An Overlay MAC Layer for 802.11 Networks

Ananth Rao, Ion Stoica

OASIS Retreat, Jun 2004

Motivation

Internet Gateway

802.11 hardware provides initial ease of deployability for many applications Mesh networks Long haul links Large Infrastructure

Networks Are these apps stretching

802.11 beyond its design goals (Wireless LANs)?

Problem 1: Different Data Rates

Data Rate Throughput

Case IA 11 Mbps 3.09 Mbps

B 11 Mbps 3.36 Mbps

Case IIA 11 Mbps 0.76 Mbps

B 1 Mbps 0.76 Mbps

R

BA

Problem 2: Unpredictability

21 3 4 5

Problem 3: Forwarding on Behalf of Others

Ethernet

1/2

1/6 1/6 1/6

1/3

1/91/9

1/9

This problem cannot be solved by local scheduling or queue management algorithms like WFQ

Case for an Overlay MAC (OML)

Several new MACs have been proposed to solve these problems We try and leverage some of these techniques in OML

Hardware vs. Software MACs Huge cost advantage over building a new MAC Flexible and easy to implement application specific policies Facilitates experimental research on MAC layer issues Accurate timing not possible at the software level Devices don’t expose all information (eg. cannot carrier-sense

and obtain result) Cannot change the physical layer (eg. spread spectrum

techniques)

Constraints Do not want to modify the MAC protocol We have no control over a packet once it is in

the card’s buffer It may be queued behind another packet

Solution: Disable buffering in the lower layers

There may be some back-off and retransmission Solution: Ensure that with high probability, there will be no

back off or retransmissions

Use reservation-based time slots to schedule transmissions

Time Slots

Assume local synchronization of clocks Use coarse-grained (compared to packet

transmission times) time slots 20ms slots, 2ms to transmit an MTU sized packet

Clocks are synchronized by estimating the 1-way delay of packets Assume back-off is negligible and there are no

retransmissions at the MAC layer Use the minimum delay from the previous 20 packets

to reduce errors

Weighted Slot Allocation

Algorithm to allocate time-slots to competing nodes Only requires knowledge of the IDs of other

competing nodes – no additional signaling Uses a pseudo-random hash function

For ease of explanation Stage 1: Assume the diameter of the network is one Stage 2: Multi-hop networks of larger diameter

WSA – Diameter One Network Each node keeps track of an active_list, the set

of nodes with active flows Each packet includes the current queue length in the

OML header

Use a pseudorandom hash function to decide the winner

The node with the highest Hi is allowed to transmit

WSA – Multi-hop Network

Use the same hash-based mechanism active_list only contains other nodes that compete

with a given node Consider only the hash values of competing nodes in

deciding the winner

Assume that nodes interfere only up to k-hops k is a tradeoff between losing possible channel reuse

opportunities, and using the underlying MAC to resolve contention

Partially Overlapping Contention Regions

ijk

Hi > Hj > Hk

•1-hop neighbors of the winner initiate an “inactivity timer” to detect when this happens

•If i is more than 1-hop away from j, it is notified using a control message Contention region of k

Contention region of j

Persistent Allocation of SlotsTime

0 ms

24 ms

48 ms

72 ms

•Slots maybe

•Available for contention

•Assigned to a particular node

•If the nodes queue goes empty, the rest of the slot is used by the node with the next highest hash

1 2 3 4 7 865

Groups of 8 slots each of length 3ms

Amortize the Cost of Contention Resolution

Time

0 ms

24 ms

48 ms

72 ms

•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

1 2 3 4 7 865

C C

C CC CC CC CC

C C

C C

C

More Details

The protocol is “optimistic” in assigning a slot to a node If a slot is assigned to more than 1 node, 802.11 will

still do its best to resolve contention If this happens very often, resource allocation goals

may not be met Queues vs. Flows

Because of TCP congestion control, queues become empty and full very often

If the queue is empty, a node will signal that other nodes may send in that slot

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 20 ms each, p=0.05

Fairness in a Multi-hop network

Nearly 40% of the flows receive less that 100 kbps, while others receive

up to 3 Mbps without OML

The disparity between 1-hop and 4-hop flows is reduced from

8 to 2 when using OML

0

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fra

ctio

n o

f flo

ws

Throughput (Mbps)

30 nodes - w/o OML30 nodes - with OML50 nodes - w/o OML50 nodes - with OML

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

1 1.5 2 2.5 3 3.5 4 4.5 5

Th

rou

gh

pu

t (M

bp

s)

Hops

30 nodes - w/o OML30 nodes - with OML50 nodes - w/o OML50 nodes - with OML

Flexibility in resource allocation

Qualitative differentiation is possible by assigning different

weights to flows

Different allocation policies can result in very different outcomes –

hence the MAC layer must be flexible

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0 0.1 0.2 0.3 0.4 0.5 0.6

Fra

ctio

n o

f flo

ws

Throughput (Mbps)

Weight 1 flowsWeight 2 flowsWeight 3 flows

0

0.1

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Fra

ctio

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Throughput (Mbps)

Allocation by nodeAllocation by flow

Test-bedHardware

•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

Click Architecture

EncapTimestamp

DecapTimestamp

TimeslotEnforcerContentionResolver

FromDevice

ToDevice

Mixed Queue

Rest of the Router

1

1Push

Pull

LIFO

FIFO

SetTimeOffset

SetOKSlots

Results (Test-bed – Data Rates)

0

2

4

6

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14

54 36 18 12 6

Data Rate of Node 2

TC

P T

hro

ug

hp

ut

(Mb

ps

)

Node 1 Node 2

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2

4

6

8

10

12

14

54 36 18 12 6

Data Rate of Node 2

TC

P T

hro

ug

hp

ut

(Mb

ps

)

Node 1 Node 2

Equal throughput – default 802.11 Equal channel access time - OML

OML can achieve fairness in terms of transmission time, or in terms of throughput

Results (Testbed – Fairness, Flexibility)

•In 26% of the experiments, one flow was completely shut out by the other

•k=2 leads to underutilization of the channel in some cases

When all nodes can hear each other, WSA guarantees weighted fairness

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Fra

ctio

n o

f flo

ws

Throughput (Mbps)

No OMLOML - 1 hop signalingOML - 2 hop signaling

Results (Test-bed)

A B

C

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 performance problems better though more measurements on the test-bed

More benchmarks of OML