Introduction to Wireless & Mobile Systems. Chapter 6. Multiple Radio Access Cengage Learning Engineering. All Rights Reserved.

Transcription

1 Itroductio to Wireless & Mobile Systems Chapter 6 Multiple Radio Access 1

2 Outlie Itroductio Multiple Radio Access Protocols Cotetio-based Protocols Pure ALOHA Slotted ALOHA CSMA (Carrier Sese Multiple Access) CSMA/CD (CSMA with Collisio Detectio) CSMA/CA (CSMA with Collisio Avoidace) Summary 2

3 Itroductio Multiple access cotrol chaels Each Mobile Statio (MS) is attached to a trasmitter or receiver which commuicates via a chael shared by other odes Trasmissio from ay MS is received by other MSs MS 3 MS 2 Shared Multiple Access Medium MS 4 MS 1 M 3

4 Itroductio (Cot d) Multiple access issues If more tha oe MS trasmit at a time o the cotrol chael to BS, a collisio occurs How to determie which MS ca trasmit to BS? Multiple access protocols Solvig multiple access issues Differet types: Cotetio protocols resolve a collisio after it occurs. These protocols execute a collisio resolutio protocol after each collisio Collisio-free protocols (e.g., a bit-map protocol ad biary coutdow) esure that a collisio ca ever occur 4

5 Chael Sharig Techiques Static Chaelizatio Medium - Sharig Techiques Schedulig Dyamic Medium Access Cotrol Radom Access 5

6 Classificatio of Multiple Access Protocols Multiple-access protocols Cotetio-based Coflict-free Radom access Collisio resolutio ALOHA, CSMA, BTMA, ISMA, etc. TREE, WINDOW, etc. BTMA: Busy Toe Multiple Access ISMA: Iteret Streamig Media Alliace FDMA, TDMA, CDMA, Toke Bus, DQDB, etc. DQDB: Distributed Queue Dual Bus 6

7 Cotetio-based Protocols ALOHA Developed i the 1970s for a packet radio etwork by Hawaii Uiversity Wheever a termial (MS) has data, it trasmits. Seder fids out whether trasmissio was successful or experieced a collisio by listeig to the broadcast from the destiatio statio. If there is a collisio, seder retrasmits after some radom time Slotted ALOHA Improvemet: Time is slotted ad a packet ca oly be trasmitted at the begiig of oe slot. Thus, it ca reduce the collisio duratio 7

8 Slotted ALOHA Node 1 Packet No trasmissio Nodes 2 & 3 Packets Retrasmissio Retrasmissio 3 Time Slot Collisio Collisio mechaism i slotted ALOHA 8

9 Throughput of Slotted ALOHA The probability of successful trasmissio P s is the probability o other packet is scheduled i a iterval of legth T P e s S th gte gt The throughput S th of pure Aloha as: where g is the packet rate of the traffic gt Defiig G gt to ormalize offered load, we have Differetiatig S th with respect to G ad equatig to zero gives ds dg th Ge S th Ge G G + e G 0 The Maximum throughput of ALOHA is 1 Smax e 9

10 Pure ALOHA Node 1 Packet Node 2 Packet Waitig a radom time Retrasmissio Retrasmissio Collisio 3 2 Time Node 3 Packet Collisio mechaism i ALOHA 10

11 Throughput of Pure ALOHA The probability of successful trasmissio P s is the probability o other packet is scheduled i a iterval of legth 2T Node 1 P P( o _ collisio) Packet s e 2gT The throughput S th of pure Aloha as: 1 S th gte S th Ge where g is the packet rate of the traffic 2G 2gT Defiig G gt to ormalize offered load, we have Differetiatig S th with respect to G ad equatig to zero gives ds dg th 2Ge 2G + e 2G The Maximum throughput of ALOHA is 1 Smax e 0 Node 2 Packet 2 Time 11

12 Throughput Throughput S Aloha Slotted Aloha G (gt)

13 Cotetio Protocols (Cot d) CSMA (Carrier Sese Multiple Access) Improvemet: Start trasmissio oly if o trasmissio is ogoig CSMA/CD (CSMA with Collisio Detectio) Improvemet: Stop ogoig trasmissio if a collisio is detected CSMA/CA (CSMA with Collisio Avoidace) Improvemet: Wait a radom time ad try agai whe carrier is quiet. If still quiet, the trasmit CSMA/CA with ACK CSMA/CA with RTS/CTS 13

