UNIT-VAd-hoc Networks

Ad-hoc Networks• Two types of wireless network:

– Infrastructured• the mobile node can move while communicating• the base stations are fixed• as the node goes out of the range of a base station, it gets into the range

of another base station– Infrastructureless or ad-hoc

• the mobile node can move while communicating• there are no fixed base stations• all the nodes in the network need to act as routers

– In Latin “ad-hoc” literally means “for this purpose only”. Then an ad-hoc network can be regarded as “spontaneous network”

• Infrastructured network

PDA

Pen computer

Radio tower

Laptop computer

Radio tower

Infrastructure(Wired line)

Desktop computer

Laptop computer

Ad-hoc Networks

• Infrastructurless (ad-hoc) network or MANET (Mobile Ad-hoc NETwork)

Ad-hoc Networks

PDA

Pen computer

Laptop computer

Laptop computer

PDA

– Single hop – nodes are in their reach area and can communicate directly

Multi hop – some nodes are far and cannot communicate directly. The traffic has to be forwarded by other intermediate nodes.

Classification of ad-hoc networksAd-hoc Networks

• Characteristics of an ad-hoc network– Collection of mobile nodes forming a

temporary network– Network topology changes frequently and

unpredictably– No centralized administration or standard

support services– Each host is an independent router– Hosts use wireless RF transceivers as network

interface – Number of nodes 10 to 100 or at most 1000

Ad-hoc Networks

Why we need ad-hoc networks? Setting up of fixed access points and backbone

infrastructure is not always viable Infrastructure may not be present in a

disaster area or war zone Infrastructure may not be practical for short-

range radios; Bluetooth (range ~ 10m) Do not need backbone infrastructure support

Are easy to deploy Useful when infrastructure is absent,

destroyed or impractical

Ad-hoc Networks

Ad-hoc Networks

Example applications of ad hoc networks: emergency search-and-rescue operations, meetings or conventions in which persons wish to

quickly share information, data acquisition operations in inhospitable terrain, local area networks in the future.

Ad-hoc NetworksMobile Ad Hoc Networking is a multi-layer problem !

Physical/Link Layer

Network Layer

Transport Layer

Application Layer

- Routing- Addressing- Location Management

- Power Control- Multiuser Detection- Channel Access

- TCP- Quality of Service

- Security- Service Discovery- Location-dependent Application

Is it possible to use standard routing protocols? Distance-vector protocols

Slow convergence due to “Count to Infinity” Problem

Creates loops during node failure, network partition or congestion

Link state protocols Use flooding technique and create excessive

traffic and control overhead Require a lot of processor power and therefore

high power consumption

Problems with Routing

Problems with Routing Limitations of the Wireless Network

packet loss due to transmission errors variable capacity links frequent disconnections/partitions limited communication bandwidth Broadcast nature of the communications

Limitations Imposed by Mobility dynamically changing topologies/routes lack of mobility awareness by

system/applications Limitations of the Mobile Computer

short battery lifetime limited capacities

DSDV

DSDV (Destination Sequenced Distance Vector) Each node sends and responds to routing control message

the same way No hierarchical structure Avoids the resource costs involved in maintaining high-level

structure Scalability may become an issue in larger networks

Basic Routing Protocol known also as Distributed Bellman-Ford

Every node maintains a routing table all available destinations the next node to reach to destination the number of hops to reach the destination

Periodically send table to all neighbors to maintain topology Bi-directional links are required!

DSDV

Intro -2

Short introduction to wireless multihop networks• Two or more nodes equipped with wireless

communications and networking capability

• Base station is not necessary

• A node can communicate directly with another node that

is immediately within radio range

• To communicate with nodes outside its own radio range

an intermediate node is used to forward the packet

• The network is self-organizing and adaptive (autonomous

distributed control is required)

• Nodes are able to detect the presence of other nodes and

join them into the network

• The nodes don’t need to be of the same type (phone, PDA,

laptop, sensor, etc.)

Application areas

• Tactical military• Emergencies• Sensor• Meetings/conferences

Challenges

• Dynamic topologies• Bandwidth-constrained, variable capacity links• Energy-constrained• Limited physical security• Scalability

Simple routing protocol example

• Propagation of routing table• Routing and transmitting

Routing table

Destination terminal

Nextnode

A AB AC ED D… …

• Each terminal has its own routing table(in proactive routing algorithms)

Position notification packet

ID of terminal which the created the packet

Timestamp for the created packet

ID of hop source terminal

Hop count

Contents of the packet:

• Used to make and update the Routing Table• Broadcasted in a limited area

Renewal of Position Notification Packet

A B D

t =1 t =2 t =3 t =4

A A A B A C

A 1 A 1 A 1 B 2 A 1 C 3

C

B B CC

Basic transmitting procedure

• Request to send (RTS)• Clear to send (CTS)

• Ready to receive (RTR)

Topology problem

• Hidden terminal problem• Exposed terminal problem

• Busy tones

Ad Hoc routing protocols

• Proactive– Large overhead

• Reactive– Delay before first packet– Doesn’t scale

• Hybrid scheme– Clusters

25

Unicast, id-centric routing• Given: a network/a graph

– Each node has a unique identifier (ID)• Goal: Derive a mechanism that allows a packet sent from an arbitrary node

to arrive at some arbitrary destination node– The routing & forwarding problem– Routing: Construct data structures (e.g., tables) that contain

information how a given destination can be reached– Forwarding: Consult these data structures to forward a given packet to

its next hop• Challenges

– Nodes may move around, neighborhood relations change– Optimization metrics may be more complicated than “smallest hop

count” – e.g., energy efficiency

26

Ad-hoc routing protocols• Because of challenges, standard routing approaches

not really applicable– Too big an overhead, too slow in reacting to changes– Examples: Dijkstra’s link state algorithm; Bellman-Ford

distance vector algorithm• Simple solution: Flooding

– Does not need any information (routing tables) – simple – Packets are usually delivered to destination – But: overhead is prohibitive ! Usually not acceptable, either

! Need specific, ad hoc routing protocols

27

Ad hoc routing protocols – classification

• Main question to ask: When does the routing protocol operate?

• Option 1: Routing protocol always tries to keep its routing data up-to-date– Protocol is proactive (active before tables are actually needed) or

table-driven

• Option 2: Route is only determined when actually needed– Protocol operates on demand

• Option 3: Combine these behaviors– Hybrid protocols

28

Ad hoc routing protocols – classification

• Is the network regarded as flat or hierarchical?– Compare topology control, traditional routing

• Which data is used to identify nodes?– An arbitrary identifier?– The position of a node?

• Can be used to assist in geographic routing protocols because choice of next hop neighbor can be computed based on destination address

– Identifiers that are not arbitrary, but carry some structure?• As in traditional routing• Structure akin to position, on a logical level?