DATA COMMUNICATION
(ELA…)
1
DATA LINK CONTROL PROTOCOLS
INTRODUCTION
 Synchronization
and Interfacing are
insufficient due to


Probability of transmission errors
Need of receiver to regulate arrival rate of data
 Layer

of logic is added
Data Link Control
 Key
mechanisms
2
FLOW CONTROL
FLOW CONTROL
Stop
Begin
No
Rcv Data from Tx
Is there buffer
capacity left?
Yes
Yes
No
More data
to Tx?
Clear Buffer
Stop Rcv’ng Data
from Tx
Process Data
Transfer Data
to next upper layer
4
MODEL OF FRAME TRANSMISSION
5
FLOW CONTROL
 Definitions

Transmission time
Time taken to emit all bits onto the medium
 Proportional to length of frame


Propagation time

Time for a bit to traverse the link
 Options


Stop-and-Wait
Sliding Window
6
STOP-AND-WAIT FLOW CONTROL

How does it work?




Source transmits frame
Destination receives frame and replies with
acknowledgement
Source waits for ACK before sending next frame
Destination can stop flow by not sending ACK
Works well for a few large frames
 Not good when fragmentation is used

7
PERFORMANCE ISSUES
Propagation Time
a=
Transmission Time
d/V
=
=
L/R
Rd
VL
Where




d  Link distance (m)
V  Velocity of propagation (m/s)
L  Length of frame in bits
R  Data Rate (bps)
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STOP-AND-WAIT – LINK
UTILIZATION
Tx Time = 1, Propagation time = a
9
a>1
a<1
SLIDING WINDOW FLOW CONTROL
 How
does it work?
Allows multiple frames to be in transit
 Receiver has buffer of length = W
 Transmitter can send up to W frames without
ACK
 Each frame is numbered
 ACK includes number of next frame expected
 Sequence number bounded by size of field (k)


Frames are numbered modulo 2k
10
SLIDING WINDOW DIAGRAM
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SLIDING WINDOW ENHANCEMENTS

Receiver can acknowledge frames without
permitting further transmission



RNR  Receive Not Ready
Must send a normal ACK to resume
If duplex, use piggybacking


If no data to send, use ACK frame
If data but no ACK to send
 Send last ACK number again, or have ACK valid flag
(TCP)
12
ERROR CONTROL
ERROR CONTROL
 Detection


and correction of errors
Lost frames
Damaged frames
 Automatic




Repeat Request
Error detection
Positive acknowledgment
Retransmission after timeout
Negative acknowledgement and
retransmission
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AUTOMATIC REPEAT REQUEST
(ARQ)

These techniques are all based on either Stopand-wait or Sliding Window flow control
techniques



Stop-and-wait ARQ
Go-back-N ARQ
Selective reject ARQ

Selective retransmission
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STOP-AND-WAIT ARQ
 Basic
Operation
t<To
T
R
F(0)
F(0)
t=T
….
o
F(1)
t=T
….
o
F(1)
F(1)
t<To
F(1)
F(1)
Duplicate detected/16
discarded
t
t
STOP-AND-WAIT ARQ

Pros and Cons
 Simple
 Inefficient

Referred to as Idle RQ
17
GO-BACK-N ARQ
Most common implementation of Sliding Window
flow control
 Uses window to control number of outstanding
frames
 If no error, ACK as usual with next frame expected
 If error, reply with rejection (REJ)
 Discard that frame and all future frames until
error frame received correctly
 Transmitter must go back and retransmit that
frame and all subsequent frames
18

GO-BACK-N ARQ
19
SELECTIVE REJECT
 Also
called selective retransmission
 Advantages



Only rejected frames are retransmitted
Subsequent frames are accepted by the
receiver and buffered
Minimizes retransmission
 Disadvantages


Receiver must maintain large enough buffer
More complex logic in transmitter
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SELECTIVE REJECT ARQ
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PERFORMANCE ISSUES
PERFORMANCE ISSUES
 Stop-and-Wait





Flow Control (No Errors)
A sends a frame to B
After receiving frame, B sends and ACK to A
Process is repeated n times
Thus, total time to transmit n frames can be
expressed as
Where
T
 F  Time to send a frame and receive
ACK
and
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PERFORMANCE ISSUES
Where TF  t prop  t frame  t proc  t prop  t ack
 tprop  Propagation time from A to B
 tframe  Time to transmit a frame
 tproc  Processing time at each station
 tack  Time to transmit and ACK
 t proc
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PERFORMANCE ISSUES
TF  t prop  t frame  t proc  t prop  tack  t proc
T  nTF  n(2t prop  t frame)
U

n  t fram e
n(2t prop  t fram e)
Since a = tprop / tframe 

t fram e
2t prop  t fram e
1
U
1  2a
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PERFORMANCE ISSUES

Sliding Window Flow Control (No Errors)
1
U
W  2a  1
W
W  2a  1
2a  1
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PERFORMANCE ISSUES
 Stop-and-Wait
 Where
ARQ (With Errors)
1
U
N r (1  2a)
Nr  Expected # of transmissions of
a frame
1
Nr 
1 P
1
1 P
U

N r (1  2a) 1  2a
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PERFORMANCE ISSUES
 Selective
U
Reject ARQ (With Errors)
1 P
W  2a  1
W (1  P)
2a  1
W  2a  1
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PERFORMANCE ISSUES
 Go-back-N
U
ARQ (With Errors)
1 P
W  2a  1
1  2aP
W (1  P)
W  2a  1
(2a  1)(1  P  WP)
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