COSC 3213: Computer Networks I Chapter 6 Handout # 7

Transcription

1 COSC 3213: Compuer Neworks I Chaper 6 Handou # 7 Insrucor: Dr. Marvin Mandelbaum Deparmen of Compuer Science York Universiy F05 Secion A Medium Access Conrol (MAC) Topics: 1. Muliple Access Communicaions: Channelizaion vs MAC 2. MAC: Random Access Proocols vs Scheduled Approaches 3. Random Access Proocols: ALOHA, Sloed ALOHA, CSMA, CSMA/CD Garcia: Secions

2 Classificaion of Neworks (1) Two basic ypes of neworks: 1. Swiched Neworks: Inerconnec hoss by ransmission lines, muliplexers, rouers, and swiches Addressing is hierarchical o scale o large sizes Muliple pahs beween he source and desinaion Rouing required: for Transmission of packes Subne Subne Rouer Rouer Hos Hos LAN LAN 2

3 Classificaion of Neworks (2) 2. Broadcas Neworks: machines conneced: a single shared ransmission medium. Addressing: fla (based on NIC). No rouing: Only one pah is available beween source and desinaion; medium access proocol (MAC): o coordinae ransmissions beween differen machines. Pros: low cos infrasrucure; complex rouing algorihms are no required Cons: A MAC proocol is needed o minimize collisions and ensure fair sharing of he medium; Transmission efficiency is low due o collisions. Compuer Cable Shared Medium (a) Token Bus (b) Token Ring Compuer Compuer 3

4 MAC Proocols MAC: Muliple users share he communicaion channel: sharing scheme mus be devised o preven collision of packes. Medium Sharing Techniques. Pariion he medium ino separae channels. Each channel is dedicaed o a ransmiing hos. Useful for seady raffic condiions. Pros: No collisions; fair Saic Channelizaion. Cons: wase of resources for bursy raffic Dynamic Medium Access Conrol Scheduling 1. Dynamic sharing of medium 2. Use-as-required basis 3. Useful when raffic is bursy Random Access 1. Transmission is scheduled 2. A cenral body ypically deermines he schedule 1. Depending upon he raffic condiion, any hos can ransmi a any ime. 4

5 Example of MAC (1) 1. Saellie Communicaions: Two frequency bands: one for uplink (5M 42M) & one for downlink (550M 750M) Each saion is allocaed a channel (2M) in he uplink frequency band Saellie is a repeaer ha changes he carrier frequency & repeas message Each saion has a channel (6M) in he downlink frequency band Uplink Downlink Aciviy 1: Under which caegory of MAC schemes does saellie communicaions fall? 5

6 Example of MAC (2) 2. Muli-drop Telephone Line: Se of M saions share an inbound and an oubound line Saions ransmi informaion o hos using inbound line, one a a ime Hos ransmis informaion o saion using oubound line Hos compuer issues polling message o saions graning permission o ransmi Inbound line Hos Oubound line Saions Aciviy 2: Under which caegory of MAC schemes does mulidrop elephone line fall? 6

7 Example of MAC (3) 3. Ring Neworks: Hoss are conneced in a ring One saion wih he oken ransmis packe in he form of bis Each conneced saion received daa, bi by bi Desinaion hos copies daa bu leaves daa on he ring, Ohers ignore Transmiing hos exracs daa from he ring Aciviy 3: Under which caegory of MAC schemes does Ring Neworks fall? 7

8 Example of MAC (4) 4. Muliapped Bus: Uses coaxial cable where a signal can propagae in boh direcions A saion lisens and if no one else is ransmiing, sars o ransmi If a collision occurs, i wais for a random duraion before ransmiing again All saions receive he ransmied message Desinaion saion acceps he message ohers rejec i Aciviy 4: Under which caegory of MAC schemes does muliapped bus fall? 8

9 Example of MAC (5) 5. Wireless LAN: Se of devices (worksaions, lapops, cordless, ec.) share a wireless medium Message ransmied have differen bi raes (hence differen BW requiremens) Differen sraegies used: A cenral auhoriy acceps all messages and redirec hem o is desinaion Messages can be communicaed direcly o each oher A combinaion of he wo May or may no be presen Cenral Auhoriy 9

