CPSC 689: Discrete Algorithms for Mobile and Wireless Systems

1. CPSC 689: Discrete Algorithms for Mobile and Wireless Systems Spring 2009 Prof. Jennifer Welch

2. Lecture 2 • Topics: • Introduction • Physical Layer • MAC Protocols part 1 • Sources: • Schiller, Ch 1-3 • Balakrishnan, Ch 11 • Vaidya, Ch 1-2, 4-5 • MIT 6.885 Fall 2008 slides Discrete Algs for Mobile Wireless Sys

3. Wireless Computing Vision • Future of computing includes • portable computers • wireless communication • Why portable computers? Many devices are designed to move with people: • cell phones, PDAs, cars, planes • Why wireless communication? • quick, for temporary purposes (e.g., tradeshow) • unintrusive, for delicate situations (e.g., historic bldg) • useful when no infrastructure (countryside, rugged terrain, after a natural disaster) Discrete Algs for Mobile Wireless Sys

4. Applications • car of the future: cars driving in same area build a local ad-hoc network, use to learn about emergencies, keep safe distance • emergencies: ambulance can send info about injuried people to hospital from accident scene • business: traveling salesman can keep laptop in constant synch with company's database • infotainment: as you travel, get up-to-date info about nearby goods and services; buy tickets, etc. Discrete Algs for Mobile Wireless Sys

5. From Vision to Reality • System support for such applications is in its infancy • Open research areas from Schiller book: • handle interference of radio transmissions • use radio frequencies more efficiently • political and social issues regarding control of the spectrum • tolerate high delays and variation in delays • security (easier to eavesdrop on wireless) • coordinate access to shared medium well • routing, service discovery, etc. scalably and reliably Discrete Algs for Mobile Wireless Sys

6. Protocol Stack • Physical layer • convert bit stream into (analog) signals and back • Data link layer • provide reliable connection between a sender and one or more receivers (w/in range) • Network layer (cf. IP) • route packets from sender to receiver (not necessarily w/in range) • Transport layer (cf. TCP and UDP) • establishes an end-to-end connection • Application layer Discrete Algs for Mobile Wireless Sys

7. Application Application Application Transport Transport Transport Network Network Data link Data link Physical Physical Protocol Stack Network Data link Physical radio radio not every node needs every layer Discrete Algs for Mobile Wireless Sys

8. Implementing the Protocol Stack • Physical layer: signals, antennas, etc. • Data link layer: various "medium access control" (MAC) protocols developed to help nodes coordinate when they transmit to reduce likelihood of interference • Network layer: Extensions to IP to deal with mobility have been developed. • addressing, routing, device location, handover between networks • Transport layer: Extensions to TCP have been developed. • quality of service, flow control, congestion control • Applications: new ones ("find closest parking place") • service location, support for multimedia, adapt to variations in transmission characteristics Discrete Algs for Mobile Wireless Sys

9. Our Use of the Protocol Stack • Use it as a rough organizing principle. • Won't focus on mobile versions of IP or TCP • Use general idea of layers of abstraction • Broader perspective of structuring mobile and wireless systems • not just mimicking wired Internet Discrete Algs for Mobile Wireless Sys

10. Physical Layer: Overview • Mobile devices communicate using radio broadcasts, over radio spectrum. • Only a limited set of frequencies for transmission. • Communicating devices must share a common medium. • Concurrent communications by nearby nodes may interfere with each other, so that a receiver may hear garbled signals. • Antennas provide the coupling between the transmitter and space, and between space and the receiver. • What is actually transmitted is an analog signal. • Discrete information has to be encoded into analog signals. Discrete Algs for Mobile Wireless Sys

11. Physical Layer • Wireless transmission uses certain frequencies of the electromagnetic spectrum • very low: submarines, underwater • infrared: connecting laptops and PDAs • Data is encoded in signals • Signals in radio transmission are usually sine waves • Amplitude, frequency and/or phase shift of a sine wave are changed to represent different information: called modulation Discrete Algs for Mobile Wireless Sys

