1.Wireless Mesh Networks

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Oct 28, 2013 (3 years and 7 months ago)

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1.

Wireless Mesh Networks


1.1.

Introduction

Wireless Mesh Network (WMN) is a promising wireless technology for
several emerging and commercially interesting applications, e.g.,
broadband home networking, community and neighborhood networks,
coordinated network
management, intelligent transportation systems. It is
gaining significant attention as a possible way for Internet service
providers (ISPs) and other end
-
users to establish robust and reliable
wireless broadband service access at a reasonable cost
.

WMNs co
nsist of mesh routers and mesh clients as shown in Fig. 1.1. In
this architecture, while static mesh routers form the wireless backbone,
mesh clients access the network through mesh routers as well as directly
meshing with each other
.

[1]


Figure 1.1 an
illustration of wireless mesh network architecture.


Wireless communication is without a doubt a very

desirable service as
emphasized by the tremendous growth in both cellular and wireless local
area networks (WLANs). The cellular networks offer wide

area
coverage,
but the service is relatively expensive and

offers low data rates, even the
third generation of cellular networks (3G) offers low data rates (2Mbps)
compared to WLANs. On the other hand, the WLANs have rather limited
coverage (and the associated
reduced mobility). Furthermore, in order to
increase the coverage of WLANs, a wired backbone connecting multiple
access points is required.

Wireless metropolitan area networks (WMANs) partially bridges this
gap, offering high data rates with guaranteed qua
lity of service to a
potentially large customer base (up to tens of miles from the base station).
The main drawback of WMANs is their (current) lack of mobility support
and the line of sight (LOS) requirement.

Wireless mesh networks (WMNs) have the potenti
al to eliminate many of
these disadvantages by offering low cost, wireless broadband Internet
access both for fixed and mobile users.
[2]


In this chapter, we will
provide

a comprehensive introduction to the
recent developments in the protocols and architec
tures of wireless mesh
networks (WMNs) and also discusses the opportunities and challenges of
wireless mesh networks
.





1.2.

WMN Architecture


WMN is quite different network architecture when comparing to the
traditional Internet, cellular networks, or WLAN.

The first widely deployed wireless data network standard has been
IEEE’s 802.11 standard. The 802.11 standard is a suite of protocols
defining an Ethernet
-
like communication channel using radios instead of
wires
.

[3]

A typical 802.11 network
has two types
of network elements: stations
(STAs) and access points (APs).Stations can be mobile devices such as
laptops,
personal digital assistants
,
IP phones
, or fixed devices such as
desktops

and
workstations

that are equipped with a wireless network
interface. Access points (APs), normally
routers
, are base stat
ions for the
wireless network. They transmit and receive radio frequencies for
wireless enabled devices to communicate with.

[4]

The 802.11 standard specifies two network architectures: Infrastructure

basic service set

BSS and Independent

basic service set

I
BSS.

Either

802.11 stations communicate directly with each
other to form an IBSS or
with the AP to form a BSS.

An extended service set (ESS) is a set of
connected BSS. Access points in an ESS are connected by a distribution
system (DS) via wired links.
Fi
gure 1.2 shows an example of an ESS.



Figure 1.2. Extended service Set

:
[5]


The drawback of this
architecture

is highly expensive infrastructure costs,
since an expensive cabled connection to the wired Internet backbone is
necessary for each AP. On the other hand, constructing a wireless mesh
network decreases the infrastructure costs, since the mesh network
requ
ires only a few points of connection to the wired network
.(1)


In wireless Mesh Networks (WMNs) APs turn into mesh access points
(MAPs). Mobile stations are sometimes referred as mesh clients. The new
IEEE 802.11s standard for WMNs introduces a third class

of nodes called
mesh points (MPs) [
6
].

MPs and MAPs support
WLAN mesh services
,
allowing them to forward packets on behalf of other nodes to extend the
wireless transmission range. Mesh clients can associate with MAPs but
not with MPs.
Mesh portals
are
MAPs connected to a distribution system
or a non IEEE 802.11 network
. [7]

There are three different types of
WMN:



Infrastructure WMN

(Figure 1.3)
:
is a hierarchical network
consisting of mesh clients, mesh routers and gateways. Mesh
routers constitute a wi
reless mesh backbone, to which mesh
clients are connected as a star topology, and gateways are chosen
among mesh routers providing Internet access.
[8]



Client WMN

(Figure 1.4): In this type the mesh clients form the
network and no MAP is required. Client WM
Ns are also known as
mobile ad
-
hoc networks (MANETs)
.

