link Control

Line Discipline
Flow Control
Error Control

Data Link Layer



FLOW CONTROL



ERROR CONTROL

Automatic Repeat Request (ARQ)
Stop and Wait
Go Back N
Selective Repeat







Flow and 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.

Error control is based on automatic repeat request (ARQ), which is the retransmission of data.


Example

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?

The bandwidth-delay product is 1  106  20  10-3 = 20,000 bitsThe 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.