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1. FLOW AND ERROR CONTROL

2. FLOW AND ERROR CONTROL
The most important responsibilities of the data link
layer are flow control and error control. Collectively,
these functions are known as data link control.
Flow Control
Error Control
Topics discussed in this section:

3. 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

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

5. PROTOCOLS
Now let us see how the data link layer can combine
framing, flow control, and error control to achieve the
delivery of data from one node to another. The
protocols are normally implemented in software by
using one of the common programming languages. To
make our discussions language-free, we have written
in pseudocode a version of each protocol that
concentrates mostly on the procedure instead of
delving into the details of language rules.

6. Taxonomy of Flow and error control protocols

7. NOISELESS CHANNELS
Let us first assume we have an ideal channel in which
no frames are lost, duplicated, or corrupted. We
introduce two protocols for this type of channel.
Simplest Protocol
Stop-and-Wait Protocol
Topics discussed in this section:

8. It is very simple. The sender sends a sequence of frames
without even thinking about the receiver. To send three
frames, three events occur at the sender site and three
events at the receiver site. Note that the data frames are
shown by tilted boxes; the height of the box defines the
transmission time difference between
the first bit and the last bit in the frame.
Simplest Protocol

9. Figure 1 Flow diagram for Simplest Protocol

10. Algorithm Sender-site algorithm for the simplest protocol

11. Algorithm Receiver-site algorithm for the simplest protocol

12. It is still very simple. The sender sends one frame and
waits for feedback from the receiver. When the ACK
arrives, the sender sends the next frame.
Stop-and-wait protocol works under the assumption
that the communication channel is noiseless and
transmissions are error-free.
Working:
•The sender sends data to the receiver.
•The sender stops and waits for the acknowledgment.
•The receiver receives the data and processes it.
•The receiver sends an acknowledgment for the above
data to the sender.
•The sender sends data to the receiver after receiving
the acknowledgment of previously sent data.
•The process is unidirectional and continues until the
sender sends the End of Transmission (EoT) frame
Stop and Wait Protocol

13. Figure 2 Flow diagram for Example 11.2
Note that in figure 2,sending two frames in
the protocol involves the sender in four events
and the receiver in two events.

14. Algorithm Sender-site algorithm for Stop-and-Wait Protocol

15. Algorithm Receiver-site algorithm for Stop-and-Wait Protocol

16. NOISY CHANNELS
Although the Stop-and-Wait Protocol gives us an idea
of how to add flow control to its predecessor, noiseless
channels are nonexistent. We discuss three protocols
in this section that use error control.
Stop-and-Wait Automatic Repeat Request
Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request
Topics discussed in this section:

17. Error correction in Stop-and-Wait ARQ is
done by keeping a copy of the sent
frame and retransmitting of the frame
when the timer expires.
Note

18. In Stop-and-Wait ARQ, we use sequence
numbers to number the frames.
The sequence numbers are based on
modulo-2 arithmetic.
Note

19. In Stop-and-Wait ARQ, the
acknowledgment number always
announces in modulo-2 arithmetic the
sequence number of the next frame
expected.
Note

20. Stop-and-Wait ARQ
1. It provides both error control and flow control
2. Error control in stop and wait ARQ is done by keeping a
copy of frame until it receives a acknowledgement
3. Sender start a timer when it send a frame. If ACK is not
received within the allocated time period , the sender
assume that the frame was lost or damage and resent
it.
4. Receiver send an ACK to sender if it receives a frame
correctly.
5. ACK number always define the number of the next
expected frame.
Stop and wait ARQ = Stop and wait + Time out + Sequence
Number (Data) + sequence number (ACK))

21. Stop-and-Wait ARQ
For identification, both data frames and
acknowledgements (ACK) frames are numbered
alternatively 0 and 1.
 Sender has a control variable (S) that holds
the number of the recently sent frame. (0 or
1)
 Receiver has a control variable ® that holds
the number of the next frame expected (0 or
1).

22. Figure3 shows an example of Stop-and-Wait ARQ.
Frame 0 is sent and acknowledged. Frame 1 is lost and
resent after the time-out. The resent frame 1 is
acknowledged and the timer stops. Frame 0 is sent and
acknowledged, but the acknowledgment is lost. The
sender has no idea if the frame or the acknowledgment
is lost, so after the time-out, it resends frame 0, which is
acknowledged.
Stop-and-Wait ARQ

23. Figure 3 Flow diagram for Example 11.3

24. Disadvantage of Stop-and-Wait
• In stop-and-wait, at any point in time, there is only one frame that is sent
and waiting to be acknowledged.
• This is not a good use of transmission medium.
• To improve efficiency, multiple frames should be in transition while
waiting for ACK.
• Two protocol use the above concept,
– Go-Back-N ARQ
– Selective Repeat ARQ