[7]



Hybrid WMN (Figure 1.5): Mesh clients can perform mesh
functions with other mesh clients as well as accessing the
network.
[9]





Figure 1.3. Infrastructure Wireless Mesh Network [10]





Figure 1.4. Client Wireless
M
esh
N
etwork
[10]





Figure 1.5. Hybrid Wireless Mesh Network [10]



Because of the system
architecture,

wireless mesh networks have
different requirements to the physical layer, MAC layer and routing
protocols than the traditional IEEE 802.11
WLAN,

in the next few
sections we will present an overview
of the WLAN protocols and which
changes have to be made f
or WMN.




1.3 Physical Layer


1.3.1 IEEE 802.11 WLAN Physical Layer


The 802.11 physical
layer

(PHY) is the interface between the MAC and
the wireless media where frames are transmitted and received. The PHY
provides three functions. First, the PHY provides an interface to exchange
frames with the upper MAC layer for transmission and reception of d
ata.
Secondly, the PHY uses signal carrier and spread spectrum modulation to
transmit data frames over the media. Thirdly, the PHY provides a carrier
sense indication back to the MAC to verify activity on the media
.

[
11]


The PHY layer is composed of physical layer convergence (PLCP) and
physical medium dependent (PMD) layers
.
PLCP is an interface to MAC
layer and PMD is equipped with a transmission interface to send and
receive files over the air.

The physical layer (PHY)
is the layer 1 element of OSI protocol stack.
IEEE 802.11 introduced three PHY standards in 1997 and two
supplementary standards in 1999:


Frequency
-
hopping spread
-
spectrum (FHSS)


Direct
-
sequence spread
-
spectrum (DSSS)


Infrared light (IR)


802.11b: H
igh
-
rate Direct Sequence (HR/DSSS)


802.11a: Orthogonal Frequency Division Multiplexing (OFDM)

[12]


To further increase the throughput a new physical layer was introduced in
802.11g, which supports up to 54 Mbps. IEEE 802.11a/g uses orthogonal
frequency
-
division multiplexing (OFDM).

The newest standard IEEE 802.11n will use OFDM in combination with
multiple antennas. Thereby data rates of more than 100 Mbit/s will be
possible. [13]


A comparison of the most used IEEE 802.11 standards is described below
i
n Table 1.1



802.11n

802.11g

802.11b

802.11a

802.11

standard

> 100 Mbps

54 Mbps

11 Mbps

54 Mbps

2 Mbps

Max. Data
Rate

2.4 or 5 GHz

2.4 GHz

2.4 GHz

5 GHz

2.4 GHz

Frequency

OFDM/MIMO

OFDM

DSSS

OFDM

FHSS/DSSS

Physical
layer

CSMA/CA

CSMA/CA

CSMA/CA

CSMA/CA

CSMA/CA

MAC Layer


Table 1.1
Comparison of wide
-
spread 802.11 standards

[13]



1.3.2 WMN Physical Layer


The key functions of physical layer techniques involve two aspects:
efficient spectrum utilization and robustness to interference, fading, and
shadowing. In order to increase capacity and mitigate the impairment by
fading, delay
-
spread, and co
-
channel inte
rference, antenna diversity and
smart antenna techniques can be used in WMNs.



It is still necessary to further improve the performance of physical
layer techniques. Multiple
-
antenna systems have been researched
for years. However, their complexity and cos
t are still too high to
be widely accepted for WMNs.



To best utilize the advanced features provided by physical layer,
higher layer protocols, especially MAC protocols, need to be re
-
designed. Otherwise, the advantages brought by such physical
layer techn
iques will be significantly compromised.
[14]



1.4 MAC Layer

1.4.1 IEEE 802.11 WLAN MAC Layer

The 802.11 MAC layer provides functionality to allow reliable data
delivery for the upper layers over the wireless PHY media. The data
delivery itself is based on

an asynchronous, best
-
effort, connectionless
delivery of MAC layer data. There is no guarantee that the frames will be
delivered successfully
.

[
11]

The 802.11 standard defines two media access protocols: DCF and PCF.
Distributed coordination
function (
DCF
) allows
multiple stations to
interact access the medium without a central control.

