Title: OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks
1OAR An Opportunistic Auto-Rate Media Access
Protocol for Ad Hoc Networks
- B. Sadeghi, V. Kanodia, A. Sabharwal, E. Knightly
- Presented by Sarwar A. Sha
2802.11b Transmission rates
- Different modulation methods for transmitting
data. - Binary/Quadrature Phase Shift Keying
- Quadrature Amplitude Modulation
- Each packs different quantities of data into the
modulation. - The highest speed has most dense data and is most
vulnerable to noise.
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Time
3Transmission Throughput
Image courtesy of G. Holland
- Why would a node ever want to slow down?
- Longer transmission distance
- More robust modulation
- Moving node rapidly changes channel conditions
- Must adapt to channel conditions based on SNR
4Background
- IEEE 802.11 multi-rate
- Support of higher transmission rates in better
channel conditions - Auto Rate Fallback(ARF)
- Use history of previous transmissions to
adaptively select future rates - Error free transmissions indicates high channel
quality - Lucent ARF implemention reduces rate after 2 lost
ACKs, then attempts to speed up after a time
interval -
- Receiver Based Auto Rate (RBAR)
- Use RTS/CTS to communicate a transmission rate
based on channel quality. Receiver determines
rate.
5Motivation
- Consider the situation below
- ARF?
- RBAR?
6Motivation
- What if A and B are both at 56Mbps, and C is
often at 2Mbps? - Slowest node gets the most absolute time on
channel?
Throughput Fairness vs Temporal Fairness
7Opportunistic Scheduling
- Goal
- Exploit short-time-scale channel quality
variations to increase throughput. - Issue
- Maintaining temporal fairness (time share) of
each node. - Challenge
- Channel info available only upon transmission
8Coherence Interval
- The time duration over which a channel is
statistically likely to remain stable. - This interval ranges from (122ms) - (5ms) based
on node motion at speeds of (1 m/s) - (20 m/s). - OAR was designed such that transmissions do not
exceed the coherence interval most of the time.
9Opportunistic Auto Rate (OAR)
- Poor connections transmit one data packet per
RTS/CTS connection. - Good connections, hence faster rate, transmit
multiple data packets. - But maintain temporal fairness between good bad
connections by balancing the time using channel,
not the number of packets. - i.e. (1 packet_at_2Mbps 5 fast packets_at_11Mbps)
- OAR Higher overall throughput, while maintaining
temporal fairness properties of single rate IEEE
802.11
10OAR Protocol
- Rates in IEEE 802.11b 2, 5.5, and 11 Mbps
- Number of packets transmitted by OAR
11OAR Protocol (RBAR Based)
- Review Receiver Based AutoRate (RBAR) Bahl01
- Receiver controls the senders transmission rate
- Control messages sent at Base Rate
12OAR Protocol (Multi-packet)
- OAR - Opportunistic Auto Rate
- Once access granted, it is possible to send
multiple packets if the channel is good
13Performance Comparison
R
D1
Transmitter
C
A
Receiver
14MAC Access Delay Simulation
- Back to back packets in OAR decrease the average
access delay - Increase variance in time to access channel
- Figure
- On the left is 2Mbps
- On the right is 5.5 Mbps
15Simulations
- Three Simulation experiments
- Fully connected networks all nodes in radio
range of each other - Number of Nodes, channel condition, mobility,
node location - Asymmetric topology
- Random topologies
- Implemented OAR and RBAR in ns-2 with extension
of Ricean fading model Punnoose et al 00
161 Fully Connected Setup
- Every node can communicate with everyone
- Each nodes traffic is at a constant rate and
continuously backlogged - Channel quality is varied dynamically
171 Fully Connected Throughput Results
- OAR has 42 to 56 gain over RBAR
- Increase in gain as number of flows increases
- Note that both RBAR and OAR are significantly
better than standard 802.11 (230 and 398
respectively) - Variation in line of sight (K), mobility, and
location distribution throughput all showed
improvements with OAR.
182 Asymmetric TopologySetup
Low speed (L)
High Speed (H)
B
A
- Asymmetric topology simulated above in 4
different combinations of channel conditions - A and B are simulated at slow (2Mbps) and fast
(11Mbps) - Each combination of slow/fast i.e. LL, HL, LH, HH
compared between A B concurrently communicating - Sender of Flow B hears A and knows when to
contend for channel, but sender in A has to
discover a time slot
192 Asymmetric Topology Results
- OAR maintains time shares of IEEE 802.11
- Significant gain over RBAR
203 Random TopologiesSetup
- A pair are moved across a communication range
- Nodes are uniformly distributed over area similar
to test setup 1
213 Random TopologiesResults
- Gains are similar as before despite changes
- Throughput is 40-50 improved as compared to RBAR
despite motion of a node pair.
22Integration with IEEE 802.11
- Options to hold the channel and send multiple
packets - Fragmentation
- A mechanism in IEEE 802.11 to send multiple
frames - Each frame/ACK acts as virtual RTS/CTS
- Use of more-fragment-flag in Data packets
- Contention window set to zero
- Packet bursting (802.11e)
- Transmit as many frames as you like up to
threshold
Method used in study
23Discussion Issues
- Not enough packets to fill a slot
- If running at Good 11Mbps with 5 packets
allowed, but only have 2 packets to send. Then
other nodes NAV tables are wrong (silent for 5
instead of 2). - Authors Fix More Fragments indicator in the
data packet. Upon hearing, nodes revert to RBAR. - Problem Hidden terminals would still have
incorrect NAV tables, and would remain silent
longer than needed. (Unless the data ACK has a
More Fragments ACK.)
24Discussion Issues
- Channel condition changes during multi-packet
transmission. - Channel gets worse
- Later packets get corrupted
- Channel gets better
- Wasted channel capacity waiting for packets to
finish - Authors propose adding RSH messages to notify
receiver of these updates and adapt the rate. - The RSH is in the header of the data packet, and
would allow changing speed mid transmission.
25Discussion Issues
- Ad Hoc Networks considerations
- Needed more variety in the network topology.
Fully connected isnt very interesting in Ad Hoc
Networks - Data traffic patterns. I.e. short bursts of
traffic vs continuous traffic. - No power considerations studied or mentioned
26Discussion Issues
- Increase variance in time to access channel
- Real-time traffic (like voice) is impacted.
Sometimes there would be more delay before you
hear something. - Short term fairness gets worse!
- Trade throughput for a higher worst case time to
access channel