Lecture 7 Overview of Wireless Mesh Networks_Part 1.pdf

Overview of Wireless Mesh
Networks (WMNs)_Part 1
Assist. Prof. Dr. Salah Abdulghani
Computer Engineering Department
University of Mosul
1
Lecture 7

Introduction
Wireless Mesh Networks (WMNs) introduce a new
model of wireless broadband Internet access by
providing high data rate service, scalability.
Obtaining high throughput for multi-cast applications
(e.g. video streaming broadcast) in WMNs is
challenging due to the interference and the change of
channel quality.
To overcome this issue, cross-layer has been proposed
to improve the performance of WMNs.
There are several new applications of WMNs such as
digital home, broadband Internet access, building
automation, health and medical systems, emergency
and disaster networking, etc.
2

6
Gateways
• Multiple interfaces (wired &
wireless)
• Mobility
– Fixed (e.g. rooftop) – most
common case
– Mobile (e.g., airplane,
busses/subway)
• Serve as (multi-hop) “access
points” to user nodes
• Relatively few are needed,
(can be expensive)
GW

7
Wireless Routers
 At least one wireless interface.
 Mobility
 Fixed (e.g. rooftop)
 Mobile (e.g., airplane,
busses/subway).
 Provide coverage (acts as a mini-
cell-tower).
 Do not originate/terminate data
flows
 Many needed for wide areas,
hence, cost can be an issue.

8
Users
• Typically one interface.
• Mobility
– Fixed
– Mobile
• Connected to the mesh
network through
wireless routers (or
directly to gateways)
• The only
sources/destinations for
data traffic flows in the
network.

9
User – Wireless Router Links
• Wired
– Bus (PCI, PCMCIA, USB)
– Ethernet, Firewire, etc.
• Wireless
– 802.11x
– Bluetooth
• Point-to-Point or Point-to-
Multipoint
• If properly designed is not a
bottleneck.
• If different from router-to-
router links we’ll call them
access links
(PCMCIA) Personal Computer Memory Card International Association
(PCI) Peripheral Component Interconnect

10
Router to Router Links
• Wireless
– 802.11x
• Usually multipoint to
multipoint
– Sometimes a
collection of point to
point
• Often the bottleneck
• If different from
router-to-user links
we’ll call them
backbone links

11
Gateway to Internet Links
• Wired
– Ethernet, TV Cable,
Power Lines
• Wireless
– 802.16
• Point to Point or
Point-to-Multipoint
• We’ll call them
backhaul links
• If properly designed,
not the bottleneck

12
How it Works
• User-Internet Data Flows
– In most applications the main
data flows
• User-User Data Flows
– In most applications a small
percentage of data flows

Introduction (cont.)
The major components of a wireless mesh network
include wireless mesh routers, wireless mesh clients,
such as; (PCs, laptops, PDAs, and cell phones), and
access points (AP) or gateways that act like both as
Internet routers and wireless mesh routers.
The mesh routers in WMNs provide multi-hop
connectivity from one host to another, or to the
Internet via the access points. Wireless mesh routers
can be access points of wireless local area network
(WLAN), source nodes of wireless sensor network, or
base stations (BS) of cellular network.
13

Introduction (cont.)
Mesh router is generally much more powerful than
client in terms of computation and communication
capabilities, and have a continuous power supply.
They normally stay in static and supply connections
and services to mesh clients.
The routers automatically establish and maintain
mesh connectivity among themselves, making
WMNs dynamically self-organized and self-
configured networks.
These features bring many benefits to WMNs such as
low installation cost, large-scale deployment,
reliability, and self-management. 14

Challenges of WMNs
There are some challenges needed to be resolved for all
protocol layers.
In MAC layer, the challenges are effective channel
allocation, efficient spectrum utilization among multiple
radios, scheduling of flows for maximum resource
utilization, seamless mobility between heterogeneous
WMNs, provisioning of multiple QoS metrics, etc.
WMNs also need the development of MAC protocols in a
multi-radio multi-channel architecture that satisfied QoS
metrics requirements such as end-to-end delay, packet
loss ratios, link quality, interference, bandwidth and delay
jitter.
15

