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Transcript
More on the link layer Logical Link Control (LLC) Medium Access Control (MAC) SMU CSE 4344 link layer functions • services • • • • • – un-ACKed connectionless – ACKed connectionless – ACKed connection oriented framing encoding error control flow control (vs. congestion control?) MAC SMU CSE 4344 point-to-point vs. broadcast media • point-to-point – PPP for dial-up access – point-to-point link between Ethernet switch and host • broadcast (shared wire or medium) – traditional Ethernet – 802.11 wireless LAN SMU CSE 4344 data link protocols (logical) • unrestricted simplex • simplex stop-and-wait • simplex for noisy channel – discussion • sliding window protocols SMU CSE 4344 sliding window protocols • piggybacked ACKs – less overhead (bandwidth, interrupts, buffering, ...) – how to deal with timeouts? • sequence numbers • sending window • receiving window SMU CSE 4344 1-frame sliding window SMU CSE 4344 1-bit sliding window (seq#, ACK#, pkt#); ACK#: last OK frame SMU CSE 4344 pipelining • idea – do not block transmitter during the roundtrip time – increase the window size • what happens with errors – go back n – selective repeat SMU CSE 4344 go back n • if error – – – – – SMU receiver discards subsequent frames no ACKs for the discarded frames receiver window size = 1 transmitter times-out, resends unACKed frames inefficient if the error rate is high CSE 4344 selective repeat • if error – – – – – SMU receiver still stores the subsequent good frames transmitter retransmits the bad frame receiver window size > 1 efficient at higher error rate trade-off between bandwidth and buffer space CSE 4344 sliding window schemes “go back n” (a) “go back n” (frames received out of sequence discarded) (b)( “selective repeat” (frames received out of sequence buffered) a (after error, receiver may NAK frame 2 to short-circuit sender timeout) SMU CSE 4344 3-bit sequence numbers SMU CSE 4344 line utilization • b = channel capacity in bits per second • f = frame size in bits • R = round-trip propagation time • frame transmission time = (f / b) seconds • line utilization = f / (f + bR) • if f < bR, then efficiency < 1/2 SMU CSE 4344 window size • sender needs n buffers for window size n • go back n – sender needs enough buffers for • timeout • RTT of frame + NACK • selective repeat – window size = floor((maxseq+1)/2) – why? • optimal window size, sequence#s SMU CSE 4344 (review this in detail) protocol specification by FSM (a) state diagram; key to state ovals: (SRC), S in {0,1}, R in {0,1}, C in {0,1,A,-} (b) transition chart (from Tanenbaum text) SMU CSE 4344 (review this in detail) Petri net for simplex “wait for ACK” • • • • [Tanenbaum 4/e, 233] places, tokens, transitions, input/output arcs tokens not conserved no composite states: sender, receiver, channel separated • transitions may be viewed as a “grammar” SMU CSE 4344 multiple access protocols • single shared broadcast channel – multiple nodes can speak at once – collisions lead to garbled data • multiple access protocol (medium access control) – distributed algorithm for sharing the channel – algorithm determines which node can transmit SMU CSE 4344 types of MAC • static bandwidth allocation: channel partitioning – – – – TDM FDM CDMA (see handout) problems? • deterministic sharing – token-passing – polling – reservations, scheduling • contention – random access: allow collisions, and then recover – ALOHA, CSMA, more SMU CSE 4344 “taking turns” MAC protocols polling token passing • master node “invites” slave nodes to transmit in turn • control token passed from one node to next sequentially • concerns: – polling overhead – latency – single point of failure (master) SMU • token message • concerns: – token overhead – latency – single point of failure (token) CSE 4344 token rings • (a) transmit token after frame is sent • (b) transmit token while frame is being stripped SMU CSE 4344 ring topology • self-healing ring (SHR) SMU CSE 4344 random access protocols • when node has packet to send – transmit at full channel data rate – no a priori coordination among nodes • >= 2 nodes transmit concurrently ➜ “collision” • random access MAC protocol specification – collision