Wireless networks - Lecture 32: Wireless mesh networks (Part 2)

Wireless networks - Lecture 32: Wireless mesh networks (Part 2). The main topics covered in this chapter include: MAC layer; network layer; QoS support at each layer; WMN standards; network self-organization; increases network capacity;...

Wireless Networks

Lecture 32 Wireless Mesh Networks Part II

Dr Ghalib A Shah

► Routing – Cross-layer design

 QoS Support at each layer

 WMN Standards

 Critical factors influencing performance

► Radio techniques, scalability, QoS, security, Ease of

Use, Mesh connectivity

MAC Layer

 MAC for WMNs is concerned with more than

one hop communication.

 MAC is distributed and cooperative and works

for multipoint-to-multipoint communication.

 Network self-organization is needed for the

MAC.

 Mobility affects the performance of MAC

 The scalability of MAC can be addressed in two

ways.

► Enhance the existing or propose new for single

channel to increase E2E throughput

► Allow transmission on multiple channels of each

network node

Basic Techniques

 Scheduled

► Fix scheduled TDMA

► Polling

► Impractical due to lack of:

• Central coordination point

• Reasonable time synchronization

 Random Access

► CSMA – simple and popular

► RTS/CTS – protects the receiver

t c

f

t c

s 2

s 3

s 1 f

t c

f

t c

f

t c

Single channel MAC

 Improving existing MAC protocol

► By changing parameters of CSMA/CA based MACs like contention

window size and backoff procedures for multi-hop

► Contention based approaches are not scalable and throughput

degrades with increase in contention

 Cross-layer design with advanced physical layer.

► MAC based on directional antenna and power control

 Innovative solutions

► Need new ideas to overcome low end-end throughput for multi-hop ad

hoc environment

► TDMA or CDMA based MAC needs to be explored

► Compatibility, cost and complexity are the important factors in

designing new protocols

 Increases network capacity

MAC – Multichannel Why?

Chain bandwidth = B

C h-1

C h-2

1 2 3

MAC – Multichannel

 Perhaps, if a new

Multi-Channel Coordination Layer

(MCCL) is introduced between

MAC and Network

 Must work within the

constraints of 802.11

 May increase the capacity of

the network

PHY 802.11 MCCL IP

Ch-1

Ch-2

3 4

C h-1

C h-2

1 2 3

Multi-channel MAC (MMAC)

 Multi-channel single transceiver

► One channel active at a time

► Different nodes may operate on different times.

► Hence, coordination is required

 Multi-channel multi-transceiver

► A radio includes multiple parallel RF chips and baseband

processing modules to support several simultaneous channels.

► On top of multiple channel in physical layer, only single MAC to

coordinate operations.

 Multi-radios MAC

► A node has multiple radios each with its own MAC and

physical.

► A virtual MAC protocol such as a multi-radio unified MAC

protocol is required to coordinate communication among all.

MMAC Functions

 Maintaining data structure of all channels in each node.

► classified into three types depending on its status of allocation.

 Negotiating channels during ad hoc traffic indication

message (ATIM) window

► Negotiations are done through a pre-defined channel known to

all nodes.

 Selecting a channel

► The criterion is to use a channel with the lowest count of

source–destination pairs that have selected the channel.

Multi-radio Unification Protocol

 Discovering neighbours

► After the discovering procedures, neighbors are classified into

MUP enabled and legacy nodes.

 Selecting a NIC

► based on one-hop round trip time (RTT) measurements MUP

selects the NIC with the shortest RTT between a node and its neighbors.

 Utilizing the selected NIC for a long period

► This period is determined by a random process and in the

order of 10–20 s.

 Switching channels

► After the random time period, all NICs are measured again

through one-hop probe messages If an NIC has a certain amount of quality improvement than the existing NIC, then it is selected for sending packets.

Open research issues

yet.

 CSMA/CA based MAC protocols solve partial problems

power consumption etc.

 Some Mesh routers integrate various wireless networking

technologies and require advance bridging functions

 Existing research focuses on capacity, throughput and fairness

But many applications may require broadband multimedia

communication in WMNs.

 MAC developed with QoS metrics

Network Layer

 WMN will be tightly coupled with internet and IP

has been widely accepted in different wireless networks.

 However routing differs from IP and cellular

 Finds and maintains

routes for data flows

 The entire performance of

the WMN depends on the

routing protocol

 May be the main product

of a mesh company

 May be missing

Routing – Wish List

► Essential for reliability

 Mobile user support

► Seamless and efficient

Existing Routing Protocols

(e.g., DSR, AODV, OLSR, CBR, TORA)

► Newcomers by

comparison with the Internet protocols

► Designed for high rates of

link changes; hence perform well on WMNs

► May be further optimized

to account for WMNs’

particularities

Ad Hoc Networks

Routing - Optimization Criteria

 Minimum Hops

 Minimum Delays

 Maximum Data Rates

 Minimum Error Rates

 Maximum Route Stability

 Power Consumption

 Combinations of the

above

 Use of multiple routes

to the same gateway

 Use of multiple

gateways

Routing – Cross-Layer Design

 Routing – Physical

► Link quality feedback is

shown often to help in selecting stable, high bandwidth, low error rate routes

► Fading signal strength can

signal a link about to fail →

► Cross-layer design essential

for systems with smart antennas

 Routing – MAC

► Feedback on link loads can

avoid congested links → enables load balancing

routing depend on each other

► MAC detection of new

neighbors and failed routes

performance at routing layer

Routing – Cross-Layer Design (cont)

► Choosing routes with

low error rates may improve TCP’s

► Especially with respect of

satisfying QoS constraints

Network Layer - Fairness

► The MAC layer’s fairness

ensures horizontal fairness

QoS Support required at every layer

► Negotiate end-to-end and

with lower layers

► Adapt to changes in QoS

WMNs 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

WMNs Standards

 802.11s WLAN Mesh

► MAC layer needs to be extended to a wireless DS to support

broadcast/multicast

► 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

WMNs Standards

 802.11s MCF Sublayer

► Routing – Cross-layer design

 QoS Support at each layer

 WMN Standards