Mobile Communications

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1 COMP61242 Mobile Communications Lecture 7 Multiple access & medium access control (MAC) Barry Cheetham 16/03/2018 Lecture 7 1

2 Multiple access Communication links by wire or radio generally provide access to several users at once. Multiple access techniques used by mobile phones include: Frequency division multiple access (FDMA), Time division multiple access (TDMA), Code division multiple access (CDMA), Orthogonal frequency division multiple access (OFDMA) Spatial division multiple access (cellular radio) Several of these are combined e.g. 2G-GSM uses FDMA, TDMA & Spatial In principle, provide connection-oriented channels that do not interfere with each other. 16/03/2018 Lecture 7 2

3 Medium access control (mac) Term reserved for wired & wireless packet switched netwrks: e.g. Ethernet where one wire is shared, e.g. Wi-Fi where one radio channel is shared. Sharing mechanism is contention at the physical layer. Much simpler than FDMA, CDMA etc Simplest version is ALOHA, invented in Hawaii: There is one wire or radio channel a broadcast channel Users can just transmit when they like! 16/03/2018 Lecture 7 3

4 Pure ALOHA (1970s) Stations transmit whenever they have data to send Success is detected at the transmitter by listening to channel while transmitting or by receiving an acknowledgement from receiver If transmission is found to be unsuccessful, send it again but not immediately. Why not? Consider why it failed probably because there was collision with somebody else s transmission. If everybody repeats immediately, collision will occur again. Insert random back-off delay after any collision This is a form of multiple access by collision sensing. 16/03/2018 Lecture 7 4

5 Transmitter Pure ALOHA (illustrated) Y R B again again Collision 2 2 again 2 Collisions 2 again 3 again t Three transmitters (Y,R & B) trying to send packets of varying length. When collisions occur, transmitters wait for random times, then re-send. 16/03/2018 Lecture 7 5

6 Transmitter Slotted ALOHA (illustrated) Y R B 50 s again 4 again 2 again t Collisions Works best when all packets have same length, say 50 microseconds. Time-domain is now divided into time-slots of 50 s. Each transmitter must wait until the start of a new slot before sending. Fewer collisions occur 16/03/2018 Lecture 7 6

7 Efficiency of ALOHA Graph reproduced from recommended text-book Assumes large number of users & equal length packets Few transmissns - few collisions Many transmissns Many collisions Pure ALOHA achieves max 18% channel utilization Slotted ALOHA achieves max 37% utilization plus 37% empty, 26% collisions 16/03/2018 Lecture 7 7

8 Explanation of previous graph If network is able to transmit fixed packets of length=p bits at B bit/s, frame time T F = P/B seconds. e.g. if P = 1000 & B = 20 Mb/s, T F = 50 microseconds We can send 20,000 frames in a second. We have 20,000 transmission opportunities per second. What percentage of these convey a successful packet? If all trans opps resulted in a successful packet it would be 100 %. Probability of successful packet per transmission opp would be 1. Call this S = (throughput per frame time) Some opps not used when there are few transmissns. Opps will be wasted when collisions occur. Successful packets may require many re-transmissions. We cannot expect S to be close to 1. 16/03/2018 Lecture 7 8

9 Attempts per packet time (G) With few transmissions, almost no re-transmissions needed: Lots of users, but they are not transmitting very often Therefore attempts per packet time G is low. Almost no collisions, but most transmit opps are not being used S is low because the network is hardly being used. As users increase their transmissions: More transmit opps are used, but number of collisions increases Therefore G increases. S rises to 0.18 for pure, but then starts to fall as more & more opps are wasted with collisions or are used for re-transmissions. Max throughput (channel utilisation efficiency) is: 18 % of its full capacity for pure ALOHA 37 % for slotted ALOHA. 16/03/2018 Lecture 7 9

10 Carrier Sense Multiple Access (CSMA) Each transmitter listens to channel before deciding whether to transmit If a carrier signal is detected, some-one else is transmitting. 1-persistent CSMA: continually monitors channel (greedy) waits until no carrier is detected, then transmits immediately. waits for random time after any collision, then starts again. non-persistent CSMA: monitors channel at random time intervals (less greedy) waits until no carrier is detected, then transmits immediately. waits for random time after any collision, then starts again. p-persistent CSMA (for slotted channels): monitors channel at each time-slot if no carrier detected, generates a random integer in range 1 to 1/p Like throwing a 1/p-sided dice. if dice shows 1 it transmits; otherwise it waits till next slot. waits for random time after any collision, then starts again. 16/03/2018 Lecture 7 10

11 Randomisation CSMA relies on randomisation in various ways. Generates pseudo-random numbers for the back-off delay. Throwing a 1/p-sided dice: Generate random integer uniformly* distributed between 1 & 1/p. That s it. (* binomially) If it is 1, we have thrown a 1, therefore transmit, otherwise wait. For 0.5 persistence, need 2 sided dice, or a coin. If it is 1 or heads we transmit if it is 2 or tails we wait 16/03/2018 Lecture 7 11