Challenges of WMNs (cont.)
The various research challenges of routing
protocol for WMNs are mentioned as follows:
 Propose modern routing metrics for new
applications.
 Design multichannel routing protocols that is
scalable, efficient, reliability that satisfied QoS
metrics.
Furthermore, many applications need multi-
casting capability. For example, in a community or
citywide network, video distribution is a common
application
16

Medium Access Control (MAC) Protocol
for Wireless Mesh Networks
 The MAC protocol is a process of sharing single communication
medium among multiple users with quality of service such as
throughput, packet loss ratio, delay, delay jitter, bit rate, and bit
error rate.
 MAC protocols can be classified into two major types,
depending on the coordination scheme, centralized MAC and
the distributed MAC. For the first one, the communication
process is controlled and coordinated by a central node (e.g.,
AP and BS), and all other nodes can communicate only under
the permission of a central node.
 The distributed MAC is preferred in multi-hop wireless
networks because the network itself is distributed .
17

MAC Protocols Classification
 Based on literature review, MAC protocols are classified into four
different perspectives.
 The First one is classified according to the functionality of the
protocols, and the problems are resolved such as collision
avoidance that included (request to send, clear to send (RTS/CTS)
handshake-based MAC, receiver initialized MAC, dual/multiple-
channel-based MAC), energy conservation, interference resistance
and rate adaptation.
 The Second classification depends on the session initiator. There
are sources (sender) initialized, and destination (receiver)
initialized MAC.
 The Third classification depends on channel division. They are
classified into a single channel and dual or multiple channels MAC.
 The Fourth way of classification is single-radio and multiple radio.
18

MAC Protocols Classification
Other researcher's mix between the third and the
fourth classifications, as single-channel single-radio
MAC protocols, multi-channel single-radio MAC
protocols, and multi-channel multi-radio MAC
protocols.
19

Single-Channel Single-Radio MAC Protocols
In this type of classification, many schemes have
been proposed such as CSMA/CA improvements,
TDMA over CSMA/CA and CDMA MAC.
CSMA/CA improvements: The MAC for WLANs is
usually implemented based on carrier sense multiple
accesses with collision avoidance (CSMA/CA).
In order to improve the performance for WMNs,
many schemes have been proposed to fine-tune
parameters of CSMA/CA such as contention window
size and modify backoff parameters.
20

Multi-Channel Single-Radio MAC Protocols
A single-channel MAC protocol (such as IEEE 802.11
DCF) does not work well in a multi-channel
environment where nodes may dynamically switch
channels.
Multiple channels can be used to resolve the capacity
limitation by interference, because the interference
range is much larger than the communication range in
the single channel. This causes to drop the network
capacity as the number of hops or as the number of
nodes increased. Multiple radios on the same node
can be implemented as multiple NICs or one NIC on
which multiple radios reside via system-on-chip (SoC)
or radio-on-chip (RoC) technique.
21

Multi-Channel Single-Radio MAC Protocols (cont.)
 A multi-channel MAC (MMAC Protocol) was proposed for Ad-
hoc wireless networks that utilize multiple channels
dynamically to improve performance. The main goal of MMAC
is to solve the multichannel hidden node problem. It solves
this problem by synchronizing the RTS/CTS based channel
negotiation process among all nodes. In this case, channel for
different pairs of communicating nodes will not interfere with
each other.
 The proposed protocol enables hosts to utilize multiple
channels by switching channels dynamically, thus increasing
network throughput, especially when the network is highly
congested. The proposed protocol requires only one
transceiver for each host, while other multi-channel, the MAC
protocols require multiple transceivers for each host.
22

Multi-Channel Single-Radio MAC Protocols (cont.)
• The MMAC achieved higher throughput and
lower average packet delay than IEEE 802.11
MAC. However, the overall design of MMAC is
far from a practical MAC protocol that can be
applied to WMNs.
23

Multi-Radio MAC Protocols
There are two advantages of multi-radio MAC
protocols over a single-radio MAC protocol. These
advantages are clear as the same node can have
simultaneous communications on different
radios.
First advantage is that the multi-radio MAC can
achieve higher network capacity and throughput
than a single radio MAC, while on the second
count, the multi-radio MAC protocol does not
need to switch channels on a wireless radio.
24

