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UCSC CMPE 257 - Wireless and Mobile Networking

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CMPE 257 Wireless and Mobile Networking Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 10 CMPE 257 Winter 11 1 Student Presentations Feb 28 March 9 Mobility management Tyler and Niosha March 2 DTNs Philip and Rance March 7 Energy management Mohamed Security Jim March 14 Security Chris and and Seth Hybrid networks Gregg and Darien CMPE 257 Winter 11 2 Student Presentation Logistics We will source from San Jose Send me your presentations ahead of time Alternatively Use MS Live Meeting CMPE 257 Winter 11 3 Schedule Exam Feb 23 5 6 45pm Project presentations March 17 Time change 5 8pm Approximately 15 minutes for each presentation CMPE 257 Winter 11 4 Today End to end protocols cont d CMPE 257 Winter 11 5 E2E Protocols Reliable point to point Reliable multipoint CMPE 257 Winter 11 6 Reliable Point2Point Transport Layer Outline TCP IP basics Impact of transmission errors on TCP performance Approaches to improve TCP performance on wireless networks Classification TCP on cellular TCP on MANETs CMPE 257 Winter 11 7 Approaches to Improve Performance of TCP in Wireless Networks CMPE 257 Winter 11 8 Classification Based on who takes the action and What kind of action taken E2E versus cross layer approaches E2E approaches connection end points try to distinguish between congestion and noncongestion losses Cross layer approaches combination of e2e and intermediate node participation CMPE 257 Winter 11 9 Infrastructure Based Wireless Earlier efforts to improve TCP s performance focused on inrastructurebased wireless environments CMPE 257 Winter 11 10 Cross Layer Approaches Link layer error recovery Link layer retransmission TCP awareness TCP unawareness Split connection CMPE 257 Winter 11 11 Link Layer Mechanisms Error Correction Example Forward Error Correction FEC can be used to correct limited number of errors Correctable errors hidden from the TCP sender FEC incurs overhead even when errors do not occur Adaptive FEC schemes can reduce the overhead by choosing appropriate FEC dynamically CMPE 257 Winter 11 12 Link Layer Mechanisms Link Level Retransmissions Link level retransmission schemes retransmit a packet at the link layer if errors are detected Retransmission overhead incurred only if errors occur CMPE 257 Winter 11 13 Link Layer Mechanisms May combine both FEC and retransmissions Use FEC to correct small number of errors Use link level retransmission when FEC capability is exceeded CMPE 257 Winter 11 14 Link Level Retransmissions TCP connection Link layer state application application application transport transport transport network network link link link physical physical physical rxmt network wireless CMPE 257 Winter 11 15 Link Level Retransmissions Issues How many times to retransmit at the link level before giving up Finite bound semi reliable link layer No bound reliable link layer What triggers link level retransmissions Link layer timeout mechanism Link level acks negative acks dupacks CMPE 257 Winter 11 16 Link Level Retransmissions Issues How much time is required to trigger link layer retransmission Small fraction of end to end TCP RTT Multiple of end to end TCP RTT Should link layer deliver packets as they arrive or deliver them in order Link layer may need to buffer packets and reorder if necessary so as to deliver packets in order CMPE 257 Winter 11 17 Link Layer Schemes Summary When is a reliable link layer beneficial to TCP performance If it provides almost in order delivery and TCP retransmission timeout large enough to tolerate additional delays due to link level retransmits CMPE 257 Winter 11 18 Cross Layer Approaches Link layer error recovery Link layer retransmission TCP awareness TCP unawareness Split connection CMPE 257 Winter 11 19 TCP Aware Link Layer CMPE 257 Winter 11 20 Snoop Protocol Balakrishnan95 Retains local recovery of Split Connection approach Link level retransmissions Differs from split connection schemes End to end semantics retained Soft state at base station CMPE 257 Winter 11 21 Snoop Protocol Per TCP connection state TCP connection application application transport transport network network link link link physical physical physical FH BS application transport rxmt wireless CMPE 257 Winter 11 network MH 22 Snoop Protocol Buffers data packets at base station Data sent by FH not yet ack d by MH Allow link layer retransmission When dupacks received by BS from MH or local timeout retransmit on wireless link if packet in buffer Prevents fast retransmit by TCP sender at FH by suppressing dupacks at BS CMPE 257 Winter 11 23 Snoop Example 35 36 TCP state maintained at link layer 37 38 40 39 38 FH 37 BS 34 MH 36 Example assumes delayed ack every other packet ack d CMPE 257 Winter 11 24 Snoop Example 35 39 36 37 38 41 40 34 39 38 36 CMPE 257 Winter 11 25 Snoop Example 37 40 38 39 42 41 40 36 39 36 dupack Duplicate acks are not delayed CMPE 257 Winter 11 26 Snoop Example 37 40 38 41 39 43 42 41 36 40 36 36 Duplicate acks CMPE 257 Winter 11 27 Snoop Example 44 37 40 38 41 39 42 43 FH 37 41 BS Dupack triggers retransmission of packet 37 from base station BS needs to be TCP aware to be able to interpret TCP headers Discard dupack MH 36 36 36 CMPE 257 Winter 11 28 Snoop Example 45 37 40 38 41 39 42 44 43 42 37 36 36 36 36 CMPE 257 Winter 11 29 Snoop Example 46 37 40 43 38 41 44 39 42 45 43 42 36 TCP sender does not fast retransmit 41 36 36 36 CMPE 257 Winter 11 30 Snoop Example 47 37 40 43 38 41 44 39 42 45 46 44 43 41 TCP sender does not fast retransmit 36 36 36 36 CMPE 257 Winter 11 31 Snoop Example 42 45 43 46 44 48 47 45 FH 44 BS 41 MH 43 36 36 36 36 CMPE 257 Winter 11 32 Performance bits sec 2000000 1600000 1200000 base TCP Snoop 800000 400000 0 no error 256K 128K 64K 32K 16K 1 error rate in bytes 2 Mbps Wireless link CMPE 257 Winter 11 33 Snoop Protocol Advantages Snoop prevents fast retransmit from sender despite transmission errors and out of order delivery on the wireless link If wireless link delay bandwidth product less than 4 packets simple TCPunaware link level retransmission scheme can suffice Since delay bandwidth product is small retransmission scheme can deliver lost packet without causing MH to send 3 dupacks CMPE 257 Winter 11 34 Snoop Protocol Advantages Higher throughput can be achieved Local recovery from wireless losses Fast retransmit not triggered at sender despite out of order link layer delivery End to end semantics retained Soft state at base station Loss of the soft state affects performance but not


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