10 Delay Bandwidh Produc Disance d meers prop = d / ν seconds A ransmis a = 0 A B A B B ransmis before = prop A deecs collision a = 2 prop A B 1. Saion A wans o alk o saion B 2. Saion A lisens o he medium; begins ransmiing as no signal is presen 3. Signal from Saion A will ake prop seconds o reach saion B and vice versa 4. Saion B lisens and begins ransmiing before prop seconds as no signal is deeced a is end. 5. Resul: collision of packes 6. Saion A will no know of collision ill 2 prop seconds 7. Saion B knows almos immediaely 8. Resoluion: Who sops ransmiing? Proocol is he one who sared ransmiing las. 10

11 Delay Bandwidh Produc (2) Disance d meers prop = d / ν seconds A ransmis a = 0 A B A B B ransmis before = prop A deecs collision a = 2 prop A B 8. Time wased in coordinaing = 2 prop seconds. 9. If ransmission rae of he medium = R bps; # of bis wased = 2 prop Rbis 10. If size of packe = L bis, efficiency in he use of channel is Efficiency = L / (L + 2 prop R) = 1 / (1 + 2a) where a = prop R / L where prop R is he delay-bandwidh and a is he raio of delay-bandwidh o average packe lengh 11. For a = 0.01, efficiency is 98%; For a = 0.5, efficiency is 50% 11

12 Delay Bandwidh Produc (3) Disance 10 Mbps 100 Mbps 1 Gbps Nework 1 m 3.33 x x x 10 0 Desk area 100 m 3.33 x x x 10 3 LAN 10 km 3.33 x x x 10 4 MAN 1000 km 3.33 x x x 10 6 WAN km 3.33 x x x 10 8 Global area Table 6.1: Delay Bandwidh produc for a number of Neworks Aciviy 5: In Eherne, he size of packes is limied o a maximum size of 1500 byes (12,000 bis). Calculae he value of a (raio of delay-bandwidh o average packe lengh) for a local area nework (LAN) a 10 Mbps, 100 Mbps, and 1Gbps using he values of he delay BW produc from he above able? 12

13 Definiions Frame Transfer Delay (X): Duraion beween he ime when he firs bi of frame leaves he MAC layer of he source o he ime when he las bi reaches he MAC layer of he desinaion. Throughpu (S ou ): Effecive rae of ransmission (based on successful deliver of frames) in frames/s across a nework Suppose ha he ransmission rae of a nework is R bps Lengh of a frame is L bis Maximum hroughpu = R / L frames/s Acual hroughpu < R / L frames/s, Why? Load (G): Load (oal number of frames) on he channel in frames per X seconds. Topology: Way a nework is srucured, i.e., ring versus bus versus sar opology. Technology: Se of proocols used for a nework o funcion. 13

14 Random Access: ALOHA (1) MAC proocols allow sharing of a common ransmission medium by several hoss. Recall MAC proocols can be divided ino wo differen caegories: Medium Sharing Techniques 1. Pariion he medium ino separae channels 2. Each channel is dedicaed o a ransmiing hos 3. Useful for seady raffic condiions Saic Channelizaion Scheduling Dynamic Medium Access Conrol Random Access 1. Dynamic sharing of medium 2. Use-as-required basis 3. Useful when raffic is bursy 1. Transmission is scheduled 2. A cenral body ypically deermines he schedule 1. Depending upon he raffic condiion, any hos can ransmi a any ime. 14

15 Random Access: ALOHA (2) Random Access: MAC proocols include 1. ALOHA: Earlies random access mehod. Developed a Universiy of Hawaii in 1970s o inerconnec universiy campuses on differen islands hrough a microwave link Transmier: Transmis he frame as soon as he MAC layer receives i Channel: If a collision occurs, frames received by he receiver will conain errors Receiver: In case of errors, no acknowledgemen is ransmied o he receiver. (Alernaively, a reques for reransmission may be made in case of errors) Transmier: If he ransmier receives no ACK wihin imeou (2 propagaion delay), i backs off for a random period of ime. On he expiry of backoff ine, he ransmier reransmis he frame. Aloha is successful for ligh raffic. Noe ha collision is differen from errors since i affecs wo hos saions. For he scheme o work, i is vial ha he hos saions wai for a random period of ime before reransmiing. If boh saions wai an equal ime before reransmiing, here will always be collisions. 15