12. Physical Layer: Antennas • Antennas convert electromagnetic energy between space and a wire. • Ideal antenna radiates equal power in all directions from a point in space • transmission: receiver gets signal with sufficiently low error rate • detection: receiver can detect signal but error rate is too high • interference: receiver cannot detect signal but signal may interfere with other xmissions by adding to background noise transmission detection interference Discrete Algs for Mobile Wireless Sys

13. Physical Layer: Attenuation • In a vacuum, received power is proportional to 1/d2, where d is distance of receiver from sender • signal travels away from sender at speed of light • signal is a wave with spherical shape • sphere keeps growing and energy is equally distributed over the sphere's surface • surface area s = 4  d2 • In non-vacuum, signal decreases even faster due to atmosphere ("path loss" or "attenuation") • exponent between 2 and 4 Discrete Algs for Mobile Wireless Sys

14. Physical Layer: Propagation • Types of propagation behaviors: • groundwave (< 2 MHz): follow earth's surface; can propagate long distances, penetrate objects (ex: submarine communication) • sky wave (2-30 MHz): waves bounce b/w ionosphere and earth's surface, traveling around world (ex: short wave radio) • line-of-sight (> 30 MHz): waves follow a straight line (ex: mobile phones, satellites) • Obstacles are problem for line-of-sight: • blocked, reflected, refracted, scattering, diffraction • solution: additional antennas to fill in coverage gaps Discrete Algs for Mobile Wireless Sys

15. Physical Layer: Propagation • Because of all these physical effects, radio signal behavior is highly variable • depends on type of antenna and environment • Example problem: 2-ray ground propagation model: Discrete Algs for Mobile Wireless Sys

16. Physical Layer: Bottom Line • Not every message sent is received • Loss due to noise and interference • Not easy to model in a realistic way • Mathematical models for propagation are not accurate representations of real channel behavior. • In practice, we want algorithms that can adapt to real channel characteristics. • Models are useful mainly for analysis and simulation: get general idea of algorithms’ behavior, in some ideal cases Discrete Algs for Mobile Wireless Sys

17. Physical Layer: Multiplexing • Share the electromagnetic spectrum w/o undue interference along several dimensions: • space, time, frequency, code • Space division: senders are so far apart they don't interfere • Ex: FM radio stations in different towns w/ same frequency (90.9) • Disadvantages: wastes space, what if senders are close to each other? Discrete Algs for Mobile Wireless Sys

18. Physical Layer: Multiplexing • Frequency division: divide spectrum into several non-overlapping frequency bands • Ex: radio stations in same town use different frequencies (90.9 vs. 89.1) • Disadvantages: wastes frequency (unless senders transmit all the time); fixed assignment of frequency to sender is inflexible and limits number of senders Discrete Algs for Mobile Wireless Sys

19. Physical Layer: Multiplexing • Time division: all senders use same frequency but at different times • Ex: different radio shows on the same station but at different times • Disadvantages: need precise synchronization; receiver has to listen at right time • Advantage: can assign more sending time to senders with heavier load Discrete Algs for Mobile Wireless Sys

20. Physical Layer: Multiplexing • Code division: • all users use same frequency at same time, each user has different "code". • With right choice of codes, transmissions can be done simultaneously • constructive interference properties of radio signals allow the codes to be separated at receives • Advantages: code space is huge, good protection against interference and tapping • Disadvantages: receiver must know code and separate the desired information from background noise; receiver must be synchronized with sender Discrete Algs for Mobile Wireless Sys

21. Data Link Layer: Overview • Medium Access Control (MAC) protocols try to synchronize when nearby nodes transmit, in order to reduce the likelihood of interference • The "medium" is the communication resource • Purpose: Achieve relatively reliable local communication of packets (fixed-length messages) among nearby devices. • Both point-to-point and broadcast. • Reasonable throughput (capacity). • Reasonable fairness to each transmitter and receiver. Discrete Algs for Mobile Wireless Sys

