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Giuseppe Bianchi Transmission & Error control ARQ.

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Presentation on theme: "Giuseppe Bianchi Transmission & Error control ARQ."— Presentation transcript:

1 Giuseppe Bianchi Transmission & Error control ARQ

2 Giuseppe Bianchi Two scenarios network End to End (transport protocol issue) Hop by Hop (datalink protocol issue)

3 Giuseppe Bianchi Coping with rx errors Forward Error Correction Extra overhead capable of CORRECTING errors Large overhead depending on protection capability Retransmission Extra overhead capable of DETECTING errors Small constant overhead (2-4 bytes) Called: »Frame Check Sequence »Cyclic Redundance Check

4 Giuseppe Bianchi Retransmission scenarios referred to as ARQ schemes (Automatic Retransmission reQuest) DATA ACK SRC DST Basic ACK idea DATA NACK SRC DST Basic NACK idea Error Check: OK COMPONENTS: a) error checking at receiver; b) feedback to sender; c) retx Error Check: corrupted DATA Automatic retransmit DATA SRC DST Basic ACK/Timeout idea Retx Timeout (RTO) DATA SRCDST DATA Error Check: corrupted DATA SRC DST DATA ACK

5 Giuseppe Bianchi Sequence numbers – a must Sender side: DATA ACK RTO DATA rtx Receiver side: DATA DATALINK or NETWORK (ACK lost) New data? Old data? Need to univocally label all packets circulating in the network between two end points. 1 bit (0-1) enough for Stop-and-wait

6 Giuseppe Bianchi Link-level model C bits/sec In e2e scenario, C approximated by bottleneck link rate Stop & Wait one frame/packet at a time Pipelining (continuous ARQ) more than one frame/packet

7 Giuseppe Bianchi Stop & Wait

8 Giuseppe Bianchi stop-and-wait time sender time receiver One way delay RTT MSG/C REMARK: throughput always lower than Available link rate! ACK/C For simplicity all MSG of same size; extension to != size use mean value

9 Giuseppe Bianchi Upper bound No processing = 0 ACK size negligible ACK = 0

10 Giuseppe Bianchi stop-and-wait: upper bound /1 MSG = 1500 bytes Under-utilization with: 1) high capacity links, 2) large RTT links

11 Giuseppe Bianchi Under-utilization with: 1) high capacity links, 2) large RTT links stop-and-wait: upper bound /2 MSG = 1500 bytes

12 Giuseppe Bianchi Dealing with errors TO senderreceiver IMPORTANT ISSUE: setting the Time Out right! Too short unnecessary rtx Too short inconsistent protocol operation Too long waste of time Ideally: TO = RTT+ACK/C (+2 ) Easy to say, but harder to do if RTT is not known (e.g. in e2e scenario)

13 Giuseppe Bianchi Inconsistent protocol operation M=1 TO senderreceiver M=1 M=2 M=3 M=2 will be NEVER received! Consequence: ACK MUST be also numbered To always guarantee consistent operation

14 Giuseppe Bianchi Performance with loss (upper bound) No processing time, negligible ACK Per packet loss probability P Assumed indipendent P(immediate success) = (1-P) P(success at second tx) = P(1-P) P(success at third tx) = P 2 (1-P)

15 Giuseppe Bianchi Pipelining (Continuous ARQ)

16 Giuseppe Bianchi Pipelining idea Up to W>1 frames can be in fly In fly = frames transmitted but not yet ACKed Sliding Window Size W Slides forward at each received ACK


18 Giuseppe Bianchi Esercizio (fatelo) MSG=500 bytes W=4 C= 2 Mbps Propagation = 16 ms How much time to transmit 6 messages? Which message size for continuous tx?

19 Giuseppe Bianchi Continuous transmission Time to transmit W frames Time to receive Ack of first frame Condition in which link rate is fully utilized We may elaborate: This means that full link utilization is possible when window size (in bits) is Greater than the bandwidth (C bit/s) delay (RTT s) product!

20 Giuseppe Bianchi Bandwidth-delay product Network: like a pipe C [bit/s] x D [s] number of bits flying in the network number of bits injected in the network by the tx, before that the first bit is rxed D C 64Kbps A (64000x0.240) bits worm in the air!! bandwidth-delay product = no of bytes that saturate network pipe

21 Giuseppe Bianchi Long Fat Networks LFNs (el-ef-an(t)s): large bandwidth-delay product Ethernet T1, transUS T1 satellite T3 transUS Gigabit transUS NETWORKRTT (ms) rate (kbps) BxD (bytes)

22 Giuseppe Bianchi Throughput for pipelining MSS = 1500 bytes

23 Giuseppe Bianchi Maximum achievable throughput (assuming infinite speed line…) W = bytes

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