Seminar 2: Multi-Channel, Multi-Radio Wireless Mesh Networks r e - - PowerPoint PPT Presentation

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 1

• Seminar 2:

Multi-Channel, Multi-Radio Wireless Mesh Networks

Andreas J. Kassler kassler@ieee.org

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 2

• Overview

Principles of Mesh Networks Multi Radio Multi Channel Conclusions

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• Overview

Principles of Mesh Networks Multi Radio Multi Channel Conclusions

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Current Wireless Networks Infrastructure based

– needs “wired” connectivity to access points. – Deployment slow and expensive

Wired Backbone Internet R

Node

• ut of

Reach

X

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Multi Hop Wireless Networks Internet R

Node Reachable! Node Reachable!

Every node is now Access Point AND Router Every node is now Access Point AND Router

Get rid of the wires!

– mesh routing backbone created by grid of wireless APs – Clients can associate with any access point. – Complete transparency: nodes forward voice, video and data traffic to and from nearby nodes wirelessly and ultimately to the internet

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Multi Hop Wireless Networks Wireless mesh networks

– Small number of wireless hops to gateway

Internet R

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Multi Hop Wireless Networks Why interesting and study?

– No Wires! – Properties:

• Robust & Fault tolerant
• Self organising
• Self configuring
• Self healing
• No centralized

management

– Empowers the individual and community

A WMN is dynamically self-organized and self-configured, with the nodes in the network automatically establishing and maintaining mesh connectivity among themselves A WMN is dynamically self-organized and self-configured, with the nodes in the network automatically establishing and maintaining mesh connectivity among themselves

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!" Broadband Internet Access for rural/urban areas Metropolitan scale mesh networks chaska.net

– City of Chaska (8000 homes, 23.000 residents) 28% uptake after 2 years – Nomadic broadband service for $17.99 per month – Based on Tropos mesh products

• $600,000 infrastructure plus 2 month deployment
• 365 mesh routers 95% coverage
• 60 backhaul links

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!" WiFi Mesh for Rural Networks

• Extend Internet access into areas

which do not have wired networking infrastructure.

• Reduced Infrastructure cost
• Typically semi infrastructured

backbone network (Mesh)

• Long distance links can be common
• Cheap, Off the shelf hardware
• Mission to support both social &

economic development

• Useful for developing areas

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# An Early Multi Hop Wireless Network What Challenges can we identify? What Challenges can we identify?

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# Channel Access

Wired networking protocols such as Ethernet perform poorly when used in wireless communication Why? Because of media dependent differences You Should know:

– Hidden terminal problem – Exposed terminal problem – Collision detection problem – Interference problem

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$ Medium Access Coordination

no parallel transmission no parallel reception

Goal for MAC layer design:

• avoid parallel interfering transmissions
• do not hinder parallel non-interfering transmissions

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$ Random Access without reservation

• IEEE 802.11 Distributed Coordination Function – DCF (CSMA CA)

– Stations have to equally compete for access to the medium – Acknowledgment scheme is used for error indication

Time Time Time Node 1 (transmit) Node 2 (receive) Node 3 (transmit) Busy Busy DIFS DIFS CW = 4 CW = 9 Data Frame Busy SIFS ACK DIFS DIFS CW = 11 CW=9-4=5 NAV

• simple, well accepted, most frequently implemented and used

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$ Hidden Node Problem

hidden nodes

Hidden Node Problem

– A mesh node is hidden for an ongoing transmission if it is not able to sense the

• ngoing transmission but its transmission would disturb the reception.

– A node not in the sensing range of the transmitter but within the interference range

• f the receiver

HN induced problems

– Throughput degradation – Unfairness

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$ Simple Reservation – Two way Handshake

IEEE 802.11 RTS/CTS

– RTS: Request to send – CTS: Clear to send

Nodes receiving RTS or CTS might not get involved in new transmissions RTS/CTS

– solves hidden node problem – induces increased overhead and delay – also virtual carrier sensing

hidden nodes RTS CTS

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$ RTS/CTS

Time Time Time sender receiver

• ther 1

Busy Busy DIFS DIFS CW = 4 CW = 9 RTS SIFS CTS NAV(RTS) SIFS DATA SIFS ACK DIFS DIFS DIFS Time

• ther 2

Busy DIFS CW = 13 NAV(CTS) DIFS sender receiver

• ther 1
• ther 2

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$ RTS/CTS Problems

• A node unable to decode the CTS might nevertheless disturb the transmission.
• Hidden node problem still exists.
• Critical if adaptive modulation and coding is used.
• Threshold on packet size for RTS/CTS usage typically maximum packet size.