detection – collision recovery • examples – ALOHA, slotted ALOHA – CSMA, CSMA/CD, CSMA/CA SMU CSE 4344 key ideas of random access • station model – n independent stations (nodes, terminals) • single channel – all transmission/reception on shared channel – nodes have equivalent ability – node priority may vary dynamically • time – continuous – slotted (discrete, timed intervals, “master clock”) SMU CSE 4344 key ideas of random access • carrier sense (or not) – listen before speaking, and don’t interrupt – check if someone else is already sending data ... – … wait till the other node is done • collision detection (or not) – if someone else starts talking at the same time, stop – if the data on the wire is garbled ... – ... another node is transmitting, too, so stop • randomness – don’t start talking again right away – wait for a random time before trying again SMU CSE 4344 pure ALOHA in pure ALOHA, frames are transmitted at completely arbitrary times temporal collisions destroy colliding frames send when data arrives; if collision, random delay, resend SMU CSE 4344 ALOHA Vulnerable period for the shaded frame. shaded frame vulnerability period modeled as two frame times intuition: collisions of partially-overlapping frames slotted ALOHA: during contention, frames collide in some single slot/frame time SMU CSE 4344 slotted ALOHA assumptions operation • all frames same size • when node obtains fresh frame, transmits in next slot • time divided into equal slots (time to transmit a frame) • nodes start to transmit frames only at start of slots • nodes are synchronized • if two or more nodes transmit, all nodes detect collision SMU • no collision: node can send new frame in next slot • collision: node retransmits frame in each subsequent slot with probability p until success CSE 4344 pure vs. slotted ALOHA Throughput versus offered traffic for ALOHA systems. max throughput: ALOHA 1/(2e) ~ 18%; slotted ALOHA 1/e ~ 37% SMU CSE 4344 CSMA: carrier sensing multiple access • 1-persistent CSMA: – – – – if idle, send; ... ; if collision, random delay, sense ... propagation delay → collision “two nodes waiting for idle” → collision idea behind Ethernet LAN protocol • non-persistent CSMA: – if idle, send; else, random delay • p-persistent CSMA (slotted time): – if idle, send with probability p; if collision, random delay – slotted transmission discipline SMU CSE 4344 persistent and non-persistent CSMA non-persistent CSMA: very good throughput under high load 0.01-persistent CSMA: best throughput tolerance for delay can enhance throughput in chaotic environments SMU CSE 4344 collision-free protocols basic bit-map protocol - contention slots are constant overhead - overhead means less as frames get large SMU CSE 4344 collision-free protocols addresses: (AND of addresses) binary countdown protocol (log2n bits); dash indicates silence. SMU CSE 4344 limited-contention protocols throughput of contention protocols under high load Acquisition probability for a symmetric contention channel. ~ 1/e - motivation for hybrid deterministic/probabilistic protocols - idea: allow contention at low load, use taking-turns at high load SMU CSE 4344 adaptive tree walk protocol tree for eight stations: depth first search, LR nodes - at idle, each station ready to send data signals 1 or not, according to a clever plan - example: only station H ready to send: slot1 = 1 (candidate sender is under 1), s2 = 0 (not under 2), s3 = 0 (not under 6), s4 = 0 (not G), s5 = 1 (H). - for binary tree, sender is chosen in O(log2(n)) time SMU CSE 4344 WDM access protocol (one example out of 100s) Wavelength division multiple access. - fixed data output & control input - tunable data input & control output - on control channel: control slots; on data channel, status slot - classes of traffic: CBR, VBR, datagram SMU CSE 4344 IEEE 802.3: Ethernet • • • • • • • [Metcalfe, Boggs, 1976]: first LAN, “the Ethernet” TCP/IP/Ethernet: a connectionless stack simple to use, reliable, cheap, scalable LAN collision domains (broadcast) (bus, hub) store-and-forward switches, point-to-point links 10 Mbps, 100 Mbps, gigabit, 10 gigabit becoming very rare for network paths not to traverse any Ethernet links SMU CSE 4344 “original