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CENG415 – Communication Networks

This lecture covers the fundamental concepts of multiple access protocols in communication networks, focusing on shared communication links. There are two primary link types: point-to-point and broadcast. We explore how protocols manage simultaneous transmissions, preventing collisions through techniques such as channel partitioning (TDMA, FDMA, CDMA) and random access protocols (ALOHA, CSMA). We discuss the implications of these methods on performance, capacity, and efficiency, emphasizing the need for effective channel sharing mechanisms among nodes.

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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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