1. Effects of Multi-Rate in Ad Hoc Wireless Networks
Baruch Awerbuch, David Holmer, Herbert Rubens
Center for Networking and Distributed Systems Computer Science Department Johns Hopkins University
Technical Report 2003

2. Contents Introduction Background Performance evaluation Conclusions
IEEE
Receiver Based Auto Rate (RBAR)
Opportunistic Auto Rate
Performance evaluation
Conclusions

3. Introduction Wireless Trend Multi-rate capability
Enable devices to operate using many different transmission rates.
Multi-rate capability
Transmission can take place
At a number of rates
According to channel condition
IEEE 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)
Multi-rate capability.
802.11b
802.11a
802.11g
HiperLAN2

4. Background (1/3) IEEE 802.11 media access RTS/CTS mechanism
Network Allocation Vector (NAV)
SIFS
SIFS
SIFS
DIFS
RTS
DATA
source
CTS
ACK
destination
NAV (RTS)
others
NAV (CTS)
Channel access
with backoff
Delayed medium access

5. Choose a rate based on heuristic and determine data rate
Background (2/3)
Receiver Based Auto Rate (RBAR)
Receiver selects transmission rate
Use latest channel condition
Additional reservation sub header (RSH)
RTS
RSH+DATA
source
destination
CTS
ACK
Choose a rate based on heuristic
Analysis of RTS & SINR
and determine data rate

6. Background (3/3) RTS RSH DATA source CTS ACK destination NAV (RTS)
SIFS
SIFS
SIFS
DIFS
RTS
RSH
DATA
source
CTS
ACK
destination
NAV (RTS)
others
NAV (CTS)
NAV (RSH)
Delayed medium access

7. 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

8. Opportunistic Auto Rate (2/4)
Fragmentation in IEEE
Fragmentation fields
more-fragments, fragment number, duration time
SIFS
SIFS
SIFS
SIFS
SIFS
DIFS
RTS
DATA/FRAG1
DATA/FRAG2
source
CTS
ACK1
ACK2
destination
NAV (RTS)
NAV (DATA)
others
NAV (CTS)
NAV (ACK)
Delayed medium access

9. Opportunistic Auto Rate (3/4)
Issues
When there are no data packets available in the interface queue
Reset the more-fragments
Reverts back to the default RBAR protocol
When channel condition significantly change during multi-packet-transmission
Continually monitor the channel quality
Use additional RSH message to notify the receiver and adapt the rate

10. Opportunistic Auto Rate (4/4)
Example
Node 1 has a good channel (11Mbps)
Node 2 has a poor channel (5.5Mbps)
11Mbps
11Mbps
11Mbps
RTS
CTS
NODE1(PKT0)
ACK0
NODE1(PKT1)
ACK1
NODE1(PKT2)
OAR
11Mbps
5.5 Mbps
RTS
CTS
NODE1
ACK
RTS
CTS
NODE2
ACK
RBAR
Random
backoff time

11. 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

12. 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 geometry
react to the existing channel
reliably

13. 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.

14. Multi-Rate Model Model is based on the 802.11b standard
NS2 simulations
Compare with RBAR and OAR
Lucent ORiNOCO PC Card

15. Assumption

16. Real world ranges are smaller
non-zero system loss
additional noise sources
Obstructions
propagation effects

17. Minimum Hop Path Throughput Loss Reliability 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

18. Throughput Phenomena MAC
Only a single transmission can occur at a time within range of the intended receiver.

19. Simulation
1472 byte UDP Packets flood across a single link.

20. In 802.11 overhead is composed of
timespent transmitting control frames
random back-off time during contention resolution
time wasted as a result of collisions.
total throughput in the wireless network
the number of non-interfering transmissions that can simultaneously occur

21. total throughput in the wireless network
the number of non-interfering transmissions that can simultaneously occur

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

23. Even though high link rate paths must traverse more links
to reach the same distance, they still provide more throughput.
0.4
2.2
Even thought high link rate path must traverse more links to reach

24. Temporal Fairness One sender send at 1M and other send at 11M
MAC Protocol attempts to provide fairness to individual senders on a per packet basis

25. 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

26. Thank You