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CENG415 – Communication Networks

CENG415 – Communication Networks. Lectures 19 Data Link layer – Multiple access links and protocols. Introduction. Two types of “links”: point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) Old-fashioned Ethernet

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CENG415 – Communication Networks

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  1. CENG415 – Communication Networks Lectures 19 Data Link layer – Multiple access links and protocols

  2. Introduction Two types of “links”: • point-to-point • PPP for dial-up access • point-to-point link between Ethernet switch and host • broadcast (shared wire or medium) • Old-fashioned Ethernet • upstream HFC (hybrid fiber coax ) • 802.11 wireless LAN multiple access protocol • Single shared broadcast channel • Two or more simultaneous transmissions by nodes: interference • collision if node receives two or more signals at the same time

  3. Multiple access protocols multiple access protocol • distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit • communication about channel sharing must use channel itself! • no out-of-band channel for coordination Ideal multiple access protocol Broadcast channel of rate R bps • When one node wants to transmit, it can send at rate R. • When M nodes want to transmit, each can send at average rate R/M • Fully decentralized: • no special node to coordinate transmissions • no synchronization of clocks, slots • Simple

  4. Multiple access protocols • We talk about MAP when a link is shared between more than 2 nodes • A mechanism is needed to manage communication between these nodes • The shared link is called Broadcast channel • Packets in the broadcast channel might he captured by any node • Collision might happen if two or morenodes broadcast at the same time

  5. MAC protocols Three broad classes: • Channel Partitioning • divide channel into smaller “pieces” (time slots, frequency, code) • allocate piece to node for exclusive use • Random Access • channel not divided, allow collisions • “recover” from collisions • “Taking turns” • Nodes take turns, but nodes with more to send can take longer turns

  6. Channel Partitioning MAC protocols Three modes to study: • TDMA: time division multiple access • FDMA: frequency division multiple access • CDMA: Code division multiple access • Capacity is divided equally between nodes • Division done in time or in frequency • TDM eliminates collision • Drawbacks: • A node is limited to part of the capacity of the link even when it is the only node with frames to send (FDMA) • a node must always wait for its turn in the transmission sequence, even when it is the only node with a frame to send (TDMA)

  7. Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access • access to channel in "rounds" • each station gets fixed length slot (length = packet transmission time) in each round • unused slots go idle • example: 6-station LAN, 1,3,4 have packets, slots 2,5,6 idle • TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

  8. Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access • channel spectrum divided into frequency bands • each station assigned fixed frequency band • unused transmission time in frequency bands go idle • example: 6-station LAN, 1,3,4 have packet, frequency bands 2,5,6 idle time frequency bands

  9. Channel Partitioning MAC protocols: CDMA • CDMA assigns a different code to each node • Each node uses its code to encode the data bits it sends • Assuming the receiver knows the sender's code • CDMA allows different nodes to transmit simultaneously and yet have their respective receivers correctly receive a sender's encoded data bits Functionality Bits have two values: -1 and 1 Each bit being sent is encoded by multiplying the bit by a signal Example: Suppose the sender code is (1, 1, 1, -1, 1, -1, -1, -1) In this case M = 8

  10. Channel Partitioning MAC protocols: CDMA

  11. Channel Partitioning MAC protocols: CDMA

  12. Random Access Protocols When node has packet to send • transmit at full channel data rate R • no a priori coordination among nodes two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: • how to detect collisions • how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: • slotted ALOHA • ALOHA • CSMA, CSMA/CD, CSMA/CA

  13. Slotted Aloha Assumptions • all frames same size • time is divided into equal size slots (time to transmit 1 frame) • nodes start to transmit frames only at beginning of slots • nodes are synchronized • if 2 or more nodes transmit in slot, all nodes detect collision Operation • when node obtains fresh frame, it transmits in next slot • no collision, node can send new frame in next slot • if collision, node retransmits frame in each subsequent slot with prob. p until success

  14. Slotted Aloha C: collision E: Empty S: Success Advantages • single active node can continuously transmit at full rate of channel • highly decentralized: only slots in nodes need to be in sync • Simple Disadvantages • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization

  15. Slotted Aloha efficiency • Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send • Suppose N nodes with many frames to send, each transmits in slot with probability p • Probability that node 1 has success in a slot= p(1-p)N-1 • Node 1 success  all the other (n-1) fail each with probability (1-p) • Probability that any node has a success = Np(1-p)N-1 • For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1 • For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37 • At best: channel used for useful transmissions 37% of time!

  16. Pure Aloha (unslotted) • unslotted Aloha: simpler, no synchronization • when frame first arrives • transmit immediately • collision probability increases: • frame sent at t0 collides with other frames sent in [t0-1,t0+1] • Remember that the unit of time is the time needed to send a frame • All frames are of equal size

  17. Pure Aloha Efficiency P(success by given node) = P(node transmits) * P(no other node transmits in [t0-1,t0]) * P(no other node transmits in [t0,t0+1] ) = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) Optimality is achieved with 1/(2e) = .18 Worse than Slotted Aloha

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