Download MANET File

Mobile and Wireless Computing
MANET
Mobile Ad Hoc Networks: Introduction
1
Mobile and Wireless Computing
Mobile Ad hoc Networks (MANET)
2

This presentation is about ways of information
exchange in a network of mobile and wireless nodes
without any infrastructural support.

Such networks are often called ad hoc networks to
emphasize that they do not depend on infrastructural
support.

The purpose of an ad hoc network is to set up
(possibly) a short-lived network for a collection of
nodes.
Mobile and Wireless Computing
Ad hoc Networks
3

Most network operations involve exchange of
information among the computers participating
in a network.

There are numerous protocols for networks with
infrastructural support , starting from LANs,
Ethernets and global networks like the Internet.

However, most of these protocols do not work
well for ad hoc networks.
Mobile and Wireless Computing
Routing in an Ad hoc Network
4

If all the wireless nodes are within the
transmission range of each other, routing is
easy. Every node can listen to all transmissions.

However, this is not true in most situations, due
to short transmission range. Hence, most ad hoc
neworks are multi-hop.

A message from a source node must go through
intermediate nodes to reach its destination.
Mobile and Wireless Computing
Routing in multi-hop networks
5

All nodes cooperate in delivering messages
across the network.

The nodes must collect local neighbourhood
information in order to make global routing
decisions.

This situation is quite different from wired
networks where the routing decisions are made
based on the infrastructure.
Mobile and Wireless Computing
Bluetooth
6

Mobility becomes much more important when
there is a need to network between several
PANs.

Bluetooth is a short-range radio technology for
wireless connectivity of PDAs and other similar
devices.

If each device is equipped with a Bluetooth
radio, it is possible to connect upto 8 such
devices into a piconet.
Mobile and Wireless Computing
Other types
7

BAN – Body Area Networks
For wearable computers (see Xybernaut)
Not yet implemented

UWB – Ultra Wide Band
Deemed a probable successor of Bluetooth
However, the present commercial
implementations were not successful, and some
UWB chip makers closed down their facilities
(inclusive the leader – Tzero Technologies)
Mobile and Wireless Computing
MANET Routing Protocols
8

We will discuss routing protocols for mobile ad
hoc networks (MANET).

Routing protocols for MANETs can be classified
as either reactive or proactive.

This classification is based on the way a protocol
tries to find a route to a destination.
Mobile and Wireless Computing
Proactive Protocols
9

Proactive protocols are based on periodic
exchange of control messages and maintaining
routing tables.

Each node maintains complete information
about the network topology locally.

This information is collected through proactive
exchange of partial routing tables stored at each
node.
Mobile and Wireless Computing
Proactive Protocols
10

Since each node knows the complete topology,
a node can immediately find the best route to a
destination.

However, a proactive protocol generates large
volume of control messages and this may take
up a large part of the available bandwidth.

The control messages may consume almost the
entire bandwidth with a large number of nodes
and increased mobility.
Mobile and Wireless Computing
Reactive Protocols
11

In a reactive protocol, a route is discovered only
when it is necessary.

In other words, the protocol tries to discover a
route only on-demand, when it is necessary.

These protocols generate much less control
traffic at the cost of latency, i.e., it usually takes
more time to find a route compared to a
proactive protocol.
Mobile and Wireless Computing
Topology based protocols

Some examples of proactive protocols are :
–
–

Some examples of reactive protcols are :
–
–
–

Dynamic Source Routing (DSR)
Ad hoc On-demand Distance Vector (AODV)
Temporally Ordered Routing Algorithm (TORA)
Some examples of hybrid protocols are:
–
–
12
Destination Sequenced Distance Vector (DSDV)
STAR
Cluster
Zone Routing Protocol (ZRP)
Mobile and Wireless Computing
Position based protocols

Protocols based on location services are:
–
–
–
–

Protocols based on forwarding strategy are:
–
–
–
–
13
Distance Routing Algorithm for Mobility (DREAM)
Quorum-based
Geographic Location Services (GLS)
Home zone, etc.
Greedy
Gready Perimeter Stateless Routing (GPSR)
RDF
Hierarchical. etc
Mobile and Wireless Computing
The Dynamic Source Routing Protocol - DSR
An Overview of the DSR Protocol
14
Mobile and Wireless Computing
The DSR Protocol (I)
15

Nodes in the network may move about, join or
leave.

All routing is automatically determined by the
protocol.

The number and sequence of intermediate hops
needed to reach any destination may change
dynamically. Hence the network topology may
be quite complex.
Mobile and Wireless Computing
The DSR Protocol (II)
16

DSR is an on-demand or reactive routing
protocol.

When a source node S wants to send a
message to a destination node D, the process
starts with a route discovery phase.

The message is sent once a route has been
discovered and S knows about the discovered
route.
Mobile and Wireless Computing
The DSR Protocol (IV)
17

Each node maintains a route cache to
remember routes that it has learnt about.