14 CSMA (Carrier Sese Multiple Access) Max throughput achievable by slotted ALOHA is CSMA gives improved throughput compared to Aloha protocols Listes to the chael before trasmittig a packet (avoid avoidable collisios) 14

15 Collisio Mechaism i CSMA MS 1 Packet MS 2 Packet MS 3 Packet MS 5 sese Delay for MS Delay for MS Collisio Time MS 4 seses 15

16 Kids of CSMA CSMA Nopersistet CSMA (o wait) Persistet CSMA (wait) Uslotted Nopersistet CSMA Slotted Nopersistet CSMA Uslotted persistet CSMA Slotted persistet CSMA 1-persistet CSMA p-persistet CSMA 16

17 p-persistet CSMA Protocols p-persistet CSMA Protocol: Step 1: If the medium is idle, trasmit with probability p, ad delay for worst case propagatio delay by oe packet with probability (1-p) Special case of p0 ad p1 Step 2: If the medium is busy, cotiue to liste util medium becomes idle, the go to Step 1 Step 3: If trasmissio is delayed by oe time slot, cotiue with Step 1 p0: opersistet ad p1: 1-persistet CSMA A good tradeoff betwee opersistet ad 1-persistet CSMA 17

18 Throughput 1.0 p0.01: persistet CSMA p0: Nopersistet CSMA Always wait for T uits Throughput S Aloha p0.1: persistet CSMA p0.5: persistet CSMA p1: persistet CSMA No wait Slotted Aloha Traffic Load G 18

19 Nopersistet/p-persistet CSMA Protocols Nopersistet CSMA Protocol: Step 1: If the medium is idle, trasmit immediately (same as p1) Step 2: If the medium is busy, wait a radom amout of time ad repeat Step 1 Radom backoff reduces probability of collisios Waste idle time if the backoff time is too log For uslotted opersistet CSMA, the throughput is give by: 2αT Ge propagatio delay Sth where α τ αg G(1 + 2α ) + e T packet trasmissio time For slotted opersistet CSMA, the throughput is give by: S th αge (1 e αg 2αT + α) 19

20 1-persistet CSMA Protocols 1-persistet CSMA Protocol: Step 1: If the medium is idle, trasmit immediately Step 2: If the medium is busy, cotiue to liste util medium becomes idle, ad the trasmit immediately There will always be a collisio if two odes wat to retrasmit (usually you stop trasmissio attempts after few tries) For uslotted 1-persistet CSMA, the throughput is give by S th [ 1+ G + αg(1 + G + αg / 2) ] G G(1 + G(1+ 2α ) αg G(1+ α ) 2α ) (1 e ) e + (1 + αg) e For slotted 1-persistet CSMA, the throughput is give by αg G(1+ α ) G(1 + α e ) e Sth αg G(1+ α ) (1 + α)(1 e ) + αe 20

21 How to Select Probability p? Assume that N odes have a packet to sed ad the medium is busy The, Np is the expected umber of odes that will attempt to trasmit oce the medium becomes idle If Np > 1, the a collisio is expected to occur Therefore, etwork must make sure that to avoid collisio, where N is the maximum umber of odes that ca be active at a time 21

22 Itroductio to Wireless & Mobile Systems, 4 th Editio p-persistet CSMA Protocol If N termials have packets to sed, Np termials will attempt to trasmit oce the medium becomes idle. If Np>1, the collisio is expected. Therefore, Np 1. Throughput S as: [ ] ' ] 1 ) (1 ' )[ (1 ) (1 ) (1 ),, ( απ α π α π α π π α α α t t e P P e p G S G s s G th where G is offered traffic rate ατ/t propagatio delay/packet trasmissio time where are give by the followig equatios: 0 ' ',,, π ad t t P P s s 0 1 ' 1 ' 0 1 ' 1 ' 1 1 ) ( ) ( π π π π π π s s s s t t t t P P P P 22

23 Itroductio to Wireless & Mobile Systems, 4 th Editio p-persistet CSMA Protocol Where:. Kroecker the is, ] ) (1 [1 } Pr{ )! ( ) ( } Pr{ } Pr{ 1, )!(1 } Pr{ ) (1 1 ) (1 ) ( 0,,! ] ) [(1,, 1 0 ' 1 ) (1 delta ad l p k t e l kg l L where k t t e e g l L p p lp P e G j i l kg l k k g g l l s G l δ δ π α π α + >

