SEEDEX: A MAC protocol for ad hoc

networks

SEEDEX: A MAC protocol for ad hoc

networksR. Rozovsky and P. R. Kumar ACM ACM MobiHoc 2001

speaker: Chien-Wen

Outline• INTRODUCTION• THE IEEE 802.11 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 802.11 PROTOCOL

THE IEEE 802.11 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'sneighbors 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 gen

erator– 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 transmitElse

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 802.11

SIMULATION RESULT ： PERFORMANCE COMPARISON OFSEEDEX-R WITH IEEE 802.11

SIMULATION RESULT ： PERFORMANCE COMPARISON OFSEEDEX-R WITH IEEE 802.11

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

LL

T R

PT L

L

L

L

PT

α /n+1

Hidden

Probability ！！

Disscusion

TR

PT

L

L

L

L

T R

PT L

L

L

L

Exposed

transmitt ！！

Disscusion

T

RPT

L

L

L

L

T R

L

L

L

Collision??

PT

PT

α /2

Disscusion

T

RPT

L

L

L

L

T R

PT L

L

L

Probability ！！

PT

α /2