16 Random Access: ALOHA algorihm (4) [Sar ] [Iniialize backoff o zero] Loop [send frame ] [wai ] if (ACK received) hen [success and end] leave loop else (ACK no received)) [wai backoff ime] if (reached backoff limi) hen [unsuccessful and abor] leave loop else [selec random backoff ime] end if end if end loop 16

17 Random Access: ALOHA hroughpu (5) Saion A Saion B A1 A2 A2 (reransmi) Frame ransfer ime (X) backoff period (B) B1 B2 B2 (reransmi) backoff period (B) Saion C C1 C1 (reransmi) backoff period (B) Saion D D1 backoff period (B) Channel 17

18 Random Access: ALOHA analysis (6) Firs ransmission Time-ou Backoff period B Reransmission if required 0 X 0 0 +X 0 +X+2 prop 0 +X+2 prop +B Vulnerable period Assumpions: 1. All frames are of equal lengh (L). 2. All hoss are similar such ha he frame ransfer ime X = L/R is he same for each hos. Assume ha a frame is ransmied a 0 seconds, hen 1. Vulnerable ime: arrival of daa o daa-link layer resuling in collisions ( 0 X 0 + X). 2. Afer ransmission, hos imes ou and wai for he ACK frame for: ( 0 + X 0 + X + 2 prop ). 3. In case no ACK is received, he hos imes ou for B seconds. 4. Reransmission is aemped a: = 0 + X + 2 prop + B seconds. 18

19 Random Access: ALOHA analysis (7) Firs ransmission Time-ou Backoff period B Reransmission if required 0 X 0 0 +X 0 +X+2 prop 0 +X+2 prop +B Vulnerable period 5. Assume ha he arrival ime of a frame o he daa link layer follows a Poisson s disribuion P[ k ransmissions in T seconds] = k ( λt ) λt where λ is he average number of frames ransmied per second. 6. Given ha he oal load on he channel is G frames per X seconds, λ = G/X. 7. Based on he Poisson s disribuion, k! e P[ k ransmissions in 2X seconds] = k ( 2G) 2G k! e 19

20 Random Access: ALOHA analysis (8) Firs ransmission Time-ou Backoff period B Reransmission if required 0 X 0 0 +X 0 +X+2 prop 0 +X+2 prop +B Vulnerable period 8. To preven collisions, here should be no ransmissions wihin he vulnerable period P[0 ransmissions in 2X seconds] = e 9. The hroughpu S is defined as he produc of he oal arrival rae and probabiliy of a successful ransmission. S = Ge Aciviy 6: Deermine he maximum value of hroughpu S and he value of G a which i is possible. 2G 2G 20

21 Random Access: ALOHA Performance (9) S = Ge 2G G 10. Mode 1 (low raffic): As we increase he load G from 0, he hroughpu increases seadily. In fac for low values of G, S = G. This maches our inuiion ha for low raffic, here will be no collisions and he hroughpu would equal he load. 11. The hroughpu S achieves is maximum value of a G = 0.5. This corresponds o an arrival rae of exacly one frame per vulnerable period. 12. Mode 2 (high raffic): As G > 0.5, he hroughpu drops. Again, his is consisen wih our inuiion ha a high load would backlogg he channel wih a large number of collisions. 21

22 Random Access: ALOHA (10) Aciviy 7: Suppose ha he ALOHA proocol is used o share a 56 kbps saellie channel. Suppose ha he frames are 1000 bis long. Wha is he maximum hroughpu of he sysem in frames/s if he propagaion ime is ignored. Soluion: Maximum hroughpu for ALOHA = frames / X seconds. Frame ransfer delay (X) = 1000/56000 = 1/56 seconds Maximum hroughpu in frames/s = , or approximaely 10 frames/sec. 22