receiving range interfering range sender receiver hidden node CTS

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$ Exposed Node Problem

• disabling of possible non interfering parallel transmissions
• nodes that only receive RTS can transmit
• nodes that only receive CTS can receive

blocked nodes RTS CTS Parallel transmissions might occur (except for ACK)

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No collision detection in wireless communications In wireless can’t listen while you send

– Generally hardware is not flexible enough – All you hear is your own signal

• Your own signal at your antenna is much stronger than anyone else’s signal
• The power law

Consequently,

– wireless can’t do collision detect like Ethernet

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d d P P         =

# How to detect Collisions?

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• WLANs operate in the following unlicensed bands (US)

– 2400 – 2483.5 MHz (2.4 GHz), – 5150 – 5250 MHz (lower U NII), – 5250 – 5350 MHz (mid U NII), and – 5725 – 5825 MHz (upper U NII)

• interference can happen based on, licensed services, other unlicensed devices, ISM

equipment, and incidental radiators

• IEEE Standards operating in these bands include:

– 802.11{b,g} in the 2.4 GHz band; 802.11a in the U NII band – 802.15 WPAN (Bluetooth) in the 2.4 GHz band – 802.16 WirelessHUMAN in the mid and upper U NII bands

• Other devices that also use these bands:

– Field disturbance sensors, cordless telephones, low power devices, and microwave ovens – Non 802.11 Part 15 devices: cordless telephones, A/V repeaters, security cameras, baby monitors, & digital data links

• Licensed services that operate in these bands include:

– Amateur radio in the 2.4 GHz and upper U NII bands; fixed microwave in the 2.4 GHz band; and satellite in the lower U NII band

# Spectrum Usage

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Phone on

TCP download from a 802.11 AP Performance worsens when there are large number of short range radios in the vicinity

Panasonic 2.4GHz Spread Spectrum Phone 5 m and 1 wall from receiver

802.11 in presence of BT

# Interference Problem

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% Overview

Principles and Challenges Multi Radio Multi Channel Conclusions

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Single Radio Single Channel

Key Issue

– MRN need to relay traffic AND serve attached clients – In single radio WMNs, clients and MRNs operate on same channel

• more MRNs more relay traffic, less user capacity, higher delay/jitter
• One large collission domain

Internet

MRN1 MRN2 MRN3 MRN4

Single radio (e.g. 802.11b) for backhaul and client access

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2 3 4 5 7 8 9 1 11 10

RTS RTS CTS CTS

6

2 packets in flight! Only 4 out of 11 nodes are active…. 2 packets in flight! Only 4 out of 11 nodes are active…. Backoff window doubles!

RTS RTS RTS

Multihop Causes More Collisions for Single Channel

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Single Radio Single Channel Mesh Throughput

Key issues:

Cannot Tx and Rx in parallel (single radio) More problems due to collisions (hidden nodes) and interference Need to serialize reception and transmission Reduces capacity

Key issues:

Cannot Tx and Rx in parallel (single radio) More problems due to collisions (hidden nodes) and interference Need to serialize reception and transmission Reduces capacity Per MN Capacity=1/N , (N=hops) Per MN Capacity=1/N , (N=hops)

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Single Radio Throughput (Best Case) 2 4 6 8 10 12 14 16 18 20 22 24 1 3 5 7 9 Hops Available Bandwidth (Mbps) 802.11b 802.11a

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Dual Radio WMNs +" mesh

– and , on " Radios

• Different frequencies (e.g. 2.4 GHz 802.11b and 5 GHz 802.11a)
• Local access not affected by backhaul traffic full speed

– BUT: Wireless Backhaul still shared All 802.11a MRNs operate on same channel

• Reduced system capacity with growing network

, Nico Bayer, Marcel Cavalcanti de Castro, Peter Dely, Andreas Kassler, Yevgeni Koucheryavy, Piotr Mitoraj and Dirk Staehle, in: Proceedings of the IEEE ICCSC 2008, Shanghai, China, May 26 28 2008.

VoIP background VoIP background

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Multi Channel Mesh Backhaul

Key Idea:

Multi-radio, multi- channel Backhaul required for Carrier- Grade Send and receive in parallel on different channels Channel qualities and traffic demand vary

• ver time, unknown a

priori How to find the “best” channel for given link? How to coordinate which channel to use between what nodes at a given time?

