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Data Link Layer. Flow Control. Flow Control

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Data Link Layer

Flow Control

Copyright by Jorg Liebeherr 98,99 2

Flow Control

n Flow Control is a technique for speed-matching

of transmitter and receiver. Flow control ensures that a transmitting station does not overflow a receiving station with data

n We will discuss two protocols for flow control:

n Stop-and-Wait n Sliding Window

n For the time being, we assume that we have a

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Copyright by Jorg Liebeherr 98,99 3

Stop-and-Wait Flow Control

n Simplest form of flow control

n In Stop-and-Wait flow control, the receiver

indicates its readiness to receive data for each frame

n Operations:

1. Sender: Transmit a single frame

2. Receiver: Transmit acknowledgment (ACK) 3. Goto 1. 4

Analysis of Stop-and-Wait

Frame ACK Frame ACK Receiver Sender propagation delay

Last bit received End of Transmit of ACK

Start Transmit End Transmit Last bit of ACK received transmission

delay

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Copyright by Jorg Liebeherr 98,99 5

Sliding Window Flow Control

n Major Drawback of Stop-and-Wait Flow

Control:

n Only one frame can be in transmission at a time n This leads to inefficiency if propagation delay is

much longer than the transmission delay

n Sliding Window Flow Control

n Allows transmission of multiple frames

n Assigns each frame a k-bit sequence number n Range of sequence number is [0..2k-1], i.e.,

frames are counted modulo 2k

Copyright by Jorg Liebeherr 98,99 6

Operation of Sliding Window

n

Sending Window:

n At any instant, the sender is permitted to send

frames with sequence numbers in a certain range (the sending window)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Frames already

transmitted

Window of frames that may be transmitted Frame sequence number Last frame transmitted Window shrinks as

frames are sent

Window expands as acknowledgements are

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Copyright by Jorg Liebeherr 98,99 7

Operation of Sliding Window

n

Receiving Window:

n The receiver maintains a receiving window

corresponding to the sequence numbers of frames that are accepted

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6

Frames already received Window of frames that areaccepted by receiver

Last frame acknowledged

Window shrinks as frames are received

Window expands as acknowledgements are

sent

8

Operation of Sliding Window

n How is “flow control” achieved?

n Receiver can control the size of the sending

window

n By limiting the size of the sending window data

flow from sender to receiver can be limited

n Interpretation of ACK N message:

n Receiver acknowledges all packets until (but not

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Copyright by Jorg Liebeherr 98,99 9

Example

Receiver Transmitter 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 F0 F1 F2 ACK43

Copyright by Jorg Liebeherr 98,99 10

Example Continued

Receiver Transmitter 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 F3 ACK3 F4 F5 F6 4

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Copyright by Jorg Liebeherr 98,99 11

Analysis of Sliding Windows

0 a 1st frame received a+1 ... 0 1 2 W 1st ACK2a+1 received Receiver Sender Wth frame received a+W W 12 n If the window size is sufficiently large the sender

can continuously transmit packets:

n W2a+1: Sender can transmit continuously

n W < 2a+1: Sender can transmit W frames every

2a+1 time units

Analysis of Sliding Windows

1 efficiency normalized = 2a 1 W efficiency normalized + =

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Copyright by Jorg Liebeherr 98,99 13

ARQ Error Control

n Types of errors: Lost frames, damaged frames n Most Error Control techniques are based on (1)

Error Detection Scheme (e.g., Parity checks, CRC),

and (2) Retransmission Scheme

n Error control schemes that involve error detection

and retransmission of lost or corrupted frames are referred to as Automatic Repeat Request

(ARQ) error control

Copyright by Jorg Liebeherr 98,99 14

ARQ Schemes

n The most common ARQ retransmission

schemes:

n Stop-and-Wait ARQ n Go-Back-N ARQ n Selective Repeat ARQ

n The protocol for sending ACKs in all ARQ

protocols are based on the sliding window flow control scheme

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Copyright by Jorg Liebeherr 98,99 15

Stop-and-Wait ARQ

n Wait ARQ is an addition to the

Stop-and-Wait flow control protocol:

n Frames have 1-bit sequence numbers (SN = 0 or 1) n Receiver sends an ACK (1-SN) if frame SN is correctly

received

n Sender waits for an ACK (1-SN) before transmitting

the next frame with sequence number 1-SN

n If sender does not receive anything before a timeout

value expires, it retransmits frame SN

16

Stop-and-Wait ARQ

Frame 1 n Lost Frame A B ACK 0 Frame 0 Timeout ACK 1 Frame 1 Frame 1 ACK 0

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Copyright by Jorg Liebeherr 98,99 17

Stop-and-Wait ARQ

Frame 1 n Lost ACK A B ACK 0 Frame 0 ACK 1 Timeout Frame 0 ACK 1

Copyright by Jorg Liebeherr 98,99 18

Go-Back-N ARQ

n

Go-Back-N uses the sliding window

flow control protocol. If no errors

occur the operations are identical

to Sliding Window

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Copyright by Jorg Liebeherr 98,99 19

Go-Back-N ARQ

n

Operations:

n A station may send multiple frames as allowed by

the window size

n Receiver sends a NAK i if frame i is in error. After

that, the receiver discards all incoming frames until the frame in error was correctly retransmitted

n If sender receives a NAK i it will retransmit frame i

and all packets i+1, i+2,... which have been sent, but not been acknowledged

20

Go-Back-N ARQ

Frame 0 n Lost Frame A B Frame 2 Frame 4 Frame 1 Frame 3 ACK 3 Frame 5 Frame 6 NAK 4

Frame 4 Frame 5 Frame 6

Frames 4,5,6 are retransmitted

ACK 6

Frames 5 and 6 are discarded

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Copyright by Jorg Liebeherr 98,99 21

Go-Back-N ARQ

Frame 0 n Lost ACK A B Frame 2 Frame 1 Frame 3 ACK 3 Frame 0 Frame 5 Frame 4 Frame 3 Frame 4

Frames 0-4 are retransmitted Timeout

Frame 1 Frame 2

ACK 3 ACK 5

Frames 3 and 4 are discarded

Copyright by Jorg Liebeherr 98,99 22

Selective-Repeat ARQ

n Similar to Go-Back-N ARQ. However, the

sender only retransmits frames for which a

NAK is received

n Advantage over Go-Back-N:

n Fewer Retransmissions.

n Disadvantages:

n More complexity at sender and receiver

n Each frame must be acknowledged individually (no

cumulative acknowledgements)

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Copyright by Jorg Liebeherr 98,99 23 only Frame 4 is retransmitted

Selective-Repeat ARQ

Frame 0 n Lost Frame A B Frame 2 Frame 1 Frame 3 ACK 2 Frame 5 Frame 6 ACK 6, NAK 4

Frame 4 Frame 7 Frame 0

ACK 7 ACK 1 ACK 3 ACK 4

Frame 4

Frames 5 and 6 are buffered

ACK 5 ACK 0 ACK 1

24

Example of Selective-Repeat

ARQ

B A 2 1 3 2 Frames received Frames waiting for ACK/NAK 3 ACK2

frame 1 is correct, send ACK 2

B A 3 2 4 3 4 NAK2

frame 2 is in error, send NAK2 1 1 2 4 3 B A 2 4 retransmit frame 2 1 3 5 5

Receiver must keep track of `holes’ in the sequence of delivered frames Sender must maintain one timer per outstanding packet

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