Capacity Scaling with Multiple Radios and Multiple Channels in Wireless Mesh Networks Oguz GOKER.

Capacity Scaling with Multiple Radios and

MultipleChannels in Wireless

Mesh Networks

Oguz GOKEROguz GOKER

IntroductionIntroduction

Emerge of wireless technologies.Emerge of wireless technologies.

Increasing Used of Wifi points Increasing Used of Wifi points everydayeveryday

ProblemsProblems

Small sized networkSmall sized network

One hop systemOne hop system

NO Interoperability on AP sNO Interoperability on AP s

Nothing for hidden terminal and Nothing for hidden terminal and other problemsother problems

ContentionContention

Mai problemsMai problems Frequency Re use.Frequency Re use.

3 orthogonal in 802 11b 3 orthogonal in 802 11b

5 orthogonal in 802 11a5 orthogonal in 802 11a

AdvantagesAdvantages

Higher degree of spatial variability of the MAI.

Significant location-dependent node throughput.

Leads toLeads to

Dynamic interference management

Topology control via techniques such as node clustering and power control.

Mesh APsMesh APs

Can increase channel Re use.Can increase channel Re use. Single Radios vs Multi RadiosSingle Radios vs Multi Radios 802.11b interconnection802.11b interconnection 802.11a client connection in multi 802.11a client connection in multi

radiosradios

SINGLE RADIO MULTI-HOP SINGLE RADIO MULTI-HOP MESH NETWORKSMESH NETWORKS

It is under study till 1970s.It is under study till 1970s.

The end-to-end throughput in such The end-to-end throughput in such single radio networks reduces with single radio networks reduces with the number of hops.the number of hops.

Operates in Half Duplex ModeOperates in Half Duplex Mode

ContinueContinue

End to end user throughput will End to end user throughput will decrease if the hop number increasedecrease if the hop number increase

So throughput is inversely So throughput is inversely proportional to hop numberproportional to hop number

One hop is the most effectiveOne hop is the most effective

Single Radio, Single Channel Mesh Single Radio, Single Channel Mesh NetworksNetworks

throughput of a single-channel multi-hop network of 802.11 nodes typically Works in 1/nWorks in 1/n

is coming from topology and traffic characteristics.is coming from topology and traffic characteristics.

Topologies like a single-channel n node linear chain Topologies like a single-channel n node linear chain t t is based on pessimistic that all network interfere with each is based on pessimistic that all network interfere with each otherother

0.5 for a purely ad-hoc topology and random choice of source-destination pairs.

In larger networks it is better.In larger networks it is better.

Single Radio, Multi-channel Mesh Single Radio, Multi-channel Mesh NetworksNetworks

We can re use C different channels. So re We can re use C different channels. So re use increases.use increases.

But works in C/nBut works in C/n is not possible because is not possible because of half duplexof half duplex

On the other hand, exploiting the multiple orthogonal channels clearly enhances aggregate 1-hop throughput vis-avis the single channel scenario but at the cost of enhancing the end-to-end delay.

Main ProblemsMain Problems

Still half duplex so there is no real Still half duplex so there is no real advantage on a network.advantage on a network.

This necessitates channel scanning, selection and switching the radio at each node causes big delay

Few milliseconds to a few hundred microseconds

Single vs Multi ChannelSingle vs Multi Channel

Multi Channel give much more Multi Channel give much more frequency re use (spatial re use)frequency re use (spatial re use)

But the final throughput totally But the final throughput totally depends on topology.depends on topology.

In a linear chain or near to that single In a linear chain or near to that single is better because of channel change is better because of channel change time on hardwaretime on hardware

ConclusionConclusion

MULTI-RADIO MESH

Multiple radio nodes are effectively full duplex

We can allocate radios to different channel.

ExampleExample

R/4 in example a.R/4 in example a. R in example bR in example b

Importance of network designImportance of network design

We have to select true type of nodes. We have to select true type of nodes. How many channel and how many How many channel and how many radios per node is very important.radios per node is very important.

MistakesMistakes

Radio/Channel assignment and routing

Classical wired network solutions are Classical wired network solutions are not efficient for mesh networks.not efficient for mesh networks.

Wired Networks use Wired Networks use both proactive (tabledriven) and reactive (on-demand)

Increased size of overhead makes classical methods not possible on mesh networks.

Cont..Cont..

Since wireless is essentially a broadcast medium, any transmission between two neighboring nodes impacts (in principle) transmissions anywhere else in the network.

Classical Shortest path algorithms may not be effective.

Why no shortest pathWhy no shortest path

Such algorithms simply select the path with the fewest hops without regard to the available link bandwidth, which can vary significantly depending on the interference environment at the local receiver.

It is simple to construct examples where paths with larger number of hops can provide shorter end-to-end delay, depending on the residual and bandwidths available on the links in the respective routes.

What is ImportantWhat is Important

In wireless network for routing In wireless network for routing important thing is channel diversity.important thing is channel diversity.

In another mean re use and empty of In another mean re use and empty of channelschannels

Cont…Cont…

In other words, longer paths (measured in number of hops) that reuse the available channels for better co-channel interference management may provide improved end-to-end throughput/delay than shorter paths which use a fewer number of hops.

So system must be channel awareSo system must be channel aware

ContCont

Unlike a wired network where each hop is assumed to be isolated from simultaneous transmissions on other links, the interaction between the link and upper layers is a vital and unavoidable element in wireless multi-hop routing.

So this creates a problem.

NP Hard ProblemNP Hard Problem

So transmission have to find a way So transmission have to find a way that’s channel are not too busy, low that’s channel are not too busy, low delay, and if possible lowest number delay, and if possible lowest number of hops. For finding this we must of hops. For finding this we must aware of all nodes and channels aware of all nodes and channels position. As system is mesh for position. As system is mesh for optimal this will create a NP hard optimal this will create a NP hard problemproblem

Some solutionSome solution

RaniwalaCentral system that’s load aware of network Central system that’s load aware of network

updates it self with the move. updates it self with the move. Synchronization needed.Synchronization needed.

KyasanurNodes can decrease traffic on channels

depending on coming traffic. Can change interface. Work done with neighbours so no synchronization and channel change ynchronization and channel change needed.needed.

Channel-Aware Path Metrics for Routing

A good channel-aware path metric should incorporate notions of

(i) total link cost (e.g., sum of the trannsmission delays along the links in a path)

(ii) path channel diversity

Weighted Cumulative Expected Transmission

Time

More common useMore common use

ProblemsProblems

WCETT doesnot consider spatial re WCETT doesnot consider spatial re use, so in a big networks spatial re use, so in a big networks spatial re use can be high and there can be use can be high and there can be some channels that congestion is some channels that congestion is low. So WCETT may miss it. It is low. So WCETT may miss it. It is better on small networksbetter on small networks

ConclusionConclusion

Right now we are using single radios Right now we are using single radios network, with the increase usage of network, with the increase usage of 802 technologies we must start use 802 technologies we must start use multi radios systems. But there is still multi radios systems. But there is still too much need for research on multi too much need for research on multi radio system considering single radio system considering single channels systems are in research channels systems are in research since 1970s.since 1970s.