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Random Access to Wireless Networks

Random Access to Wireless Networks. Jean-Paul M.G. Linnartz Nat.Lab., Philips Research. Random Access. Many terminals communicate to a single base station Fixed multiple access methods (TDMA, FDMA, CDMA) become inefficient when the traffic is bursty. Random Access works better for

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Random Access to Wireless Networks

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  1. Random Access to Wireless Networks Jean-Paul M.G. Linnartz Nat.Lab., Philips Research

  2. Random Access • Many terminals communicate to a single base station • Fixed multiple access methods (TDMA, FDMA, CDMA) become inefficient when the traffic is bursty. • Random Access works better for • many users, where .. • each user only occasionally sends a message Jean-Paul Linnartz

  3. Suitable Protocols • ALOHA • Carrier Sense • Inhibit Sense • Collision Resolution • Stack Algorithm • Tree Algorithm • Reservation methods • Reservation ALOHA • Packet Reservation Multiple Access Jean-Paul Linnartz

  4. ALOHA Protocol • Any terminal is allowed to transmit without considering whether channel is idle or busy • If packet is received correctly, the base station transmits an acknowledgement. • If no acknowledgement is received by the mobile, 1) it assumes the packet to be lost 2) it retransmits the packet after waiting a random time Jean-Paul Linnartz

  5. Any terminal is allowed to transmit without considering whether channel is idle or busy If packet is received correctly, the base station transmits an acknowledgement. If no acknowledgement is received by the mobile, 1) it assumes the packet to be lost 2) it retransmits the packet after waiting a random time, usually with probability Pr in every slot. i t b ALOHA Protocol Jean-Paul Linnartz

  6. ALOHA Protocol • Unslotted ALOHA: transmission may start anytime • Slotted ALOHA: packets are transmitted in time slots • Critical performance issue: "How to choose the retransmission parameter?" • Too long: leads to excessive delay • Too short: stirs instability Instability: Number of terminals in backlog grows without bounds Jean-Paul Linnartz

  7. CollisionResolution • Adaptation of random retransmission • global control of Pr (by base station) • doubling the retransmission interval after every failure • Dynamic frame length ALOHA • Stack algorithm • Tree algorithm Jean-Paul Linnartz

  8. Carrier Sense Multiple Access : CSMA • " Listen before talk " • No new packet transmission is initiated when the channel is busy • This reduces the number of collisions • Performance is very sensitive to delays in Carrier Sense mechanism • CSMA is useful if channel sensing is much faster than packet transmission time • satellite channel with long roundtrip delay: just use ALOHA Hidden Terminal Problem: • mobile terminal may not be aware of a transmission by another (remote) terminal. • Solution: Inhibit Sense Multiple Access (ISMA) • Decision Problem: how to distinguish noise and weak transmission? • Solution: Inhibit Sense Multiple Access (ISMA) Jean-Paul Linnartz

  9. Inhibit Sense Multiple Access : ISMABusy Tone Multiple Access : BTMA If busy, base station transmits a "busy" signal to inhibit all other mobile terminals from transmitting Collisions still occur, because of • Signalling delay • New packet transmissions can start during a delay in the broadcasting of the inhibit signal, • Persistent terminals • After the termination of transmission, packets from persistent terminals, awaiting the channel to become idle, can collide. Jean-Paul Linnartz

  10. Throughput - Offered Traffic (S-G) Relation Jean-Paul Linnartz

  11. Common Performance Analysis Assumptions • All packets are of uniform duration, • unit of time = packet duration + guard time • Acknowledgements are never lost • Steady-state operation (stability) • Poisson distributed attempts Steady-state operation: Random waiting times need to be long enough to ensure uncorrelated interference during the initial and successive transmission attempts. This is an approximation: dynamic retransmission control is needed in practice N.B. ALOHA without capture, with infinite population is always unstable Jean-Paul Linnartz

  12. WIRELESS RANDOM-ACCESS Probability of successful reception depends on • Receiver capture performance • Modulation method • Type of coding • Signal processing at the receiver (diversity, equalization, ...) • Propagation: • Distance from the central receiver, path loss, Shadowing • Channel fading and dispersion • Channel noise • Traffic: • Contending packet traffic (from same cell) • Interference from co-channel cells • Initial Access protocol: slotted ALOHA, Carrier Sense (CSMA) or Inhibit Sense Multiple Access (ISMA) • Retransmission policy Jean-Paul Linnartz

