Scheduling Transmissions in 802.11 Networks

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Scheduling Transmissions in 802.11 Networks

• Ananth Rao
• SAHARA Retreat, Jan 2004

2
Motivation

• Multi-hop networks based on IEEE 802.11 standards
  have a lot of potential
• Hardware is inexpensive
• Many interesting applications
• Recently, lots of thrust from the research
  community (MIT, MSR, Intel..)

Internet Gateway
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Problem

• 802.11 MAC does not perform well for multi-hop
  networks
• Real test-beds have been plagued with performance
  problems (Roofnet, MSR Testbed)
• Throughput (poor contention resolution)
• Fairness (longer routes get very low throughput)
• Goal Improve multi-hop 802.11 throughput and
  fairness

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Constraints

• Do not want to modify the MAC protocol
• Takes a lot of time and effort to standardize
• Must be compatible with the 802.11 MAC
• Use readily available low-cost hardware
• The only control we have is When to ask the
  card to send a packet
• Do some form of scheduling on top of 802.11

Use an Overlay for the MAC Layer
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Overhead of Contention-Resolution
Sender
Receiver
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Hidden Terminal Problem

• CTS may not be received clearly for the following
  reasons
• Node is within interference but outside
  communication range
• Another transmission interferes with reception of
  CTS (loss rates as high as 60 seen in
  simulations)

S
R
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Fairness Problems

• The 802.11 MAC gives roughly equal number of
  transmission opportunities to competing stations
• This results in undesirable outcomes when
• Senders use different packet sizes
• Senders are transmitting at different rates
• Senders are forwarding traffic from other nodes

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Different Data Rates
Data Rate Throughput
Case I A 11 Mbps 3.09 Mbps
Case I B 11 Mbps 3.36 Mbps
Case II A 11 Mbps 0.76 Mbps
Case II B 1 Mbps 0.76 Mbps
R
B
A
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Forwarding on Behalf of Others
Ethernet
1/2
1/2
1/6
1/6
1/6
1/6
1/6
1/6
This problem cannot be solved by local scheduling
or queue management algorithms like WFQ
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Related Work and Challenges

• Collision-free MACs
• A Channel Access Scheme for Large Dense Packet
  Radio Networks (1998), Timothy J. Shepard
• Channel Access Scheduling in Ad Hoc Networks with
  Unidirectional Links (2001), Lichun Bao, J.J.
  Garcia-Luna-Aceves
• New Challenges
• Accurate timing not possible at the software
  level
• Devices dont expose all information (eg. cannot
  carrier-sense and obtain result)
• Senders from other networks might interfere
  Polling messages might be lost
• No changes to physical layer (spread spectrum
  techniques)

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Time Slots on Top of 802.11
Time

• Assume local synchronization of clocks
• Use coarse-grained (compared to packet
  transmission times) time slots
• Slots maybe
• Available for contention
• Assigned to a particular node
• If the nodes queue goes empty, the rest of the
  slot is open to all

1
2
3
4
7
8
6
5
0 ms
24 ms
48 ms
72 ms
Groups of 8 slots each of length 3ms
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Amortize the Cost of Contention Resolution

• Nodes that transmitted successfully in the
  previous slot with index i own the slot with
  probability (1-p)
• Cost is amortized because
• A time-slot is much longer than a packet
  transmission
• Nodes compete for an average of 1/p slots at a
  time
• Orthogonal to method used to resolve contention
  for a slot

Time
1
2
3
4
7
8
6
5
C
2
C
X
0 ms
2
24 ms
48 ms
72 ms
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Synchronization of Clocks
Every packet in encapsulated in a new header
which contains a timestamp
IP hdr .
... MAC hdr
ts
tsAuth
tsDist
Initialize() myTsAuth myNodeId myTsDist
0 Recv(Packet p) tDiff estimatedTransitTime(p
) if(p.tsAuth lt myTsAuth (p.tsAuth
myTsAuth p.tsDist lt myTsDist)) myTime
p.ts tDiff myTsAuth p.tsAuth myTsDist
p.tsDist1
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Competing for and Relinquishing a Slot

• Use 2-hop broadcasts to request a slot or to
  announce giving up a slot
• The probability of winning a slot is based on the
  current of slots owned and the weight of the
  competing node
• Compete for a slot on
• Receiving a relinquish message
• Think slot is free and no packets are seen for
  0.5ms after start of a slot
• Immediate neighbors may stop the broadcast if it
  is somebody elses slot

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Simulation Results

• Qualnet Network Simulator
• Commercial software from www.scalable-networks.com
  
• Packet level simulator similar to ns2, but faster
  and more scalable
• Models collisions, interference and contention
• Use 802.11a at 54 Mbps
• 20 slots of 3 ms each, p0.05

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Performance in a Chain
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Performance in a Multi-hop Network (Throughput
Fairness)
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Performance in a Multi-hop Network (Collisions)
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Testbed

• Hardware
• ASUS Pundit barebones system
• Celeron 2.4 Ghz, 256 MB
• Netgear WAG511, 802.11a

• Software
• RH 9.0, Kernel 2.4.22
• Madwifi driver for Atheros
• Click modular router

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Click Architecture
Push
1
Pull
FromDevice
DecapTimestamp
ToDevice
ContentionResolver
TimeslotEnforcer
EncapTimestamp
Rest of the Router
1
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Results (Testbed Data Rates)
With Scheduling
Without Scheduling
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Results (Testbed Chain)
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Conclusions and Future Work

• Coarse-grained scheduling on top of 802.11 is a
  very powerful technique to
• alleviate inefficiencies of the MAC protocol in
  resolving contention
• overcome the lack of flexibility of assigning
  priorities to senders
• Future work
• Understand the performance problems in multi-hop
  networks better using the test-bed
• Further refine the algorithms for allocation of
  slots and the implementaion