Chapter 2 Review olf Cornterntiorn Basedl MAC...

Chapter 2

Review olf Cornterntiorn Basedl MAC Protocols

Contention based Medium Access Control protocols for wireless network proposed in

the literature or implemented work on the principle of collision avoidance by means

of handshake between sender and receiver. Based on this principle the contention

based protocols are classified as Sender initiated and Receiver initiated protocols. In

this chapter, the review of Sender initiated and Receiver initiated MAC protocols is

presented. Being only available protocol in practice, the review of various models

developed for performance evaluation of IEEE 802.11 is also presented. Finally, the

review of contention based protocols :s being presented in this chapter.

2.1 Sender Initiated MAC Protocols

Kam [ 49], proposed a Multiple Access Collision A voidance (MACA) protocol. This

protocol uses two short control packets RTS and CTS for collision avoidance on the

shared channel instead of carrier sensing (see Fig 2.1). In MACA a ready node

transmits an RTS packet to inform the receiver to get ready for the receiving the data.

The receiver node replies by transmitting a CTS packet to inform the sender that it is

ready for receiving the data. Both RTS and CTS carry the duration of data

transmission. The neighbor nodes of a sender differ their transmissions upon hearing

Review ofContention Based MAC Protocols 15

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the RTS packet for the duration of CTS period. Similarly, the neighbor nodes of

receiver also differ their transmissions upon hearing the CTS packet for the duration

of data receiving. Therefore, MACA overcomes both the hidden and exposed terminal

problem. MACA was proposed to resolve the hidden terminal and exposed terminal

problems, but collisions occur in MACA especially during the RTS-CTS phase (see

Fig 2.2), if CTS is not received within specified period. Other shortcomings of

MACA protocol are discussed as follows. MACA does not provide any

acknowledgement of data transmission. If a collision takes place the sender node

differs its transmission according to Binary Exponential Back-off (BEB) algorithm. In

BEB, after every collision Backoff window size gets doubled. Therefore, nodes may

get starved. To overcome this problem a protocol with modification was proposed by

Bharghawan et al. [61] namely MACAW (MACA for wireless).

MACAW was developed to overcome the shortcomings ofMACA. This protocol uses

five steps RTS-CTS-DS-DATA-ACK exchange. It includes two extra control packets

i.e. DS (Data-Sending) and ACK. Availability of ACK packet allows faster error

recovery in MACAW since error recovery in MACA is dealt by transport layer. The

DS control packet is used to prevent the collision that may occur due data

transmission by the exposed terminal while CTS is being received by the sender. A

sender node transmits DS packet, which carries the expected time period of data

transmission between sender and receiver. MACAW protocol uses one more control

packet RRTS (Request-for-Request-to-Send). This control packet is transmitted by a

receiver on behalf of sender to save it from starvation.

Review a/Contention Based MAC Protocols 16

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Neighbor Sender Receiver Neighbor

RTS

DATA DATA

Figure 2.1 Packet transmissions in MACA [8]

Node A Node B Node C NodeD

Figure 2.2 Illustration of collision in RTS-CTS exchange [8]

Fullmer and Garcia [10] proposed Floor Acquisition Multiple Access (FAMA)

protocol that uses non-persistent carrier sensing and control packet exchange between

sender and receiver. In this protocol, each ready node has to compete for the channel

(the floor) before they can use the channel to transmit the data. In F AMA protocol

control packets may collide with each other, but data packets are transmitted in


Chapter 2

collision free manner. There are two variations of the F AMA, One uses the RTS-CTS

exchange with ALOHA and the other uses RTS-CTS exchange along with non

persistent carrier sensing. This protocol does not guarantee that there will not be any

collision and is not able to deal with the hidden terminal problem as well.

The Busy Tone Multiple Access (BTMA) protocol [15] was a first attempt to solve

the hidden terminal problem by introducing a separate channel carrying busy tone.

BTMA was primarily designed for centralized network operation, i.e. a network with

base stations results in low collisions in Ad Hoc Network. But it provides poor

bandwidth utilization in ad hoc network. Therefore, Dual Busy Tone Multiple Access

(DBTMA) [35], a modified form of BTMA is used in ad hoc networks. In DBTMA

two out-of-band busy tones are used to notify neighboring nodes of any on-going

transmission. The single shared channel is further split into data and control channels

used to transmit data packets and control packets, respectively. The sender and

receiver use separate busy tone signals with different sine wave frequencies.

