Ad Hoc Networks

Ad-Hoc Wireless Network

Sandeep NagarI-Mtech

Computer Science(08559)

Road Map

• Introduction Cellular & Ad Hoc Wireless Network

Application Of Ad Hoc Wireless Network

• Issues In Ad Hoc Wireless Network Medium Access Scheme

Routing

Multicasting

Security

• Classification Of MAC Protocol Contention-Based Protocols

Contention-Based Protocols with R Reservation Mechanism

Contention-Based Protocols with R Schedule Mechanism

Introduction

• The 1st innovative communication system was devised in 500 BC by the king of Persia for sending the messages(from his capital to remote provinces of his empire). This system was 25 times faster then the available at that time.

• In 1994 Swedish communication equipment maker Ericsson proposed to develop a short range, low power, low complexity & inexpensive radio interface & that device called BLUETOOTH.

• The principal behind the Ad Hoc n/w is multi hop relaying.

Cellular & Ad Hoc Wireless Network

• Cellular Wireless Network

• In Cellular wireless n/w the p presence of base stations I msimplifies routing & resource mmanagement.

• Routing direction are made in a a centralized manner with m o more information about the d destination.

Cellular & Ad Hoc Wireless Network…

• Ad Hoc Wireless Network

• In Ad Hoc wireless n/w routing & resource management are done in a distributed manner.

• This require each node to be more intelligent, so that it can function both as n/w host for transmitting & receiving data.

Cellular Vs Ad Hoc Wireless Network

Cellular Wireless Network• Fixed Infrastructure-Based• Single-Hop Wireless links• Centralized routing• Seamless connectivity(low call

drops during handoffs)• Reuse of frequency spectrum

through Geographical channel reuse.

• High cost of network maintenance( backup, power sour, staffing & etc…)

Ad Hoc Wireless Network• Infrastructure-less• Multi-Hop Wireless links• Distributed routing• Frequent path breaks due to

mobility• Dynamic frequency reuse

based on carrier sense mechanism

• Self-organization and maintenance properties are built into the network.

Cellular & Ad Hoc Wireless Network…

Cellular Wireless Network

Ad-Hoc Wireless Network

Wireless Mesh

Network

Wireless Sensor

Network

Hybrid wireless network



Hybrid Wireless Network

Wireless Mesh Network Wireless Sensor Network


Hybrid Wireless Network• It is the combination of CELLULAR network & Ad-Hoc network .• It support Multi-Hop Cellular Network.• Primary concept behind cellular network is geographical channel reuse.

MAIN ADVANTAGES o Higher Capacityo Increased Flexibility & Reliable Routingo Better Coverage & Connectivity in Holes


Wireless Mesh Network• Mesh networking is a way to route data, voice and instructions between nodes• Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad.


Wireless Sensor Network • A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations.• The development of wireless sensor networks was originally motivated by military applications such as battlefield surveillance.




Routing

Multicasting

Issues In Ad Hoc Wireless Network

Medium Access Scheme• The primary responsibility of MAC(Medium Access Control)

Protocol in ad-hoc network is distributed arbitration for the shared channel for transmission of packet.

• Major issues in MAC Protocol are:– Distributed Operating System :-> Ad-Hoc wireless n/w needs to operate in

environment where no centralized co-ordination is possible.– Synchronization :-> MAC protocol design should take into account the

requirement of time synchronization. It is mandatory for TDMA based system for management of transmission & reception slots.

– Capability for Power Control :-> The transmission power control reduce the energy consumption at the node, causes a decrease in interface at neighboring nodes & increase frequency reuse.


Routing The responsibility of routing protocol are

Exchange the route information, finding the feasible path from source to destination(such as Hop-length)

Minimum power required & it save wireless link life time(till the network is there).

The major challenges that a routing protocol faces are Quick route reconfiguration:- The unpredictable changes in the topology of

the network require that the routing protocol be able to quickly perform route reconfiguration.

Loop-Free Routing :- This is the fundamental requirement of any routing protocol to avoid unnecessary wastage of nodes.


Multicasting• It plays an important role in typical network application of ad-hoc

network. • For example: Emergency search & rescue operation, Military

communication & etc… In such environment certain tasks that require are:o Point to multipoint(voice & data communication)o Multipoint to multipoint(voice & data communication)

• The arbitrary movement of nodes changes the topology dynamically in unpredictable manner.

