Wireless Mesh Networks: Theory & Practicegiambene/reti_di_telecomunicazioni_II/... · Wireless Mesh...

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Center for REsearch And Telecommunication Experimentation for NETworked communities

Wireless Mesh Networks:Theory & Practice

Daniele Miorandi, PhD

Area Head of PervasiveCREATE-NET

[email protected]/~dmiorandiwww.create-net.org/pervasive

Joint work with R. Riggio & T. RasheedSponsored by the WING project (www.wing-project.org)

http://www.create-net.org/pervasive



Outline

• Motivation & basic concepts• Application scenarios & business models• Architectures and technologies• WMN protocols (in particular: routing)• Open issues & research directions• How to build your own WMN



Motivation

• What do (most) users want?– ubiquitous connectivity– Internet services– cheap prices

• What do (most) industries want?– networks easy to deploy & manage– networks with low deployment costs



Do we need new solutions?

• Plenty of nets already in place• Fiber getting closer to the home• Cellular coverage almost ubiquitous• Scenarios?

– Home broadband Internet access

– Community networks

– Enterprise networking

– Wireless Metropolitan Area Networks

This presentation is property of CREATE-NET and is protected by Copyright © www.create-net.org14/06/08


Basics of WMNs

• Wireless Mesh Networks (WMNs) rely on a multi-hop wireless backbone for delivering services to the users • Very limited fixed infrastructure to be deployed: no need to wire APs!• Self-organization capabilities: limit the impact of network planning, allows incremental deployment (~ ad hoc nets)• Can be operated in unlicensed bands (ISM bands)• Can be operated with cheap technologies (e.g., WiFi)



Wireless Mesh Networks•Main Features

– Goal: intended as access architecture

– Heterogeneity of the devices: there might be dedicated devices acting as pure wireless routers

– Self-organizing features: allowing the deployment of unplanned wireless networks

•Example: A wireless interconnection of hotspots providing enhanced coverage without the need of having all of them wired to the Internet



Conventional single-hop

Infrastructure Network(Internet)

PSTN Network

Access point

Base station

http://images.google.com/imgres?imgurl=http://www.eba.com.hk/upload/Blackberry%25207230.jpg&imgrefurl=http://www.eba.com.hk/english/viewnews.asp%3Fnewsid%3D1891&h=1230&w=828&sz=248&tbnid=6fRqBccGWe8J:&tbnh=148&tbnw=100&start=5&prev=/images%3Fq%3Dblackberry%26hl%3Den%26lr%3D

http://images.google.com/imgres?imgurl=http://www.goodexperience.com/broken/i/03/10/rim.s.gif&imgrefurl=http://broken.typepad.com/b/product_design/&h=324&w=285&sz=35&tbnid=uEomKGz9YPoJ:&tbnh=113&tbnw=100&start=17&prev=/images%3Fq%3Dblackberry%26hl%3Den%26lr%3D

http://images.google.com/imgres?imgurl=http://ci.fort-worth.tx.us/cmo/pio/citypage/2001/cell_tower.gif&imgrefurl=http://ci.fort-worth.tx.us/cmo/pio/citypage/2002/cp20020107.asp&h=179&w=150&sz=9&tbnid=Q9UcRYzAMyYJ:&tbnh=94&tbnw=79&start=81&prev=/images%3Fq%3Dcell%2Btower%26start%3D80%26hl%3Den%26lr%3D%26sa%3DN

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Wireless Mesh Networks

•Consist of several nodes, interconnected via wireless links to the Internet through one or multiple gateway(s)

•Nodes roles in WMNs:–Mesh routers–Mesh clients

•Most traffic is user-to-gateway



WMNs Building Blocks



Gateways• Multiple interfaces (wired &

wireless)• Mobility

– Stationary (e.g. rooftop) – most common case

– Mobile (e.g., airplane, busses/subway)

• Serve as (multi-hop) “access points” to user nodes

• Relatively few are needed, (can be expensive)

GW



Wireless Routers

• Multiple interfaces (wired & wireless)

• Mobility– Stationary (e.g. rooftop) –

most common case– Mobile (e.g., airplane,

busses/subway)• Serve as (multi-hop) “access

points” to user nodes• Relatively few are needed, (can

be expensive)



Clients

• Typically one interface.

