Effects of Multi-Rate in Ad Hoc Wireless Networks

Baruch Awerbuch, David Holmer, Herbert Rubens

Center for Networking and Distributed SystemsComputer Science Department

Johns Hopkins UniversityTechnical Report 2003

Contents

Introduction Background

IEEE 802.11 Receiver Based Auto Rate (RBAR) Opportunistic Auto Rate

Performance evaluation Conclusions

Introduction Wireless Trend

Enable devices to operate using many different transmission rates.

Multi-rate capability Transmission can take place

At a number of ratesAccording to channel condition

IEEE 802.11 supports it at the physical layer MAC mechanisms are required to exploit it

ARF (Auto Rate Fallback)RBAR (Receiver Based Auto Rate)OAR (Opportunistic Auto Rate)

Background (1/3) IEEE 802.11 media access

RTS/CTS mechanism Network Allocation Vector (NAV)

RTS

CTS

DATA

ACK

NAV (RTS)

NAV (CTS)others

destination

source

SIFS SIFS SIFS DIFS

Channel accesswith backoff

Delayed medium access

Background (2/3)

Receiver Based Auto Rate (RBAR) Receiver selects transmission rate Use latest channel condition Additional reservation sub header (RSH)

source destination

Choose a rate based on heuristic

RTS

Analysis of RTS & SINR and determine data rate

CTS

RSH+DATA

ACK

Background (3/3)

RTS

CTS

RSH

ACK

NAV (RTS)

NAV (CTS)others

source

SIFS SIFS SIFS DIFS

Delayed medium access

destination

DATA

NAV (RSH)

Opportunistic Auto Rate (1/4)

Key idea Based on RBAR Opportunistically exploit high quality channel

when they transmit multiple packets Must limit the extent of holding the channel Use fragmentation mechanism of IEEE802.11

Opportunistic Auto Rate (2/4) Fragmentation in IEEE 802.11

Fragmentation fieldsmore-fragments, fragment number, duration time

RTS

CTS

DATA/FRAG1

ACK1

NAV (RTS)

NAV (CTS)others

destination

source

SIFS SIFS SIFS DIFS

Delayed medium access

DATA/FRAG2

ACK2

SIFS SIFS

NAV (DATA)

NAV (ACK)

Opportunistic Auto Rate (3/4) Issues

When there are no data packets available in the interface queueReset the more-fragmentsReverts back to the default RBAR protocol

When channel condition significantly change during multi-packet-transmission Continually monitor the channel qualityUse additional RSH message to notify the receiver

and adapt the rate

Opportunistic Auto Rate (4/4) Example

Node 1 has a good channel (11Mbps) Node 2 has a poor channel (5.5Mbps)

RTS

CTS

NODE1(PKT0)

ACKRBAR

OAR NODE1(PKT1)CTS ACK0 ACK1 NODE1(PKT2)

RTS NODE1 ACKRTS NODE2CTS

11Mbps 11Mbps 11Mbps

11Mbps 5.5 Mbps

Random backoff time

Multi Rate Problem

Distance is the primary factor Multi-rate devices must have protocols that

select the appropriate rate for a given situation.

High transmission rate Effective transmission range

High Speed Long Range

Introduction

Infrastructure based networks Single rate nodes have the ability to select the

best access point based on the received signal strength.

Multi-rate only need to add selecting the actual rate used to communicate.physical geometryreact to the existing channelreliably

Introduction

Ad hoc multi-hop wireless networks the routing protocol must select from the set of

available links to form the path between the source and the destination

Long distance = Few hops, but low speed. Short links = High rates, but more hops.

Multi-Rate Model

Model is based on the 802.11b standard NS2 simulations Compare with RBAR and OAR Lucent ORiNOCO PC Card

Assumption

Minimum Hop Path

Throughput Loss The selection of minimum hop paths typically

results in paths where the links operate at low rates.

Reliability Loss Small broadcast packets to establish/maintain

routing, device will have high bit error rate

Throughput Phenomena

MAC Only a single transmission can occur at a time within

range of the intended receiver.

Simulation

1472 byte UDP Packets flood across a single link.

Hops vs. Throughput Trade-off Highest transmission speed more hops

Even though high link rate paths must traverse more linksto reach the same distance, they still provide more throughput.

Temporal Fairness

One sender send at 1M and other send at 11M

Conclusion

Multi Rate could enable high network throughput.

The Multi-rate protocol need to consider more phenomena in routing deccisions. Hop path Throughput Quantitative Fairness

Thank You