23 Random Access: Sloed ALOHA (1) 1. The maximum hroughpu of ALOHA is frames per frame ransfer ime. 2. The firs aemp o increase he hroughpu is called he Sloed ALOHA, which includes he following modificaions o ALOHA. Divide he ime ino slos. Any saion is only allowed o ransmi a he beginning of a slo. Res of he procedure is same as for ALOHA. 3. Drawback: Addiional complexiy in he proocol as saions mus be synchronized wih he beginning of slos. 4. Advanage: Number of collisions are reduced as frames would collide only a he beginning of a ime slo. 23

24 Random Access: Sloed ALOHA hroughpu (2) Saion A Saion B Saion C A1 A2 A2 (reransmi) Frame ransfer ime (X) backoff period (B) B1 B2 B2 (reransmi) backoff period (B) C1 C1 (reransmi) backoff period (B) Saion D D1 backoff period (B) Channel 24

25 Random Access: ALOHA analysis (6) Firs ransmission Time-ou Backoff period B Reransmission if required 0 = kx (k+1)x (k+2)x (k+2)x+2 prop (k+2)x+2 prop +B Vulnerable period Assumpions: 1. All frames are of equal lengh (L). 2. Time is divided ino ime slos such ha each slo duraion equals he frame ransfer ime (X). 3. Frames are ransmied only a he sar of a slo. Assume ha a frame is ransmied a 0 = KX seconds, hen 1. Vulnerable ime: arrival of daa o daa-link layer resuling in collisions kx (k+1)x 2. Assuming a Poisson s disribuion for he arrival of daa P[ k ransmissions in X seconds] = k ( G) G k! e 25

26 Random Access: ALOHA analysis (8) Firs ransmission Time-ou Backoff period B Reransmission if required 0 = kx (k+1)x (k+2)x (k+2)x+2 prop (k+2)x+2 prop +B Vulnerable period 3. To preven collisions, here should be no ransmissions wihin he vulnerable period P[0 ransmissions in X seconds] = e 4. The hroughpu S is defined as he produc of he oal arrival rae and probabiliy of a successful ransmission. S = Ge 5. The maximum value of hroughpu S = 1/e = for G = 1. G G 26

27 Random Access: ALOHA vs Sloed ALOHA S Ge -G Ge -2G G 10. Sloed ALOHA doubles he maximum hroughpu of he channel by reducing he number of collisions. 11. Sloed ALOHA adds an addiional waiing period, where each frame has o wai ill he sar of he nex slo before ransmission. Aciviy 10: Compare he maximum hroughpu of ALOHA and sloed ALOHA for a radio sysem using 9600 bps channel and a frame lengh of 120 bis. 27

28 Random Access: CSMA (1) Carrier Sensiive Muliple Access (CSMA): An improvemen over ALOHA by providing carrier sense o he saion. Before ransmiing, he saion senses if any carrier (or signal) is presen on he shared medium. If a carrier is presen, he saion wais and ransmis again when he medium indicaes no carrier. Depending upon how he duraion of he wai period is decided, CSMA can be classified in differen caegories: a) 1-Persisen CSMA: If he channel is busy, he saion ransmis immediaely as soon as he medium becomes idle (free of any carrier). b) Non-persisen CSMA: If he medium is busy, saion runs a backoff algorihm o reschedule a fuure sensing period. I senses again afer a cerain wai and ransmis only if he medium is free. c) P-persisen CSMA: Saion senses for carrier. If he carrier is absen, i ransmis wih a p probabiliy. If busy, i persiss wih sensing unil he medium is free. 28

29 Random Access: CSMA-CD Random Access: MAC proocols include 3. Carrier Sensiive Muliple Access wih Collision Deecion (CSMA-CD): An improvemen over CSMA by giving he saion capabiliy of deecing collisions Procedure is same as CSMA excep ha if a collision is deeced, he saion sops immediaely wihou ransmiing he complee packe. 29