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Capacity Comparison

Internet

MRN1 MRN2 MRN3 MRN4

Internet

MRN1 MRN2 MRN3 MRN4

Internet

MRN1 MRN2 MRN3 MRN4

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• %

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Per MRN Capacity (different WMN types)

1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 10 Number of MRNs (first one wired to internet) Per MRN Access Capacity (Mbps)

Single Radio Per-MRN Capacity (Mbps) Dual Radio Per-MRN Capacity (Mbps) Multi Radio Per-MRN Capacity - String (Mbps) Multi Radio Per-MRN Capacity - Loop (Mbps)

Internet

MRN1 MRN2 MRN3 MRN4

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Exploit Diversity Multiple Channels Large number of channels available Large number of channels available

Today’s US Spectrum Map – 300 MHz to 30 GHz

Utilizing multiple channels in backhaul

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Multi Channel Mesh Backhaul

• With sufficient radios and

sufficient channels, interference can be completely eliminated.

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• Channel assignment

becomes crucial and influences topology

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• Routing

Routing Channel Assignment Channel Assignment Influences interference Influences Topology and Capacity

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Channel Separation 5 2 1 Non-overlapping channels, A = 1, B = 6 Partially Overlapped Channels, A = 1, B = 3 Partially Overlapped Channels, A = 1, B = 2 Same channel, A = 1, B = 1 LEGEND 3 4 5 6 10 20 30 40 50 60 Distance (meters) UDP Throughput (Mbps)

3 435 + 6789

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• For given channel distance there is a dedicated physical distance that maximizes

throughput, can observe minimal distance

• This should depend on SNR and noise level
• Can be used for channel assignment if SNR, noise level known.
• , Noise level depends on interfering traffic

Overlapping Channels Do Work

Banerjee-SIGMETRICS-2006

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Exploit Diversity Multiple Channels Utilizing multiple channels in backhaul

– Manageability:

• Different networks on different channels avoids interactions between networks

– Contention mitigation:

• Fewer nodes on a channel reduces MAC layer contention

– Better performance via use of more spectrum

How to best utilize multiple channels in an Mesh network with limited hardware? How to best utilize multiple channels in an Mesh network with limited hardware?

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c 1 m 1 m m m+1

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Multi Radio, Multi Channel Capacity

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network density increases per node capacity decreases

– Use more channels shall be beneficial for dense networks – BUT: Cannot add infinite number of radios – What improvements are there when adding more channels? – How many radios are then needed?

General capacity constraints

– Available spectrum bandwidth – Connectivity constraints [Gupta Kumar]

• Availability of route places constraint on transmission power

– Interference constraints [Gupta Kumar]

• Limits spatial reuse among simultanous transmissions

;"$ #,3#0<= ;"$ #,3#0<=

Backup

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Multi Radio, Multi Channel Capacity Capacity constraints specific for c<m

– Interface constraint [Kyasanur Vaidya]

• Throughput is limited by number of Radios in a !"#of n nodes

– Destination bottleneck constraint [Kyasanur Vaidya]

• Node may be destination of multiple flows
• Per node throughput shared by all incoming flows

total throughput ≤ > > total throughput ≤ > >

1

• 1
• f flows

Node throughput ≤ > Per flow throughput = ? Node throughput ≤ > Per flow throughput = ?

Backup

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Conflict Graph Conflict graph

– captures link interference between pair of links, which

• changes dynamically and with nodes entering and leaving

– Helps in

• capacity estimation, routing, channel assignment, power management

– Can use to model fractional interference and variable traffic

Conflict Graph requires knowledge of

– packet transmission from nodes that are not “visible” – physical location of nodes within the network – whether or not multiple transmissions increase or decrease interference 1 2 6 4 5 3

1 - 4 1 - 2 2 - 3 4 - 5 2 - 5 3 - 6 5 - 6

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Conflict Graph

Connectivity Graph:

1 2 6 4 5 3

Ch=1 Ch=6 Ch=11

1 2 6 4 5 3

1 - 4 1 - 2 2 - 3 4 - 5 2 - 5 3 - 6 5 - 6

Conflict Graph:

[Subramanian08]

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3#*" Multi Channel, #*" Radio Card can switch channels dynamically

– Today: 4 ms with optimisations – Possible: 80 microsec

Centralized Channel Assignment: Compute channel assignments using global knowledge Hard! Distributed: Use a modified RTS/CTS sequence to negotiate channels

– RTS: Potential channels to be used – CTS: Receiver tells sender which channel to use

Problem:

– How does the sender know which channel the receiver is listening on?