• Mobility

– Stationary

– Mobile

• Connected to the mesh network through wireless routers (or directly to gateways)

• The only sources/destinations for data traffic flows in the network.



Application Scenarios•Outdoor:

– Community networks

– Broadband wireless access

– City/county/municipality networks

– Government, public safety, law enforcement

•Indoor:– Home networking

– Office environments



Community Networks

•Broadband Internet Access

•Several neighbors may share their broadband connection

•Not run by ISP

•FON-like, but leveraging multihop

•Driven by social interests

•Business model: economy of sharing



Community Networks (8+)

•Manchester Wireless–www.manchesterwireless.net•Champaign-Urbana Community Wireless Network–www.cuwireless.net•SeattleWireless–www.seattlewireless.net•Bay Area Wireless Users Group (BAWUG)–www.bawug.org/•Southampton Open Wireless Network–www.sown.org.uk/•NYC Wireless–www.nycwireless.net/•Personal Telco–www.personaltelco.net/static/index.html•FreeNetworks–www.freenetworks.org/

http://www.manchesterwireless.net/

http://www.cuwireless.net/

http://www.seattlewireless.net/

http://www.bawug.org/

http://www.sown.org.uk/

http://www.nycwireless.net/

http://www.personaltelco.net/static/index.html

http://www.freenetworks.org/



Broadband Internet Access




• Business model: wireless ISP• Advantages: low market entrance barrier, low

cost of deployment, possible incremental deployment

• Drawbacks: limited bandwidth, no real provisioning of QoS

• In general: model interesting, but should rely on advanced services (not just pure connectivity)



Metro WMN

• Variation of the preceding one• Focus: not Internet access but wireless

coverage• End-users/owners: municipalities• Used for management/control of

communications among, e.g., police units, firefighters, first aid etc.etc.

• Now questioned heavily from economists



Extend WLAN Coverage

Source: www.belair.com Source: www.meshdynamics.com

http://www.belair.com/



http://www.meshdynamics.com/

http://www.meshdynamics.com/wIntegration.html



Extended WLAN coverage

• Business model: WMN as a cheap solution to build LANs (no need for wiring), technology providers (owners control access to Internet through 3rd parties)

• Limited potential (niche market)• Interesting for private users (e.g., apartment

complex with students willing to share Internet connection)



Public Safety

[Source: www.meshdynamics.com]



Emergency Management

• WMNs used to put in place quickly an ad hoc network for crisis/emergency management

• Currently done with GSM/GPRS & “mobile BSs”• Alternative: TETRA • Niche market, limited sets of end-users

(municipalities, DoD, civil protection etc.)




CableDSL

WMAN(802.16)

Cellular(2.5-3G)

WMN

Bandwidth VeryGood

VeryGood Limited Good

Upfront Investments

VeryHigh

High High Low

Total Investments

VeryHigh High High Moderate

Market Coverage Good Good GoodModest



WLAN Coverage

802.11 WMN

WiringCosts LowHigh

Number of APs As needed Twice as many

Cost of APs HighLow

Bandwidth GoodVeryGood



Mobile Internet Access

Cellular2.5 – 3G WMN

UpfrontInvestments LowHigh

Geolocation Limited Good

Bandwidth GoodLimited

UpgradeCost LowHigh

Source: www.meshnetworks.com

(now www.motorola.com).

http://www.meshnetworks.com/

http://www.motorola.com/

http://www.intel.com/update/contents/nc11032.htm



Emergency Response

Cellular2.5 – 3G WMN

Source: www.meshdynamics.com

TETRA

Availability Reasonable Good Good

Bandwidth GoodLimited Poor

Geolocation Poor LimitedPoor

http://www.meshdynamics.com/



Architectures

• 3 possible types of network architectures:– Flat: 1 single level. All devices act as both wireless

mesh routers & clients. Main limit: scalability– Hierarchical: 2 distinct level. Devices are either

WMRs or WMCs. WMRs as dedicate devices.– Hybrid: 2 mixed levels. Like hierarchical but

communications between clients allowed.• In the remainder, we focus on hierarchical architectures



Protocol Architectures

• Main question: where to perform routing?• Approach #1: Level-3 routing

– routing using IP & modified routing protocols– '+': many solutions deployed for ad hoc

networks to be re-used, limited overhead– '-': requires modifications of the protocol

stack, does not consider the access nature of WMNs



Protocol Architectures (2)

• Approach #2: Level 2.5 routing:– performs routing below IP– mesh seen as a subnet– from the external world: looks a large

Ethernet switch– '+': does not require modification at the OS

level, better scalable– '-': requires solutions to be developed, adds

overhead



Which one to choose?

