effects of multi-rate in ad hoc wireless networks baruch awerbuch, david holmer, herbert rubens...
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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
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.
Even though high link rate paths must traverse more linksto reach the same distance, they still provide more throughput.
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