Solutions mostly based on MAC layer extensions

– Receive on all channels simultaneously costly – Use a dedicated control channel – Use a synchronized hopping protocol – Provide multiple rendezvous opportunities

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3#*" Dedicated Control Channel

Negotiating Channel with RTS / CTS Before starting, Node C sends RTS on Control Channel C1 to D including list of potential channels to be used. Node D replies with channel selected. Nodes tune to the selected channel

B C D E G H I A K J

RTS (C1,C3,C7) RTS (C3,C5,C7,C11) CTS (C11) CTS (C3)

F C2 C2 C3 C11 C1 10 nodes are active, 5 packets in flight, 150% improvement! 10 nodes are active, 5 packets in flight, 150% improvement!

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3#*" Multi Channel Hidden Terminal Problem

Let C1 be the control channel C can hear traffic on C11 only, doesn’t hear the CTS from B consequently doesn’t know anything about traffic on C6 (D is too far to hear anything from B)

C1 C1 C11

RTS RTS Data on C6 Data on C6 Data on C6

Collision

CTS (C6) CTS (C6) Time

C6 C1 C6 C6 C6 C1

A B C D

Possible solution: Use multiple radios

So-MobiHoc-2004

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3#*" More Problems

B does not hear the RTS from A on C1. As node B has its interface on C11 it doesn’t hear the RTS from A. Also, A cannot sense the carrier as it is on different channel. A falsely concludes that B is not reachable.

A B C

C1 C11 C11

R T S Time R T S

Deafness Problem Deafness Problem

A B C D

Channel Deadlock Channel Deadlock

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All nodes send RTS in circular fashion to

• neighbors. Deadlock may be

resolved but system capacity is degraded.

Backup

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3*" Observations

• Can apply single radio solutions to multi radio

– number of channels typically greater than the number of radios

• Single radio solutions are more power efficient

– but power is not the primary concern in most mesh networks

• Single radio solutions are less costly than multi radio solutions

– but radios are fairly inexpensive – However, cannot add radios at will – How many cards give a good speedup at a reasonable cost?

• Switching speed is a problem in single radio solutions

– but switching speeds are being reduced

• When distance between nodes is large,

can use partially overlapping channels

• No Need to implement MAC Co ordination

mechanisms for concurrent transmissions

– Nodes can send and recieve in parallel using different Radios – Several links can operate in parallel at different nodes

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3*" Issues in Multi Radio WMNs

• Multi radio backhaul mesh use multiple channels/radios for backhaul

– Example: MeshDynamics MD4000

• Compatibility options

1. Use standard 802.11 based hardware (BUT: need multiple interfaces). 2. Use 802.11, but customized hardware. 3. Develop minor extensions to 802.11 (AKA layer 2.5) 4. Design new MAC protocol.

• Observations

– Interface can only use a given channel at a time – For two nodes to communicate they need to share acommon channel – Using multiple Radios, deafness, multi channel hidden terminal and channel deadlock problems can be mitigated – Channel re assignments might be required to improve capacity, minimize interference from external networks, etc – Network Partition Problems might arise

Network poorly connected

A B C D

1,3 2,4 1,2 3,4

A B C D

1,3 2,4 1,2 3,4 1,2 Some channels not used

A B C D 1,2

1,2 1,2 1,2

A B C D 1,2

1,2 1,2

A B C D 1,2

1,2 1,2

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3*" Interference in Multi Radio WMNs

• Question: Do we still get improvement if we use 2 radios or more on

Overlapping channels?

7

• 5
• +

A B C D

5 10 15 20 25 30 35 64,64 60,64 56,64 52,64 Channels Throughput (Mb/sec) Netgear: A to B Hop Netgear: C to D Hop

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 Hop1 = A, Hop2 = G Hop1 = G, Hop2 = A

TCP Throughput (Mb/Sec) Hop 2 Hop 1

Same channel or channel separation of 4 causes 46% 49% reduction in overall throughput 802.11a link causes a 22% reduction in overall throughput, and a 63% reduction in throughput on the 802.11g link.

Interference is significant, RF hardware shielding work is beneficial Interference is significant, RF hardware shielding work is beneficial

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3*" Multiple Radios – Channel Assignment Issues How should we assign channels to each interface?

– Connectivity, Spectrum Utilization, Load Awareness, External Interference?

Which interface should we send the packet on?

– Routing determines traffic load on the virtual links – Need to consider channel, range, data rate diversity.

Potential Problems

– Network Partition Problem – Channel Dependency Ripple Effect Channel Re assignment potentially needs Coordination – Topology Change Routing should be aware of Re assignment – Non Convergent behaviour during Channel Re assignment 0$ : $ 0$ : $

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3*" Multiple Radios – Channel Assignment Issues

Channel asignment strategies for the radios

– Classification according the timescale where channels are re assigned – ! #

• One channel per radio all the time

– 69+$ ! #

• Channels assigned dynamically (e.g. every 5 minutes) to match traffic

patterns and/or to reduce internal or external interference.