On the other hand,
the point coordination function (PCF) lets a central entity

the point
coordinator (or PC, usually located in the AP)

control the medium. The
PC is respon
sible for managing access to the medium.

All 802.11
-
compliant devices must support DCF, whereas the support for
PCF is optional. However, PCF has never been, in fact, implemented by
any 802.11 equipment manufacturers. [3]


Carrier senses

multiple
access
:


The basic access mechanism, called DCF is typically the carrier sense
multiple access with collision avoidance (CSMA/CA) mechanism.
CSMA protocols are well known and Ethernet is the most famous one,
which is a protocol based on the CSMA/CD access mechanis
m (CD for
collision detection). Contrarily to CSMA/CD mechanism which is based
on the collision detection, the CSMA/CA allows an access to a shared
medium by avoiding collisions
.

[
15]

The CSMA
-
CD scheme works well for a wired medium. However, this
approach

is unsuitable for the wireless medium for multiple reasons.



Implementing
collision

detection would require the implementation of
a full
-
duplex radio (capable of transmitting and receiving at the same
time).



Two, the wireless medium is inherently open to i
nterference from a
wide variety of sources, especially since 802.11 operates in the
unlicensed frequency spectrum
[3]



The Hidden node problem

(Figure 1.6)
:

The hidden node problem that is unlikely to occur in a wired LAN is
another challenge for WLANs. If
two stations (A, C) are unreachable and
if there is a station (B) in the middle of t
hose two that is reachable from
both [12], CSMA requires that, before starting transmission, a terminal
“senses” the medium to ensure that the medium is idle and therefore
available for transmission. In our case, assume that A is already
transmitting data to B. Now, C also wishes to send data to B. Before
beginning transmission, it senses the medium and finds it idle since it is
beyond the transmission range of A. It therefo
re begins transmission to B,
thus leading to collision with A’s transmission when the signals reach B.

[3].


Figure 1.6 Hidden Node Problems
[12]




The Exposed Node Problem
(Figure 1.7)
:

Consider what happens when B wants to send data

to A and C wants to
send data to D (Figure 4.10). As is obvious, both communications can go
on simultaneously since they do not interfere with each other. However,
the carrier
-
sensing mechanism r
a
ises

a false alarm in this case. Suppose B
is already sendi
ng data to A. If C wishes to start sending data to D, before
beginning
it senses the medium and fi
nds it busy (due to B’s ongoing

transmission). Therefore, C delays its transmission unnecessarily. [3]



Figure 1.7 Exposed

Node Problems [12]



Interframe Spaces (Spaces
between

s
uccessive
f
rames):


IEEE 802.11

standard defines four types of IFS timers classified by
ascending order, which

are used to define different priorities:


Short interframe space (SIFS)

is the smallest
time required to give
priority to the completion of a frame exchange sequence, since other
STAs wait longer to seize the medium.
[12]

, SIFS is specific to PHY
layers.


Point coordination IFS (PIFS) is used by the AP (called coordinator in
this case) to
gain the access (to the medium before any other station. It
reflects an average priority to transmit the time
-
bounded traffic.
[3]

PIFS = SIFS + 1 Slot time.


Distributed IFS (DIFS) is the IFS of weaker priority than the two
previous;

it is used in the case

of data asynchronous transmission.
[3]

DIFS = SIFS + 2 Slot time.


Extended IFS (EIFS) is the longest IFS. It is used by a station receiving a
packet which is corrupted by collisions to wait more time than the usual
DIFS in order to avoid future collisions
.

[
3]


Slot Time
:
Time is quantized in slots. Slot time is specific to PHY layers.

[12].


Table 1.2 shows the main parameters of SIFS and Slot Time for different
P
HY

layer
technologies.


Value

Physical layer

Parameters

50

FHSS

Slot Time

20

DSSS, HR/DSSS



9

OFDM



8

IR








28 +/
-

10 %

FHSS

SIFS Time

10

DSSS, HR/DSSS



16

OFDM



10

IR




Table
1.2 Main Parameters
(Values are in microseconds).
[15]



Distributed Coordination Function (DCF)

The primary 802.11 MAC function is the
so
-
called Distributed
Coordination Function (DCF). The DCF is a random access scheme
based on the Carrier Sense Multiple Access with Collision Avoidance
protocol (CSMA/CA)
. [
17]

The DCF will be implemented in all STAs, for use within both IBSS and
infrastr
ucture network configurations.
In this protocol, the

STA, before
transmitting, senses the medium. If the medium is free for a specified
time, called
distributed interframe space
(DIFS), the STA executes the
emission
of its data [18].
When a station receives a unicast frame it waits
for the duration of SIFS (which is shorter than DIFS) and sends back an
acknowledgement
message (ACK).

