data link and flow control networks and protocols prepared by: tgk first prepared on: last modified...

Data Link and Flow Control

Networks and Protocols

Prepared by: TGK First Prepared on: Last Modified on:

Quality checked by: Copyright 2009 Asia Pacific Institute of Information Technology



Slide 2 of 27

Topic & Structure of the lesson

Overview of Data Link Control and Flow Control

MechanismsFlow Control TechniquesError Detection TechniquesError Control Techniques



Slide 3 of 27

Learning Outcomes

At the end of this module, YOU should be able to:

• Discuss Asynchronous and Synchronous communications• Discuss Stop-and-Wait and Sliding Windows flow-control

techniques• Discuss the Parity check and Cyclic Redundancy Check

approaches for error detection• Discuss error control techniques of Automatic Repeat Request -

Stop-and-Wait, Sliding Window and Selective Reject



Slide 4 of 27

Key Terms you must be able to use

If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:

• Flow Control• Error Detection• Error Control



Slide 5 of 27

Main Teaching Points

• SYNC characters• START and STOP Bits• ACK status• Sliding Windows• ODD and EVEN parity• Automatic Repeat Request• Stop-and-Wait ARQ• Sliding Window• Selective Reject



Slide 6 of 27


Mechanisms

• Synchronous Communications

• Receiver clock operates in synchronism with the

received signal

• The clocking signal is embedded into the transmitted

bit stream and subsequently extracted by the

receiver (SYN characters)

• Asynchronous Communications

• The receiver’s clock runs asynchronously with

respect to incoming signal

• A Byte is framed by a START and STOP bit



Slide 7 of 27

• The START bit changes the signal to Non-Idle state,

this warns receiver a byte is arriving

• Eight (8) data bits follow

• The STOP bit follows the data bits and returns the

signal to the Idle state


Mechanisms



Slide 8 of 27

Flow Control Techniques

• Stop-and-Wait

• Source transmits frame

• Destination receives frame and replies with

acknowledgement

• Source waits for ACK before sending next frame

• Destination can stop flow by not send ACK

• Works well for a few large frames

• Sliding Windows

• Allow multiple frames to be in transit

• Receiver has buffer of W long/length

• Transmitter can send up to W frames without ACK



Slide 9 of 27

• Allow multiple frames to be in transit

• Receiver has buffer W long

• 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 2 k

• Following figure shows the sliding-windows

technique operation:




Slide 10 of 27




Slide 11 of 27

• Sliding Windows Enhancements

• Receiver can acknowledge frames without

permitting further transmission (Receive Not

Ready)

• Must send a normal acknowledge to resume

• If duplex, use piggybacking

If no data to send, use acknowledgement frame If data but no acknowledgement to send, send

last acknowledgement number again, or have ACK valid flag (TCP)




Error Detection Techniques

• Parity Check

• Additional bits added by transmitter for error

detection code

• Parity

• Value of parity bit is such that character has even

(even parity) or odd (odd parity) number of ones

• Even number of bit errors goes undetected

• Example : Odd parity Check1+ 11011 111011(data will be accepted)1+ 11011 110011(reject because single bit error)



Slide 13 of 27

• Disadvantage:

• 1+11011 111000(data will be accepted) but

the message has burst error

• Cyclic Redundancy Check

• For a block of k bits transmitter generates n bit

sequence

• Transmit k+n bits which is exactly divisible by some

number

• Receive divides frame by that number

• If no remainder, assume no error

Error Detection Techniques



Slide 14 of 27

Error Control Techniques

• Automatic Repeat Request

• Detection and correction of errors

• Lost frames

• Damaged frames

• Features:

• Positive acknowledgment

• Retransmission after timeout

• Negative acknowledgement and retransmission

• Techniques adopted:

• Stop and Wait ARQ

• Sliding window

• Selective reject (selective retransmission)




• Stop and Wait ARQ

• Source transmits single frame

• Wait for ACK

• If received frame damaged, discard it

• Transmitter has timeout

• If no ACK within timeout, retransmit



Slide 16 of 27

• If ACK damaged transmitter will not recognize it

• Transmitter will retransmit

• Receive gets two copies of frame

• Use ACK0 and ACK1

• Go-Back-N ARQ

• Based on sliding window

• If no error, ACK as usual with next frame expected

• 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




Slide 17 of 27

• Transmitter must go back and retransmit that frame

and all subsequent frames

• Go-Back-N ARQ - Damaged Frame

• Receiver detects error in frame i

• Receiver sends rejection-i

• Transmitter gets rejection-i

• Transmitter retransmits frame i and all subsequent

• Go-Back-N ARQ - Lost Frame

• Frame i lost

• Transmitter sends i+1

• Receiver gets frame i+1 out of sequence




Slide 18 of 27


• Receiver send reject i

• Transmitter goes back to frame i and retransmits

• Selective Reject ARQ

• 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



Slide 19 of 27

Quick Review Question



Slide 20 of 27

Follow Up Assignment



Slide 21 of 27

Summary of Main Teaching Points



Slide 22 of 27

Q & A

Question and Answer Session



Slide 23 of 27

Topic and Structure of next session

• Routing

Routing characteristics Routing strategies Routing algorithms

Next Session

data link and flow control networks and protocols prepared by: tgk first prepared on: last modified...

Documents

flow control slide

flow control networks

ack slide

protocols data link

data bits

window selective reject

w frames

multiple frames