• Interference patterns can change, network may get disconnected

– ;$,"! #

• One channel to one radio for all time

– Channel might change on large timescale according to traffic demand

• for all other radios, channels are assigned dynamically to match traffic

patterns and/or reduce interference.

• Most flexible.

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3*" Multiple Radios – Static Interface Assignment Characteristics for Static Interface Assignment

– A given interface fixed to a given channel

– E.g. C1 assigned to Radio 1, C2 to Radio 2, etc.

• Benefit: no dynamic coordination needed, stable connectivity, better survivability

– All nodes use common set of channels used by Mesh Connectivity Layer [Draves04] or MUP

• Drawback: cannot use all channels, cannot consider traffic load

11 1 1 11 11 1 1 11 11 11 1

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3*" Multi Radio Unification Protocol (MUP) Problem: If two or more cards use different channels, which one to use at any given time? Idea: Nodes make independent decisions about channel selection based only on locally available information about channel load.

– Implemented at Link Layer – Exposes a single virtual MAC address – All nodes use common set of channels – Neighbor discovery and classification is done by ARP, channel selection (CS), and the MUP table – Broadcast over all interfaces – Determines channel quality to each neighbor using periodic probing over all channels – Switch to a new (better channel) based on SRTT measurements after time interval

Adya 2004

Backup

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3*" Multi Radio Unification Protocol (MUP) Benefits:

– Optimizes locally use of spectrum – Does work with standard hardware – No need for global topology info. – Can use standard protocols – Reduce delay by 40 50%

Disadvantages:

– Estimating channel quality can be hard – Need to couple with routing and routing metrics to utilize end to end

• Ch. 0
• Ch. 1

1 MUP Enabled

Backup

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3*" Multiple Radios – Static Interface Assignment Different Approaches, also using diverse set of channels

– [Marina 05]Treat Channel Assignment as Topology Control Problem, – use conflict graph to model interference – Assign Channels to minimize maximum conflict weight – [DAS05] Use ILP to maximize # concurrent transmissions given connectivity constraints – [Tang05] Statically bind interface channels by minimizing interference among links

3%

Drawback: longer routes required Mostly Centralized Approaches

60 52 2 1 64 60 Not possible 52 56 52 60

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3*" Multiple Radios – Semi Dynamic Interface Assignment Characteristics for Semi Dynamic Interface Assignment

– Re Assign channels at slow time scales – External Interference Aware, Centralized [Ramachandran06] – Load Awareness – Centralized [Raniwala04] – Distributed [Raniwala05]

3%

Drawback: longer routes required

60 52 2 1 64 60 Not possible 52 56 52 60

2 radios / node, 4 channels

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3*" Interference Aware Channel Assignment Interference aware channel assignment

– External interference can severely degrade performance

• A radio per node monitors

data and control traffic

• Channels ranked from least

interfered to most interfered

• Ranking sent to centralized

channel assignment server (CAS)

– Internal interference between mesh links should be avoided Assign orthogonal channels using Conflict Graph Concept

• Gather Neighbor information on delay to each neighbor and interference

estimates for all channels supported by the router’s radios – Channel sensing and assignment can break network connectivity

• Use dedicated radio per router tuned to a common channel to ensure connectivity

– Uses Multi Radio Conflict Graph (MCG) to model interference between mesh links

[Ramachandran06]

Backup

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3*" Interference Aware Channel Assignment CAS Uses BFS (Breadth first search)

– CAS ranks channels according to their interference levels – With the links adjacent to the gateway as the starting points, the algorithm traverses vertices in MCG in BFS order – Each vertex is assigned the currently highest ranked channel that is not assigned to its adjacent vertices in MCG. – Considering the MCG constraint is to reduce the interference among the mesh nodes – Considering the channel rankings of the external interference level is to reduce the interference between the mesh and the external wireless networks – If such a channel is not available, then randomly assign a channel to this vertex.

CAS notifies all nodes about assignment using broadcast Problems:

– Interference can change rapidly, Depends on existence of traffic patterns to determine interference can lead to incorrect channel assignment – Assumes most traffic towards gateway

Backup

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3*" Traffic Aware Channel Assignment

10 50 40 10 50 40

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How to develop traffic aware channel assignment algorithms? How to estimate traffic that varies over time? How to estimate the interference graph? How to handle non binary interference through RSS variation? How much does traffic awareness improve network performance and when is it beneficial? How to develop traffic aware channel assignment algorithms? How to estimate traffic that varies over time? How to estimate the interference graph? How to handle non binary interference through RSS variation? How much does traffic awareness improve network performance and when is it beneficial?