22. Challenge for MAC Protocols • Communication provided by physical layer is not very reliable: • Messages might not get delivered, because of noise or interference (collisions). • MAC layer should improve the reliability. • Won’t make it perfect, in spite of many tricks. • Main job of MAC layer: Manage contention among nearby transmitters and receivers. • Q: What are reasonable statements of the guarantees of a MAC layer? • Probabilistic delivery guarantees? Conditional? • Layer should be efficiently implementable. • Should support higher-level programming. Discrete Algs for Mobile Wireless Sys

23. Reinventing the Wheel? • Most of the complication is with schemes based on time division multiplexing. • There are MAC protocols in wired networks (ex: Ethernet) • Must they be changed to work in wireless networks? If so, how? Discrete Algs for Mobile Wireless Sys

24. CSMA/CD MAC Protocol for (Wired) Ethernet • Carrier Sense Multiple Access with Collision Detection • sender senses the medium (wire) to see if it is free • if busy, sender waits until it is free • once medium is free, sender starts transmitting data and continues to listen • if sender detects a collision while sending, it stops and sends a jamming signal • Ethernet hardware allows collision detection by sender Discrete Algs for Mobile Wireless Sys

25. A B C Anomaly: Hidden Terminals • Consider three wireless devices, A, B, and C, such that • A and C can both reach B, • A and C cannot reach each other, or even detect each other. • A and C can both start transmitting to B. • Since A and C cannot hear each other, they will not know anything is wrong. • But B receives both transmissions, garbled. • A and C are “hidden” from each other. • Problem does not occur if detection range is > 2X transmission range. Discrete Algs for Mobile Wireless Sys

26. A B C D Anomaly: Exposed Terminals • Consider four wireless devices, A, B, C, and D, such that • B can reach A, C can reach D, • B and C can detect each other. • A cannot detect C, D cannot detect B. • B might want to transmit to A, and C to D. • Since they hear each other’s transmissions, they might decide they should not both transmit. • But this would be OK: they would not interfere. • B and C are “exposed” to each other. Discrete Algs for Mobile Wireless Sys

27. Wireless MAC Protocol: Fixed TDM • Algorithm: • allocate time slots in a fixed pattern • just wait for your time slot to send • Evaluation: • Gives fixed bandwidth: inefficient for bursty data or asymmetric connections • Requires time synchronization between nodes Discrete Algs for Mobile Wireless Sys

28. Time Synchronization • For wireless local area networks (LANs), IEEE 802.11 standard: • local nodes synchronize to one node, the beacon • if there is infrastructure, beacon can be specified • in ad-hoc case, use random backoff so that only one node attempting to be a beacon succeeds • Gives local synchronization, which is enough for a LAN • Later we will see more general, and more complicated, methods Discrete Algs for Mobile Wireless Sys

29. Transmission Window of vulnerability Trivial MAC protocol: Pure Aloha • When packet arrives at transmitter, transmitter immediately sends. • Resolving collisions is responsibility of higher layer • Q: What is the probability of a transmission succeeding? • Window of vulnerability for a transmission: • Interval in which a transmission from another sender can interfere with reception. • 2L, where L is the length of the transmission interval. • Assumes negligible propagation delay. Discrete Algs for Mobile Wireless Sys

30. Transmission Window of vulnerability Slotted Aloha • Assumes every transmission takes time L. • Instead of transmitting at arbitrary points in time, divide time into slots of length L and transmit only on slot boundaries. • Reduces window of vulnerability by a factor of 2, to one slot: • Requires synchronized clocks. • If they are approximately synchronized, with bounded skew, we must increase the slot size to compensate. • Synchronized clocks are not so easy to achieve in large ad hoc networks. Discrete Algs for Mobile Wireless Sys