• Most open-source solutions & academic testbed based on layer-3 routing (large availability of routing protocols from ad hoc nets research)

• Most commercial solutions based on layer-3 routing (proprietary hardware!)

• Main layer-2.5 solutions: MSR, MIT RoofNet• My personal opinion: layer 2.5 solutions (better

scalable, makes more sense from architectural perspective)



Technologies

• Mainly 2 technologies used in current deployments:– IEEE 802.11x (WiFi)– IEEE 802.16x (WiMax)



WiFi for WMNs

• Main advantages: large availability of components, stable hardware/firmware, very low cost of devices, no need for licenses

• Drawbacks: no performance guarantee (unlicensed bands), no built-in support for mesh functionalities, no QoS support (partial in 802.11e)

• Most WMN deployments currently use WiFi• WiFi for mesh: 802.11s under standardization



WiMax for WMNs

• Main advantages: QoS support, easy to control & manage, guaranteed performance (licensed bands)

• Drawbacks: limited set of devices available on the market, high cost of hw, licenses needed

• Operators look at WiMax as 'the' technology for metro-scale WMN deployments

• WiMax for mesh: 802.16j under standardization (current trend: no real mesh, only 2 hops)



WiFi vs. WiMax

• WiFi will remain the solution for grassroot deployments & all situations where cost plays a key role

• WiMax will be deployed by operators ('light infrastructure'), complementing cell networks & offering SLAs

• My opinion: plenty of space for working with WiFi, will represent the de facto standard for the next 5-8 yrs



WMN Protocols

• Key aspect: routing• L1-L2: use standardised technologies• L4 upwards: use standard protocol stacks

(TCP/SIP-IMS/HTTP-FTP)• Problem: how to effectively route packets on

the mesh?



Uniqueness of the problem

• Why is it different from standard routing?– Hops are different! Use link quality indicators– Limited bandwidth (wireless devices)– Rule of thumb: no more than 3 hops

(otherwise connections time out)– Unreliable channels, changing conditions– Most traffic to/from GWs– Possible support of users' mobility



Taxonomy of WMN routing protocols

• Following similar classifications in ad hoc networks:– Proactive: maintain routing tables at each

node– Reactive: create routing entries upon specific

connection requests• In reality, the distinction is more philosophical

than practical. Looking at the code, there are many similarities



Proactive Routing

• Nodes maintain global state information• Consistent routing information are stored

in tabular form at all the nodes• Changes in network topology are

propagated to all the nodes and the corresponding state information are updated

• Routing state maintenance could be flat or hierarchical



Destination Sequenced Distant Vector (DSDV) Routing

• Table-Driven algorithm based on Bellman-Ford routing mechanism

• Every node maintains a routing table that records the number of hops to every destination

• Each entry is marked with a sequence number to distinguish stale routes and avoiding routing loops

• Routes labeled with most recent sequence numbers is always used

• Routing updates can be incremental or full dumps



OLSR (Optimized Link State Routing)

• Version of LSR optimized for wireless networks• Accounts for link metric in taking forwarding

decisions• Widely used in academic testbeds• “Standardised” as an IETF RFC• Still instability problems with multi-gateway



Routing Protocols for Mesh Networks• TBRPF

– Topology broadcast based on reverse-path forwarding– PacketHop Inc. and Firetide Inc. Mesh routers

• AODV– Ad hoc On-demand Distance Vector Routing– Kiyon Inc.’s Autonomous Network

• DSR – Dynamic Source Routing– MSR’s Mesh testbed

• ExOR– Extremely Opportunistic Routing– RoofNet project of MIT



On-demand (Reactive) Routing

• A path is computed only when the source needs to communicate with a destination

• The source node initiates a Route Discovery Process in the network

• After a route is discovered, the path is established and maintained until it is broken or is no longer desired

• In real implementation, a timer is associated to paths, so that routes are cached even if not in use



Ad-hoc On-demand Distance Vector Routing (AODV)

• When a source desires to send a message to any destination, and if the route table does not have a corresponding entry, it initiates a route discovery process.

