improving the scalability of mac protocols in wireless mesh networks mthulisi velempini (mr.)

Improving the scalability of MAC protocols in Wireless Mesh

Networks

Mthulisi Velempini (Mr.)

UCT-COE Seminar000 May 3, 2023

Introduction

WMN are composed of a wired backbone A Wireless Back haul of static routers Mobile mesh clients Broadband wireless communication system It is scalable and easy to deploy Offers a cost effective last mile solution


Mesh Architecture

BackboneBack Haul

Mobile Mesh Clients


Research Question and Arguments

1. Is CSMA/CA suitable for WMN? 2. CSMA/CA can not be scaled up to meet

new challenges in multi channel multi hop WMN

3. CSMA/CA is not suitable for WMN


IEEE 802.11 MAC

Consists of two schemes: PCF and DCF PCF is centralized, real time data scheme DCF is a CSMA/CA based decentralized

scheme designed for non real time data PCF needs the services of a PC There is no PC in Ad Hoc networks PCF is not functional in WMN Requirements of multimedia data are not

guaranteed


IEEE 802.11 MAC DCF

It is a primary medium access protocol It is contention based It uses CSMA/CA for carrier sensing CSMA/CD can not be implemented due to

transmission/reception constraint CSMA/CA suffers from packet collisions


CSMA/CA

Consists of three processes: Virtual Sensing – NAV Random back off interval ( ) Contention period Collision result in retransmission of packets Back off interval is doubled Delayed ACK is associated with congestion Degrades significantly as network grows

2n – 1


Continuation

CCA procedure

Contention Window

Reset parameters CW = 0?

DATA

RTS


Multi hop Communication

IEEE 802.11 performs badly in Multi hop It is a single hop protocol Fails to deal with Hidden and exposed nodes Relay packets degrade throughput Listen before you talk results in delays Collisions lead to Hold off and retransmissions Lack of coordination affects throughput HOL problem degrades delay bounded data


Hidden and Exposed Nodes

Y is exposed to WX flow, this problem affects spatial reuse

Z is hidden to XW flow and will interfere with data reception at W

. . . . X W Y Z


Asymmetric Views

Nodes have different views of the channel Lead to unfairness and starvation of

disadvantaged flows The asymmetric problems are IA and FIM In IA, an advantaged flow knows when to

contend In FIM, a middle node continues to sense

outer nodes and to hold off A node can suffer combinational effects of

FIM and IA


Continuation

ZY

W X

W X A Y Z

B


Multi Channel

IEEE 802.11 MAC is single channel It is not suitable for multi channel Transmission/reception constraint Hidden and exposed nodes Missing receiver problem Virtual sensing has limitations RTS/CTS fail to notify neighbours Lack of coordination is a major problem


Deafness Problem Relates to channel assignment There is a need to trade off deafness and

Interference Common channel is required for connectivity Channel sharing leads to interference Minimal connectivity may be a solution

X Y ZChannel 1 Channel 2


Related Work

MAC can be optimized for real time data IEEE 802.11 MAC is scalable It has to be modified to ensure compatibility Use of multi channels should be coordinated Per flow models to be designed Back offs, hidden and exposed nodes to be

addressed MAC should guarantee throughput, capacity,

latency, reach and QoS requirements


Summary MAC promotes unfairness and starvation Deferred transmissions cause starvation Radios have transmit/reception constraints Distributed schemes are preferred Designs to optimize channel utilization To improve end to end flow fairness against

interference This leads to the scalability of WMN Heterogeneity between MR and MC

improving the scalability of mac protocols in wireless mesh networks mthulisi velempini (mr.)

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