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lec8 - ECE 333 Introduction to Communication Networks Fall...

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ECE 333: Introduction to Communication Networks Fall 2002 Lecture 8: Data Link Layer IV ARQ algorithms In the last lecture, we began looking at ARQ protocols. Recall our assumptions are: 1. Packets on both the forward or reverse link may be delayed arbitrarily long and may not arrive at all (packets that are found to be in error are considered to not have arrived) 2. If packets do arrive they arrive in the order they were sent. Given these assumption we want to develop a protocol to provide a reliable packet delivery service, meaning that each packet is delivered to the next layer correctly, in order and only one time. Last time we proposed that the receiver send an Acknowledgement packet (ACK) to the transmitter for every correct packet received, and we saw that the transmitter needs to number the packets that are sent.

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Where Do We Stand Now? Current protocol: Transmitter: 1. Set sequence number, SN = 0 2. Wait until packet available to be sent (from higher layer) 3. Send current packet in frame with number SN, start timer. 4. If time-out, go to 3. 5. If (error free) ACK received, set SN=SN+1 (mod 2), go to 2. Receiver: 1. Set current number to be received, RN= 0. 2. Wait until packet arrives error free then: a. Send ACK to transmitter. b. If SN=RN, release packet to higher layer, set RN=RN+1(mod2). 3. Go to 2. Is our protocol now reliable? A Problem How do we decide on the value to use for the timer? § If it is too long, we waste time waiting to retransmit. § What if it is too short? Consider the following possibility: (what should the transmitter do next ?) The ACK that arrives after packet 2 may be for the second packet 1 as shown, or it could be for packet 2 (if one of the ACKs for packet 2 was lost). Depending on which event occurred the transmitter should either resend packet 2 or send packet 3. Ack Ack 1 1 2 ? Transmitter Receiver
The solution to this is to include in the ACK packet the sequence number of the next frame expected. (Equivalently, the sequence number of the frame just received could be used, but in practice this is not done.)

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