Problems in Ad Hoc channel Access

8/10/2019 Problems in Ad Hoc channel Access

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AbstractIn this term paper , I am going to discuss in

detail the various parameters related to An ad-hoc

network. Ad-hoc network is a self-organized and

distributed entity consisting of a number of mobile

stations (MS) without the coordination of any centralized

access point. Clustering is one of the fundamental

problems in ad hoc networks. In this context, we describea distributed clustering algorithm for multihop ad hoc

networks. We first propose a randomized control channel

broadcast access method to maximize the worst-case

control channel efficiency, based on which a distributed

clustering algorithm is proposed. Both theoretical

analysis and simulations indicate that the proposed

clustering algorithm takes much less time and overhead to

cluster a given network with more stable cluster

structure, while incurring very small maintenance

overhead in a dynamic resulting from the mobility of the

MS. Network. Three types of collision-free channelaccess protocols for ad-hoc networks are presented.

These protocols are derived from a novel approach to

contention resolution that allows each node to elect

deterministically one or multiple winners for channel

access in a given contention context (e.g., a timeslot),

given the identifiers of its neighbors one and two

hopsaway. The new protocols are shown to be fair and

capable of achieving maximum utilization of the channel

bandwidth.The delay and throughput characteristics of

the contentionresolution algorithms are analyzed, and the

performance of the three types of channel access

protocols is studied by simulations

Introduction

In this term paper we are going to discuss about the

various problems faced in adhoc networks. HiddenTerminal Problem in which Two nodes transmit

concurrently data to the same receiver which causes the

collision between the nodes. It can be prevented by using

control messages. Another problem faced in the ad-hoc

network is Exposed node problem in which overhearing

data transmission from neighboring nodes

take place. To overcome this problem we use separate

control and data channels or directional antennas.there

are many more problems faced in ad-hoc networks such

as Receiver-Initiated MAC protocols in which receiver

informs sender that it is ready to receive data but there

are possibilities that no way of knowing for sure that

sender has data to send.Other problem such as Sender-Initiated MAC protocols in which sender informs receiver

it has data to send receiver confirms it is ready to receive

but control messages: RTS-CTS.

Problems in Ad Hoc Channel Access

Hidden Terminal Problem

This is a well-known problem found in contention-

based protocols, such as pure ALOHA, slotted

ALOHA, CSMA, IEEE 802.11, etc. Two nodes are

said to be hidden from one another (out of signalrange) when both attempt to send information to the

same receiving node, resulting in a collision of data

at the receiver node (seeFigure 4.1).

Taufeeq Malik

Lovely Professional University

Problems in Ad Hoc channel Access


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Solution Hidden Terminal Problem

To avoid collision, all of the receiver's neighboring

nodes need to be informed that the channel will be

occupied. This can be achieved by reserving the

channel using control messages, that is, using a

handshake protocol. An RTS (Request To Send)

message can be used by a node to indicate its wish

to transmit data. The receiving node can allow this

transmission by sending a grant using the CTS

(Clear To Send) message. Because of the broadcastnature of these messages, all neighbors of the sender

and receiver will be informed that the medium will

be busy, thus preventing them from transmitting and

avoiding collision. Figure 4.2illustrates the concept

of the RTS-CTS approach.

Figure 4.2. Using an RTS-CTS handshake to

resolve hidden node problems

Shortcomings of the RTS-CTS Solution

The RTS-CTS method is not a perfect solution to

the hidden terminal problem. There will be cases

when collisions occur and the RTS and CTS control

messages are sent by different nodes. As shown in

Figure 4.3, node B is granting a CTS to the RTS

sent by node A. However, this collides with the RTS

sent by node D at node C. Node D is the hidden

terminal from node B. Because node D does not

receive the expected CTS from node C, itretransmits the RTS. When node A receives the

CTS, it is not aware of any collision at node C and

hence it proceeds with a data transmission to node

B.

Fi gure 4.3. The incompleteness of the RTS-CTS method.


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Another problematic scenario occurs when multiple

CTS messages are granted to different neighboring

nodes, causing collisions. As shown in Figure 4.4,

two nodes are sending RTS messages to different

nodes at different points in time. Node A sends an

RTS to node B. When node B is returning a CTS

message back to node A, node C sends an RTS

message to node B. Because node C cannot hear theCTS sent by node B while it is transmitting an RTS

to node D, node C is unaware of the communication

between nodes A and B. Node D proceeds to grant

the CTS message to node C. Since both nodes A

and C are granted transmission, a collision will occur

when both start sending data.

F igure 4.4. Another il lustration of the RTS-CTS

problem.

Exposed Node Problem

Overhearing a data transmission from neighboring

nodes can inhibit one node from transmitting to

other nodes. This is known as the exposed node

problem. An exposed node is a node in range of the

transmitter, but out of range of the receiver. This is

illustratedFigure4.5.