flavor” 10 Mbps Ethernet common kinds of Ethernet cabling only twisted pair and fiber are still being deployed (except in specialized environments) SMU CSE 4344 10 Mbps (“plain”) Ethernet PHY (a) and (b) (10base5, 10base2) seldom deployed (c) 10baseT hub is a collision domain SMU CSE 4344 10 Mbps Ethernet PHY topologies - broadcast medium - same logical topologies - no loops (rings) allowed SMU -no path has > 4 repeaters - network diameter <= 2500 m CSE 4344 intermission SMU CSE 4344 Ethernet MAC sublayer protocol - no two nodes are farther apart than A and B - τ is the diameter of the network, the one-way propagation time between the farthest nodes SMU CSE 4344 Ethernet: CSMA/CD (Collision Detection) contention slot time = 2τ = max(signal round-trip time) (10 Mbps Ethernet slot = 51.2 microsec = 512 100-nanosec-wide bits) contention period: series of slot-length collisions/jamming frames half-duplex: cannot receive while listening for own transmission collision sense; if idle, send; if collision, abort, random delay SMU CSE 4344 performance of Ethernet efficiency of 10 Mbps Ethernet, 512-bit slot times SMU CSE 4344 switched Ethernet vs. hub half-duplex “collision domain” full-duplex point-to-point links SMU CSE 4344 100 Mbps (“fast”) Ethernet cabling T4: 4 each, “cat 3” unshielded twisted pairs, 3 pairs simplex forward, 1 pair simplex reverse (dynamic) ternary encoding (trits, not bits) TX: 2 each cat 5 unshielded twisted pairs (opposing simplex) FX: 2 multimode fiber (opposing simplex), point-to-point links only SMU CSE 4344 gigabit Ethernet cabling 100 m and 25 m copper segments used, e.g., in data center or POP of ISP SMU CSE 4344 Ethernet MAC sublayer framing (a) DIX (Digital, Intel, Xerox) (b) IEEE 802.3 preamble: for receiver clock sync dest addr: 0* unicast, 1* multicast, all 1s broadcast type: network protocol to call at dest, OR length: # data bytes (“type” embedded in data) pad: frame length (without preamble) >= 64 bytes = 512 bits SMU CSE 4344 Why MAC is a sublayer • Ethernet (one of the MAC protocols) interfaces directly to network layer (IP) • the Ethernet MAC offers best-effort, no-guarantee, datagram service • this is great for TCP/IP, nothing else is needed • but, other network layer protocols expect link layer error control and flow control services • IEEE 802.2 (LLC) supports these services, built on various MAC sublayers (e.g., Ethernet) SMU CSE 4344 IEEE 802.2: logical link control - Ethernet MAC sends best effort datagrams - LLC supports flow-control & error-control PHY PHY (a) network layer sees the same LLC, regardless of type of MAC (b) LLC encapsulates network layer packet, MAC encapsulates LLC frame before passing to PHY SMU CSE 4344 wireless LANs • IEEE 802.11 (“WiFi”) • Distributed Coordination Function (DCF ) – CSMA-CA • Point Control Function (PCF) – centrally controlled by basestation (access point) • short-range RF (rooms, battlefields) • ad hoc and basestation flavors • many PHY layer options – 802.11, 802.11a, 802.11b, 802.11g, 802.11n (pre-std) SMU CSE 4344 The 802.11 Protocol Stack 1-2 Mbps 54 Mbps 11 Mbps 54 Mbps 1997 SMU 1999 CSE 4344 2001 CSMA fails off the wire why CSMA falls short in packet radio networks and mobile ad hoc networks C wants to send to B C wants to send to D channel sounds clear to C channel sounds busy to C C cannot hear A D cannot hear B the “hidden station problem” the “exposed station problem” (not “unhidden station problem”) SMU CSE 4344 wireless LAN protocols (CSMA-CA) MACA(W), multiple access collision avoidance: A sends RTS to B, B sends CTS to A all potential “interrupters” hear B's CTS, wait for frame (“W”ireless: sense first, ACK every frame, sophisticated backoff) SMU CSE 4344 802.11 MAC sublayer protocol The use of virtual channel sensing using CSMA/CA. - C hears A’s RTS … D hears B’s CTS (NAV: transmitter quiet time) - notice how politely C and D each set aside time for A and B SMU CSE 4344 802.11 MAC sublayer protocol A fragment burst. - A sends a burst to B … C and D wait for it - after each data or control frame, a system of delay intervals … SMU CSE 4344 802.11 delay timing • how can PCF and DCF protocols coexist? • end of frame or ACK starts series of timers • {Short, PCF, DCF, Expanded} InterFrame Spacing • Short for burst fragment, receiver ACK, or CTS • PCF for central control (beacons, polling) • DCF for contention (RTS) • Extended for bad frame reporting (NAK) SMU CSE 4344 802.11 MAC sublayer protocol PCF and DCF coexist in a single collision domain: Interframe spacing in 802.11. fragment CTS FRAME dead ACK central ctl RTS NAK ACK the starting guns go off at different times for different frame types SMU CSE 4344 RF signaling • • • • • RF signal strength at node's own antenna 2-antenna implementation? shared channel RF signal strength from distant antennae how to detect? – interference – fading – multipath SMU CSE 4344 IEEE 802.11 • access point infrastructure largely 802.11 – 100 Million 802.11 chipsets per annum (out of date statistic) – strong application development efforts • IEEE 802.11 spec: CSMA-CA – RTS/CTS channel reservation, ACK • explicit ACK – CSMA sender will not hear interference, fading, multipath • contention: short RTS frames, collisions waste less • if sender’s CTS times out, it knows the RTS failed – random backoff (countdown proceeds while channel is idle) SMU CSE 4344 802.11 distribution services • association (connect to access point cell) – beacons in the “jungle”; “there can only be one” – next, DHCP discovery • disassociation • reassociation (cell-to-cell handover) • distribution (how to route frames) • integration (802.11 external network format) SMU CSE 4344 802.11 intracell services • authentication (challenge frame, key, encryption) – if invoked, a pre-condition for association • deauthentication • privacy (data encryption) • data delivery (best effort) SMU CSE 4344 802.11 data frame - type: data, ctl, mgt; subtype: RTS, CTS, probe (scanning for new AP) - to DS/from DS: activation of address 3, 4 for “distribution system” APs - MF: “more frags”; retry; more (frames) - duration, sequence SMU CSE 4344 broadband wireless comparison ... • 802.11 (WiFi) – mobile Ethernet LAN – centralized infrastructure (APs and cell architecture) – or, distributed architecture • ad hoc • mobile ad hoc (MANET) – – – – SMU best effort delivery short range, half-duplex power concerns limited budget (commodotized) CSE 4344 ... broadband wireless comparison • 802.16 (WiMax) – wireless “local loop” for buildings – metro area coverage, full duplex – many users aggregated per endpoint – connection oriented – FEC (Hamming codes), security – base station control (centralized control) – fixed, directional antennas SMU CSE 4344 802.16 Protocol Stack modulation schemes vary with range to end-points (what is wrong with this picture?) SMU CSE 4344 The 802.16 Physical Layer The 802.16 transmission environment. SMU CSE 4344 The 802.16 Physical Layer - shown: TDD (time division duplexing) PHY frames - not shown: FDD (frequency division duplexing) - millimeter RF waves are not omnidirectional SMU CSE 4344 The 802.16 Frame Structure (a) A generic frame. SMU (b) A bandwidth request frame. CSE 4344 802.16 MAC sublayer protocol service classes • constant bit rate service (regular slots) • real-time variable bit rate service (regular polling) • non-real-time variable bit rate service (frequent polling) • best effort service (contend for request slots) • aggregator switch at subscriber building may negotiate with base station for all users, and arbitrate received bandwidth between users SMU CSE 4344 personal area networks (PANs) • Bluetooth (Ericsson, IBM, Intel, Nokia, Toshiba) • “cable replacement” • specifies complete networking stack • TDM; 10 m; 2.4 Ghz FHSS; 79 1-MHz channels • IEEE 802.15 – only PHY & LL • interferes with 802.11 (2.4 GHz) • master/slave “piconets” • slaves can bridge piconets to form “scatternets” SMU CSE 4344 remarks • networks run on various link layer technologies – point-to-point links vs. shared media – wide varieties within each class • link layer performs key services – encoding, framing, and error detection – optionally error correction and flow control • shared media introduce interesting challenges – decentralized control over resource sharing – partitioned channel, taking turns, and random access – Ethernet as a wildly popular example • next: switches and bridges SMU CSE 4344 summary/glossary SMU CSE 4344