However
If

the medium is busy because another STA is transmitting, the
STA defers its transmission, and
then it executes a backoff algorithm
within a
contention window
(CW). This behavior of the CSMA/CA
protocol is sketched in the Figure
1.8
.

The backoff mechanism used in the DCF is discrete and the time
following a DIFS is divided into temporal
slots
.[18]




Figure 1.8 IEEE 802.11 unicast data transfer. [13]

Distributed Coordination Function (DCF)

with RTS/CTS

In order to solve the hidden and exposed terminal problems in CSMA,
researchers have come up with many protocols, which are contention
based

but involve some forms of dynamic reservation collision resolution.
Some schemes use the Request
-

To
-
Send/Clear
-
To
-
Send (RTS/CTS)
control packets to prevent collisions
. [20]

The principle operation of the mechanism is described as follows:



A station
wanting to send frames begins by initially transmitting a
short control packet called request to send (RTS), which contains the
source, the destination and duration of the transmission
(ie.NAV)
.
[15]



The destination station answers (if the medium is free)
with a control
packet called clear to send (CTS), containing the same duration
information
(ie.NAV)
.
[15]



All stations receiving either the RTS and/or the CTS will set their
Virtual Carrier Sense
indicator (
called NAV, for Network Allocation
Vector), for th
e given duration, and will use this

information together
with the Physical Carrier Sense when sensing the medium.

Therefore
,
these nodes consider the channel busy for the NAV mentioned
.
[21]



If the source does not receive the CTS packet, it assumes that a
c
ollision occurred and will retransmit the RTS packet after a random
waiting period.
[15]



If the recipient receives the CTS packet correctly, the source emits an
acknowledgement to announce to the recipient that the packet CTS
was received. Then the communic
ation will be able to take place.
[15]


Figure 1.9 shows a transmission between two stations A and B, and the
NAV setting of their neighbors.





Figure
1.9 CSMA
/CA with
RTS
/CTS
mechanism. [
15
]

Point Coordination Function (PCF)

As an optional
access method, the 802.11 standard defines the PCF,
which enables the transmission of time
-
sensitive information. With PCF,
a point coordinator within the access point controls which stations can
transmit during any give period of time. Within a time perio
d called the
contention free period, the point coordinator will step through all stations
operating in PCF mode and poll them one at a time. For example, the
point coordinator may first poll station A, and during a specific period of
time station A can tra
nsmit data frames (and no other station can send
anything). The point coordinator will then poll the next station and
continue down the polling list, while letting each station to have a chance
to send data
. [
16]


1.4.2 WMN MAC Layer


In general, the
shared wireless medium in

W
MN
s

requires the use of
appropriate MAC protocols to mitigate the medium contention issues as
well as to allow for efficient use of the limited bandwidth. Specialized
MAC protocols could also help alleviate the hidden/ exposed te
rminal
problem
. [
10]

Distributed coordination Function DCF can be used in WMN, However it

shows bad performance,

it

do
es

not solve the hidden and exposed
terminal problem and
do
es

not provide fairness
.

As a consequence MAC protocols for single
-
hop WLANs do

not work
properly in WMNs. Consequently specialized MAC protocols should be
used in WMNs.


Single channel MAC
protocols

Even though multiple non
-
overlapped channels exist in the 2.4GHz and
5GHz spectrum, most IEEE 802.11
-
based multi
-
hop ad hoc networks
to
day use only a single channel
.

[
22]


There are three approaches in this case:




Improving existing MAC protocols. Currently several MAC
protocols have been proposed for multi
-
hop ad hoc networks by
enhancing the CSMA/CA protocol. These schemes usually
adjust
parameters of CSMA/CA such as contention window size and
modify backoff procedures. They may improve throughput for one
-
hop communications. However, for multi
-
hop cases such as in
WMNs, these solutions still reach a low end
-
to
-
end throughput,
becaus
e they cannot significantly reduce the probability of
contentions among neighboring nodes.[23]



Cross
-
layer design with advanced physical layer techniques.