Solution

A solution to the exposed node problem is the use of

separate control and data channels or the use of

directional antennas. The former will be discussed in

the PAMAS and DBTMA sections. Figure 4.6a

shows that a mobile node using an omni-directional

antenna can result in several surrounding nodesbeing "exposed," thus prohibiting them from

communicating with other nodes. This lowers

network availability and system throughput.

Alternatively, if directional antennas are employed,

this problem can be mitigated. As shown in Figure

4.6b, node C can continue communicating with the

receiving palm pilot device without impacting the

communication between nodes A and B. The

directivity provides spatial and connectivity isolation

not found in omni-directional antenna systems.

Figure 4.6. Using a directional antenna to resolve

the exposed node problem.


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CTS packet is overheard so as to temporarily limit

power output when a CTS packet is overheard.

Figure 4.9. An illustration of the control

handshake used in MACA

Collisions do occur in MACA, especially during the

RTS-CTS phase. There is no carrier sensing in

MACA. If two or more stations transmit an RTS

concurrently, resulting in a collision, these stations

will wait for a randomly chosen interval and try

again, doubling the average interval on every

attempt. The station that wins the competition willreceive a CTS from its responder, thereby blocking

other stations to allow the data communication

session to proceed.

MACA-BI (By Invitation)

A shift from the classic three-way handshake MAC

protocol is MACA-BI (By Invitation). Invented by

Fabrizio Talucci, MACA-BI uses only a two-way

handshake, as shown in Figure 4.10. There is no

RTS. Instead, the CTS message is renamed as RTR

(Ready To Receive). In MACA-BI, a node cannot

transmit data unless it has received an invitation

from the receiver. Note that the receiver node does

not necessarily know that the source has data to

transmit. Hence, the receiver needs to predict if

indeed the node has data to transmit to it. Thetimeliness of the invitation will, therefore, affect

communication performance.

Fi gure 4.10. An ill ustration of MACA-BI control

handshake.

The author suggested the estimation of packet queue

length and arrival rate at the source to regulate the

transmission of invitations. One possible way to

accomplish this is to piggyback such information

into each data packet so that the receiver is aware of

the transmitter's backlog. Hence, for constant bitrate (CBR) traffic, the efficiency of MACA-BI will

be high since the prediction scheme will work fine.

However, for bursty traffic, MACA-BI performance

will be no better than MACA. To enhance the

communication performance of MACA-BI under

non-stationary traffic situations, a node may still

transmit an RTS if the transmitter's queue length or

packet delay exceeds a certain acceptable threshold

before an RTR is issued.


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Power-Aware Multi-Access Protocol with

Signaling (PAMAS)

The Power-Aware Multi-Access Protocol with

Signalling for ad hoc networks (PAMAS)[17] is

based on the MACA protocol with the addition of a

separate signalling channel. RTS-CTS dialogue

exchanges occur over this channel. PAMASconserves battery power by selectively powering off

nodes that are not actively transmitting or receiving

packets.

In PAMAS, nodes are required to shut themselves

off if they are overhearing other transmissions not

directed to them. In addition, each node makes an

independent decision about whether to power off its

transceiver. The conditions that force a node to

power off include:

a. If a node has no packets to transmit, it

should power off if one of its neighboring

nodes is transmitting.

b. If a node has packets to transmit, but at least

one of the neighboring nodes is transmitting

and another is receiving, then it should

power off its transceiver.

Dual Busy Tone Multiple Access (DBTMA)

The use of a busy tone [18] was first proposed by

Professor Fouad Tobagi from Stanford University.

He proposed Busy Tone Multiple Access (BTMA)

to solve the hidden terminal problem. However,

BTMA relies on a wireless last-hop network

architecture, where a centralized base station servesmultiple mobile hosts. When the base station is

receiving packets from a specific mobile host, it

sends out a busy tone signal to all other nodes within

its radio cell. Hence, hidden terminals sense the busy

tone and refrain from transmitting.

(1) Fi gure 4.13. The pri nciple of Dual Busy Tone

Multi ple Access (DBTMA).

In DBTMA (Dual Busy Tone Multiple Access)[19]

[20], two out-of-band busy tones are used to notify

neighboring nodes of any on-going transmission. In

addition, the single shared channel is further split

into data and control channels. Data packets are sent

over the data channel, while control packets (such as

RTS and CTS) are sent over the control channel.Specifically, one busy tone signifies transmit busy,

while another signifies receive busy. These two busy

tones are spatially separated in frequency to avoid

interference.


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Conclusion

We discussed main issues of MAC layer and Routing

protocols of network layer. These two layers play most

important role in improving the performance of MANET.

