Wireless MAC Protocols for Ad-Hoc Networks

Derek J CorbettSupervisor: Prof. David Everitt

Introduction

■ Wireless Ad-Hoc Networks A network that is formed spontaneously by devices

locating eachother, lacking any kind of fixed infrastructure.

Characterised by their high volatility as it is envisaged that mobile devices are able to join and depart at any time.

■ Wireless MAC Protocols Allow for the efficient allocation of the limited resources

of a shared medium, i.e. the RF Channels available in Wireless Ad-Hoc Networks

Wireless MAC Protocols for Ad-Hoc Networks

■ Wireless MAC Protocols for Ad-Hoc Networks differ from other Wireless MAC protocols

There are no predefined base stations to co-ordinate channel access.

Centralized MAC protocols will not work.

■ Major Problems in Ad-Hoc Networks Hidden Nodes Exposed Nodes

Hidden Nodes

AA B C

Node A Transmits a message to Node B

10001001

Node C Cannot hear the transmission from Node ANode C Senses the Medium

??

Node C Thinks the Channel is IdleNode C Starts a Transmission

01010001X

This Transmission Collides At Node BNode C is a Hidden Node to Node AA Hidden Node causes Collisions on Data Transmission

Exposed Nodes

A B C

Node B is Transmitting to Node A

101101010

Node C Can Hear the Transmission from Node B to Node ANode C Senses the Channel

??

The Channel Appears Busy So Node C Doesn’t Transmit

X

A Transmission by Node C would not Reach Node Aand hence would not Interfere with Data Reception at Node A Node C could therefore have a Parallel Conversation with Another Terminal Out of Range of Node B and in Range of Node C

D

Node C is an Exposed Node to Node B. If the Exposed Nodes are not Minimized, the Bandwidth is Underutilized.

MAC Protocols for Ad-Hoc Networks

■ MAC protocols for Ad Hoc networks can be divided into three different categories

Contention Protocols Allocation Protocols Hybrid Protocols (which are protocols which combine

elements of the two previous classes)

Contention Protocols

■ Contention protocols use an asynchronous, random-access style, of communication.

■ Contention protocols use direct competition between nodes to determine channel access rights.

■ Collisions during contention are resolved through randomized back off periods before retransmission.

■ Collision avoidance is key to the design of these protocols.

■ Contention protocols perform well at low traffic loads, when there are few collisions.

■ Protocol performance degrades as traffic load increases due to increasing numbers of collisions.

MACA

■ MACA Multiple access with collision avoidance uses a hand

shaking protocol to avoid collisions. The hand shake dialogue alleviates the hidden node

interference, whilst minimizing the number of exposed nodes.

A B

Node A sends a request to send (RTS) control packet

RTS

If Node B receives the control packet it responds with a clear to send (CTS) control packet

CTS

All Nodes within hearing distance of Node B will have heard its CTS response and will be silent to avoid data collision at Node B

1010101010101

Node A Can now send data to Node B for However long the channel was acquired

MACAW

■ Similar to MACA Uses positive acknowledgement (ACK) to aid in the

rapid retransmission of lost packets. To protect the ACK from collision the source transmits

a data sending (DS) control packet to alert exposed nodes of its impending arrival.

A B

Data transmission from Node A to Node B finishesNode A sends a data sending (DS) control packet to alert exposed Nodes to impending data arrival at Node A

DS

Node B then sends an acknowledgement (ACK) control packetIf it has correctly received data sent from Node A

ACK

Allocation Protocols

■ Allocation protocols use a synchronous communication model.

■ Allocation protocols use a scheduling algorithm to map timeslots to nodes. These mappings define which node has access in a certain timeslot.

■ Allocation protocols have collision free transmission schedules.

■ Scheduling can be done on either a static or dynamic basis.

■ Allocation protocols perform well at medium to high traffic loads as all slots are likely to be utilized.

■ They perform poorly at low traffic loads as the slotted nature of the channel introduces an artificial delay, as not all slots are likely to be utilized.

Static Allocation Protocols

■ Static allocation protocols do not scale for large networks. They require a-priori knowledge of the global system parameters.

■ A simplistic TDMA static allocation protocol builds its schedules according to the number of nodes N in a network and then has a schedule of N timeslots, one for each node to transmit.

Dynamic Allocation Protocols

■ Five-Phase Reservation Protocol (FPRP) Designed to be Arbitrarily Scalable

■ Five Phases 1. Send a Request 2. Feedback provided by surrounding nodes 3. Reserve the slot on Success 4. All nodes within 2 hops of the source are notified of

the reservation 5. Transmission

■ Advantages Collision Free Schedules

■ Disadvantages Large control overhead, compounded by lots of

hardware switching from transmission to reception

Hybrid Protocols

■ Hybrid protocols are able to combine the performance capabilities of Contention protocols at low traffic loads, and scale to handle medium to high traffic loads in the way Allocation protocol based do.

■ AGENT Integrates the unicast capabilities of ADAPT with the multicast capabilities of ABROAD

AGENT

RTS CTS RTS CTS data packet

1 2 N

TDMA frame

priority interval

contention interval

transmission interval

Each Node is assigned a TDMA SlotIf a Node wishes to transmit in its slot it does a RTS/CTS handshake in the priority slotIf there is no signaling in the priority interval Nodes can use the contention interval to contend for transmission

On a multicast no CTS is returned, however if a reservation collisionIs detected a Negative CTS (NCTS) sent. This avoids CTS collsions

Data can then be sent in the transmission interval

Specialised MAC Protocols

■ S-MAC designed specifically for Ad-Hoc Sensor Networks

■ Different Design Criteria Minimise Energy Consumption Self Configuration Less importance placed on Fairness and Latency Avoid overhearing Node sleep cycle

■ Application importance is held above the performance of Individual nodes in the network

■ All Nodes in the network are assumed to be part of some grander scheme

Design Concerns

■ Synergy between the MAC Layer and the Routing Algorithm

■ Table Driven Routing Algorithms have been shown to be almost unaffected in efficiency by MAC layer changes

■ On Demand Routing protocols such as AODV, have been shown to be quite sensitive of the functionality of the MAC layer.

Future Directions

■ Exploring Synergy between MAC protocols and Routing Protocols

■ Exploring Location Aware MAC protocols and Routing

■ Exploring Location Information Storage■ Exploring the Application layer ■ Exploring Commercial Models for the deployment

of Ad-Hoc Networks