Improving the scalability of MAC protocols in Wireless Mesh Networks

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21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Improving the scalability of MAC
protocols in Wireless Mesh
Networks

Mthulisi Velempini (Mr.)

UCT
-
COE Seminar

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2

November 21, 2013

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

UCT
-
COE Seminar

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3

November 21, 2013

Mesh Architecture

Backbone

Back Haul

Mobile Mesh Clients

UCT
-
COE Seminar

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4

November 21, 2013

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



UCT
-
COE Seminar

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5

November 21, 2013

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

UCT
-
COE Seminar

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6

November 21, 2013

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

UCT
-
COE Seminar

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7

November 21, 2013

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

2
n



1

UCT
-
COE Seminar

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8

November 21, 2013

Continuation

CCA procedure


Contention Window

Reset parameters

CW = 0?

DATA

RTS

UCT
-
COE Seminar

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9

November 21, 2013

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

UCT
-
COE Seminar

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10

November 21, 2013

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


UCT
-
COE Seminar

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11

November 21, 2013

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

UCT
-
COE Seminar

000 Page
12

November 21, 2013

Continuation

Z

Y

W

X

W

X

A

Y

Z

B

UCT
-
COE Seminar

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November 21, 2013

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


UCT
-
COE Seminar

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14

November 21, 2013

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

Z

Channel 1

Channel 2

UCT
-
COE Seminar

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15

November 21, 2013

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

UCT
-
COE Seminar

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16

November 21, 2013

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