31. Success Probability in Slotted Aloha • Simplifying assumptions: • Each transmission takes one slot. • n nodes • Each has probability p of transmitting at each slot. • Probability that a given transmission succeeds: (1-p)n-1. • Probability that, in a given slot, a given node transmits successfully: p (1-p)n-1 . • Throughput = expected number of successful transmissions per slot: n p (1-p)n-1 . • Maximize throughput when p = 1/n, limiting value is 1/e. Discrete Algs for Mobile Wireless Sys

32. Success Probability in Unslotted Aloha • Probability that a given transmission succeeds: (1-p)2(n-1) Because each other sender must avoid 2 “slots”. • Probability that, in a given slot, a given node transmits successfully: p (1-p)2(n-1) • Throughput = expected number of successful transmissions per slot: n p (1-p)2(n-1) . • Maximize throughput when p = 1/(2n-1), limiting value is 1/(2e). • Moral: Synchronizing sending on slot boundaries doubles the throughput. Discrete Algs for Mobile Wireless Sys

33. Techniques for Improving Throughput • Carrier sensing • Busy-tones • Link-layer Acks • Reservation-based strategies • Acks and reservations • Reducing collision probability • p-persistence • Backoff strategies • TDMA Discrete Algs for Mobile Wireless Sys

34. Carrier Sensing • Determine whether the channel seems to be busy, before starting to transmit. • Sense the energy on the channel, see if it exceeds the Carrier Sense (CS) Threshold. • Choice of threshold is flexible: • Higher: • More spatial reuse • Smaller signal-to-noise ratio (SNR), so reliability of communication is worse. • Lower: • Less spatial reuse • Larger SNR, so reliability is better. • Tradeoff: Overall network throughput depend both on spatial reuse and channels’ reliable transmission rates. • Used in practical protocols like 802.11. Discrete Algs for Mobile Wireless Sys

35. A B C D A B C Problems with Carrier Sensing • Doesn’t avoid all collisions. • Hidden terminals: • C does not detect that A is transmitting. • Could lower threshold, but would get more “false positives”. • Exposed terminals: C detects that B is transmitting. Discrete Algs for Mobile Wireless Sys

36. Problems with Carrier Sensing • The problem: What we really care about is collisions at the receiver, but carrier sensing checks for collisions at the sender. • In wired networks, like Ethernet, these are pretty much the same, but they’re different in wireless networks. Discrete Algs for Mobile Wireless Sys

37. Busy-Tones • A simple strategy for avoiding collisions at a receiver. • Receiver who is successfully receiving a transmission broadcasts a special “busy tone” on a separate channel. • Other nodes that receive the busy tone delay transmission. • Requires complex hardware to receive message and broadcast busy-tone at the same time. Discrete Algs for Mobile Wireless Sys

38. A B C Busy-Tones • Solves hidden terminal problem: • When B is receiving from A, its busy tone reaches C. • Still get collision if A and C start transmitting at just the same time. • But not very likely. Discrete Algs for Mobile Wireless Sys

39. Link-Layer Acks • Messages sometimes fail to get through. • So it makes sense to retransmit. • But how does a sender know whether its message got through? • If the message has single intended target node, can use an Ack-based protocol: • Receiver sends an Ack message immediately after receiving a message. • Sender sets timeout to a little more than the normal round-trip time, resends if timeout expires. • Use sequence numbers for repeated receipts of the same data packet. • Retransmit only a bounded number of times. • To maintain some predictability for message delays. • Also, because in mobile setting, the receiver could have moved away. Discrete Algs for Mobile Wireless Sys

40. Protecting the Acks • Need collision avoidance for Ack messages as well as the data messages. • Loss of an Ack causes the data message to be retransmitted, costly. • Solution 1: Defer for longer • Nodes that sense the channel as busy (using physical carrier sensing) defer sending for a while after the channel becomes free. • Enough time for the sender to receive an Ack. • Solution 2: Busy-tones for Acks • Nodes that sense the channel as busy (using physical carrier sensing) defer sending for a very small time after the channel becomes free. • Also, recipient of an Ack sends busy-tones, and anyone hearing a busy-tone defers sending, as usual. Discrete Algs for Mobile Wireless Sys