24 CSMA/CD (CSMA with Collisio Detectio) I CSMA, if 2 termials begi sedig packet at the same time, each will trasmit its complete packet (although collisio is takig place) Wastig medium for a etire packet time CSMA/CD: Step 1: If the medium is idle, trasmit Step 2: If the medium is busy, cotiue to liste util the chael is idle the trasmit Step 3: If a collisio is detected durig trasmissio, cease trasmittig (detectio ot possible by wireless devices) Step 4: Wait a radom amout of time ad repeats the same algorithm 24

25 CSMA/CD i Etheret (Cot d) T 0 A begis trasmissio A (τ is the propagatio time) B T 0 +τ-ε B begis trasmissio A ε B T 0 +τ+τ cd B detects collisio A B T 0 +2τ+τ cd +τ cr -ε Time A detects collisio just before ed of trasmissio A B 25

26 Throughput of Slotted Nopersistet CSMA/CD α τ /Τ 0 Throughput S α 1 α 0.1 α Traffic load G 26

27 CSMA/CA (CSMA with Collisio Avoidace) All termials liste to the same medium as CSMA/CD Termial ready to trasmit seses the medium If medium is busy it waits util the ed of curret trasmissio It agai waits for a additioal predetermied time period DIFS (Distributed iter frame Space) The picks up a radom umber of slots (the iitial value of backoff couter) withi a cotetio widow to wait before trasmittig its frame If there are trasmissios by other MSs durig this time period (backoff time), the MS freezes its couter It resumes cout dow after other MSs fiish trasmissio plus DIFS. The MS ca start its trasmissio whe the couter reaches to zero 27

28 CSMA/CA (Cot d) MS A s frame Delay: B MS B s frame Delay: C MS C s frame Time MSs B & C sese the medium MSs B reseses the medium ad trasmits its frame MSs C starts trasmittig MSs C reseses the medium but defers to MS B 28

29 CSMA/CA Explaied DIFS Cotetio widow DIFS Cotetio widow Medium Busy Next Fame Time Defer access Slot Backoff after defer DIFS Distributed Iter Frame Spacig 29

30 Radom Delay helps CSMA/CA MS A 25 wait MS A 5 data MS B 20 CW 31, data wait Aother request MS B 15 MS B 10 MS A ad MS B are backoff itervals at odes A ad B MS A ad MS B are the backoff itervals of MS A ad MS B We assume for this example that CW 31 MS A ad MS B have chose a backoff iterval of 25 ad 20, respectively MS B will reach zero before five uits of time earlier tha MS A Whe this happes, MS A will otice that the medium became busy ad freezes its backoff iterval curretly at 5 As soo as the medium becomes idle agai, MS A resumes its backoff coutdow ad trasmits its data oce the backoff iterval reaches zero 30

31 CSMA/CA with ACK for ad hoc etworks Immediate Ackowledgemets from receiver upo receptio of data frame without ay eed for sesig the medium ACK frame trasmitted after time iterval SIFS (Short Iter-Frame Space) (SIFS < DIFS) Receiver trasmits ACK without sesig the medium If ACK is lost, retrasmissio doe 31

32 CSMA/CA/ACK Source MS DIFS delay Data Time SIFS ACK DIFS Cotetio widow Other MSs Next Frame Defer access Backoff after defer SIFS Short Iter Frame Spacig 32

33 Hidde Termial Problem Radio trasmissio rage R R R A B C Nodes A ad C are hidde with respect to each other 33

34 CSMA/CA with RTS/CTS Trasmitter seds a RTS (request to sed) after medium has bee idle for time iterval more tha DIFS Receiver respods with CTS (clear to sed) after medium has bee idle for SIFS The Data is exchaged RTS/CTS is used for reservig chael for data trasmissio so that the collisio ca oly occur i cotrol message 34

35 CSMA/CA with RTS/CTS DIFS RTS SIFS Data Time Source MS SIFS SIFS Destiatio MS CTS ACK DIFS Cotetio widow Other MSs Next Frame Defer access Backoff 35

36 RTS/CTS MS A MS B Propagatio delay This helps avoid hidde termial problem i Ad hoc etworks 36

37 Exposed Termial Problem Radio Trasmissio rage R R R A B C D R Trasmissio at Node A forces Node C (Exposed) to stop trasmissio to Node D 37