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Transcript
TELE3118 extras For week 3 Looking deeper into ARQ “frames” • “Frame” vs “ack”: – Both are frames (groups of bits); “Frame” means carrying payload data – Both ends might want to transmit data; more complicated. – May be distinguished by a type field in packet header (and/or time of transmission – e.g. WiFi acks) • “Frame” & “ACK” both contain check fields – Frame: To detect bit errors – ACK: s.t. unlikely noise can produce a valid ACK • Sequence number – Definitely needed in data – Avoidable in ACK for stop-and-wait over link? • Necessary for general use: sliding window or network path. Broadcast & flooding • Videos seem to flip these terms • Concepts: – All devices (in an area) should receive the info – Info should be sent on all links of network • All receive => send everywhere, but send everywhere !=> all receive – e.g. switching to destination with unknown location • Elsewhere – “Broadcast addresses” (MAC & IP) – all receive [4-6.7] – 802.1D (Spanning Tree) “floods” – send on all outputs – Flooding link state advertisements [5-5] Wifi retransmission & acks • Ack quickly (0.06ms) when receiver is not broadcast. • Retransmit (in 0.4 to 0.5 ms) if no ACK • Quirks • Frames 10-12: destination is broadcast, but not receiver • Non-binary sequence # & retry flag. Perhaps to allow retransmission of only errored fragments. • See more: http://uluru.ee.unsw.edu.au/~tim/zoo/index.html#wifi_retx 5 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability Slide from Kurose and Ross Copyright © May-17, Tim Moors 6 CSMA with collision detection (CD) Longer detection time gives more confidence interference is due to collision rather than noise => should back off exponentially. Need to ensure that all stations are aware of the collision => transmit a “jam signal” (32 or 48 arbitrary bits) after detecting the collision to ensure that other nodes also detect it and back off. => non-zero abort time. Copyright © May-17, Tim MoorsFigure from Kurose and Ross Ethernet frame size limits • Shared media Ethernet is rare today, but 1970s design choices persist in today’s frame sizes • Minimum: Want to detect collision while transmitting. (= Want to conclude transmission with last bit) => 64B minimum • Maximum: 1500B to limit size of expensive RAM on Network Interface Cards – Extended by later versions (“jumbograms”?) but path MTU discovery rarely uses it 8 Derivation of minimum frame length Consider the first IEEE standard: 10base5 Ethernet: Calculate geographical span: Maximum segment length = 500m Concatenate segments by using repeaters. Maximum span: 5 segments (passing through 4 repeaters) => Maximum LAN span = 2500m Translate into round-trip time: Signal propagates at c2/3 = 2E8m/s Round-trip time = 5km/2E8 = 25s Fudge factor: Double to account for repeater delays: 50s Translate into frame length: 50s = time to transmit 500b conservatively round up to 512b => minimum frame length = 64B Assuming preamble is part of physical layer, DA+SA+Type+Null Data+CRC=18B => pad null data with 46B Copyright © 20/03/14, Tim Moors Efficiency of contentionbased MAC N nodes, each transmitting with probability p P(successful transmission) = Np(1-p)N-1 Differentiate wrt p => optimal p = 1/N • Transmission probability should vary with N! lim (1-1/N)N-1 = 1/e N→∞ i.e. 37% efficiency! (Pure Aloha only 18%!) 63% of time wasted in collisions or all nodes waiting Copyright © May-17, Tim Moors 9 10 Ambiguity • Sender didn’t receive ACK. o Was data frame lost? o Was ACK lost? o A: Don’t care. Add sequence # to enable receiver to determine. • MAC: Why didn’t my transmission work? o Due to collision? => back-off o Due to bit error? => just retransmit immediately o Ethernet: sustain collision through jamming to differentiate from probably transient noise o Wifi: can’t differentiate • TCP: Why didn’t my packet get through? o Due to congestion? => slow down o Due to bit error => retransmit immediately o Critical on wireless networks; ECN might help before congestion causes loss Copyright © May-17, Tim Moors Official 802.11 definition • P. 829 (of 2793!) of 802.11-2012 NAV = Network Allocation Vector – indicates period when channel is known to be busy IFS = InterFrame Space. Short IFS (SIFS) separates ack from data, giving time for propagation & radio turnaround (tx<DCF (Distributed Coordination Function = random access) IFS (DIFS) is longer than SIFS Exposed terminals hear CTS and defer. Chipsets vary in what options they provide Windows 8 on another machine Windows Vista on one machine Link layer addresses • 48bit (usually) – MAC / IEEE / Link layer address – Yes! • EUI-64, Bluetooth etc – Physical/hardware [ipconfig] – No! – Network address [driver] – No!