Announcements. Tuesday April 15 covers material from chapters: 1-3, 5-6 emphasis on material since last midterm

Transcription

1 Announcements Reading Today: 4.1 & 4.2 (skip and 4.2.5) Second Midterm: Tuesday Apri 15 covers materia from chapters: 1-3, 5-6 emphasis on materia since ast midterm CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 1

2 Medium Access Layer Broadcast Networks share a common resource for communication bus, wire, air, etc. need to coordination access to this resource Limits of Static Channe Aocation suitabe for constant rate traffic of simiar speeds however, bursty traffic resuts in poor channe utiization consider one queue vs. separate queues for each person n queues with bursty arriva have mean deay n times 1 queue Dynamic Aocation ony use channe when have something to send need to contro access to the channe CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 2

3 Shared Channe Mode Station mode N independent stations each wants to send λ frames per second a station may not send another frame unti the first is sent Singe Channe Assumption a stations communicate over a singe shared channe Coisions: two stations attempt to send at once neither transmission succeeds Time continuous time: frame transmissions can start anytime discrete time: cock ensures a sends initiate at the start of a sot Carrier Sense stations can te if channe is in use before sending stations must wait to know if channe was in use CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 3

4 Aoha Stations ground based radio stations on isands Pure Aoha send data a wi, coisions wi happen on coision, wait a random amount of time & try again use standard, fixed size packets what is channe efficiency? assume S new frames per frame time assume G tota frames trying to be sent per frame time S = G P 0 probabiity of k frames generated during a frame time Pr[k] = G k e -G / k! P o = e -2G, so S= Ge -2G CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 4

5 Performance of Aoha Coides with the start of the shaded frame t Coides with the end of the shaded frame t o t o + t t o + 2t t o + 3t Time Vunerabe From: Computer Networks, 3 rd Ed. by Andrew S. Tanenbaum, (c)1996 Prentice Ha. CMSC S97 (ect 18) S (throughput per frame time) 0.40 Sotted ALOHA: S = Ge G Pure ALOHA: S = Ge 2G G (attempts per packet time) From: Computer Networks, 3 rd Ed. by Andrew S. Tanenbaum, (c)1996 Prentice Ha. copyright 1997 Jeffrey K. Hoingsworth 5

6 Aoha (cont.) Sotted Aoha Use a centra cock Each station ony sends at the start of frame Reduces coision window by 1/2 S = G e -G CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 6

7 Carrier Sense Mutipe Access ook before you eap! don t send if someone ese is sending coisions are sti possibe propagation deay induces uncertainty into sensing possibe two hosts both start sending at the same time persistence: when to send after detecing channe in use 1-persistent as soon as the channe is free, starting sending nonpersistent CSMA if channe is sensed busy, wait a random time and try again p-persistent CSMA if sot is ide send with probabiity p, ese wait for next ide sot CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 7

8 Impact of Carrier Sense S (throughput per packet time) Pure ALOHA Sotted ALOHA 0.5-persistent CSMA 1-persistent CSMA 0.01persistent CSMA Nonpersistent CSMA 0.1-persistent CSMA G (attempts per packet time) From: Computer Networks, 3 rd Ed. by Andrew S. Tanenbaum, (c)1996 Prentice Ha. CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 8

9 Coision Detection If a sender senses a coision stop sending at once appy random backoff contention period after contention period, there wi be no coision send for for 2τ (max propagation deay) need 2τ since might be a coision at far end at τ ε CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 9

10 Coision Free Protocos Use an aocation scheme must be dynamic (based on oad) or we are reduced to TDM Bit Map Reservation Protoco round of aocation (contention period) everyone who indicated a desire to send goes in turn requires an overhead of one bit per per station per round Binary Countdown reservation round send your host address uses a wired or to compute winner as soon as a station senses a 1 where it sent 0 it backs off winner sends packet gives higher priority to higher numbered hosts can rotate station number after successfu transmission CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 10

11 Wireess Shared Channes A B C Every node may node be in range of every other node a is in range to send to b, but not c b can send to a or c c can send to b Coisions carrier sense wi not work due to range must avoid any host sending that is in rang of sender or receiver CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 11

12 Wireess Networks (MACA) Stations send data into the air not a stations can see a other stations Need to avoid coisions between sender an receiver possibe for the sender to not be abe to sense coision Use a two stage protoco send a RTS (request to send) receiver responds CLS (cear to send) Hosts that hear a RTS or CLS wait and don t send coisions sti possibe since two RTS frames may coide CMSC S97 (ect 18) copyright 1997 Jeffrey K. Hoingsworth 12