Wireless Media Access Control (MAC)

Transcription

1 Wireless Media Access Control (MAC) Li-Hsing Yen National University of Kaohsiung What is it all about? Consider an audioconference where if one person speaks, all can hear if more than one person speaks at the same time, both voices are garbled How should participants coordinate actions so that the number of messages exchanged per second is maximized time spent waiting for a chance to speak is minimized This is the multiple access problem 1

2 Some Simple Solutions Use a moderator a speaker must wait for moderator to call on him or her, even if no one else wants to speak what if the moderator s connection breaks? Distributed solution speak if no one else is speaking but if two speakers are waiting for a third to finish, guarantee collision Designing good schemes is surprisingly hard! Contexts for the Multiple Access Problem Broadcast transmission medium message from any transmitter is received by all receivers Colliding messages are garbled Goal maximize message throughput minimize mean waiting time 2

3 Solving the Problem First, choose a base technology to isolate traffic from different stations can be in time domain or in frequency domain Then, choose how to allocate a limited number of transmission resources to a larger set of contending users Design Choices Centralized vs. distributed design is there a moderator or not? in a centralized solution one of the stations is a master and the others are slaves in a distributed solution, all stations are peers Circuit-mode vs. packet-mode do stations send steady streams or bursts of packets? with streams, doesn t make sense to contend for every packet with packets, makes sense to contend for every packet to avoid wasting bandwidth 3

4 Constraint: Spectrum Scarcity radio spectrum is hard to come by only a few frequencies available for longdistance communication multiple access schemes must be careful not to waste bandwidth Constraint: Radio Link Properties radio links are error prone fading multipath interference hidden terminals transmitter heard only by a subset of receivers capture on collision, station with higher power overpowers the other lower powered station may never get a chance to be heard 4

5 Performance Metric: Normalized Throughput (Goodput) fraction of link capacity used to carry nonretransmitted packets example with no collisions, 1000 packets/sec with a particular scheme and workload, 250 packets/sec goodput = 0.25 Performance Metric: Stability with heavy load, is all the time spent on resolving contentions? Yes unstable with a stable algorithm, throughput does not decrease with offered load if infinite number of uncontrolled stations share a link, then instability is guaranteed but if sources reduce load when overload is detected, can achieve stability 5

6 Performance Metrics: Mean Delay and Fairness Mean delay amount of time a station has to wait before it successfully transmits a packet depends on the load and the characteristics of the medium Fairness no single definition no-starvation : source eventually gets a chance to send Base Technologies Isolates data from different sources Three basic choices Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA) 6

7 Frequency-Division Multiple Access (FDMA) Each user is assigned a different radio frequency (channel) Channel information may be transmitted in a dedicated frequency Frequency... guard band FDMA Simplest Best suited for analog links Each station has its own frequency band, separated by guard bands Receivers tune to the right frequency Number of frequencies is limited reduce transmitter power; reuse frequencies in nonadjacent cells 7

8 Time-Division Multiple Access (TDMA) Users are assigned time slots to share a physical channel All stations transmit data on same frequency, but at different times => needs time synchronization guard time frame Time TDMA: Pros and Cons Pros users can be given different amounts of bandwidth mobiles can use idle times to determine best base station can switch off power when not transmitting Cons synchronization overhead greater problems with multipath interference on wireless links 8

9 TDMA Over FDMA Cellular systems always overlays TDMA on top of an FDMA structure Frequency Time Spread Spectrum Modulation Schemes spread spectrum ( 展頻 ): uses a wider spectrum than is strictly necessary for carrying the transmitter s signal DSSS Direct Sequence Spread Spectrum FHSS Frequency Hopping Spread Spectrum Users separated both by time and frequency Send at a different frequency at each time slot OFDMA Orthogonal Frequency Division Multiple Access 9

10 Code Division Multiple Access (CDMA) Frequency-hopping CDMA (FH/CDMA) each station transmits at a particular frequency for a short duration, then hops to another frequency Direct-sequence CDMA (DS/CDMA) each bit of the transmitter is replaced by a codeword, i.e., a sequence of bits (chips) orthogonal codewords must be chosen to prevent too much confusion at a receiver Direct-Sequence CDMA: Basic Principle Break each bit into k chips according to a fixed pattern specific to each user Bit data rate D kd chips per second Example: the code for A: c A = 1,-1,-1,1,-1,1 k =6 User A 1 0 1,-1,-1,1,-1,1-1,1,1,-1,1,-1 (complement) Decoding function Σ=6 Σ=-6 10