• The source broadcasts a route request (RREQ) packet to its neighbors, which in turn, forward it to their neighbors, and so on, until either the destination node or an intermediate node with a valid route to the destination is located.

• The intermediate nodes set of a reverse route entry for the source node in their routing table.

• The reverse route entry is used for forwarding a route reply (RREP) message back to the source.

• An intermediate node while forwarding the RREP to the source, sets up a forward path to the destination



AODV -2

Backwards learningRREQ

F?

F?

S

B

A C

D

E

F

F?

F?

F?

F?

F?

F?

I am F

F ?



AODV -3

RREP

-> F

S

B

A C

D

E

F

Backwards learning

-> F-> F

To F,Next-hop

is B



Dynamic Source Routing (DSR)

• On-demand source-based routing approach

• Packet routing is loop-free• Avoids the need for up-to-date route

information in intermediate nodes• Nodes that are forwarding or overhearing

cache routing information for future use• Two phases: Route Discovery and Route

Maintenance



DSR: Route Discovery

• Route discovery is initiated if the source node does not have the routing information in its cache

• The source node broadcasts a route request packet that contains destination address, source address, and a unique ID

• Intermediate nodes that do not have a valid cached route, add their own address to the route record of the packet and forwards the packet along its outgoing links



DSR: Route Reply• Route reply is generated by the

destination or a node that has a valid cached route

• The route record obtained from the route request is included in the route reply

• The route is sent via the path in the route record, or from a cached entry, or is discovered through a route request

• Route maintenance is accomplished through route error packets and acknowledgments



DSR

S

B

A C

D

E

F

F ?

F?SA

CE

F?S

F?SF?SB

F?SA F?SAC

F?SA

CF?SBD

I am FRoute1: SACEFRoute2: SBDF

Choose Route2

RREP Unicast

To F, routeis SBDF



Proactive vs. Reactive

• Proactive protocols seem an obvious option in networks with low dynamism

• But: in WMNs traffic is only node<-->GW!– proactive maintains a lot of useless state

information (scalability!)– patch: use a 2-level hierarchy in order to

limit state information• My personal take: reactive (coupled with a

caching mechanism) as the way to go



The MIT Roofnet Project

• MIT developed an open-source software toolkit for building WMNs using WiFi– software released as modules for click (a sw

router)– routing performed at level 2.5 (below IP)– uses a modified version of DSR, which

accounts for link quality by using the ETX metric



MIT Roofnet (cont'd)

• The sw works only with Atheros chipsets (WiFi cards are operated in the monitoring mode)

• Most of the team which developed the sw are now with a CA-based startup



Open Issues

• Open issues at (almost) all OSI layers• Most promising/interesting ones:

– PHY: smart antennas– MAC: multimode/multichannel protocols– NET: multiRAT support, mobility

management, multi-GW association– APP: network management/monitoring– x-layer: support for multimedia services



PHY – Smart Antennas

• Background

– Implemented as an array of omnidirectional antennas

– By changing the phase, beamforming can be achieved

– The result is a software steered directional antenna

Omnidirectionalantenna

Variabledelay

Signal totransmit

Radiation Pattern

Directionchanged bythe delays



PHY-Smart AntennasAdvantages

• Low power transmissions– Battery not a big

concern in many applications

– Enables better spatial reuse and, hence, increased network capacity



PHY-Smart AntennasAdvantages (cont)

• Punch-through links– Better delays (?)– Less packet loss (?)– Better data rates (?)– Less power (?)



MAC Protocols

• Current MAC protocols (standardized ones) do not support well multimode/multichannel operations

• Many research papers appeared in the last few years

• Limitation: given the low cost of cards, easier to integrate 2 cards on 1 motherboard than to design a whole new MAC protocol



Routing Protocols

• Support of multi Radio Access Technologies (e.g., WiFi & WiMax).

