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On the advantage of multi-hop extensions to IEEE 802.11

infrastructure modeSathya Narayanan

PanasonicPei Liu

Shivendra PanwarPolytechnic University

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Overview We study the effect of using multi-hopping

Instead of reducing the transmission rate of nodes use multi-hop (two-hop) transmissions at highest rate (11 Mbps).

The SIR (Signal-To-Interference Ratio) experienced by nodes when the transmission power is reduced at edge nodes to reach an intermediate node, rather than the access point directly. This improves the throughput of nodes at the edge of the network.

Important note: Advantages discussed from both forwarding nodes and source nodes perspective.

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IEEE 802.11 MAC

Infrastructure mode and ad hoc mode

DCF Operation Multiple rates

11 Mbps, 5.5. Mbps, 2 Mbps, 1 Mbps Single hop MAC

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Advantages of multi-hop extension

Studying the effect of multi-hop extensions w.r.t The network throughput in a multi-

rate network The SIR experienced by edge nodes

Initial study demonstrates possible improvements in both area Result in next few slides …

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Simulation

24 nodes – AP in the middle Manual rate selection High speed nodes at 11 Mbps Low speed nodes at 1 Mbps

Increase the number of slow nodes to study impact on throughput

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Avg. throughput per node (Total 24 nodes)

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Number of nodes at 1 Mbps

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Analytical

Simulation

Average Throughput per node drops for all nodes when slower nodes are introduced in the network

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Total useful throughput of the network

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1 3 5 7 9 11Number of nodes at 1 Mbps

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Total througput of thenetwork with slownodes

Total througput of thenetwork usingforwarding with allnodes @ 11 Mbps

Total useful throughput of the network increases when multi-hop (two) is used maintaining higher data rate at all nodes

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Forwarding node

Increase in total throughput is not an incentive for the forwarding node to participate in multi-hop scheme.

What is the effect on the throughput experienced by forwarding node?

Assumption: Each forwarding node only has to support one slow node

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Avg. traffic per forwarding node - Total 24 nodes

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Number of nodes at 1 Mbps

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Average data trafficper node with slownodes

Average traffic forforwarding nodes (and slow nodes) -current MAC

Average traffic forforwarding nodes (and slow nodes) -new opportunisticMAC

Current MAC: The available throughput at the forwarding node is equally shared with the slow node.

Opportunistic MAC: The total available throughput of the network is equally shared among all nodes

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Bandwidth improvement for the forwarding node

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-40000

-20000

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Number of nodes at 1 Mbps

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Bandwidthimprovement -currentMAC

Bandwidthimprovement - newOpportunistic MAC

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Percentage improvement

-60-40-20

020406080

100120

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Number of nodes at 1 Mbps

Per

cent

age

incr

ease

Percentageimprovement - currentMAC

Percentageimprovement-newopportunistic MAC

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A

B

A

C

A

C

B

C

A

B

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C

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d

ii

i

Rayleigh fading model

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Simulation

C Program Calculate interference at each

point in the cell from proximal cells at the same channel Each point represented by polar co-

ordinates R = 1:100:1 Theta = 1:360:1

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b

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Downstream Coverage

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Downstream coverage with reduced transmission power at edge node

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Upstream coverage

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Upstream coverage using multi-hop and reduced transmission power

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Review comments

Simple answer to most of the comments: Work in progress ;)

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Reviewer 1: Not clear on: 1. how the network topology would impact the

results and in general the effects of multi-hop extensions.

2. What is the performance v.s. cost tradeoffs of the proposed solution.

3. how would things change if each forwarding node can be responsible for more than one slower node.  Would there still be enough benefits for the forwarding nodes?

4. Results are based on analysis/simulation. Any verification using experiment done or planned in the future? How would that be done?

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Reviewer 1: Some response

Not clear on:

1. how the network topology would impact the results and in general the effects of multi-hop extensions.

2. What is the performance v.s. cost tradeoffs of the proposed solution.

3. how would things change if each forwarding node can be responsible for more than one slower node.  Would there still be enough benefits for the forwarding nodes?Response: That was a simplifying assumption. As long

as the forwarding node shares only a part of its available bandwidth it will have the benefits. If more than one slow node use the same forwarding node, the slow nodes could probably loose their benefit.

4. Results are based on analysis/simulation. Any verification using experiment done or planned in the future? How would that be done?

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Reviewer 1: Some response

Not clear on:

1. how the network topology would impact the results and in general the effects of multi-hop extensions.

2. What is the performance v.s. cost tradeoffs of the proposed solution.

3. how would things change if each forwarding node can be responsible for more than one slower node.  Would there still be enough benefits for the forwarding nodes?

4. Results are based on analysis/simulation. Any verification using experiment done or planned in the future? How would that be done?Answer: More open questions left in the

simulation/analysis area (some raised here). Plan is to address those questions first. No plan for experimental study right now.

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Reviewer 2: The capacity and coverage improvement of

multi-hop networks has been well documented. The paper presents a opnet simulation of 802.11 systems in a multi-hop system.

1. The problem with multi-hop systems  however are additional delays and additional collisions.

2. Instead of restricting themselves to just throughput and coverage benefit analysis, the authors should also discuss the added multi-hop delays and scalability issues to make their message more powerful.

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Reviewer 2: Some response

The capacity and coverage improvement of multi-hop networks has been well documented. The paper presents a OPNET simulation of 802.11 systems in a multi-hop system. Answer: We don’t know of any coverage improvement

work in a cellular, 3 frequency band environment.Our contribution is in demonstrating incentive for

forwarding nodes participation.1. The problem with multi-hop systems  however are

additional delays and additional collisions.2. Instead of restricting themselves to just throughput and coverage benefit analysis, the authors should also discuss the added

multi-hop delays and scalability issues to make their message more powerful.

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Reviewer 2: Some response

The capacity and coverage improvement of multi-hop networks has been well documented. The paper presents a opnet simulation of 802.11 systems in a multi-hop system.

1. The problem with multi-hop systems  however are additional delays and additional collisions.Answer: Actually the delay should go down because of

the higher transmission rates maintained in a two hop situation. Additional collisions is possible only when transmission power reduction is used – we need study the effect of proposed reduction factors on the carrier-sense threshold.

2. Instead of restricting themselves to just throughput and coverage benefit analysis, the authors should also discuss the added multi-hop delays and scalability issues to make their message more powerful.