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3*" Traffic Aware Channel Assignment Traffic aware channel assignment

– Link load determined by routing algorithms Joint channel assignment routing? – Having complete information about network topology and traffic matrix (how?), the traffic aware channel assignment problem is NP hard

Idea: Distribute Load uniformly across Channels

– Form multiple spanning trees, each one rooted at a Gateway – Each node joins one (or more) gateways – Routing Metrics: Gateway link capacity, Path capacity – Re assign channels to balance traffic load across channels – Coordinate with direct neighbors

Raniwala05

# $@ *# !"@ $≥ " 2 =

• !"
• "

" 5

• A*

Backup

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 55

3*" Traffic Aware Channel Assignment Hyacinth Neighbor Interface Mapping

– Each interface is used to communicate with multiple neighbors. – Each node balances its children associations across DOWN NICs.

Interface Channel Mapping

– Each node periodically exchanges its channel utilization with neighbors – constructs a neighborhood channel usage map – Periodically re assigns DOWN NICs’ channels to balance traffic load across – Adapt channel assignment to traffic patterns

Channel Usage Metric

– A weighted combination of aggregated traffic load and number of nodes using the channel

[Raniwala05]

Start with single channel routing for initial load estimation Iterate over channel assignment and routing until convergence Factor 6 to 7 throughput improvement Start with single channel routing for initial load estimation Iterate over channel assignment and routing until convergence Factor 6 to 7 throughput improvement

Ch=36 Ch=36

Backup

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 56

3*" Multiple Radios – Dynamic Interface Assignment Characteristics for Dynamic Interface Assignment

– Interface can switch channel when needed [So MobiHoc 2004 , Bahl04]

• Any channel can be used at any given time
• All Methods for Single Radio Multiple Channels can be used (SSCH, MMAC, ...)
• Benefit: no limitations on channel usage
• Drawback: coordination required, deafness problem

( 11 1 1 11 11 1 1 11 11 11 1Backup

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3*" Multiple Radios – Hybrid Interface Assignment Common Control Channel [Jain01]

– One common control channel (e.g. On radio 1), many data channels (switchable, e.g. on radio 2) – Control channel used to negotiate, which data channel to use – Advantages:

• All nodes aware of busy channels
• No need for time synchronisation

– Disadvantages

• Nodes contend for control channel Bottleneck
• When few channels available, spectrum efficiency is low
• Increased cost due to dedicated channel for control

Control Channel: 1 Control Channel: 1 Data Channel: 2 4 Data Channel: 2 4

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3*" Multiple Radios – Hybrid Interface Assignment Hybrid Interface Assignment: Fixed/Switchable Approach Net X

Each node has at least 1 fixed, 1 switch able interface Connectivity is maintained, all channels used Every node picks a channel as it’s fixed channel Different nodes use different fixed channels Once a “connection” is made, there may not be a reason to switch channels again for that particular flow Per Channel Packet Queue

A

Fixed (ch 1) Switchable

A

Fixed (ch 1) Switchable

B

Fixed (ch 2) Switchable

B

Fixed (ch 2) Switchable

C

Fixed (ch 3) Switchable

C

Fixed (ch 3) Switchable

1 2 3 2

B D C

• Ch. 3
• Ch. 4

A

• Ch. 1

B D C

• Ch. 3
• Ch. 4

A

• Ch. 1

Packet to D Packet to C

• Ch. 4
• Ch. 3

Packet to C arrives buffer packet Interface switches to channel 3

[Kyasanur06]

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3*" Multiple Radios – Hybrid Interface Assignment Hybrid Interface Assignment: Fixed/Switchable Approach Net X

Multi channel broadcast support, Scheduling for channel switching Hybrid Multichannel Control Protocol (HMCP)

Challenge: Create and maintain channel diversity Fixed Channel Selection Protocol Semi Dynamic

On startup each node picks a random fixed channel Periodically send a “hello” pkt. containing fixed channel & 1 hop neighbors info.