12 How can collisions occur with CSMA? Does CSMA eliminates possibility of collisions? Unfortunately not. Two transmitters, A & B say, may sense the channel clear and start transmitting at exactly the same time. Transmission from A will take some time to reach B. And vice-versa. Because of these delays A, B or both may have started transmitting before they realise that the other one has started also. Therefore a collision occurs. Therefore, must continue to use the random back-off provision that was invented for ALOHA collisions. 16/03/2018 Lecture 7 12

13 Persistent and non-persistent CSMA Graph reproduced from recommended text-book. Assumes many users & equal length packets. Comparison of the channel utilization versus load for various random access protocols. 16/03/2018 Lecture 7 13

14 Comment Seem to be getting close to 100% efficiency with very low persistence. But we have not considered how long it takes packets to get through - delay 0.01 persistent CSMA only uses 1 opp out of every 100. Network seems efficiently used for a lot of users. But to any individual user it will be V E R Y S L O W. 16/03/2018 Lecture 7 14

15 CSMA with collision detection Transmitter must monitor its own transmissions Abort transmission as soon as a collision is detected. Saves time, power & transmission capacity. Used by wired Ethernet. Carrier detection is physical layer process. Result used by Medium Access Control sub-layer It is part of the data-link layer. MAC sub-layer makes decisions about when to transmit, when to back-off, etc. 16/03/2018 Lecture 7 15

16 CSMA with collision avoidance CSMA/CD is fine for wired systems but not for wireless. Wireless transmitter cannot easily monitor its own transmissions. It is either transmitting or receiving, but not both at same time Also, it is not appropriate, since what matters is collision at seen the receiver. This may be different from what is observed at the transmitter. With wireless, emphasis is on avoiding collision. IEE supports 2 transmission modes: Non-contention mode (PCF) using centralised control Contention mode (DCF) using CSMA/CA PCF is optional & not widely used. Concentrate on DCF. 16/03/2018 Lecture 7 16

17 Wi-Fi contention mode Uses real channel sensing, i.e. Transmitter senses channel, & if it is free just starts transmitting. Collisions sensed at receiver at end of transmission. Receiver sends acknowledgement (ACK) for correct packet. Transmitter re-sends (with back-off delay) when ACK not received Also virtual (RTS/CTS) channel sensing, i.e. If A wants to transmit to B, it sends a short RTS (request to send) control frame If B is ready, it sends back a CTS (clear to send) control frame. A now sends its message frame & starts an ACK timer When B receives frame, sends an ACK. If A does not receive an ACK in time, it re-transmits. When other users hear RTS or CTS, they set a network allocation vector flag (NAV) for a period of time. No transmissions allowed while NAV is set. Devices can sleep. 16/03/2018 Lecture 7 17

18 Network allocation vector (NAV) NAV is a virtual carrier sensing flag. The other devices are not sensing the carrier They are told to assume that the channel is busy. RTS/CTS also solves hidden & exposed device problems 16/03/2018 Lecture 7 18

19 Hidden device (station) problem A B C Range of transmissions from C C is hidden from A (by distance) A wants to transmit to B, but will not hear any transmissions from C. It may just assume channel is free and transmit. Messages from A and C will cause collision at B. This is bad. But if A sends RTS to B while B is talking to C, B won t hear it. So B will not send CTS, so A will not transmit until B is ready. 16/03/2018 Lecture 7 19

20 Exposed device (station) problem A B C Range of transmissions from C D C is exposed to A because it needlessly prevents B from transmitting to A. B wants to transmit to A who will not hear transmissions from C or D. C may be transmitting to D. B hears C & will assume channel is not free & that it cannot transmit. But, in fact, B can transmit to A while C is transmitting to D. C will not hear B because it is transmitting & D is too far away to hear B. 16/03/2018 Lecture 7 20

21 Bluetooth Architecture Basic unit of a Bluetooth system is a pico-net Consists of a master node And up to 7 slaves within 10 m. Two piconets can be connected to form a scatternet See recommended text-book Tanenbaum PP in Version 4. PP in Version 5 16/03/2018 Lecture 7 21

22 Text-book 16/03/2018 Lecture 7 22

23 Power & spatial multiplexing Remember, for mobile wireless communications transmit power affects battery life excessive transmit power also increases interference with other users e.g. in cellular system with small cells use the minimum power necessary better error correction = lower power reduce power until correctable errors occur? Trade-off of extra bits for better FEC against lower energy/bit 16/03/2018 Lecture 7 23

24 Summary Multiple access mechanisms for cellular telephony: FDMA, TDMA, CDMA & OFDMA Also spatial since transmissions have limited range. Mechanisms (MAC) for computer networks: pure & slotted ALOHA, CSMA/CD for wired PCF (non-contention mode for WiFi: optional) CSMA/CA (contention mode for WiFi) RTS/CTS (MACAW solves hidden station problem) For spatial multiplexing, transmission power must be carefully controlled Can reduce power until correctable errors occur. 16/03/2018 Lecture 7 24

Chapter 2 Overview. Duplexing, Multiple Access - 1 -

Chapter 2 Overview. Duplexing, Multiple Access - 1 - Chapter 2 Overview Part 1 (2 weeks ago) Digital Transmission System Frequencies, Spectrum Allocation Radio Propagation and Radio Channels Part 2 (last week) Modulation, Coding, Error Correction Part 3