A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks

By Gavin Holland, Nitin Vaidya and Paramvir Bahl

Presented by: Helal chowdhury

Telecommunication Laboratory, university of oulu

Contents

• Introduction

• Receiver Based Auto Rate (RBAR)

• RBAR implementation

• Simulation

• Conclusions

• References

Introduction

• IEEE802.11– Supports DSSS, FHSS , and IRDA at the physical layer.– RTS/CTS hand-shake.– Transmission rate 10Mbits/s.

• Rate Adaption– Rate adaption is the process of dynamically switching data rates to match the channel

conditions. There are two aspects to rate adaption:– Channel quality estimation

• By Sender• By receiver-> RBAR(Receiver Based Auto rate)

– Rate Selection• By Sender ->ARF(Auto rate Fallback)• By Receiver -> RBAR(Receiver Based Auto rate)

• Why receiver based rate adaption– The goal of rate adaption is to provide optimum throughput. The motivations for RBAR

• Rate selection can be improved by proving more timely and more complete channel quality.• Channel quality information is best acquired at the receiver.• Transmitting channel quality information to the sender can be costly.

RBAR modified DCF Protocol

DRTS: Reservation time (IEEE 802.11)

DCTS: Reservation time

DRSH: Final reservation Time

DCF: To coordinate the transfer of data packet.

NAV: To announce the duration of packet.

DRTS: Tentative reservation time (RBAR)

RBAR EVENT FLOW• S choose a data rate r1, using some heuristic,

and sends r1 and the size of the data packet n in the RTS to R.

• A, overhearing the RTS, uses r1 and n to calculate the duration of the reservation, marking it as tentative.

• R, having received the RTS, uses some channel quality estimation and rate selection technique to select the best rate r2 for the channel conditions, and sends r2 and n in the CTS to S.

• B, overhearing the CTS, calculates the reservation using r2 and n.

• S responds to the CTS by placing r2 into the header of the data packet and transmitting the packet at the selected rate. If r1≠r2, S uses a unique header signaling the rate change.

• A, overhearing the data packet, looks for the unique header. If it exists, it recalculates the reservation to replace the tentative reservation it calculated earlier.

S R Br1, nr1, n

r2, n

r2, n r2, n

r2, n

ACK

A

RBAR MAC Header

Framl

control

Duration Dest.

Address

Source

Address

BSSID Sequnce

control

Body FCS

Framl

control

Duration Dest.

Address

Source

Address

BSSID Sequnce

control

HCS Body FCS

IEEE 802.11 MAC Header

RBAR MAC Header

RBAR Reservation SubHeader

RBAR RTS/CTS Implementation

Frame

control

Duration Dest.

Address

Source

Address

FCS

IEEE 802.11 RTSRBAR RTS

Rate &

Length

Frame

control

Duration Dest.

Address

FCS

IEEE 802.11 CTSRBAR CTS

Rate &

Length

In RBAR, instead of carrying the duration of the reservation , the packets carry the modulation rate and the size of the data packet.

If there is rate mismatch between sender and receiver DRTS refer to as tentative reservation.

Final reservations are confirmed by the presence or absence of Reservation SubHeader (RSH).

RBAR PLCP Header

Sync SFD Signal Service Length CRC

802.11 PLCP header

Data

Rate

RSH

Rate

RBAR PLCP header

In standard 802.11, the PLCP header contains an 8 bit signal field.

In RBAR, the PLCP header has been divided into two 4 bit rate subfields.

Thus, the PLCP transmission protocol is modified as follows: when the MAC passes a packet down to the physical layer, it specifies two rates, one for the subheader and one for the remainder of the packet.

Simulation Model

• Error Model– Fast fading Channel model– Slow Fading Channel model

• Movement Model– random waypoint mobility pattern

• Trafic Model– CBR– FTP– ON/OFF Pareto source generating traffic

Slow fading Channel

Fast Fading Channel

Variable Traffic Source

Multi-Hop Performance

Conclusions

• Showed that RBAR improve network throughput.• RBAR outperforms ARF.

References

• Gavin Holland, Nitin Vaidya, Paramvir Bahl, ” A Rate-Adaptive MAC Protocol for Wireless Networks,” Technical Report TR00-019, Dept. of Computer Science, Texas University.

• wsl.stanford.edu/~ee360/adaptiveMAC_Jie.ppt• B. Sadeghi, V.Kanodia, A.Sabharwal, and E. Knightly,”Opportunistic

Media Access for Multirate Ad Hoc Networks”, Department of Electrical and Computer Engineering, Rice University.