One of the main advantages of DSR as opposed
to a table driven protocol like DSDV is that the
number of control messages is much smaller.

Hence DSR is more energy-efficient and does
not congest the network with too many control
messages.
Mobile and Wireless Computing
An Example when a Route is Discovered Completely
AB
A
B
A
A is
18
ABCD
C
ABC
trying to find a route to E
Each
E
D
intermediate node appends its ID at the end.
knows the reverse route and sends a route reply.
E
Mobile and Wireless Computing
An Example when a Route is Discovered Partially
D
A
AB
B
A
A is
19
C
trying to find a route to E
C
already has a route to E in its route cache.
C
sends back the complete route to A.
E
Mobile and Wireless Computing
Assumptions
20

All nodes wishing to communicate with other
nodes in the ad hoc network participate fully in
running the DSR protocol.

The diameter of the network is more than 1, but
usually a small number.

In other words, a message usually goes through
only a small number of hops.
Mobile and Wireless Computing
Assumptions
21

The nodes may move at any time without notice
and may even move continuously.

However, we assume that the speed with which
the nodes move is moderate with respect to the
packet transmission latency or wireless
transmission range of the network.

If the nodes move extremely fast, the only
possible protocol is flooding.
Mobile and Wireless Computing
Assumptions
22

Each node selects a single IP address by which
it is known in the network.

This is necessary for assigning a unique ID for
each node.

Each node may or may not work in the
promiscuous mode. However, most wireless
interface cards support this.
Mobile and Wireless Computing
Overview of the DSR Protocol



23
Route discovery and route maintenance operate
entirely on-demand.
There is no need to broadcast periodically to
update routing information in individual nodes.
DSDV requires such periodic broadcasts.
The number of overhead packets is much
smaller in DSR. The number of overhead
packets drop to zero when the nodes are static
and all routes have been discovered.
Mobile and Wireless Computing
Overview of the DSR Protocol



24
When nodes are mobile and/or communication
pattern changes, the number of overhead
packets increase proportionately.
It is necessary to discover new routes in these
situations and hence the new route discovery
packets are the overhead packets.
Note that, a node may receive multiple routes to
a destination in response to a route discovery
request.
Mobile and Wireless Computing
Overview of the DSR Protocol



25
A node may store multiple routes to a
destination in its route cache.
A node can react to changes in network topology
much more rapidly by taking advantage of
cached routes.
For example, if one route to a destination is
broken, the source node can choose another
route to the destination from its route cache.
Mobile and Wireless Computing
DSR Route Discovery
26

Consider the case when a source node S wants
to send a packet to a destination node D.

In a ‘good situation’, S already knows a route to
D from its route cache.

In this case, S will add the sequence of hops to
D in the header of the packet. Then S will send
the packet to the first node in this sequence.
Mobile and Wireless Computing
DSR Route Discovery
27

In the ‘bad case’ S will not find any route to D in
its route cache.

S will initiate the route discovery protocol. In this
case, we call S the initiator and D the target of
this protocol.
Mobile and Wireless Computing
DSR Route Discovery




28
Node S is trying to discover a route to node D.
S broadcasts a route request message to its
neighbours. This message is received by all
nodes within the transmission range of S.
Each route request message contains the
initiator and target of the route discovery.
Also, each route request is stamped with a
unique ID assigned by the initiator.
Mobile and Wireless Computing
An Example Route Discovery
Example:
D
A
AB
ABCD
B
A
29
C
ABC
E
Mobile and Wireless Computing
How a Node Deals with a Route Request Message
B
D
S
If
B has a route to D in its route cache, B sends back
that route to S.
B
D
S
If
B does not have a route to D, B broadcasts the route
request message.
30
Mobile and Wireless Computing
DSR Route Request



31
When a Route Request message reaches a
node, the accumulated route indicates the nodes
through which it has passed.
This accumulated route is used by a node to
send a Route Reply message back to the
initiator.
The Route Reply message can be sent either by
the destination node or by an intermediate node
that finds a route to the destination in its route
cache.
Mobile and Wireless Computing
How Often Should a Node Initiate a Route Discovery?
32

Suppose S is the initiator of a route discovery
message to a node D.

If S does not receive a route reply message, S
would like to initiate another route request
message.

However, S should not send route request
messages frequently, because D may be
unreachable at the moment.
Mobile and Wireless Computing
How Often Should a Node Initiate a Route Discovery?
D
S
P
P
S
may have a better chance of finding a route to D later.
S should not flood the network with route requests.
33
Mobile and Wireless Computing
How Fast Should a Node Send a Route Reply?
34

A node can send a route reply if it finds a route
to the target in its route cache.

However, if many nodes try to send route reply
for the same destination, it may result in a route
reply storm.

