1. Chapter 7 – Data Link Control Protocols
Eighth Edition
by William Stallings
Data Link Control Protocols
need layer of logic above Physical
to manage exchange of data over a link
frame synchronization
flow control
error control
addressing
control and data
link management

2. FRAMING
The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing. The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves as the delimiter.
Figure A frame in a character-oriented protocol

3. Byte stuffing is the process of adding 1 extra byte whenever there is a flag or escape character in the text.
Figure Byte stuffing and unstuffing

4. Figure 11.3 A frame in a bit-oriented protocol
Figure Bit stuffing and unstuffing
Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data, so that the receiver does not mistake the pattern for a flag.

5. FLOW AND ERROR CONTROL Flow 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
Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.
ensure sending entity does not overwhelm receiving entity
by preventing buffer overflow
influenced by:
transmission time
time taken to emit all bits into medium
propagation time
time for a bit to traverse the link
assume here no errors but varying delays
Error control in the data link layer is based on automatic repeat request, which is the retransmission of data.

6. 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.

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.

8. Model of Frame Transmission

9. Stop and Wait source transmits frame
destination receives frame and replies with acknowledgement (ACK)
source waits for ACK before sending next
destination can stop flow by not send ACK
works well for a few large frames
Stop and wait becomes inadequate if large block of data is split into small frames

10. Stop and Wait Link Utilization

11. Utilization under SW Flow Control
Utilization = U = frame time / total time
U = 1/1+2a
a = Propagation Time / Transmission Time
a = Medium length in bits / Frame length in bits
Station s1 sends f1
Station s2 sends an acknowledgment
Station s1 sends f2 .
Station s1 sends fn
CALCULATION OF U

12. CALCULATION OF U Total time to send the data=T=nTf
Tf=time to send and receive an acknowledgment
Tf=Tframe+Tprop+Tproc+ Tack + Tprop+Tproc
Tprop=propagation time from s1 to s2
Tframe=time to transmit a frame
Tproc=processing time (negligible)
Tack=time to transmit an acknowledgment(very small as compared to data frame)
T=2Tprop+Tframe
For n frames T=n(2Tprop+Tframe)
Of that time only n x Tframe is actually spent transmitting data and the rest is overhead,therefore the utilization or efficiency of the line is
U= (n x Tframe )/n(2Tprop+Tframe)
= Tframe/(2Tprop+Tframe)
If a=Tprop/Tframe then
U=1/(1+2a)

13. Utilization with Sliding Window Flow Control
N > 2a + 1
for
N < 2a + 1

14. Sliding Windows Flow Control
allows multiple numbered frames to be in transit
receiver has buffer W long
transmitter sends up to W frames without ACK
ACK includes number of next frame expected
sequence number is bounded by size of field (k)
frames are numbered modulo 2k
giving max window size of up to 2k - 1
receiver can ack frames without permitting further transmission (Receive Not Ready)
must send a normal acknowledge to resume
if have full-duplex link, can piggyback ACks

15. Sliding Window Diagram

16. Sliding Window Example

17. Error Control detection and correction of errors such as: lost frames
damaged frames
common techniques use:
error detection
positive acknowledgment
retransmission after timeout
negative acknowledgement & retransmission

18. Automatic Repeat Request (ARQ)
collective name for such error control mechanisms, including:
stop and wait
go back N
selective reject (selective retransmission)

19. Stop and Wait source transmits single frame wait for ACK
if received frame damaged, discard it
transmitter has timeout
if no ACK within timeout, retransmit
if ACK damaged,transmitter will not recognize it
transmitter will retransmit
receive gets two copies of frame
use alternate numbering and ACK0 / ACK1
pros and cons
simple
inefficient

20. Go Back N based on sliding window if no error, ACK as usual
use window to control number of outstanding frames
if error, reply with rejection
discard that frame and all future frames until error frame received correctly
transmitter must go back and retransmit that frame and all subsequent frames
Damaged Frame
error in frame i so receiver rejects frame i
transmitter retransmits frames from i
Lost Frame
frame i lost and either
transmitter sends i+1 and receiver gets frame i+1 out of seq and rejects frame i
or transmitter times out and send ACK with P bit set which receiver responds to with ACK i
transmitter then retransmits frames from i

21. Go Back N Diagram

22. Go Back N - Handling Damaged Acknowledgement Damaged Rejection
receiver gets frame i, sends ack (i+1) which is lost
acks are cumulative, so next ack (i+n) may arrive before transmitter times out on frame i
if transmitter times out, it sends ack with P bit set
can be repeated a number of times before a reset procedure is initiated
Damaged Rejection
reject for damaged frame is lost
handled as for lost frame when transmitter times out

23. Selective Reject also called selective retransmission
only rejected frames are retransmitted
subsequent frames are accepted by the receiver and buffered
minimizes retransmission
receiver must maintain large enough buffer
more complex logic in transmitter
hence less widely used
useful for satellite links with long propagation delays

24. Selective -Reject Diagram