Multi-Radio MAC Protocols (cont.)
 Several researchers have proposed multi-radio communications for
WMNs, but most of them do not consider how the MAC protocol
was designed.
 On the other hand, other researcher’s have focused on how the
channels are assigned on different radios. One of the protocols that
was proposed is a multi-radio unification protocol (MUP) for IEEE
802.11 Wireless Networks .
 The main goal is to optimize local spectrum usage via intelligent
channel selection in a multi- hop wireless network. This protocol
enables scalable multi- hop wireless networks. Nodes in a MUP
enabled multi-radio multi-hop network to achieve a 70% increase in
throughput and 50% improvement in delay.
25

There are two types of channels in multi-radio
multi channel wireless networks.
 The first one is the control channel for passing
system control and channel setup messages.
The second type is the data channel which is for
data frame transmission after a connection has
been established successfully. The control channels
should always be on, but those for the data
channels can be switched on when needed.
26

However, there are several issues remain
unresolved such as; the channel switching
mechanism is not justified.
RTT measurement does not reflect traffic load.
 The hidden node issue is not solved.
The MAC addresses of a neighbor may not be
always detected.
Channel allocation on each NIC is not optimal.
Packet rearrange is needed when channels are
switched
27

Routing Protocols for Wireless Mesh Networks
 More significantly, WMNs nodes are fixed and do not need
batteries to operate. Therefore, WMNs routing protocols must
focus on reliability and performance improvement rather than
mobility or energy consumption.
 It is quite difficult to determine, which routing protocol performs
best under a number of different network scenarios and
applications, such as traffic, load balance, capacity, delay, and
increasing node density.
28

Routing Protocols for Wireless Mesh Networks (cont.)
In a large network, route updates consume part of
the available bandwidth and increase channel
congestion.
Therefore, the traditional distance vector and link
state routing protocols do not scale in large mobile
ad hoc networks (MANETs).
 To overcome these problems which are associated
with the link state and distance vector protocols, a
number of routing protocols have been proposed
for MANETs.
29

Routing Protocols for WMNs
Ad-hoc routing protocols for WMNs are
classified into three kinds:
(1) Proactive (Table-driven)
(2) Reactive (On Demand)
(3) Hybrid
30

Proactive Routing Protocols
 In general, proactive routing protocols such as OLSR (Optimized Link
State Routing) and DSDV (Destination-Sequenced Distance Vector)
are more suitable for a stationary network.
 In these types of protocol, the routes to all destinations are
determined at the startup, and maintained by using a periodic path
update process. However, reactive routing protocols such as AODV
(Ad hoc On-demand Distance Vector), DYMO (Dynamic MANET On-
demand Routing), and DSR (Dynamic Source Routing) are better for
mobile networks with a high mobility.
 The routes are determined only when they are required by the
source using a route discovery process. In contrast, hybrid routing
protocols, such as ZRP (Zone Routing Protocol) and ZHLS (Zone-
Based Hierarchical Link- State) combine the basic properties of the
first two classes of protocols into one
32

Proactive Routing Protocols (cont.)
For several applications, a client in WMNs is
mobile, but it can vary between being mobile
and being stationary.
Therefore, use the hybrid routing approach
with the ability to adapt client mobility can
improve the performance and scalability of
WMNs by partitioning a network into several
different zones.
33

Proactive Routing Protocols (conti.)
 This type of protocol operates like a traditional routing protocols on
wired networks. This means the routes maintain at least one route to
any destination in the network.
 Typical of these routing protocols, every node in a network ensures a
routing table to store route information so that the routes are always
available when needed.
 The main advantage of this category of the protocol is to enable the
node quick access to route information and also, to establish a
session promptly.
 However, they have serious disadvantages of increasing routing
overhead and waste bandwidth. This is because each node
periodically sends the routing information throughout the entire
network, and as such, keeps the routing information in different
tables.
 These tables are periodically updated if the network topology
changes. 34

Lecture 7 Overview of Wireless Mesh Networks_Part 1.pdf

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