Too many replies for the same route request
may flood the network.
Mobile and Wireless Computing
Route Reply Storm
B
D
C
A
E
Each
35
G
F
of the nodes B, C, D, E and F knows a route to G.
Mobile and Wireless Computing
Waiting Before Replying
36

Simultaneous replies from all these nodes will
result in network congestion and packet
collisions.

Each node should wait for a random amount of
time and listen to the traffic.

If there is a better (shorter) route reply, or the
initiator (node A in this example) starts using
another route, there is no need to reply.
Mobile and Wireless Computing
Caching Overheard Routing Information
37

An important aspect of DSR is to maintain an
up-to-date route cache.

A good and current route cache helps a node to
find (i) routes for itself faster, and (ii) reply fast to
route request messages.

If the nodes overhear other messages, they
should analyse these messages and update
their own route cache if necessary.
Mobile and Wireless Computing
Caching Overheard Routing Information
C
A
E
B
D
P
P
overhears the route A-B-C-D-E when B is sending a
packet to C. This packet was originally sent by A.
P
stores this route in its route cache for future use. If P
receives a route request for any of the destinations C,D or
E, P can use this information.
38
Mobile and Wireless Computing
Route Request Hop Limit
39

Sometime it is not good to propagate a route
request message throughout the network.

Suppose S is the source of a route request
message for a destination D.

In case D is in the neighbourhood of S, the route
request message from S should not propagate
too far away.
Mobile and Wireless Computing
Route Request Hop Limit

If D is near S, propagating the route request
message too far will result in too many
unnecessary route reply messages in future.
S
D
40
Mobile and Wireless Computing
Restricted Propagation of Route Request
41

A better strategy is to propagate route request
messages with increasing hop count.

Initially, send the route request to a distance of 2
hops. If no route reply is received after
sometime, send the route request to a distance
of 4 hops and so on.

This reduces network congestion by reducing
the number of route reply messages.
Mobile and Wireless Computing
DSR Route Maintenance
42

Route maintenance is important for correct
delivery of messages.

When forwarding a packet, each node should
ensure that the packet reaches the next hop.

A packet reaches its correct destination if each
node ensures this correct delivery along the
path.
Mobile and Wireless Computing
Ensuring Correct Delivery
D
B
A
C
Node
E
A is sending a packet to E, using the route A-B-C-
D-E.
A is
B
responsible for the correct delivery of the packet to B
is responsible for the correct delivery of the packet to C
and so on.
43
Mobile and Wireless Computing
Active or Passive Acknowledgment
44

The correct delivery can be ensured through an
active acknowledgment.

An active acknowledgment may be part of the
MAC protocol in use. IEEE 802.11 standards
provide such link level active acknowledgment.

In a passive acknowledgment, node A may
overhear the forwarding of the packet by node B
and knows that the packet has been received by
B.
Mobile and Wireless Computing
Route Error Message
D
B
A
C
E
When A initiates
a message to E, there are two parts in
the message. The route from A to E and the actual
message.
Each
intermediate node tries to forward the mesage by
looking up the next hop from the route.
45
Mobile and Wireless Computing
When a message cannot be forwarded
D
B
A
C
A node
E
like C tries to forward the message and waits for
acknowledgment. C will retransmit the message a fixed
number of times if no acknowledgment arrives.
After
46
that, C will initiate a route error message.
Mobile and Wireless Computing
Route Error back to the Initiator
D
B
A
C
In
E
this example, C will initiate a route error message back
to A indicating that the link to D is currently broken.
A will
remove this route from its route cache and try
another route to E, if it has one. Or, A may start a new
route discovery.
47
Mobile and Wireless Computing
Packet Salvaging
48

After sending a route error message, a node may try to
send the packet that caused the route error.

In the previous example, C may try to find a route to E
from its own route cache.

If C can find a route to E, it will replace the previous
route by the new route and send the packet to E.

C should also indicate that this packet has been
salvaged, so that other nodes do not try to salvage it.
Mobile and Wireless Computing
Automatic Route Shortening

Some routes may become unnecessarily long
when nodes move around.
C
A
B
F
G
E
C
overhears the transmission when A sends the packet to
B. Recall that A sends the complete route to B.
C
49
informs A that B,F,G can be removed from the route.
Mobile and Wireless Computing
Spreading of Route Error Message
D
B
A
C
E
When A receives
a route error message from C, A knows
that the link C-D is broken.
 A removes
this route from its route cache. In future, A
may try a new route or try to discover a new route.
A piggybacks
the route error message so that other
nodes know that the link C-D is broken.
50
Mobile and Wireless Computing
Advantages and Disadvantages
51

DSR is a simple and efficient routing protocol
with low overhead of control messages.

However, DSR has relatively high latency in
finding routes.

DSR is not very scalable since packet size
increases with increasing hop numbers in a
route.