SEEDEX: A MAC protocol for ad hoc networks R. Rozovsky and P. R. Kumar ACM ACM MobiHoc 2001 speaker: Chien-Wen

Outline INTRODUCTION THE IEEE PROTOCOL THE SEEDEX MAC PROTOCOL SIMULATION CONCLUSION

INTRODUCTION to avoid collisions without making explicit reservation to employ a random schedule pseudo-random number generator exchanging the seeds within a two- hop neighborhood

THE IEEE PROTOCOL

the wasteful scenarios –two neighborhoods are silenced –four way handshake –Backoff scheme

SEEDEX: CHOOSING A RANDOM SCHEDULE preface –all nodes could publish their schedules –two states L: listening PT: possibly sending

choosing a slot as follows –Node T is in state PT –Node R is in state L –All of node R's neighbors are in state L SEEDEX: CHOOSING A RANDOM SCHEDULE

use an i.i.d. Bernoulli sequence With probability p to be PT state using random schedule SEEDEX: CHOOSING A RANDOM SCHEDULE

THE CENTRAL IDEA OF SEEDEX the i.i.d. Bernoulli schedule –the use of a pseudo-random number generator –initial value is called the “ seed ” publishing random schedules by exchanging seeds

THE CENTRAL IDEA OF SEEDEX exchanging seeds –two-hop neighborhood –fan-in and fan-out fan-out –broadcasts the seeds to all its neighbors fan-in –hearing a similar broadcast from each of its neighbors

SEEDEX: CHOOSING A RANDOM SCHEDULE

WHEN DOES A NODE TRANSMIT A PACKET? Waiting for a slot –Sender is in PT state –Receiver is in L state –if Other neighbors of Rx are in L state then transmit Else Min{α/n+1, 1} //n is the num. of others which are in PT state

WHEN DOES A NODE TRANSMIT A PACKET? The use of α –a Possibly Transmit state –in light traffic α ≒ 2.5 –in heavy traffic α ≒ 1.5

WHAT IS A GOOD CHOICE OF P? node T is in PT state

WHAT IS A GOOD CHOICE OF P? node R is in L state

WHAT IS A GOOD CHOICE OF P? j other neighbors are in PT state, and the remaining neighbors are in L state

WHAT IS A GOOD CHOICE OF P? Only node T decides to trans.

WHAT IS A GOOD CHOICE OF P? When N=6, p =0.246

WHAT IS A GOOD CHOICE OF P? Optimal p =0.21

SIMULATION RESULT ： SOME PERFORMANCE NUMBERS Conducted on NS 100 nodes chooses a random neighboring recipient for each packet

SIMULATION RESULT ： SOME PERFORMANCE NUMBERS

USING SEEDEX FOR RTS RESERVATIONS using SEEDEX only on the RTS packet

SIMULATION RESULT ： PERFORMANCE COMPARISON OF SEEDEX-R WITH IEEE

CONCLUSION Avoid making reservations Does not employ backoff scheme Does not require silencing both neighbors of Tx and Rx

Futher Investigation Fan-in procedure The adaption of α Impact of topology

Disscusion T R PT L L L L L T R L L L L α /n+1 Hidden Probability ！！

Disscusion T R PT L L L L T R L L L L Exposed transmitt ！！

Disscusion T R PT L L L L T R L L L Collision?? PT α /2

Disscusion T R PT L L L L T R L L L Probability ！！ PT α /2