The future is PERVASIVE MANET[12]. cross-layer

policies is a very promising direction, which can be

further explored. Cross-layering can tackle the traffic in

better manner on ad hoc networks by sharing information

from different layers . Moreover, information collected at

a particular layer (e.g., a route failure) can be exploited

by different layers to tune the protocol behavior. The

future of ad hoc networks is really appealing, giving the

vision of anytime, anywhere and cheap

communications. Before those imagined scenarios come

true, huge amount of work is to be done in both research

and implementation. At present, the general trend in

MANET is toward mesh architecture and large scale.

Improvement in bandwidth and capacity is required,

which implies the need for a higher frequency and betterspatial spectral reuse. Propagation, spectral reuse, and

energy issues support a shift away from a single long

wireless link (as in cellular) to a mesh of short links (as in

ad hoc networks). Large scale ad hoc networks are

another challenging issue in the near future which can be

already foreseen. As the evolvement goes on, especially

the need of dense deployment such as battlefield and

sensor networks, the nodes in ad hoc networks will be

smaller, cheaper, more capable, and come in all forms. In

all, although the widespread deployment of ad hoc

networks is still year away, the research in this field will

continue being very active and imaginative. Futureresearch makes it possible to Imagine a wireless mesh of

rooftop-mounted ad hoc routers; an ad hoc network of

cars for instant traffic and other information; sensors and

robots forming a multimedia network that allows remote

visualization and control; multiple airborne routers (from

tiny robots to blimps) automatically providing

connectivity and capacity where needed like in a football

game; in an ad hoc network of spacecraft around and in

transit between the Earth and mars .These may seem like

science fiction, but a lot of work is in Process seriously

by the ad hoc research

Bibliography

http://www.jatit.org/volumes/research-

papers/Vol8No1/2Vol8No1.pdf

http://www.sandilands.info/sgordon/doc/jayasuriya2004-

hidden.pdf

https://www.google.co.in/webhp?sourceid=chrome-

instant&ion=1&espv=2&ie=UTF-

8#q=future%20scope%20of%20ad%20hoc%20network

http://rimtengg.com/coit2007/proceedings/pdfs/27.pdf

http://en.wikipedia.org/wiki/Wireless_ad_hoc_network

http://www.journals.elsevier.com/ad-hoc-networks/

http://www.techterms.com/definition/adhocnetwork

http://www.slideshare.net/somarka/efficient-route-

discovery-in-mobile-ad-hoc-network

http://technav.ieee.org/tag/72/ad-hoc-networks
http://www.jatit.org/volumes/research-papers/Vol8No1/2Vol8No1.pdfhttp://www.jatit.org/volumes/research-papers/Vol8No1/2Vol8No1.pdfhttp://www.jatit.org/volumes/research-papers/Vol8No1/2Vol8No1.pdfhttp://www.sandilands.info/sgordon/doc/jayasuriya2004-hidden.pdfhttp://www.sandilands.info/sgordon/doc/jayasuriya2004-hidden.pdfhttp://www.sandilands.info/sgordon/doc/jayasuriya2004-hidden.pdfhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttp://rimtengg.com/coit2007/proceedings/pdfs/27.pdfhttp://rimtengg.com/coit2007/proceedings/pdfs/27.pdfhttp://en.wikipedia.org/wiki/Wireless_ad_hoc_networkhttp://en.wikipedia.org/wiki/Wireless_ad_hoc_networkhttp://www.journals.elsevier.com/ad-hoc-networks/http://www.journals.elsevier.com/ad-hoc-networks/http://www.techterms.com/definition/adhocnetworkhttp://www.techterms.com/definition/adhocnetworkhttp://www.slideshare.net/somarka/efficient-route-discovery-in-mobile-ad-hoc-networkhttp://www.slideshare.net/somarka/efficient-route-discovery-in-mobile-ad-hoc-networkhttp://www.slideshare.net/somarka/efficient-route-discovery-in-mobile-ad-hoc-networkhttp://technav.ieee.org/tag/72/ad-hoc-networkshttp://technav.ieee.org/tag/72/ad-hoc-networkshttp://technav.ieee.org/tag/72/ad-hoc-networkshttp://www.slideshare.net/somarka/efficient-route-discovery-in-mobile-ad-hoc-networkhttp://www.slideshare.net/somarka/efficient-route-discovery-in-mobile-ad-hoc-networkhttp://www.techterms.com/definition/adhocnetworkhttp://www.journals.elsevier.com/ad-hoc-networks/http://en.wikipedia.org/wiki/Wireless_ad_hoc_networkhttp://rimtengg.com/coit2007/proceedings/pdfs/27.pdfhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttps://www.google.co.in/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=future%20scope%20of%20ad%20hoc%20networkhttp://www.sandilands.info/sgordon/doc/jayasuriya2004-hidden.pdfhttp://www.sandilands.info/sgordon/doc/jayasuriya2004-hidden.pdfhttp://www.jatit.org/volumes/research-papers/Vol8No1/2Vol8No1.pdfhttp://www.jatit.org/volumes/research-papers/Vol8No1/2Vol8No1.pdf

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Problems in Ad Hoc channel Access