Two
major schemes exist in this category: MAC based on directional
antenna and MAC with power control

[23].

The

first set of
schemes eliminates exposed
nodes,

the second set of schemes
reduce them .However hidden nodes still exist and may become
worse.



Proposing innovative MAC protocols.
[23]


New introduced WMN MAC protocols mainly try to provide QoS
mechanisms and enhance fairness.


Multi
-
channel MAC protocols


Despite significant advances in physical layer technologies, today's
wireless LAN still cannot offer the same level of sustained bandwidth as
their wired brethren. The advertised 54 Mbps bandwi
dth for IEEE
802.11a/g wireless LAN interface is the peak link
-
layer data rate. When
all the overheads
,

MAC contention, 802.11 headers, 802.11 ACK, packet
errors are accounted for the actual goodput available to applications is
almost halved. In addition,
the maximum link layer data rate falls quickly
with increasing distance between the transmitter and the receiver. The
bandwidth
problems is further aggravated for multi
-
hop ad hoc networks
due to interference from adjacent hops on the same path as well as
from
neighboring paths .Fortunately, the IEEE 802.11b/g standards and IEEE
802.11a standard provide

3 and 12 non
-
overlapped frequency channels
respectively, which could be used simultaneously within a neighborhood.
The a
bility to utilize multiple channels
substantially increases the
effective bandwidth available to wireless network nodes. [22]


A multi
-
channel MAC may belong to one of the following categories:



Multi
-
channel single
-
transceiver
MAC: Since only one transceiver
is available, only one channel is

active at a time in each network
node. However, different nodes may operate on different channels
simultaneously in order to improve system capacity.




Multi
-
channel multi
-
transceiver MAC:
In this scenario, a radio
includes multiple parallel RF front
-
end c
hips and baseband
processing modules to support several simultaneous channels. On
top of the physical layer, there is only one MAC layer to coordinate
the functions of multiple channels.




Multi
-
Radio MAC:

In this scenario, a network node has multiple
radio
s each with its own MAC and physical layers.
Communications in these radios are totally independent. Thus, a
virtual MAC protocol such as the multi
-
radio unification protocol
(MUP)
is

required on top of MAC to coordinate communications
in all channels. In
fact one radio can have multiple channels.
However, for simplicity of design and application, a single channel
is used in each radio.

[23]


Wireless mesh network standard IEEE 802.11s support an optional multi
-
channel single
-
transceiver MAC protocol called

Common Channel
Framework
(CCF)

[24]

The CCF assumes that each node is equipped with a single half
-
duplex
tra
n
sceiver and

nodes in the network share a common control channel.
Using the CCF, node
pairs, select

a different channel and switch to that
channel for a short period of time, after which they return to the

common
channel. During this time, node exchange one or more frames. The
channel coordination itself is carried out on the common channel by
exchangin
g control frames or management frames that carry information
about the destination channel. As shown in Figure 1.10, mesh points are
synchronized to each other and utilize the common control channel .once
on the common channel, an arbitrary MP can initiate

transmission by
sending
request
-
to
-
switch (RTX) frame carrying information of the
destination data channel on which the communication will take place.

The destination MP accepts this request by transmitting a clear
-
to
-
switch
(CTX) frame carrying the same
destination data channel .if the receiving
MP accepts the RTX request, the MP pair switches to the destination
channel together,

which causes the channel switching delay.

Then the sender transmits the data and the receiver responds with an
ACK.

To increase

the utilization of the common channel a channel
coordination window (CCW) is defined, in which the common channel is
solely used for RTX/CTX. Outside the CCW the common channel can
also be used for data transfers. [24, 25]




Figure 1.10
Common
Channel Frameworks in IEEE 802.11s [24]




1.5 Routing Protocols


1.5.1 Routing Protocols overview

Routing is the process of selecting paths in a network along which to send
network traffic. Routing is performed for many kinds of networks,
including the
telephone network
,
electronic data networks

(such as the
Internet
), and
transportation networks
. This section is concerned
primarily with routing in data networks using
packet switching

technology.

Routing schemes differ in their delivery
semantics
:



U
nicast

delivers a message to a single specified node;



Broadcast

delivers a message to all nodes in the network
.



Multicast

delivers a message to a group of nodes that have
expressed interest in receiving the message
.



Anycast

delivers a message to any one out of a group of nodes,
typically the one nearest to the

source
.