11 DS/CDMA: Multiple Users User A User B User C c A c B c C User B 1, 1,-1,-1, 1, Decoding with c A c A Σ=0-1, 1, 1,-1, 1, User A Decoding with c c B B Σ=0 c A and c B are orthogonal DS/CDMA Example: k = 8 A: ( ) B: ( ) C: ( ) D: ( ) C B+C A+B A+B +C A+B+C+D A+B+C +D S 1 = ( ) S 2 = ( ) S 3 = ( ) S 4 = ( ) S 5 = ( ) S 6 = ( ) 用 C 的 code 解碼 S 1 C = S 1 C = k S 2 C = S 2 C = k S 3 C = S 3 C = 0 S 4 C = S 4 C = k S 5 C = S 5 C = k S 6 C = S 6 C = -k C 傳 0 C 傳 1 C 沒傳 11

12 Theoretical Foundation A C = 0 and A C = 0 if A and C are orthogonal X (A+B+C +D) = X A + X B + X C + X D If X = C then X A = X B = X D = 0 X (A+B+C +D) = X C Pros CDMA hard to spy immune from narrowband noise no need for all stations to synchronize no hard limit on capacity of a cell all cells can use all frequencies 12

13 CDMA Cons implementation complexity need for power control to avoid capture need for a large contiguous frequency band (for direct sequence) problems installing in the field OFDMA Assign subsets of subcarriers to individual users Allows simultaneous low data rate transmissions from several users 13

14 OFDM Orthogonal Frequency Division Multiplexing Direct signal. Original reflected signal. Longer reflected signal. In OFDM, 10 (say) serial bits are converted into 10 parallel bits, each of which modulates its own radio carrier. Each carrier is now carrying a bit rate that is 1/10 th the bit rate of the original. A reflected signal path needs to be 10 times longer to cause the same interference. Longer paths are more attenuated so the strength of the interference is also less. FDD and TDD Two ways of converting a wireless medium to a duplex channel In Frequency Division Duplex, uplink and downlink use different frequencies In Time Division Duplex, uplink and downlink use different time slots Can combine with FDMA/TDMA Examples TDD/FDMA in second-generation cordless phones FDD/TDMA/FDMA in digital cellular phones 14

15 Classification of Multiple- Access Schemes Access Schem es Centralized Distributed Circuit M ode Packet Mode Packet M ode Cellular telephony Polling Reservation Decentralized polling CSM A BTM A and M ACA ALOHA Centralized Access Schemes One station is master, and the other are slaves slave can transmit only when master allows Natural fit in some situations wireless LAN, where base station is the only station that can see everyone cellular telephony, where base station is the only one capable of high transmit power 15

16 Centralized Access Schemes Pros simple master provides single point of coordination Cons master is a single point of failure need a re-election protocol master is involved in every single transfer => added delay Centralized Circuit-Mode Access Schemes When station wants to transmit, it sends a message to master using packet mode Master allocates transmission resources to slave Slave uses the resources until it is done No contention during data transfer Used primarily in cellular phone systems EAMPS: FDMA GSM/IS-54: TDMA IS-95: CDMA 16

17 Centralized Packet-Mode Access Scheme: Polling master asks each station in turn if it wants to send (roll-call polling) inefficient if only a few stations are active, overhead for polling messages is high, or system has many terminals Centralized Packet-Mode Access Scheme: Probing stations are numbered with consecutive logical addresses assume station can listen both to its own address and to a set of multicast addresses master does a binary search to locate next active station BS polls 0** 00* 01* Station replies collide collide Station transmits (assumed addresses: )

18 Centralized Packet-Mode Access: Reservation-Based Schemes master coordinates access to link using reservations Some time slots devoted to reservation messages can be smaller than data slots => minislots Stations contend for a minislot (or own one) Master decides winners and grants them access to link Packet collisions are only for minislots, so overhead on contention is reduced Minislots minislots data slots data slots data slots data slots data slots data slots reserved Reservation requests 18