• Definition of vertical handover strategies and suitable signalling mechanisms

• Support of multi-GW association & optimization thereof

• Mobility management: let a path track users' mobility in an efficient manner



Network Monitoring/Management

• Current network monitoring/management (SNMP) designed and built for centralized architecture

• Problems of scalability, implementation in a distributed environment cumbersome

• Solution: go distributed!• Need to build highly distributed network

monitoring & management framework



Cross-layer stuff

• WiFi-based WMNs support badly multimedia flows

• Problems rooted in WiFi: designed for bulk data transfer

• Many x-layer solutions for enhancing multimedia support in WiFi networks

• Porting to WMNs not straightforward! (Especially in terms of signalling protocols)

• E.g.: packet concatenation for VoIP



More on net monitoring/management

• Need to automate operations, and embed monitoring/management in the network itself (in-network management)

• Need to envision “autonomic solutions”• Novel solutions envisioned for WMNs can

potentially represent the root for building the Future Internet



Second part: hands-on WMNs

• Now you basically know what you need to know (from a student perspective) on WMNs

• However, technology is not just about knowing it, it's also about playing with it

• So the natural question is: how do I build a mesh network?

• Constraints: (very) low cost– re-using available hw, open-source sw



What for?

• Most common case: student apartment complex, willingness to share an Internet connection

• Not-so-common case (but you never know): someone willing to run a WISP



Keep in mind• You cannot understand technology until you

don't play with it• Even from a research perspective, need to start

from experiments (experimentally-driven research)

• Many things work on paper, just a few in the real world

• After your thesis, you'll have to play with things, not just to study them



What do I need (minimal)?

• a set of PCs with a WiFi card• an Internet connection to share (and one PC to

act as gateway)• a bit of hands-on experience



Option 1: friends of Bill Gates

• Not my favourite option, but many people just know Windows

• Go to the MSR Website and search for “wireless mesh”• Download the MSR Mesh Toolkit (it's free!) & install it• It builds a 2.5-level WMN using a modified DSR• The mesh is seen as a “virtual” Ethernet card• Note: it changes the setting of your wireless card!



Option 2 (geeky): Linux addicted

• If you know how to work with Linux (a bit), you have plenty of opportunity

• My suggestion: our software• Developed in the framework of the WING project• Based on the MIT Roofnet software, enhanced with a

few modules– works on any WiFi card, not just Atheros– limited support of QoS for multimedia flows– enhanced stability



Option 2 (cont'd)

• Howto– go to www.wing-project.org– download the code (requires click) & install– documentation available

http://www.wing-project.org/



More on option 2

• It seems strange, but you can also build WMNs using APs and not PCs

• Most APs have enough flash memory to install an OS for embedded devices (e.g., Voyage Linux)

• More stable (and cheaper), slightly worse performance

• On the WING project site you'll find the x-compiled version



More & more on option 2

• If you need something more (and have some cash)– buy embedded PCs (e.g., Soekris), ~100E

each– based on x86 architecture– stable & powerful– install the software there!



End of the seminar

• What if you have liked it?• Come and work on it!



•International Research Institute founded in 2003• More than 200 partners worldwide

• Promoting globalization of knowledge and research• Mission: European excellence, competitiveness,

innovation and research exploitation through a multidisciplinary approach

Budapest University of

Technology and Economics

Society InnovationResearch



Building the global networked society of the future

– CREATE-NET brings a multidisciplinary approach bridging researchers,industry and communities

– Focusing on ICT-driven technology research and transfer for the second half of 21st century

– By exploiting ICT new opportunities and pushing technologies to adapt to communities' needs CREATE-NET aims to improved quality of life and economic growth

– Accelerating innovation through sustainable ICT centered society platforms

– With Next Gen Internet as the next Industrial Revolution, CREATE-NET spearheads global research excellence in support of the “missing ingredient”



R&D Organization

For further information please visit www.create-net.org

Open Systems for COmmunities

Security

Wireless and Broadband

Communications

Engineering Competence Center

PervasiveMultimedia,

Interactions andSmart

Environments

Society

Research

Innovation

http://www.create-net.org/



Pervasive @ CN

• Team working on Pervasive Computing/Communication Environments

• Currently 5 people in the team, average age ~30yrs

• Running 2 EU projects (BIONETS, SMART-NET), 1 MIUR Project (WING), 1 MAE Project (TeraNets), 2 local projects (TRITON, A3)

• Annual budget ~400kE



Working @ CN• Current openings for Pervasive

– Junior researcher on WMNs (testbed, algorithms for network management)

– Junior researcher on WMNs (multi-RAT, mobility management)

– Junior researcher on WSN (middleware, integration with legacy networks)

– SW developer for mobile applications (middleware, J2ME, opportunistic comms)

– various MS thesis opportunities...



Working @ CN

• How to apply?• Check out: www.create-net.org• Contact myself:

[email protected]

http://www.create-net.org/

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