Wireless Mesh Networks

Introduction

Wireless mesh network architecture Why Wireless mesh network? When Wireless mesh networks? Routing background Problems in existing routing protocols Problems in frequency allocation

Existing wireless networking technologies

Wireless Mesh Networks

A wireless mesh network (WMN) is a communications network made up of radio nodes organized in a mesh topology.

Wireless mesh networks often consist of– mesh clients– mesh routers

Mesh routers contain additional routing functionality due to the presence of wireless interface card in them

Nodes have two functions:– Generate/terminate traffic– Route traffic for other nodes

Characteristics of Wireless mesh networks

Multihop Wireless network. Support for adhoc networking and capability of self

forming, self healing and self organization. Mobility dependence on the type of mesh node. Multiple types of network access. Dependence of power consumption constraints on

the type of mesh nodes

Why WMN?

Multi-hop wireless network Support for ad-hoc networking, and capability of

self-forming, self healing and self organization. Multiple types of network access Mobility dependence on the type of mesh nodes Compatibility and interpretability with existing

wireless technologies

Compatibility and inter operatability with existing wireless networks

Dedicated routing and configuration Mobility

Types of WMN

Infrastructure/ Backbone WMN Client WMN Hybrid WMN

Infrastructure/Backbone WMN

Mesh routers form an mesh infrastructure among themselves.

Provides backbone for clients and enables integration of WMNs with existing wireless networks and Internet through gateway/bridge functionalities.

Clients connect to mesh router with wireless link or Ethernet

Infrastructure/Backbone WMN

Client WMN

Client nodes constitute peer-to-peer network, and perform routing and configuration functionalities as well as provide end-user applications to customers, ”mesh routers are not required”

Multi-hop routing.

Client nodes have to perform additional functions such as routing and self-configuration.

Client WMN

Hybrid WMN

A combination of infrastructure and client meshing.

Infrastructure provides connectivity to other networks such as the Internet, Wi-Fi, WiMAX, cellular, and sensor networks;

Mesh clients can access the network through mesh routers as well as directly meshing with other mesh clients.

The routing capabilities of clients provide better connectivity and coverage

Hybrid WMN

Protocol Design

Physical Layer Mac Layer Network Layer Transport Layer Application Layer

Physical Layer

Orthogonal frequency multiple access (OFDM) has significantly increased the speed of IEEE 802.11 from 11 mbps to 54 mbps.

Ultra-wide band (UWB) can achieve much higher rate for short-distance applications.

MIMO can increase system capacity by three times or even more.

Frequency agile or cognitive radios can achieve much better spectrum utilization.

MAC Layer

Differences between WMNs MACs and Wireless Networks MACs

MACs for WMNs are concerned with more than one hop communication

MAC must be distributed and collaborative, and must work for multipoint-to-multipoint communication.

Network self-organization is needed for better collaboration between neighboring nodes and nodes in multi-hop distances.

Mobility affects the performance of MAC.

Routing Layer

Features of routing protocol for WMNs: Multiple Performance Metrics

– Hop-count is not an effective routing metric. – Other performance metrics, e.g., link quality and round trip

time (RTT), must be considered.

Scalability– Routing setup in large network is time consuming. – Node states on the path may change. – Scalability of routing protocol is critical in WMNs.

Routing Layer

Robustness– WMNs must be robust to link failures or congestion. – Routing protocols need to be fault tolerant with link failures and

can achieve load balancing.

Adaptive Support of Both Mesh Routers and Mesh Clients– Mesh routers : minimal mobility, no constraint of power consumption,

routing is simpler

– Mesh clients : mobility, power efficiency, routing is complicated

– Need to design a routing protocol that can adaptively support both mesh routers and mesh clients.

Transport layer: research issues

Cross-layer Solution to Network Asymmetry– Routing protocol can select an optimal path for both data and ACK

packets.– MAC layer and error control may need to treat TCP data and ACK packets

differently.

Adaptive TCP– WMNs will be integrated with the Internet and various wireless networks

such as IEEE 802.11, 802.16, 802.15, etc.

– Same TCP is not effective for all networks.

– Applying different TCPs in different networks is a complicated and costly approach, and cannot achieve satisfactory performance.

Application layer

Applications supported by WMNs:

Internet Access– Advantages of WMNs: low cost, higher speed, and easy installation.

Distributed Information Storage and Sharing– Data sharing between nodes within WMNs– Query/retrieve information located in distributed database servers.

Information Exchange across Multiple Wireless Networks.– Cellular phone talks Wi-Fi phone through WMNs, – Wi-Fi user monitors the status of wireless sensor networks.

WMN Standards

WPAN: Bluetooth, Zigbee WiFi: 802.11a, b, g, n WiMAX: 802.16

WMN Standards

IEEE 802.16a WMAN Mesh– “mesh mode” in addition to the point-to-multipoint(PMP)

mode defined in IEEE 802.16. – Operating in the licensed and unlicensed lower frequencies

of 2–11 GHz, allowing non-line-of-sight (NLO) communications, spanning up to a 50 km range.