• n all channels (using the switchable interface) High Overhead

Maintain a NeighborTable containing fixed channels being used by neighbors Select the channel with fewest nodes as a candidate Not traffic aware! Change fixed channel to candidate channel probabilistically to avoid oscillations

A

Fixed (ch 1) Switchable

A

Fixed (ch 1) Switchable

B

Fixed (ch 2) Switchable

B

Fixed (ch 2) Switchable

C

Fixed (ch 3) Switchable

C

Fixed (ch 3) Switchable

1 2 3 2

B D C

• Ch. 3
• Ch. 4

A

• Ch. 1

B D C

• Ch. 3
• Ch. 4

A

• Ch. 1

[Kyasanur06]

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3*" Multiple Radios – Routing Issues In Multi Channel

– Select routes that have channel diversity WCETT

Need to consider Switching Cost

– Switching interfaces results in packets being queued and delayed – If a node is on more routes, might require more switching – Try to minimize the amount of switching while maximizing channel diversity

2 1 2 1 Which Route is better? A-B-D or A-C-D? 3

4 +

• @

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3*" Hybrid Interface Assignment – Routing Metric New Metric: MCETT = include switching penalty

Intuition: balance switching overhead with channel diversity For each channel (i): measure InterfaceUsage(i), average over 1s interval Measure probability that switchable interface is on different channel i != j when packet arrives on channel j Switching Cost Path Metric

∑

≠

=

j i s

i sage InterfaceU j p ) ( ) ( elay switchingD j p j SC

s

* ) ( ) ( =

( )

        + + − =

≤ ≤ =

∑

c j j n i i i

X c SC ETT MCR

1 1

max ) ( ) 1 ( β β

ci = channel used on i th hop

2 1 2 1 3

4 +

• @

[Kyasanur06]

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2 4 6 8 10 12 14 16 1 2 3 4 5 6 7 8 9 10 Normalized throughput Topology number (2,2) (2,5) (5,5) (2,12) (12,12)

(m,c)

3*" Multi Channel Performance: CBR – Random topology

50 nodes, 50 flows, 500m x 500m area

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3*" KAUMesh

• KAUMesh

– Based on Net X, Linux 2.6, Cambria Platform (Gateworks) – Three 802.11a radios per mesh node (m = 2), Legacy clients with 1 radio 802.11b/g – Nagios Network Management Platform

http://www.cs.kau.se/cs/prtp/pmwiki/pm wiki.php?n=Resources.MeshTestbed http://www.cs.kau.se/cs/prtp/pmwiki/pm wiki.php?n=Resources.MeshTestbed

Multi Channel Routing, Hybrid Channel Assignment

Interface and Channel Abstraction Layer, Aggregation IP Stack QoS Interface Device Driver User Applications ARP QoS Interface Device Driver

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3*" Multiple Radios – Protocol Issues Link Network Transport Physical Layer Upper layers

1-

1 4 3 2 3 2 4 2 4 2

Route A B C in use D needs route to F Route D E F better

Separation on Timescale

– Routing larger time scale

• channel aware route selection channel diverse routes
• Routing metric should include channel diversity, bandwidth,

loss rate, etc...

• Need to take into account cost of switching

– Brown route ”better” than green

• Broadcast packets need to be sent on all channels

– Can use separate broadcast channel needs additional radio

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% Overview

Principles Multi Radio Multi Channel Conclusions

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Conclusions and Project Proposals

• "B6*09

– Traffic and Interference Aware Channel Assignment

• What solutions exist and what are their limitations?
• How to determine available capacity ?
• How to classify/predict Traffic Demand
• How to quantify interference among flows? What about mesh external

interference?

• What metrics can quantify Channel load, how good are they?
• "B69

1. For centralized channel assignment, implement selected subset of channel assignment strategies (e.g. BFS, ILP based, etc) and evaluate in KAUMesh

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Discussion Questions (to be answered by you!) &

• Why in dual radio Mesh deployments, latency/delay cannot be predicted?
• Derive formulas for the per Mesh Relay Node capacity versus number of MRNs deployed of

single radio, dual radio, multi radio mesh deployments (slide 63), under the assumption that available capacity is 5 Mbps (for 802.11b, single radio) and 23 Mbps for 802.11a backhaul for dual radio and multi radio. Assume interference range = 2 * transmission range. Also assume a string topology where the MRN providing internet access is located at one end of the string and all clients transmit data towards nodes in the internet. In Dual and Multi Radio deployments, assume that 802.11b radio is used for client access and 802.11a for backhaul.

• For the paper “Routing and Link layer Protocols for Multi Channel Multi Interface Ad Hoc

Wireless Networks”, Pradeep Kyasanur and Nitin H. Vaidya, in SIGMOBILE Mobile Computing and Communications Review, Volume 10, Number 1, pages 31 43, January 2006, discuss the following two questions:

– Authors compare their approach against the case where One Channel is used. Do you think this comparison is fair? Provide arguments for each potential answer. If you think it is not fair, against what solution should it be compared to make a more fair comparison? – For the chain topology Evaluation results (Figure 5), why the throughput does not increase linearly with the number of channels used? Discuss several reasons.

