Communication Networks NETW 501 Lecture 7 Data Link Layer: Error Control & Flow Control Course Instructor: Dr.-Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C3.220

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

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

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

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

(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

Stop and Wait Protocol

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

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

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

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

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

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

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

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