2.2 Receiver Initiated MAC Protocols

In case ofRI-BTMA [7] (Receiver Initiated Busy Tone Multiple Access) is similar to

BTMA except that the busy tone is transmitted by the receiver in response to the

transmission started by sender. In this protocol, data packet is divided in two parts, a

preamble and actual data. A sender starts transmission by sending preamble part.

Upon receiving the preamble, the receiver sets busy tone on the control channel. After

hearing the busy tone, the actual data transmission takes place. RI-BTMA works well

under low load, but it becomes unstable under high load.


Chapter 2

Control channel around S

RTS DATA s

R

Control channel around R BT,

Figure 2.3 Packet transmissions in BTMA

F. Talluci and M. Gerla [21] proposed MACA-BI (by invitation), which is a receiver

initiated protocol. In this protocol receiver requests the sender for the data by

transmitting control packet RTR (Ready-to-Receive) instead of the RTS and the CTS

control packets as used in MACA. After receiving the R TR, sender node transmits

data. Therefore, it reduces the number of control packets used for handshaking prior

to actual data transmission. There should be some mechanism to decide that when the

receiver transmits RTR to the sender. The authors have suggested a traffic prediction

based algorithm through which a receiver determines when to make a request for data

packet from the sender. To know about data arrival rate at the sender, control packets

are modified to carry the information regarding the backlogged flows at the sender,

number of packets queued and packet length. In case an R TR packet is not received

by the sender within stipulated time, the sender is allowed to declare its backlog by

transmitting an R TS control packet.


Chapter 2

The authors in [3 7] have introduced some other receiver initiated protocols based on

non-persistent carrier sensing, data driven polling, and with some modifications to·

MACA-BI. The modifications proposed in MACA-BI are as follows:

o Another control signal, namely No-Transmission-Request (NTR) has been

introduced along with RTR. An additional collision-avoidance waiting period

of s seconds is required at a polled node prior to answering an R TR.

o The data packets are transmitted by a polled node, if they are addressed to the

polling node.

The protocol after above modifications is called RIMA-SP (Receiver Initiated

Multiple Access with Simple Polling). In this protocol, if channel is idle during

collision-avoidance waiting period, the polling node (receiver) sends an NTR, to

inform the polled node not to send any data. Otherwise, the polled sender transmits its

data. The working ofRIMA-SP is as follows:

In RIMA-SP, initially every node goes to START state and moves to PASSIVE state.

In the PASSIVE state a node senses the carrier. If the carrier is busy the node transits

to the REMOTE state to defer its action for the duration of the ongoing transmissions.

In the REMOTE state a node must spend sufficient time for allowing a complete

successful handshake to take place. While in the passive state, if a node detects noise

in the channel, it moves to BACKOFF state. From PASSIVE state a node moves to

RTR state, if it has an RTR control packet for the sender. When a sender node

receives RTR successfully, it starts to transmit data after waiting for s seconds by

entering into XMIT state. After receiving data packet the receiver transmits ACK

Review of Contention Based MAC Protocols 20

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packet. If ACK packet is not received correctly by sender, the receiver transits to

BACKOFF state. After sending RTR packet, a receiver senses the channel.

Meanwhile, if it detects other activity on the channel, it sends an NTR packet to the

sender to stop it from transmitting data. Further, RIMA-SP is modified by using dual

purpose polling resulting into RIMA-DP (Receiver Initiated Multiple Access with

Dual Polling). After receiving R TR, if a sender does not have data to transmit, it

transmits a CIS. One more receiver initiated protocol namely RIMA-BP (Receiver

Initiated Multiple Access with Broadcast Polling), which is also a modified form of

RIMA-SP introduced by the authors [37]. In this protocol RTR is broadcast so that

multiple neighbors can receive and decode the packet at the same time. Another

modification is also made which uses R TS prior to the transmission of a data packet,

to ensure that the transmissions which collide, last for a short period.

In the literature, analytical performance of all the above mentioned protocols has been

evaluated in terms of throughput against offered load. It is observed that the

performance of RIMA-DP is much better than other protocols (F AMA-NCS, RIMA-

SP and RIMA-BP).

2.3 IEEE 802.11 MAC Protocol

Authors in [32] have evaluated the performance of asynchronous data transfer

methods of IEEE 802.11 MAC protocol in the presence of hidden terminals and

captured the effect. To access the medium, IEEE 802.11 MAC protocol uses Carrier

Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism. Since the

nature of different nodes in wireless medium is non-identical, evaluation of the impact

of spatial characteristics like station location and traffic pattern has been studied. The


Chapter 2

performance for the both modes of Distributed Coordination Function (DCF) i.e.

basic mode as well as R TS/CTS mode for critical length and variable length frames

has been evaluated in the literature. Renewal theory has been used as a modeling

technique, where an instance of completion of a successful transmission and/or a

collision is a renewal point. Comparative results for basic access mode and R TS/CTS

mode have been discussed with varying parameters like hidden terminal probability

etc.