19 Distributed Schemes Compared to a centralized scheme more reliable have lower message delays often allow higher network utilization but are more complicated Almost all distributed schemes are packet mode (why?) Decentralized Polling Just like centralized polling, except there is no master Each station is assigned a slot that it uses if nothing to send, slot is wasted Also, all stations must share a time base 19

20 Decentralized Probing Also called tree based multiple access without a master (BS) Works poorly with many active stations, or when all active stations are in the same subtree (assumed addresses: ) All 0** stations transmit packets All 00* stations transmit packets station 000 transmits packet station 001 transmits packet All 01* stations transmit packets collide collide ALOHA and Its Variants one of the earliest MAC schemes Just send it! Wait for ACK Unlike polling/probing, a node with something to send doesn t have to wait for a master, or for its turn in a schedule Collisions occur when two or more stations transmit at the same time On collisions, try again after a random waiting time The range of random is fixed (no exponential backoff) channel utilization around 18% 20

21 Pros Evaluating ALOHA useful when a is large, so carrier sensing doesn t help satellite links simple no carrier sensing, no token, no timebase synchronization independent of a Cons under some mathematical assumptions, goodput is at most 18% at high loads, collisions are very frequent sudden burst of traffic can lead to instability unless backoff is exponential The Parameter a The number of packets sent by a source before the farthest station receives the first bit propagation delay a = transmission time 21

22 Slotted ALOHA A simple way to double ALOHA s capacity The time is divided into a number of time slots When a station has information to send, it waits for a time slot and then transmits window of vulnerability L Earliest start time for colliding packet 2L ALOHA L time Latest start time for colliding packet L L Slotted ALOHA L time Packet Generation Time Vs. Packet Transmission Time In Aloha, packet generation time = packet transmission time In Slotted-Aloha, packet generation time packet transmission time in most cases Slot i Slot i+1 L L time Only packets generated in this time period will collide with L All packets generated in this time period will be sent in the next slot 22

23 Reservation ALOHA (R-ALOHA) Combines slot reservation with slotted ALOHA Contend for reservation minislots using slotted ALOHA Stations independently examine reservation requests and come to consistent conclusions Simplest version divide time into frames = fixed length set of slots station that wins access to a reservation minislot using S- ALOHA can keep slot as long as it wants station that loses keeps track of idle slots and contends for them in next frame Evaluating R-ALOHA Pros supports both circuit and packet mode transfer works with large a simple Cons arriving packet has to wait for entire frame before it has a chance to send cannot preempt hogs variants of R-ALOHA avoid these problems 23

24 Carrier Sense Multiple Access (CSMA) A fundamental advance over Aloha: check whether the medium is busy before sending a packet (i.e carrier sensing) Aloha does not perform carrier sensing If medium is idle, then send If medium is busy, then wait the probability of collisions is reduced Carrier Sense: Medium Idle B wants to send to C at this time A B C The medium is idle so B starts sending Data time 24

25 Carrier Sense: Medium Busy Data B wants to send to C at this time A B C The medium Is busy so B defers sending Data B starts sending only after the medium is free time Persistent and Non-Persistent CSMA The sender sends a frame as soon as the medium becomes idle If, on sensing busy, the sender waits for idle persistent CSMA sets a timer and tries later non-persistent CSMA Problem with persistent: two stations waiting to speak will collide 25

26 Problem With Persistent CSMA B B wants to send to A at this time B senses signal so it waits too A busy C wants to send to A at this time C senses A s signal so it waits C B starts sending C starts sending as as soon as the collide soon as the medium medium becomes idle becomes idle time Reducing Collision Probability: p-persistent CSMA Basically persistent, but a waiting station does not launch a packet immediately when the medium becomes idle p: the probability of sending a packet when the medium becomes idle hard to choose p if p small, then wasted time if p large, more collisions Best p depends on the number of contending nodes 26

27 Simple Math. If p-persistent CSMA is used, and 5 stations are contending (waiting) for the channel, what is the probability that only 1 station successfully transmits if p = 0.25? What if p = 0.75? Handling the Collision Problem: Exponential Backoff Collisions are still possible in CSMA Each station, after transmitting a packet, checks whether the packet transmission is successful on collision, choose timeout randomly from doubled range backoff range adapts to number of contending stations no need to choose p need to detect collisions: collision detect circuit => CSMA/CD 27