Small networks may involve manually configured routing tables (
static
routing
), while larger networks involve complex
topologies

and may
change rapidly, making the manual construction of routing tables

unfeasible.
Adaptive routing

attempts to solve this problem by
constructing routing tables automatically, based on information carried by
routing protocols
, and allowing the netw
ork to act nearly autonomously
in avoiding network failures and blockages

[4].

In traditional wired networks either
distance
-
vector protocols
or
link
-
state
protocols
are used.

Distance Vector protocols determine best path on how
far the destination is,
Distance can be hops or a combination of metrics
calculated to represent a distance value. While Link State protocols are
capable of using more sophisticated methods taking into consideration
link variables, such as bandwidth, delay, reliability and load.


Distance
-
vector routing protocols are simple and efficient in small
networks, and require little, if any management. However, they do not
scale well, and have poor convergence properties
.

Link State Routing
protocols provide greater flexibility and sophis
tication than the Distance
Vector routing protocols.

Some of the link
-
state routing protocols are the OSPF, IS
-
IS and EIGRP
,

RIP

and BGP are well
-
known distance
-
vector
protocols [
26].


Routing protocols can be classified as interior routing protocols or
exterior routing protocols.

Most known interior routing protocols are:



Routing Information Protocol (RIP)



Interior Gateway Routing Protocol (IGRP)



Open Shortest Path First (OSPF)



Intermediate System to Intermediate System

(IS
-
IS)

Most known exterior routin
g protocols are:



Border Gateway Protocol (BGP)



Constrained Shortest Path First

(CSPF)


Another classification of routing protocols
is useful

to
WMN;

pro
-
active
routing

protocols

maintain

fresh lists of destinations and their routes by
periodically distributing routin
g tables throughout the network, while
reactive routing protocols find a route on demand by flooding the
network with Route Request packets.
The

Hybrid routing protocol
combin
es the advantages
of proactive and r
eactive routing , The routing is
initially established with some proactively prospected routes and then
serves the demand from additionally activated nodes through reactive
flooding.[4]






1.5.2 Routing in Wireless
Mesh Network


Routing metrics in wireless mesh network


The cost of a route is calculated using what are called
routing metrics
.
Routing metrics are assigned to routes by routing protocols to provide
measurable values that can be used to judge how useful
(how low cost) a
route will be. Routes may have more than one metric and the metrics used
may be exchanged between routers, or it may be entirely local to that one
router. Routes may have more than one metric and the metrics used may
be exchanged between r
outers, or it may be entirely local to that one
router.
[27]


Expected Transmission Count (ETX)

This metric calculates the expected number of transmissions (including
retransmissions) needed to send a frame over a link, by measuring the
forward and reverse
delivery ratios between a pair of neighboring nodes.
To measure the delivery ratios, each node periodically broadcasts a
dedicated
link probe
packet of a fixed size. The probe packet contains the
number of probes received from each neighboring node during
the last
period. Based on these probes, a node can calculate the delivery ratio of
probes on the link to and from each of its neighbors. The expected
number of transmissions is then calculated as:

ETX

=






Where
d
f
and
d
r
are the forward and
reverse delivery ratio, respectively.
With ETX as the route metric, the routing protocol can locate routes with
the least expected number of transmissions.
[1]


Expected Transmission Time (ETT)

ETT estimates the MAC layer duration needed for successfully
tr
ansmitting a packet
.
ETT is a bandwidth
-
adjusted ETX, and is generated
by multiplying the link bandwidth to obtain the time spent in transmitting
the data packet. ETT has the form of

ETT =

ETX *




, where S denotes
the size of the data packet and

B

is th
e
data transmission rate of the
link.[28]



Weighted Cumulative Expected Transmission Time

(WCETT)

WCETT is a path metric that is calculated as the sum of the ETT's of all
the hops on the path. This gives an estimate of the end
-
to
-
end delay
experienced by a packet traveling along the path based on the loss rate
and bandwidth. Thus WCETT for a path with
n
hops is given by:


Where k is the number of channels in the network and
X
j
is the sum of
transmission times of hops on channel j.

The WCETT

is a measure of the quality of a path and hence it serves as a
suitable metric for choosing packet sizes. In a set of paths between a
source and destination, the path with the lowest WCETT value is most
likely to deliver the maximum number of packets with

least delay.