The authors in [25] evaluated performance of the CSMA/CA MAC protocol for IEEE

802.11 WLAN and also suggested enhancement to the protocol. Authors have

proposed an Adaptive Contention Window mechanism, where the size of optimal

Backoff window determined on the basis of number of contending stations. The

working of IEEE 802.11 DCF is discussed for both the modes [18, 26, 28, 41]. The

performance of basic access method and RTS/CTS method with adaptive contention

window has been evaluated in terms of saturation throughput for the offered load,

number of stations etc. The effect of hidden terminals on CSMA/CA has also been

studied. It is observed that the Adaptive Contention Window mechanism gives better

result than traditional Backoff algorithm. F. Cali, M. Conti and E. Gregori [18] have

also worked on dynamic tuning of contention window to maximize the throughput.

Taking into view the network configuration and dynamic tuning of the contention

window, a theoretical throughput limit (maximum protocol capacity) has been worked

out. They have also developed an analytical model to study the throughput of the p-

persistent IEEE 802.11 protocol. In case of p-persistent IEEE 802.11 the back-off

interval is sampled from a geometric distribution with parameter p. With the help of

analytical model optimal value of p is calculated to maximize the capacity of the


Chapter 2

protocol. For the analysis of adaptive back-off mechanism the same authors have used

Markov chain based modeling and observed the working of the protocol under

stationary traffic and network conditions as well as for transient conditions.

G. Bianchi [24] presented a Markov chain based modeling and analysis of the

CSMA/CA exponential back-off algorithm of IEEE 802.11 in terms of saturation

throughput. The analysis is approached with the assumption of finite number of

terminals and ideal channel conditions. The model suited to both the access methods

i.e. for basic access and RTS/CTS access method, as well as combination of the both.

The results show better performance of RTS/CTS method than basic access method

for various parameters. Optimal transmission probability has been derived, which

gives maximum saturation throughput performance within a considered network

scenario. G. Bianchi and I. Tinmirello [23] have given another approach to evaluate

the throughput and delay performance of IEEE 802.11 DCF. This approach evaluates

the performance based on conditional probability instead of two dimensional Markov

chain. Delay has been derived with conditional probability and Little's formula [11].

P. Chatzimisios et al.[47] have done the analysis ofiEEE 802.11 DCF by modifying

the Bianchi's model. In this model contention window is set at the maximum size, and

it is also extended to determine the frame delay analysis under maximum load. They

have observed that for small frame size and small number of stations, the basic access

method gives better results. However, the performance of basic access method

depends on the number of stations.


Chapter 2

H. Wu et al.[30] have studied the performance of reliable transport protocol over

IEEE 802.11. They have proposed a scheme namely DCF+ to make reliable transport

protocol perform better. Analysis has been done on the basis of Bianchi's model. The

authors have concluded that DCF+ can be used to enhance the performance of TCP

over IEEE 802.11 for three metrics i.e. throughput, fairness index and delay.

Z. Hadzi-velkov and B.Spasenovski [69] have analytically evaluated the performance

of saturated IEEE 802.11 over an error prone channel. Again the authors have used

two dimensional Markov chain based modeling of back-off window size by

considering the frame error rate and maximum retry limit. F. Jing et al. [ 19] have

modified the back-off scheme for ad hoc networks based upon IEEE 802.11. This

scheme is renamed as Adaptive minimum contention Window Binary Exponential

Back-off (A WBEB) algorithm. In the paper the authors have analyzed throughput and

delay of A WBEB algorithm using 2-D Markov chain model. The results obtained for

throughput and delay are better than the other models.

2.4 Design and Modeling of MAC Protocols

Kleinrock and Tobagi [14] have presented Carrier Sense Multiple Access (CSMA) as

a technique to broadcast the data over a single channel by multiple transmitting nodes

for packet radio networks. P-persistent and non-persistent CSMA along with slotted

version of both the techniques have been discussed and their performance has also

been traced. For high load, ALOHA and CSMA show unstable behavior along with

their slotted versions. They have also identified the detection of hidden terminal

problem in carrier sensing. The hidden terminal problem degrades the performance of

CSMA as poorly as the pure ALOHA protocol. Due to this drawback of CSMA in


Chapter 2

detecting the hidden terminals, the transmission of packets is prone to the collision.

