8.MAC Schemes for Ad Hoc Networks

8/6/2019 8.MAC Schemes for Ad Hoc Networks

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Seminar on Medium Access Control Protocols in

Wireless Communications 2010/11 1

MAC schemes for ad-hoc networks

Khalid Shabala

11.07.2011



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Outline

Introduction

Ad hoc Networks

Mac schemes for ad hoc networks

Examples of using ad hoc in existing real-world systems Summery

References


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Wireless ad hoc networks are decentralized wireless networks .

Introduction

WirelessNetworks

Decentralized Centralized


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These networks rely on nodes Instead of preexisting infrastructures (access points)to forward the data.

In an infrastructure network ,when two nodes want to communicate, all data gothrough access points first .

While in ad hoc networks ,nodes work as routers and communicate with each other

directly Considered as multi-hop wireless networks where all nodes cooperate to maintain

network connectivity These type of networks are useful in a situation where temporary network

connectivity is needed

Ad-hoc networks


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Random Access Protocols

In random access protocols each packet is subject to collision, since no resourcereservation is adopted

The advantages of this family of protocols :1. Each terminal can transmit information to the neighbor one with low

Power consumption.

2. Random access protocols provide low delays, since packets aretransmitted immediately. The disadvantages : The low stability with traffic load; When the traffic increases, the probability of

collision increases as well, and the number of lost packets increases.This reduces the throughput and increases the delay, since lost packets must beeventually retransmitted.

mechanisms are used to reduce the negative effect of collisions.


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The simplest random access protocol is ALOHA .It was Developed in 1970 atUniversity of Hawaii.

ALOHA does not require any action by terminals before they transmit a packet.

A checksum is added at the end of each packet . The receiving terminal uses thechecksum to evaluate if the packet was received correctly or was corrupted bycollision.

In case of collision the packet is discarded. Retransmission of discarded packets is accomplished based on an Automatic

Repeat on ReQuest (ARQ) protocol, that resend packets after a random delay.

ALOHA Protocol


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Pure ALOHA

If you have data to send, send the data If the message collides with anothertransmission, try resending " later.

Pure ALOHA does not check whether the channel is busy before transmitting.

Boxes indicate frames. Shaded boxesindicate frames which have collided.


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Slotted ALOHA

An improvement of the original ALOHA protocol was "Slotted ALOHA. It introduces discrete timeslots and increases the maximum throughput. A node can send only at the beginning of a timeslot, and thus collisions are reduced.

In this case, we only need to worry about the transmission-attempts within oneframe-time and not two consecutive frame-times, since collisions can only occur

during each timeslot.

Boxes indicate frames. Shaded boxesindicate frames which are in the same slots.


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o Slotted Aloha reduces the collision ,thus the throughput increases comparing with PureAloha as shown in the figure.

Slotted ALOHA


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Carrier Sense Multiple Access (CSMA)

CSMA Is a random-access method that reduces the chance of collisions . In this method a node listens to the channel first and delays transmitting when it

senses that the channel is busy.

o "Carrier Sense " describes that a transmitting node senses (hears) if there is acarrier wave in the medium before trying to send

If a carrier is sensed, the node waits for the transmission in progress to finishbefore initiating its own transmission.

o Multiple Access " describes the fact that multiple stations send and receive on the

medium.Transmissions by one node are generally received by all other stations using themedium.


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Carrier Sense Multiple Access (CSMA)


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Hidden Terminal Problem

Two nodes, which have connectivity to a third node, in hidden terminal case, can nothear each other. Consider the situation in Figure 1. Node A wants to transmit to node B. Because of an obstruction ,Node A can not listen that node C is sending to node B. collisions Occur


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A is transmitting to node B, while node C wants to transmit to node D. Following the CSMA protocol, node C listens to the medium, hears that node

a transmits and defers from accessing the medium. There is no reason why node C cannot transmit concurrently with the

transmission of node A, as the transmission of node C would not interferewith the reception at node B due to the distance between the two.