[29]




Ad
-
hoc Routing Protocols



Ad hoc On
-
demand Distance vector Routing (AODV)

The Ad hoc On
-
Demand Distance Vector (AODV) algorithm enables
dynamic, self
-
starting, multi
-
hop routing between participating mobile
nodes wishing to
establish and maintain an ad hoc network. AODV
allows mobile nodes to obtain routes quickly for new destinations
, and

does not require nodes to maintain routes to destinations
that are

not in
active communication.

Route Requests (RREQs), Route Replies (RR
EPs), and Route Errors
(RERRs) are the message types defined by AODV.
When a route to a
new destination is needed, the node broadcasts a RREQ to find a route to
the destination.
A route can be determined when the RREQ reaches either
the destination itself,

or an intermediate node with a 'fresh enough' route
to the destination.

A 'fresh enough' route is a valid route entry for the
destination whose associated sequence number is at least as great as that
contained in the RREQ.

The route is made available by u
nicasting a
RREP back to the origination of the RREQ. Each node receiving the
request caches a route back to the originator of the request, so that the
RREP can be unicast from the destination along a path to that originator,
or likewise from any intermedi
ate node that is able to satisfy the
request.
[30]





Figure 1.11 AODV
R
oute
D
iscovery



Figure 1.11 shows an example of AODV route discovery in a network
that contain
s

10 nodes. The source
node want

to
find
a route

to
the
destination
node,

so
source node
broadcasts
a RREQ

request

to
the
destination,

which

is
re
-
broadcast
ed by
other nodes
.
When

the RREQ
reaches the
destination,

the destination node

replies with a unicast RREP
message.

The AODV routing protocol is designed fo
r mobile ad hoc netwo
rks with
populations of tens to thousands of m
obile nodes. AODV can handle
low
, moderate, and relatively high
mobility rates, as well as
variety of
data traffic levels.








Hybrid Wireless Mesh Protocol (HWMP)

802.11s defines a default mandatory routing protocol called Hybrid
Wireless Mesh Protocol. HWMP is inspired by a combination of
Radio
Metric
ad hoc on
-
demand Distance Vector Routing Protocol
(RM
-
AODV)
and tree
-
based routing Protocol.

IEEE 802.11s denotes H
WMP
as path selection protocol instead of routing protocol because it uses layer
2 addressing schemes. [24]

The word “Hybrid” in this protocol refers to the fact that it supports both
Reactive and Proactive routing.

HWMP supports two operation modes
:

On de
mand: The mode is used in situations where there is no root MP
configured. If no root portal is configured, RM AODV is used. For
destinations within the mesh the route discovery works like normal
AODV. If the destination is outside the mesh, the source rec
eives no
RREP upon a RREQ. Therefore it sends the messages to the route portal
after a timeout. The portal forwards them to the connected network [24]

Proactive:
Route is discovered before any request or demand and as a
result when request arrived for a pa
rticular destination node it is fulfilled.
Root Portals (also called

Mesh Portal) are configured to send announcement called root
announcement (RANN) periodically.

The root MP periodically floods a
RANN message into the network. The information contained i
n

the
RANN is used to disseminate path metrics to the root MP. Upon
reception of a RANN, each MP

that has to create or refresh a path to the
root MP sends a unicast path request (PREQ) to the root

MP via the MP
from which it received the RANN. The unicast
PREQ follows the same
processing

rules defined in the on demand mode. The root MP sends path
reply PREP in response to each

PREQ. The unicast PREQ creates the
reverse path from the root MP to the originating MP, while

the PREP
creates the forward path from

the MP to the root MP. When the path from
an MP to a

root MP changes, it may send a PREP with the addresses of
the MPs that have established a path to

the root MP through the current
MP.

A mesh portal connects mesh networks to outside network like int
ern
et. A
designated mesh portal
(MPP) is selected as designated root MPP. This
selection is done either by configuration or by

selection process. As a
result we have a
tree
structure with a root and thus it allows
proactive
routing toward MP. [24]
Figure 1.12

shows a HWMP route discovery
example.








Figure 1.12 HWMP Route Discovery


1.5.3 Summary

Traditional routing protocols are not feasible for IEEE 802.11s WMN.

AODV is not ideal for WMNs, since it uses hop
-
count as a routing
metric. Also layer 3 routing does not fit into the concept MAPs, which
are layer 2 devices. HWMP eliminates these shortcomings.














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