Occurrence of the collisions and the presence of exposed node problem make

bandwidth under utilized. The authors of [15] have suggested BTMA (Busy Tone

Multiple Access) as solution to the hidden terminal problem and discussed the

operation of the protocol. The throughput of CSMA has been measured in the

presence of hidden terminals. In the part III [16] these authors have given polling

based or reservation based channel allocation technique. These techniques are known

as centrally controlled. Further, the issue of allocation of a single channel among N

number of stations is addressed. They have also considered the throughput and delay

metrics to compare the polling and Split-Channel Reservation Multiple Access

(SRMA) with fixed assignment and random access techniques. Authors have also

discussed Packet Switching in Radio Channels: part IV [17] which is dedicated for the

stability considerations and dynamic control in CSMA. They have considered

throughput, delay and stability for the analysis of non-persistent CSMA. The focus

has been on non-persistent CSMA because of its simplicity in analysis as well as its

relatively high efficiency. The conditions under which the channel exihibits stable

behavior have also been discussed. For the analysis purpose slotted version of the

non-persistent CSMA is considered, where the slot size is equal to the propagation

delay. Further, the analysis of the protocol is done with the help of Markov chain.

Tobagi et.al in [22] have given the modeling and measurement techniques for packet

communication networks. The authors have discussed many modeling techniques like

queuing theory and tools based on the theory of stochastic processes. These tools are

renewal theory, semi-Markov and regenerative processes, and Markov decision


Chapter 2

theory. Many other optimization techniques like linear, non-linear and integer

programming techniques have been briefly reviewed. An optimal capacity assignment

problem has also been illustrated with the application of Lagrangian multipliers. The

authors have focused on suggesting the techniques for single hop environments. They

have shown difficulties in extending the single hop environment to multi-hop

environment. Barry M. Leiner et. al. in [3] have discussed the design issues for Packet

Radio Network. They have discussed many important issues like efficient methods for

sharing the common radio channel, determining the connectivity and accordingly

finding the routes for data delivery in the network. They have also discussed the

reliability of communications in the presence of noise, and methods for managing and

controlling the distributed networks. The authors have explained the design problems

for physical layer of ISO model including data link layer.

In [22] many design questions are answered for routing, packet forwarding, and link

control that are very important in the design of Packet Radio Networks. Issues like

gateways, network access, addressing and naming involved in the connectivity of the

network with external world are also considered. The important metrics for evaluation

of any network from the perspectives of user and network operator are discussed.

H. Takagi and L. Kleinrock in [29] have determined optimal transmission radii for the

randomly distributed multi-hop packet radio nodes with different slotted protocols and

network configurations. The nodes are possibly mobile, geographically distributed

and require multi-access to channel on the sharing basis as per Poisson distribution.

The nodes are distributed in two dimensions with homogeneous or inhomogeneous

density. This paper has considered slotted ALOHA (with and without FM capture)

and non persistent CSMA. After modeling, optimization problem formulated for each


Chapter 2

protocol and optimal transmission range is calculated. Under various assumptions for

the modeling, the performance of the protocols has been evaluated. It has been

observed by the authors that the non persistent CSMA performs better than other

protocols.

Yu Wang and Garcia [68] have evaluated the performance of sender initiated collision

avoidance schemes for multi-hop ad hoc networks in the presence of hidden terminals

using analytical approach. The multi-hop environment has been modeled with the

help of two-dimensional Poisson distribution with density A. The number of hidden

terminals and the level of congestion is determined by varying density A. The

performance analysis of slotted protocols has been done with the assumption that each

slot is of equal length. This includes propagation delay as well as overheads like

transmit-to-receive tum-around time, carrier sensing delay and processing time. The

authors have shown that the performance of sender initiated protocols in terms of

saturation throughput is better than CSMA. The numerical results are calculated based

on the short sized as well as long sized data packets, and varying number of nodes. L.

Wu and P.K.Varshney [45] have analyzed throughput performance of CSMA and

various variants of BTMA protocols in multi-hop networks for single channel. The

channel is modeled with the help of Markov chain having two states (IDLE,and

BUSY), and analysis has been made with steady state probability. The authors have

measured the saturation throughput for non-persistent CSMA, ID-BTMA and C

BTMA for different values of packet generation probability p, number of nodes N

and slot size a. The numerical results show that the performance of BTMA is better

than the CSMA. Among the variants of BTMA, ID-BTMA has better performance

than the C-BTMA protocol under light load.


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