• For the paper “Protocols and architectures for channel assignment in wireless mesh networks”,

Jorge Crichigno, Min You Wub, Wei Shu, In: Ad Hoc Networks 6 (2008) 1051–1077, discuss the following questions:

– Explain the Channel Dependency Problem and the Ripple Effect. – For the Receiver Fixed Hybrid Channel Assignment strategy (4.2.4), is the Ripple Effect an Issue? What about the Non convergent behavior problem?

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More Literature

• [Banerjee SIGMETRICS 2006] Arunesh Mishra, Vivek Shrivastava, Suman Banerjee, William A. Arbaugh: Partially overlapped channels not

considered harmful. , Proceedings of the joint international conference on Measurement and modeling of computer systems, Saint Malo, France 2006

• [Subramanian08] Anand Prabhu Subramanian, Himanshu Gupta and Samir Das: Minimum Interference Channel Assignment in Multi Radio

Wireless Mesh Networks, IEEE Transactions on Mobile Computing (TMC), Vol 7. Number 11. November 2008.

• [So MobiHoc 2004] J. So and N. Vaidya. Multi Channel MAC for Ad Hoc Networks: Handling Multi Channel Hidden Terminals Using A Single
• Transceiver. In Proc. ACM MobiHoc, Tokyo, Japan, May 2004.
• [Draves04] R. Draves, J. Padhye and B. Zill, "Comparison of Routing Metrics for Multi Hop Wireless Networks", Proceedings of ACM SIGCOMM

2004.

• [Adya 04] Atul Adya, Paramvir Bahl, Ranveer Chandra, Lili Qiu: Architecture and techniques for diagnosing faults in IEEE 802.11 infrastructure
• networks. MOBICOM 2004: 30 44
• [Marina 05]M. Marina, S. Das, A topology control approach for utilizing multiple channels in multi radio wireless mesh networks, in: 2nd

International Conference on Broadband Networks (Broadnets 2005), Boston, Massachusetts – USA, October 2005.

• [DAS05]A. Das, H. Alazemi, R. Vijayakumar, S. Roy, Optimization models for fixed channel assignment in wireless mesh networks with multiple

radios, in: 2nd IEEE Communications Society Conference on Sensor and Ad Hoc Communications and Networks (SECON), Santa Clara, California – USA, September 2005.

• [Tang05]J. Tang, G. Xue, W. Zhang, Interference aware topology control and QoS routing in multi channel wireless mesh networks, in: 6th ACM

International Symposium on Mobile Ad Hoc Networking and Computing (Mobihoc 2005), Urbana Champaigne, Illinois – USA, 2005.

• [Ramachandran06] K. Ramachandran, E. Belding, K. Almeroth, M.Buddhikot, Interference aware channel assignment in multi radio wireless mesh

networks, in: 25th Conference on Computer Communications (Infocom 2006), Barcelona – Spain, April 2006.

• [Raniwala04] A. Raniwala, K. Gopalan, T. Chiueh, Centralized channel assignment and routing algorithms for multi channel wireless mesh

networks, Mobile Computing and Communications Review 8 (2) (2004) 50–65.

• [Raniwala05] A. Raniwala, T. Chiueh, Architecture and algorithms for an ieee 802.11 based multi channel wireless mesh network, in: 24th

Conference on Computer Communications (Infocom 2005), Miami, Florida – USA, March 2005.

• [Bahl04] P. Bahl, R. Chandra, J. Dunagan. SSCH: Slotted seeded channel hopping for capacity improvement in ieee 802.11 adhoc wireless

networks, in: 10th ACM International Conference on Mobile Computing and Networking (MobiCom 2004), Philadelphia, Pennsylvania – USA, 2004

• [Jain01]N. Jain, S. Das, A. Nasipuri, A multichannel csma mac protocol with receiver based channel selection for multihop wireless networks, in:

10th International Conference on Computer Communications and Networks (ICCCN 2001), Scottsdale, Arizona – USA, 2001.

• [Kyasanur06] P. Kyasanur, N. Vaidya, Routing and link layer protocols for multi channel multi interface ad hoc wireless networks, SIGMOBILE

Mobile Computing and Communications Review 10 (1) (2006) 31–43.

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 69

Ex Jobb

Possible to do Ex Jobb related to Mesh together with

– CRL in Kalmar (www.crl.se) – FOI in Linköping (www.foi.se) – Deutsche Telekom Laboratories (Berlin)

In case you are interested, please contact me

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n d r e a s J . K a s s l e r Topics in Computer Networks 2009 • Seminar 3 70

Thank you!

Andreas J. Kassler kassler@ieee.org