28 CSMA/CD Collision Detection Easy to implement in cooper media The signal amplitude on the media increases Collision starts. 1 st stops transmitting Cannot be used in wireless stations because transmitter overwhelms co-located receiver 28

29 Using ACK Frames to Detect Collisions B A Data 1 C Data 7 ACK 1 Data 2 Data 1 OK No ACK 7 Data 2 No ACK 2 CSMA/CA (Collision Avoidance) Used in wireless LANs Using explicit ACKs to detect collisions makes collisions expensive try to avoid potential collisions (collision avoidance) by random backoff (different from exponential backoff) at the beginning 29

30 Collision Avoidance: Random Backoff B When B senses no signal it starts a timer B senses signal at this time A busy When C senses no signal it starts a timer Timer value is determined by random C time CSMA/CA Algorithm First check if medium is busy Carrier sense If so, wait for medium to become idle Wait for interframe spacing (IFS) Set a contention timer to an interval randomly chosen in the range [1, CW] On timeout, send packet and wait for ack If no ack, assume packet is lost Collision avoidance try again, after doubling CW If another station transmits while counting down, freeze contention timer and unfreeze when packet completes transmission 30

31 Hidden Terminal Problem B collide A C C A 傳輸的同時 B 因聽不到 C 的訊號而送出訊號給 A A B C connectivity A B C Carrier sense 無法避免此種 collisions Hidden Terminal Problem Cannot Be Resolved by Carrier Sense B Data B s signal range A C wants to send to A at this time Data C B s a hidden terminal to C and vise versa collision C senses no signal so it starts sending time 31

32 Exposed Terminal Problem D C B deferred transmission A A B connectivity A B C D C D 不會干擾 B A 的傳送, 但 C 因聽到 B 目前的傳送而不傳送 C D Carrier sense 造成效能下降 Dealing with Hidden and Exposed Terminals CSMA/CA works when every station can receive transmissions from every other stations Not always true In both cases, carrier sense doesn t work with hidden terminal, collision because signal not detected with exposed terminal, idle station because carrier incorrectly detected 32

33 Busy Tone Multiple Access (BTMA) uses a separate busy-tone channel when station is receiving a message, it places a tone on this channel everyone who might want to talk to a station knows that it is busy even if they cannot hear transmission that that station hears this avoids both problems Problem with BTMA BTMA requires us to split frequency band more complex receivers (need two tuners) Separate bands may have different propagation characteristics scheme fails! Instead, use a single frequency band, but use explicit messages to tell others that receiver is busy (MACA) 33

34 Multiple Access Collision Avoidance (MACA) In MACA, before sending data, send a Request to Sent (RTS) to intended receiver Station, if idle, sends Clear to Send (CTS) Sender then sends data RTS/CTS contains a time duration If station overhears RTS, it waits long enough for a CTS to be sent by a receiver If a station overhears CTS, it waits long enough for the data packet to be transmitted CTS/RTS in MACA C knows A is ready to receive B B knows A is to receive C s data in d A C wants to send to A RTS(k) C overheard CTS(d) CTS(d) k d B won t send A any data in d Data the duration in RTS is for the reception of CTS RTS time 34

35 MACAW RTS and CTS both announces the duration of the transfer When receiver successfully receives data, it responds with ACK for acknowledgement Sender will retransmit RTS if no ACK is received If ACK rather than data was lost, the receiver will respond with ACK when receiving this RTS CTS/RTS in MACAW C knows A is ready to receive B B knows A is to receive C s data in d A C wants to send to A RTS(k) C overheard CTS(d) CTS(d) d B won t send A any data in d RTS Data ACK k the duration in RTS is for the transfer time 35

36 PAMAS: Power Aware Multi-Access with Signaling Improve energy efficiency from MACA Avoid overhearing by putting node into sleep Control channel separates from data channel RTS, CTS, busy tone to avoid collision Probe packets to find neighbors transmission time Increased hardware complexity Two channels need to work simultaneously, meaning two radio systems. 36