– Supporting multihop communications.

WMN Standards

802.11s WLAN Mesh– Multi-hop capability added to

802.11g/a/b– Auto configure on power up– Multi-channel multi-radio operation– Topology discovery– MAC Path selection protocol– Modified forwarding for QOS and

mesh control

Routing Layer

Features of routing protocol for WMNs: Multiple Performance Metrics

– Hop-count is not an effective routing metric. – Other performance metrics, e.g., link quality and round trip

time (RTT), must be considered.

Scalability– Routing setup in large network is time consuming. – Node states on the path may change. – Scalability of routing protocol is critical in WMNs.

When WMN?

Broadband home networking Community and neighborhood networking Enterprise networking Wireless mesh networks Transportation systems Building automation Health and medical systems Security surveillance systems

Mesh vs. Ad-Hoc Networks

Multihop Nodes are wireless,

possibly mobile

May rely on infrastructure

Most traffic is user-to-user

Ad-Hoc Networks Wireless Mesh Networks Multihop Nodes are wireless,

some mobile, some fixed

It relies on infrastructure

Most traffic is user-to-gateway

Mesh vs. Sensor Networks

Bandwidth is limited (tens of kbps)

In most applications, fixed nodes

Energy efficiency is an issue Resource constrained

Most traffic is user-to-gateway

Wireless Sensor Networks Wireless Mesh Networks Bandwidth is generous

(>1Mbps) Some nodes mobile, some

fixed Normally not energy limited Resources are not an issue Most traffic is user-to-gateway

Ad Hoc Networks

An ad-hoc network is a wireless local area network (LAN) that is built spontaneously as devices connect.

Instead of relying on a base station to coordinate the flow of messages to each node in the network, the individual network nodes forward packets to and from each other.

Contd…

Formed by wireless hosts which may be mobile.

Don’t need a pre-existing infrastructure/backbone.

Routes between nodes pottentially contain multiple hopes.

Why MANET??

Ease, speed of deployment Decreased dependence on infrastructure. Can be used in many scenerios where

deployment of wired network is impossble Lots of military applications

History of Ad Hoc networks

In situations where networks are constructed and destructed in ad-hoc manner, mobile ad-hoc networking is an excellent choice.

The idea of mobile ad-hoc or packet radio networks has been under development since 1970s. Since the mid-90s, when the definition of standards such as IEEE802.11 (what we think of as WiFi or just 802.11) helped cause commercial wireless technology to emerge, mobile ad-hoc networking has been identified as a challenging evolution in wireless technology.

Characteristics of Ad Hoc networks

Every node is responsible for forwarding packets to other nodes

Nodes themselves implement security function among themselves

Topology changes continuously as nodes are highly mobile.

Purpose Specific Dynamic No master-slave relationship (Every node is a router)

Types of MANET’s

Fully symmetric environment

Asymmetric characteristics

Asymmetric responsibilities

Fully symmetric environment

All nodes have identical capabilities and respoonsibilities

Asymmetric capabilities

Transmission range and radios may differ Battery life at different nodes may differ Processing capacity may be different at

different nodes Speed of movement different

Asymmetric responsibilities

Only some nodes may route packets Some nodes may act as leader for nearby

nodes e.g. cluster haed

Other variants

Traffic characteristics may differ (bandwidth,realibility, unicast/multicast/broadcast )

Mobility patterns may be different (Little/ Highly mobile)

Mobility characteristics may differ (speed, direction of movement, pattern of movement)

Challenges

Limited wireless transmission range Broadcast nature of wireless medium Packet losses due to transmission errors Environmental issues Mobility induced route changes Mobility induced packet losses Battery constraints

Characteristics of Ad Hoc networks

Connectivity among the hosts changes with time

Nodes are low power devices, low CPU process capability, and low memory.

Due to above reasons; the existing routing protocols are highly unstable.

Routing protocols

Proactive: – Determine route independent of traffic pattern– Used in traditional wired network

Reactive: – Discover/ maintain routes only if needed.

Tradeoff of proactive vs reactive

Proactive has low while reactive has high latency

Reactive have low overhead while proactive have high overhead

Metrics for Ad Hoc routing

Number of hops Distance Latency Load balancing for congested loads cost

Wireless Standards for Mobile Ad Hoc networks

802.11b 802.11a 802.11g 802.11n

8002.11b

Developed in July 1999 Maximum bandwidth=11 Mbps Uses 2.4 GHZ Frequency range Low cost

802.11a

Developed in 2001 Maximum bandwidth= 54 Mbps Uses 5 GHZ frequency band Much faster than 802.11b

802.11g

Developed in 2003 Modified version of 802.11b Maximum bandwidth =54Mbps Uses frequency range=2.4 GHZ

802.11n

Used for faster and long distance communication

Not formally published and approved yet.

Applications

Personal area network Civilian environment Emergency operations Sensor networks