Exposed Terminal Problem


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In general, the hidden terminal problem reduces the capacity of a network due toincreasing the number of collisions

the exposed terminal problem reduces the network capacity due to theunnecessarily deferring nodes from transmitting.

A proposed solution for the hidden terminal problem is The Collision Avoidance (CA)CA-based control could be classified to :

1. In-band CA protocols :Only one channel is used for data traffic and the Collision Avoidance protocols.Example: Medium Access with Collision Avoidance (MACA)

2. Out-of-Band CA protocols :Two channels are used ,one for the data and another for the Collision avoidance protocol.Example: Busy Tone Multiple Access (BTMA).

Hidden& Exposed Terminal Problem


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CSMA/CD is a modification of CarrierSense Multiple Access (CSMA).

Collision detection is used to improveCSMA performance by terminatingtransmission as soon as a collision isdetected, and reducing the probabilityof a second collision on retry.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)


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The Multiple Access Collision Avoidance (MACA) Scheme

In Multiple Access Collision Avoidance (MACA) RTS/CTS dialogue is usedfor collision avoidance on the shared channel.

This scheme reduces the probability of data packet collisions caused byhidden terminals .

It reduce also expose terminal problem.

The RTS/CTS dialogue isdemonstrated in the Figure .


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The node A wants to transmit a packet B, it sends a short control packet - Request To Send(RTS) packet. All nodes that hear the RTS defer from accessing the channel for the duration of the RTS/CTS

dialogue. The destination -node B-upon reception of the RTS responds with another short control

packet Clear To Send (CTS)-packet All nodes that hear the CTS

packet defer from accessing thechannel for the duration of thecontrol packet transmission.

After reception CTS ,node Aacknowledges that the RTS/CTSdialogue has been successfuland node B starts to transmitthe actual data packet.

The Multiple Access Collision Avoidance (MACA) Scheme


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The Floor Acquisition Multiple Access (FAMA) Scheme

In FAMA scheme, each ready node has to acquire the channel (the floor'') beforeit can use the channel to transmit the data packets.

Both carrier sensing and RTS/CTS dialogue are used to ensure the acquisition of the "floor'' and the successful transmission of the data packets.

To acquire the floor, the transmitting node sends an RTS to the intended node . The receiver sends a CTS after receiving an RTS addressed to it. FAMA performs as well as MACA, when hidden terminals are present and as

well as CSMA otherwise.


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The Busy Tone Multiple Access (BTMA) scheme

In addition to the use of an RTS packet, an out-of-band busy tone is used tonotify neighbor nodes of the channel status.

When a node is ready to transmit, it sends out RTS packet to its intendedreceiver.

On reception of the RTS packet, the receiver sets up a busy tone and waits forthe incoming data packet.

When the other neighbors nodes hear the tone ,they defer from sending data


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Scheduled Access Protocols

The probability of collisions of control or data packets in any random-basedscheme increases with the over load, which degrades channel utilization andfurther reduces battery life.

This motivates the need for establishing transmission schedules to allownodes to receive data packets without collisions.

Scheduled access protocols adopt mechanisms that avoid more than oneterminal to transmit at a given time .

Data packets are never subject to collision, since at any time all terminals inthe network are made aware of which terminal in the network is allowed totransmit.

The scheduled-access MAC protocol uses a combination of TDMA and FDMAor CDMA for accessing the channel. The main drawback of this scheme isthat, like most fixed scheduling mechanisms, time slots are wasted if a nodedoes not have any data to send to the intended receiver.


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The Traffic-Adaptive Medium Access (TRAMA)

is introduced for energy- efficient collision-free channel access in wirelesssensor networks.

TRAMA reduces energy consumption by ensuring that the transmissions have nocollisions, and by allowing nodes to switch to a low-power, idle state wheneverthey are not transmitting or receiving. TRAMA assumes that time is slotted.