So for such type of network MESH-SHAPED structure is very beneficial because it work in high mobility environment.




Routing

Multicasting

Security

• Classification Of MAC Protocol Contention-Based Protocols

Contention-Based Protocols with R Reservation Mechanism

Contention-Based Protocols with R Schedule Mechanism

Contention-Based Protocols

• In this protocol, node does not make any resource reservation priori. But whenever it receives a packet to be transmitted it compete with its neighbor nodes for access to the share channel.

• This protocol cannot provide QoS(Quality of Service)In this Protocol we discuss MACAW(Medium Access Protocol for Wireless LAN) mechanism.

• MACAW in this we use Back-off mechanism.

Contention-Based Protocols cont….

BACK-OFF MECHANISM Back-Off mechanism is a method for reducing the collision.

Start

Set Back-off to zero

Send the frame

Collision?

Success

Send jam

signal

Increment back-off

Back-off limit

abort

Wait back-off time

No

yes

yes

no


• Back-off continue… The Station wait time= 2N x maximum_propagation_time N is the no. of attempted transmissions. maximum_propagation_time is the time needed for a bit

r reach the end of network.


• In MACAW use control packets RTSCTSDSDATAACK RTS-Request to send CTS-Clear to send DS-Data sending(30 bytes) ACK-Acknowledgement

Sender Receiver

RTS

CTSDS

DATA

ACK

Contention-Based Protocols with Reservation Mechanism

• Ad-Hoc wireless n/w sometime need to support real-time traffic, which require QoS(Quality Of Service) guarantees to be provided.

• In content-based protocol nodes are not provide QoS. So for support real-time traffic we need to reserving bandwidth.

Contention-Based Protocols with Reservation Mechanism cont…

• D-PRMA(Distributed Packet Reservation Multiple Access Protocol)– Basically PRMA (Packet Reservation Multiple Access) was proposed for

voice support in wireless LAN with base station.

– But now PRMA use with distributed system that’s why it is called D-PRMA.

– D-PRMA is based on TDMA(Time Division Multiple Access). In this the channels are divided into fixed & equal-sized frames along with time access.


• Frame Structure In D-PRMA

– Each frames is composed of slots ‘s’ & each slots consist of mini slots ‘m’

– Each mini slots can be further divided into two control field• RTS/BI(Request to Send/Busy Indication)• CTS/BI(Clear to Send/Busy Indication)• The control fields RTS & CTS are used for slots reservation and overcoming the

hidden terminal problem.

Slot 1 Slot 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slot s

Mini Slot 1 Mini Slot 2 . . . . . . . . . . . . . . . . . Mini Slot m

RTS/BI CTS/BI

Frame Length


• Hidden Terminal Problem:

• In this situation Receiver can receive packets from two or many nodes which cannot here each other.

• Receiver may be bombard by both the sender, resulting Collision & Reducing Throughput.

• SOLUTION for this problem is :• When all nodes having packets ready for transmission are compete for the first

minislot of each slot. The remaining (m-1) minislot are granted to the node that wins the competition.

RS1

S2

Contention-Based MAC Protocols with Scheduling Mechanism

• This protocol focus on packet scheduling at nodes & also scheduling nodes to access to channel.

• This Protocol proposed two mechanism- for providing quality of service(QoS) support for connection in ad-hoc wireless n/w.– DPS(Distributed Priority Scheduling)– Multi-Hop Co-ordination

• DPS(Distributed Priority Scheduling)– It is based on 802.11 IEEE distributed co-ordinate function.– DPS use same basic RTS->CTS->DATA->ACK packet exchange mechanism.– The RTS packet transmitted by a ready node then it carries the priority tag.

Contention-Based MAC Protocols with Scheduling Mechanism cont…

• DPS Continue…• It build the schedule table(ST) in which 3 columns are

RTS CTS ACKDATA

RTS CTS ACKDATA

NAV

Source(node 1)

Destination(node 2)

Neighbor Node(node 4)

Scheduling Table Update at neighbor Node 4S - Source ID, D - Destination ID, P - Priority Index

Contention-Based MAC Protocols with Scheduling Mechanism cont…

• Multi-Hop Co-ordination

– It Just extend the DPS Scheme. It scheduling over the multi-hop paths.

– In this case each node calculate the priority index of DATA packet in the recursive fashion, based on the received value of the priority index.

Ad Hoc Networks

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