  13. Capture effect and Near-far effect • Not all packets in a collision are lost • Nearby terminals have higher probability of success • Unfairness Jean-Paul Linnartz

  14. Jean-Paul Linnartz

  15. Scenario 1: Two separate channels Let’s assume we do time division: Total traffic per cell is G packets per slot Every available slot has 2G traffic because it carries traffic that arrives in two slot periods System Throughput S = 2G exp(-2G) NB: Receiver is idle 50% of time Scenario 2: Common Channel Total traffic per cell is G packets per slot Every available slot has 2G traffic because there are two cells System Throughput S = 2G exp(-2G) or better if receivers can catch different packets Cellular ALOHA Jean-Paul Linnartz

  16. Throughput versus offered traffic in wireless channel • ALOHA (orange), 1-persistent CSMA (green), non-persistent CSMA (blue) Jean-Paul Linnartz

  17. Cellular aspects of packet transmission Circuit-switching: • Performance criterion is outage at cell boundary Packet-switching: • Performance criterion is packet delay • Collisions from within the same cell, and • Interference from outside the cell • Optimum cell reuse” C=1 Jean-Paul Linnartz

  18. DS-CDMA ALOHA Network Under ideal signal separation conditions, DS-CDMA can enhance the capacity Make a fair comparison! • Spreading by N in the same transmit bandwidth implies slot that are N times longer. The arrival rate per slot is N times larger Assumption for simple analysis: • All packets in a slot are successful iff the number of packets in that slot does not exceed the speading gain. Jean-Paul Linnartz

  19. Intuition • Compare the ALOHA system with an embarkation quay • People arrive with Poisson arrival rate l < 1 person per unit of time • Boats of seat capacity N at regular intervals of duration N • Thus: total seat capacity is 1 person per unit of time • The boat sinks and the passengers drown if the number of people exceedsN 6p 1p Jean-Paul Linnartz

  20. “Ideal” CDMA Narrowband Throughput Offered Traffic Intuition and Analysis Case I: N = 1 (ALOHA without spreading) • Boats arrive very frequently • Probability of survival is exp{-G} Case II: Large N (CDMA) • Fewer but larger boats arrive • Average waiting time is N times larger • Probability of survival is larger, because of the law of large numbers • Probability of success = Prob(n£N) = Jean-Paul Linnartz

  21. 6p 1p Jean-Paul Linnartz

  22. 6p 1p Jean-Paul Linnartz

  23. 6p 1p Jean-Paul Linnartz

  24. 6p 1p Jean-Paul Linnartz

  25. 6p 1p Jean-Paul Linnartz

  26. 6p 1p Jean-Paul Linnartz

  27. 6p 1p Jean-Paul Linnartz

  28. 6p 1p Jean-Paul Linnartz

  29. Packet Delay versus Traffic Jean-Paul Linnartz

  30. Slow Frequency Hopping • Narrowband (unspread) transmission of each packet • Receiver threshold remains unchanged • N parallel channels, each with rate 1/N • Traffic load per slot remains unchanged • Advantages: • Frequency diversity: • fading on different carrier uncorrelated • capture probabilities independent • improved performance • Less Intersymbol Interference • Disadvantages • longer delay than transmission at high rate Jean-Paul Linnartz

  31. Conclusions • Wireless channel affects performance of random access scheme • Packets lost due to fading, inter and intracell interference • Signal capture enhances throughput and stability • Packet and circuit-switched data should be treated differently • DS-CDMA in ISM bands does not necessarily help Jean-Paul Linnartz

  32. Bluetooth packet transmission • 1 Mbit/sec • Slots of 625 microsecond • Short packets • Frequency hopping 0 .. 2745 bits 72 54 access code packet header payload Jean-Paul Linnartz

  33. Bluetooth random access scheme • MULTI-SLOT PACKETS 625 s f(k) f(k+1) f(k+2) f(k+3) f(k+4) f(k+5) f(k) f(k+3) f(k+4) f(k+5) f(k) f(k+5) Jean-Paul Linnartz

  34. Physical Link Definition SYNCHRONOUS CONNECTION-ORIENTED (SCO) LINK • circuit switching • symmetric, synchronous services • slot reservation at fixed intervals • packet switching • (a)symmetric, asynchronous services • polling access scheme ASYNCHRONOUS CONNECTION-LESS (ACL) LINK Jean-Paul Linnartz

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