Data Link Control

1
Data Link Control

• General principles

2
Data Link Control

• Coordinating between two end-systems across a
  Link to send/receive data correctly
• Who should send and when Medium Access
• Coordinating sending and receiving Line
  Discipline involves Link establishment etc
• What happens if received data is in error Error
  control
• Error correction
• Retransmission
• Processing capacity Buffer and sending and
  receiving processes Flow Control

3
Line Discipline

• No Device should send unless it has ascertained
  that the receiver is ready to receive
• Link Establishment logical check
• Two Basic Schemes
• Enquiry /Ack (peer to peer)
• Poll /Select (Master /Slave)

4
T
R
ENQ
SIMPLE ENQ/ACK protocol (e.g., "peer-to-peer",
dedicated line)
Establish
ACK
data
Transfer
ACK
data
ACK
ENQ - enquiry ACK - Acknowledgement EOT - end of
transmission
data
ACK
Terminate
EOT
5
T
R
POLL
POLL protocol (e.g., multipoint)
Establish
NAK
Secondary stations
POLL
Transfer
data
ACK
Primary
data
POLL - "have anything?"ACK - AcknowledgementEOT
- end of transmission Address the secondary ?
ACK
EOT
Terminate
POLL
6
T
R
SEL
SELECT protocol (e.g., multipoint)
Establish
NAK
Secondary stations
SEL
Transfer
ACK
data
Primary
ACK
SEL - "ready to receive?"ACK -
AcknowledgementNAK - Negative AcknowledgementEOT
- end of transmission
data
ACK
Terminate
EOT
7
Flow Control

• How fast and How much data to send?
• Tx rx speed physical layer
• Memory storage at sender and receiver
• Processing capacity at sender and receiver
• Two most popular schemes
• STOP WAIT
• SLIDING WINDOW

8
T
R
data
STOP and WAIT
Establish
WAIT TIME
ACK
data
Transfer
WAIT TIME
ACK
data
WAIT TIME
ACK
ACK - Acknowledgement EOT - end of
transmission Frames can be numbered 0,1
data
WAIT TIME
ACK
Terminate
EOT
9
T
R
Data,0
SLIDING WINDOW(without error control)
Establish
WAIT TIME
Data,1
Data,2
Data,3
Transfer
WAIT TIME
ACK,4
Data,4
WAIT TIME
Data,5
data, n - data, frame sent ACK, n - ACK, expected
frame EOT - end of transmission
Data,6
WAIT TIME
ACK, 7
Terminate
EOT
10
T
R
Data 0
STOP and WAIT ARQ lost frame
TIME OUT!!
Data 0
TIME OUT!!
Data 0
ACK 1
ACK - Acknowledgement
Data 1
ACK 0
...
11
T
R
Data 0
STOP and WAIT ARQ lost ACK
TIME OUT!!
ACK 1
TIME OUT!!
Data 0
ACK 1
ACK - Acknowledgement
...
12
T
R
Data 0
STOP and WAIT ARQ damaged frame
WAIT TIME
ACK 1
Data 1
WAIT TIME
ACK 0
ERROR!
Data 0
WAIT TIME
NAK
ACK - Acknowledgement NAK - Negative
Acknowledgement
Data 0
WAIT TIME
ACK 1
...
13
T
R
Data,0
SLIDING WINDOW ARQ Go-Back-nDamaged Frame
Data,1
Data,2
Data,3
ERROR!
ACK,3
Data,4
Data, n - data, sent frameACK, n - ACK, expected
frameNAK, n - NAK, damaged frameEOT - end of
transmission
DiscardedDiscarded
NAK,3
Data,5
Data,3
ACK, 3
14
T
R
Data,0
SLIDING WINDOW Go-Back-nLost Frame
Data,1
Data,2
Data,3
LOST!
Data,4
Data,5
DiscardedDiscarded
NAK,3
Data, n - data, sent frameACK, n - ACK, expected
frameNAK, n - NAK, damaged frameEOT - end of
transmission
Data,6
Data,3
ACK, 4
15
T
R
Data,0
SLIDING WINDOW Select-RejectLost Frame
Data,1
Data,2
Data,3
LOST!
Data,4
Data,5
NAK,3
Data,6
Data, n - data, sent frameACK, n - ACK, expected
frameNAK, n - NAK, damaged frameEOT - end of
transmission
Data,3
ACK,7