ECEN 621 600 Mobile Wireless Networking Course Materials

ECEN 621 -600 “Mobile Wireless Networking” Course Materials: Papers, Reference Texts: Bertsekas/Gallager, Stuber, Stallings, etc Grading (Tentative): HW: 20%, Projects: 40%, Exam-1: 20%, Exam-II: 20% Lecture notes and Paper Reading Lists: available on-line Class Website: http: //ece. tamu. edu/~xizhang/ECEN 621/start. php Research Interests and Projects: URL: http: //ece. tamu. edu/~xizhang Instructor: Professor Xi Zhang E-mail: xizhang@ece. tamu. edu Office: WERC 331

Medium Access Control Protocols and Local Area Networks Part I: Medium Access Control Part II: Local Area Networks Part III: Wireless Local Area Networks

Medium Access Control Protocols, Local Area Networks, and Wireless Local Area Networks Medium Access Control Protocol: Slotted ALOHA: Throughput/Delay Performance Modeling and Analysis

Slotted ALOHA Time is slotted in X seconds slots Stations synchronized to frame times Stations transmit frames in first slot after frame arrival Ø Backoff intervals in multiples of slots Ø Ø Ø Backoff period k. X (k+1)X Vulnerabl eperiod t 0 +X+2 tprop B t t 0 +X+2 tprop+ B Time-out Only frames that arrive during prior X seconds collide Prof. Xi Zhang

Throughput of Slotted ALOHA 0. 368 S 0. 184 Ge-G Ge-2 G G Prof. Xi Zhang

Application of Slotted Aloha cycle . . . Reservation mini-slots X-second slot Reservation protocol allows a large number of stations with infrequent traffic to reserve slots to transmit their frames in future cycles Ø Each cycle has mini-slots allocated for making reservations Ø Stations use slotted Aloha during mini-slots to request slots Ø Prof. Xi Zhang

Carrier Sensing Multiple Access (CSMA) Ø A station senses the channel before it starts transmission Ø If busy, either wait or schedule backoff (different options) Ø If idle, start transmission Ø Vulnerable period is reduced to tprop (due to channel capture effect) Ø When collisions occur they involve entire frame transmission times Ø If tprop >X (or if a>1), no gain compared to ALOHA or slotted ALOHA Station A begins transmission at t=0 Station A captures channel at t = tprop A A Prof. Xi Zhang

CSMA Options Ø Transmitter behavior when busy channel is sensed Ø Ø Ø 1 -persistent CSMA (most greedy) Ø Start transmission as soon as the channel becomes idle Ø Low delay and low efficiency Non-persistent CSMA (least greedy) Ø Wait a backoff period, then sense carrier again Ø High delay and high efficiency p-persistent CSMA (adjustable greedy) Ø Wait till channel becomes idle, transmit with prob. p; or wait one mini-slot time & re-sense with probability 1 -p Ø Delay and efficiency can be balanced Sensing Prof. Xi Zhang

1 -Persistent CSMA Throughput S 0. 53 0. 45 Ø Better than Aloha & slotted Aloha for small a Ø Worse than Aloha for a > 1 a = 0. 01 0. 16 a =0. 1 G a=1 Prof. Xi Zhang

Non-Persistent CSMA Throughput a = 0. 01 S 0. 81 Ø Higher maximum throughput than 1 persistent for small a Ø Worse than Aloha for a > 1 0. 51 a = 0. 14 G a=1 Prof. Xi Zhang

CSMA with Collision Detection (CSMA/CD) Ø Monitor for collisions & abort transmission Ø Stations with frames to send, first do carrier sensing Ø After beginning transmissions, stations continue listening to the medium to detect collisions Ø If collisions detected, all stations involved stop transmission, reschedule random backoff times, and try again at scheduled times Ø In CSMA collisions result in wastage of X seconds spent transmitting an entire frame Ø CSMA-CD reduces wastage to time to detect collision and abort transmission Prof. Xi Zhang

CSMA/CD reaction time A begins to transmit at A t=0 B A detects collision at A t= 2 tprop- B B begins to transmit at t = tprop ; B detects collision at t = tprop It takes 2 tprop to find out if channel has been captured Prof. Xi Zhang