Medium Access Links and Protocols Three types of “links”: • point-to-point (single wire, e.g. PPP, SLIP) • broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) Physical Layer • switched (e.g., telephone systems, switched Ethernet, ATM etc) Srinidhi Varadarajan Point-to-Point protocols Modems: Signaling • Modems: • Telephone networks – Work over low bandwidth telephone lines (3000 – Switched hierarchy. Hz) – Local Loop is the last mile interface to customer • Signaling schemes: (why not just use digital premises equipment. (generally referred to in the networking world as the source of all evil) bit patterns?) – Originally involved a physical connection between the – Possible choices: sender and the receiver. • Amplitude modulation (AM) – Nowadays, telephone networks use circuit switched • Frequency modulation (FM or FSK) medium access control • Phase modulation (PSK) • Modems: Digital interface to the world of telephony Modems Signaling RS-232C, RS449: Point-to-Point Communication • RS-232C and RS449 specify physical layer point- • Modern modems use a combination of PSK and to-point serial communication AM • 25 or 9 pin connectors, 15m cable length • Create charts called constellation patterns. – <-3V = 1, >+4V=0, – Multiple bits encoded per signal. – BW: 20Kbps (originally, upgraded now to up to 115Kbps) – Trellis encoding is used to minimize the chance of error. – Main communication occurs using the RTS/CTS Errors cause loss of several bits protocol. • Echo cancellation/suppression • RS-449 is an upgraded RS-232C with 2 modes of – Needed for long-haul voice communication. communication – Prevents full duplex – Unbalanced mode, physically is similar to RS-232C, with – In-band signaling at 2100 Hz is used to inhibit echo common ground signaling. cancellation circuitry. – Balanced mode uses independent ground. Data rate – Newer solution uses end-point resources for echo 2Mbps with lengths up to 60m suppression. 1
Multiple Access protocols Multiple Access protocols • single shared communication channel • two or more simultaneous transmissions by nodes: interference • claim: humans use multiple access protocols – only one node can send successfully at a time all the time • multiple access protocol: • class can "guess" multiple access protocols – distributed algorithm that determines how stations share channel, i.e., determine when station can transmit – multiaccess protocol 1: – communication about channel sharing must use channel itself! – what to look for in multiple access protocols: – multiaccess protocol 2: • synchronous or asynchronous – multiaccess protocol 3: • information needed about other stations • robustness (e.g., to channel errors) – multiaccess protocol 4: • performance MAC Protocols: a taxonomy Channel Partitioning MAC protocols: TDMA Three broad classes: TDMA: time division multiple access • Channel Partitioning • access to channel in "rounds" – divide channel into smaller “pieces” (time slots, • each station gets fixed length slot (length = pkt trans frequency) time) in each round – allocate piece to node for exclusive use • unused slots go idle • Random Access • example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle – allow collisions – “recover” from collisions • “Taking turns” – tightly coordinate shared access to avoid collisions Goal: ef f icient , f air, simple, decent ralized Channel Partitioning MAC protocols: FDMA Channel Partitioning (CDMA) FDMA: frequency division multiple access CDMA (Code Division Multiple Access) • channel spectrum divided into frequency bands • unique “code” assigned to each user; ie, code set • each station assigned fixed frequency band partitioning • unused transmission time in frequency bands go idle • used mostly in wireless broadcast channels (cellular, • example: 6-station LAN, 1,3,4 have pkt, frequency bands satellite,etc) 2,5,6 idle • all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data t i m e • encoded signal = (original data) X (chipping sequence) f r equency bands • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 2
CDMA Encode/Decode CDMA: two-sender interference Slotted Aloha Random Access protocols • time is divided into equal size slots (= pkt trans. • When node has packet to send time) – transmit at full channel data rate R. • node with new arriving pkt: transmit at beginning – no a priori coordination among nodes of next slot • two or more transmitting nodes -> “collision”, • if collision: retransmit pkt in future slots with • random access MAC protocol specifies: probability p, until successful. – how to detect collisions – how to recover from collisions (e.g., via delayed retransmissions) • Examples of random access MAC protocols: – slotted ALOHA – ALOHA – CSMA and CSMA/CD Success (S), Collision (C), Empty (E) slots Slotted Aloha efficiency Pure (unslotted) ALOHA Q: what is max fraction slots successful? • unslotted Aloha: simpler, no synchronization A: Suppose N stations have packets to send • pkt needs transmission: – each transmits in slot with probability p – send without awaiting for beginning of slot – prob. successful transmission S is: • collision probability increases: – pkt sent at t 0 collide with other pkts sent in [t 0 -1, by single node: S= p (1-p) (N-1) t 0 +1] by any of N nodes At best : channel S = Prob (only one transmits) use f or usef ul t r ansmissions 37% = N p (1-p) (N-1) of t ime! … choosing optimum p as n -> infty ... = 1/e = .37 as N -> infty 3
Pure Aloha (cont.) CSMA: Carrier Sense Multiple Access P(success by given node) = P(node transmits) . CSMA : listen before transmit: P(no other node transmits in [p 0 -1,p 0 ] . P(no other node transmits in [p 0 ,p 0 +1] • If channel sensed idle: transmit entire pkt = p . (1-p) . (1-p) • If channel sensed busy, defer transmission P(success by any of N nodes) = N p . (1-p) . (1-p) – Persistent CSMA: retry immediately with … choosing optimum p as n -> infty ... S = t hr oughput = “goodput ” probability p when channel becomes idle (may = 1/(2e) = .18 0.4 cause instability) 0.3 (success r at e) – Non-persistent CSMA: retry after random pr ot ocol const r ains Slot t ed Aloha ef f ect ive channel interval 0.2 t hr oughput ! • human analogy: don’t interrupt others! 0.1 P ur e Aloha 0.5 1.0 1.5 2.0 G = of f er ed load = Np CSMA collisions CSMA/CD (Collision Detection) spat ial layout of nodes along et her net CSMA/CD: carrier sensing, deferral as in CSMA collisions can occur: propagat ion delay means – collisions detected within short time t wo nodes may not year – colliding transmissions aborted, reducing channel hear each ot her’s wastage t ransmission – persistent or non-persistent retransmission • collision detection: collision: – easy in wired LANs: measure signal strengths, ent ire packet t ransmission compare transmitted, received signals t ime wast ed – difficult in wireless LANs: receiver shut off while not e: transmitting role of dist ance and • human analogy: the polite conversationalist propagat ion delay in det ermining collision prob. CSMA/CD collision detection “Taking Turns” MAC protocols channel partitioning MAC protocols: – share channel efficiently at high load – inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols – efficient at low load: single node can fully utilize channel – high load: collision overhead “taking turns” protocols look for best of both worlds! 4
“Taking Turns” MAC protocols Reservation-based protocols Token passing: Distributed Polling: Polling: • time divided into slots • control token passed from • master node “invites” • begins with N short reservation slots one node to next sequentially. slave nodes to – reservation slot time equal to channel end-end propagation • token message transmit in turn delay • concerns: • Request to Send, – station with message to send posts reservation Clear to Send msgs – token overhead – reservation seen by all stations – latency • concerns: • after reservation slots, message transmissions ordered by known – single point of failure (token) – polling overhead priority – latency – single point of failure (master) 5
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