1. Communication Networks NETW 501
Lecture 7
Data Link Layer: Error Control & Flow Control
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Data Link Layer Network LLC MAC Physical
Data Link Layer provides a service for Network Layer (transfer of data from the network layer of a sender to the network layer of a receiver)
Data Link Layer uses the Physical Layer to transmit bits of Data Link Frames over the physical medium
LLC
MAC
Network
Physical

3. Bit errors in Communication Systems
At the physical layer, bit errors are inevitable to occur with small but non zero probability, example:
Bit error probability in the order of 10-6 for systems using copper wires
Bit error probability in the order of 10-9 for modern optical fiber systems
High bit error probability in the order of 10-3 for wireless transmission systems
Some services are tolerant to relatively high bit error rates such as digital speech transmission
Some applications must experience error-free communications such as electronic funds transfer

4. Error Control
Error Control is a system to deal with errors that occur due to disturbances on the physical channel.
Components of an error control system:
Error Correction and Detection
Acknowledgement (ACK) & Non- Acknowledgement Control Messages (NAK)
Timers
Frame Sequencing
Sender
Receiver
Data Frame
No Errors
Detection/
Correction
ACK
Frame is Good 1
Errors
Timer 1
Detection/
Correction 1
No Errors 1
Detection/
Correction
ACK
Frame is Good

5. Error Control Mechanisms
Forward Error Correction (FEC)
Detection of erroneous frames or packets
Processing of received frame bits in attempt to
correct the errors
Automatic Retransmission reQuest (ARQ)
Retransmission of erroneous frames with the hope that no errors would occur in the next attempt

6. (Error Control & Flow Control )
Automatic Repeat reQuest (ARQ) protocols
Purpose:
To ensure a sequence of information packets is delivered in order and without errors or duplications despite transmission errors & losses
(Error Control & Flow Control )
We will look at:
Stop-and-Wait ARQ
Go-Back N ARQ
Selective Repeat ARQ

7. Stop and Wait Protocol

8. Stop-and–Wait ARQ Stop after Transmitting a Packet
Wait for an Acknowledgement
H : Header
CRC : Cyclic Redundancy Check (Error Detection)

9. Importance of Sequence Numbers
Sender
Receiver
Sender
Receiver
Frame 0
Frame 0
ACK
ACK
Frame 1
Frame 1
ACK
Timeout
Timeout
Frame 1
Frame 1
ACK
ACK
Frame 2
Frame 2
In Scenario B, the receiver accepts Frame 1 Twice
If the frame number is not included in the header, how could the receiver distinguish whether or not it has received Frame 1 correctly?
THEREFORE: IT IS ESSENTIAL TO INCLUDE THE FRAME SEQUENCE NUMBER IN THE HEADER

10. BY INCLUDING THE SEQUENCE NUMBER IN THE ACK
Sequence Numbers in ACK Frames
The sender may mistakenly interpret ACK as it assumes that the second ACK was for Frame 1
How Could the Sender Know which Frame does an ACK correspond to?
BY INCLUDING THE SEQUENCE NUMBER IN THE ACK
Sender
Receiver
ACK
Frame 0
Frame 1
Frame 2 C
Timeout

11. Stop–and–Wait ARQ Operation
Machine A
Machine B
Physical Channel
First Packet-Bit enters Channels
First Packet-Bit arrives at B
Last Packet-Bit enters Channels
Last Packet-Bit arrives at B
Channel is Idle
Processing Time for Error Detection
Last ACK-Bit Arrives at A
The Channel Idle Time (Wait for ACK) is an Inefficiency because the channel is not used

12. t0=Total Time to Transmit 1 Frame
Stop–and–Wait ARQ Efficiency
Machine A
Machine B
Physical Channel
t0=2tprop+2tproc+tf+ta
t0=2tprop+2tproc+Lf/R+La/R
tprop
tf=Lf/R
t0=Total Time to Transmit 1 Frame
tproc
ta=La/R
tprop
tprop Propagation Time
tproc Processing Time
tf Frame Transmission Time
ta ACK Transmission Time
Lf No. of Bits/Frame
La No. of Bits/ACK
tproc

13. Effective Transmission Rate Transmission Efficiency
Error-Free Channel
Effective Transmission Rate
Headers and CRC
Effect of Header
Effect of ACK
Effect of Delay Bandwidth Product
Transmission Efficiency

14. The Expected Efficiency
Noisy Channel
Pf is the probability of Frame Error
If k Transmissions are Required
The Efficiency is
The Expected Efficiency

15. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International