Automatic Retransmission ReQuest (ARQ)By Ahmed Kremid

To ensure delivery of information from transmitter to receiver unchanged, error free and in order.

To over come the effect of channel, typically noisy, on information.

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Why ARQ?

Stop and Wait ARQ Go Back N ARQ Selective Repeat ARQAll three protocols usually use some form of sliding window protocol to tell the transmitter determine which packets (messages) need to be retransmitted

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ARQ Protocols

It is the simplest kind of ARQ method. A stop and wait ARQ sender sends one frame at a time and waits for Ack signal from receiver.

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Stop and Wait ARQ

In case of frame loss receiver would not send Ack, and time out occurs, transmitter resend.

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Stop and Wait ARQ contd

A

B

In case of Ack loss timeout occurs, transmitter resend, receiver receives duplicates.

Frame sequencing is necessary for receiver to detect duplicates.

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Stop and Wait ARQ contd

T = Tf + Tp + Tpc + Tack + Tp + TpcTf = time to transmit a frameTp = propagation time between A and B (either direction)Tpc = processing time at each stationTack = time to transmit an acknowledgement Assuming Tpc and Tack relatively negligible   T = Tf + 2Tp Throughput(Utilization), U U = Tf / T = Tf / (Tf + 2Tp ) = 1(/1 + 2a)in which a = Tp / Tf

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Stop and wait ARQ utilizationwith no error

If Nx = number of transmissions required to successfully send a frame

T = Nx(Tf + 2Tp)

Therefore throughput/utilization, UU = Tframe / T = 1(/Nx(1 + 2a))Nx = E(transmission) = S( i * Pr[I transmissions])

= S i P i-1(1-P) = 1/ (1 - P)Thus,U = (1-P)/(1 + 2a)

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Stop and wait ARQ utilizationwith error

BiSync: Character oriented data link control.

Xmodem: modem file transfer protocol. Trivial file Transfer protocol: file transfer over UDP.

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Applications of Stop and Wait ARQ

Sender transmits more than one frame (window size).

When frame loss or error detected request for retransmission from that frame and after.

Keeps channel busy by keep sending. More efficient than stop and wait.

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Go Back N ARQ

Frame 3 is lost time outs, receiver requests for frame 3 and after, sender retransmits.

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Go Back N ARQ

With window size w1. Without error: U = 1 if W > 2a + 1U = W/(2a+1) if W < 2a +12. With error:

Here each erroneous frame requires retransmission of K frames; Nx = 1 - K + K/ (1 - P)

U = 1/ Nx(1 + 2a)

U = 1/ ((1 - K + K/ (1 - P))(1 + 2a))note K ~= 2a + 1 for W > 2a + 1 and K = W for W < 1+2a Thus:

U = (1 - P)/ (1 + 2aP) for W > 1 + 2aU = W(1 - P)/((1 + 2a)(1 - P + WP)) for W < 1 + 2a

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Utilization of Go Back N ARQ

HDLC (High-Level Data Link Control)bit oriented data transfer.

V.42 Modem: error control over telephone networks.

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Applications of Go Back N ARQ

Retransmits only the affected frame. Frames maybe delivered out of sequence.

When frame Ack is received new frame is buffered.

More efficient than Go Back N because there is no need to retransmit set of frames while only a frame is affected.

Needs additional circuitry for reordering frames at the receiver.

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Selective Repeat ARQ

When frame 2 lost or corrupted, receiver sends negative Ack about F2, and even though keeps receiving other frames.

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Selective Repeat ARQ

With window size w

1. Without error: U = 1 if W > 2a + 1U = W/(2a+1) if W < 2a +12. With error, Let Nx is the number of retransmission

required to successfully send a frame, thenNx = S P i-1(1-P)Nx = 1/ (1 - P)

Thus,U = 1-P if W > 2a + 1U = W(1-P)/(2a+1) if W < 2a +1

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Utilization of Selective Repeat ARQ

TCP (Transmission control protocol): Transport layer uses variation of selective repeat to provide reliable stream service.

Service specific connection oriented protocol: error control for signalling messages in ATM networks.

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Applications of Selective Repeat ARQ

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Impact of Bit Error on Efficiency

•Selective repeat outperforms GoBackN,and however, Stop and Wait.•Efficiency increases as bit error decreases.

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ARQ Efficiency