TRAMA employs a traffic adaptive distributed election scheme that selectsreceivers based on schedules announced by transmitters

Nodes using TRAMA exchange their two hop neighborhood information and thetransmission schedules specifying which nodes are the intended receivers of their traffic in chronological order, and then select the nodes that should

transmit and receive during each time slot.


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Accordingly, TRAMA consists of three components:1. the Neighbor Protocol (NP):

propagates one-hop neighbor information among neighboring nodes during therandom access period using the signaling slots, to obtain consistent two-hoptopology information across all nodes.

2. the Schedule Exchange Protocol (SEP):allows nodes to exchange two hop neighbor information and their schedules.

3.the Adaptive Election Algorithm (AEA):uses neighborhood and schedule information to select the transmitters andreceivers for the current time slot, leaving all other nodes in liberty to switch to

low-power mode.



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TRAMA assumes a single, time-slotted channel for both data and signalingtransmissions. The Figure shows the overall time-slot organization of theprotocol.

Time is organized as sections of random and scheduled-access periods. We refer to random-access slots as signaling slots and scheduled-access slots as

transmission slots .


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Flow-Aware Medium Access (FLAMA )

FLAMA is an energy-efficient medium-access control (MAC) protocol designed for wirelesssensor networks.

Energy efficiency is achieved by preventing idle listening, data collisions and transmissionsto a node that is not ready to receive packets.

It is simple enough so that it can be run by nodes with limited processing, memory,communication, and power capabilities.

FLAMA outperforms TRAMA, the first traffic adaptive, schedule-based MAC proposed forsensor networks ( Small delays comparing to TRAMA with significant improvement inenergy savings).


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During random access, neighbor discovery, time synchronization and traffic informationexchange are performed.

Data transmission happens during scheduled access. Using periodic random-access periodsallows FLAMA to adapt to topology and traffic changes in the network.

Unlike TRAMA , FLAMA does not require scheduling announcements during scheduled accessperiods.

Alternatively, traffic information is exchanged among nodes during random access to reflectthe driving traffic patterns, or flows. This allows FLAMA to still adapt to changes in trafficbehavior and topology (e.g., node failure). FLAMA uses flow information to establishtransmission schedules for each node.

Additionally, FLAMA achieves traffic adaptive by assigning slots to a node depending on theamount of traffic generated by that node. This is accomplished by assigning node weightsbased on the incoming and outgoing flows.

Nodes with more outgoing flows are given higher weights (i.e., more slots); the net effect isthat nodes that produce/forward more traffic are assigned more slots.

Flow-Aware Medium Access (FLAMA )


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Examples of using ad hoc Networks

tactical operation for fast establishment of military communication during thedeployment of forces in unknown and hostile terrain; rescue missions for communication in areas without adequate wirelesscoverage; national security for communication in times of national crisis, where theexisting communication infrastructure is non-operational due to a natural disaster

or a global war; commercial use for setting up communication in exhibitions, conferences, orsales presentations. Civil and Environmental Engineering ApplicationsSensors can be used for civil engineering applications. Research has to developsensor technology to develop smart structures witch is attractive for

earthquake-active zones ,wildfire Instrumentation and air pollution monitoring


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Wireless Communications 2010/11


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Mac schemes for ad hoc networks:

Pure & slotted aloha.CSMA&CSMA/CDMACAMACAWFAMABTMA&DBTMAPAMASTRAMA&FLAMA

Delay ,Bandwidth efficiency , Stability ,Throughput, power consumption ,Fairness andQuality of service

Summary


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Wireless Communications 2010/11


References

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Andrew S. Tanenbaum ,Computer Networks, Fourth Edition.Holger, Karl and Adreas, Willig, Protocols and Architectures for Wireless SensorNetworks Wiley 2005.

Stallings , Data Communications , Chapter 8.1/8.2/8.3.Rajendran , Aceves, Energy-Efficient, Application-Aware Medium Access for SensorNetworks.Sohraby , Minoli and Znati ,Wireless Sensor Networks :Technology, Protocols, andApplications

8.MAC Schemes for Ad Hoc Networks

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