RELIABILITY ENHANCEMENT IN MULTIHOP MANETP. Ashok,Assistant Professor,Department of ECE,Prince ShriVenkateshwara Padmavathy Engineering College, Ponmar, Chennai 600127.

Abstract—A mobile ad-hoc network (MANET) consists of a group of mobile nodes and itenables communications between participating nodes without the burden of anybase stations. To increase the capacity of wireless network, multipletransceivers can be used. Multiple transceivers increase the cost of theequipment. So generally for data transmissions, a single transceiver is used ineach node. But single transceiver is difficult to implement in multichannelenvironment. This problem can be solved by Ad-hoc Multichannel NegotiationProtocol (AMNP). For improving reliability, further Reliable BroadcastAlgorithm (RBA) is introduced.

Simulation analysis in NS-2 based on thecombination of AMNP – RBA   givescomparatively a better performance.Keywords — MANET, Multihop, MAC, Multichannel,AMNP and RBA.I Introduction      Nowadays there is tremendous increase inusage of mobile laptops and PDA’s but we have only limited amount of radiospectrum. Within the available radio spectrum we have to effectivelycommunicate between the nodes.

Existing works have dedicated to using multiplechannels to increase the capacity of wireless communication by dividing theradio spectrum into number of channels.      Most of the mobile devices are equippedwith single transceivers and it operates in single-channel mode hence moreamount of bandwidth is wasted. To mitigate this problem, all mobile nodes haveto be equipped with multiple transceivers.

 Enhancement of the present MAC protocol can give better performance onmultichannel with single transceiver.     In 1 Jain proposes a CSMA based mediumaccesses control protocol for multihop wireless network. In which channelselection is based on signal to interference and noise ratio at the receiver.Although this method increases the throughput up to 50% there is delay inperformance due to high packet transmission. In 2, Nasipuri propose a newCSMA protocol for ad-hoc networks. In which the CSMA protocol divides theavailable bandwidth into several channels and selects the channel randomly.

Itemploys “soft channel reservation” that gives preference to the channel thatwas used for last successful transmission.    In 11 Chen proposes a AMNP protocol thatreduces the collision and interruption probabilities, and it uses the sameframe format of IEEE 802.11 with some slight modifications but it lacks in reliablebroadcast transmission. In 12 Lou proposes RBA (Reliable broadcastTransmission) with selected forward nodes to avoid broadcast storm and reducebroadcast redundancy.IIProblem statementA. Singletransceiver constraint      In IEEE 802.11 DCF theMAC protocol is designed for sharing a single channel between the nodes.

Nowadays most of the wireless devices are equipped with one half-duplextransceiver to transmit or to receive data. The transceiver can operate onmultiple channels dynamically but it can either transmit or receive data fromone channel at a time. So a node cannot communicate with other nodes when iscommunicating with another node in another channel concurrently. While usingmultiple channels IEEE 802.

11 DCF will not be suitable because it maydynamically switch channels. B. Multichannelhidden terminal problem The node whichcannot hear the radio signal from the transmitter node and may disturb theongoing data transmission is called hidden terminal nodes. Even though IEEE802.11 provides RTSCTS handshaking signals, in multichannel environment thenodes still may collide with each other.C.

 Broadcasttransmission problem       Broadcasting is an important activity inmulti hop MANET. Broadcasting a message in single channel is easy, because allthe mobile nodes in a network use a single channel so the message can bedelivered.  But in multichannelenvironment a node may miss the broadcast frame when is currently transmittingor receiving data from other nodes. iii Amnp – RBA  implementation     In IEEE 802.11 the sender and the receiver should perform a four wayhandshaking mechanism: Request-to-send /clear-to-send (RTS/CTS), data, andacknowledgment (ACK) when they have data to transmit in the same channel. Fig.

1 Anillustration of AMNP            In fig.1, which C0 representsthe contention/reservation channel and C1 and C2 represent thedata channels. The identifier BB represents the broadcast beacon, the BWTrepresents the broadcast waited time and the CST is the channelswitching/settling time, respectively.Fig. 2 Frameformat of MRTS, MCTS and CRI control frames     If mobile nodes equip with only one transceiver, some nodes will nevercommunicate with each other at the same time. As a result, few data frames willbe transmitted in the multichannel environment.

If we assign mobile nodes toaccess channels dynamically, a complicated and distributed channel schedulingmechanism has to be provided for MANETs. It will be more difficult in theMANET.        Instead of employing such complicatedscheme, AMNP allocates a dedicated contention or broadcast channel for allmobile nodes to contend. The remaining channels are served as data channelspermanently. Fig 1 illustrates the channel usage of AMNP in which channels C1–Cn-1 represent data channels, and channel C0 serves asthe dedicated contention channel or broadcast channel. Since there is nostationary node for supporting centralized multichannel control in MANETs, thedistributed negotiation protocol, which can provide ad hoc multichanneltransmission, is needed. To solve the above-mentioned problems, we employ theconcept of IEEE 802.

11 RTS/CTS handshaking mechanism to fulfill themultichannel negotiation and transmission mechanism in multi-hop MANETs. Wename the RTS/CTS mechanism as MRTS/MCTS in the AMNP. Unlike IEEE 802.11 RTS/CTSmechanism, we need more information to indicate the usage of other datachannels.      When two nodes communicate, first a node has to completea MRTS/MCTS handshaking in the contention channel to acquire the access rightof the expected data channel if it has a packet to transmit. The main purposeof the MRTS control frame is to inform its direct receiver and neighbours thepreselected data channel to indicate a virtual carrier sensing delay namednetwork allocation vector (NAV) this will prevent the exposed and hidden nodeproblems in the preselected channel.

Likewise, the MRTS also carries the neweststatus information of data channels to notify other mobile nodes within itstransmitting range for information updating.      The frame format of MRTS is shown in Fig2 where the frame control, receiver address, transmitter address andframe check sequence fields are the same as the description in the IEEE 802.11standard. In order to be compatible with the IEEE 802.11 standard, we use thereserved value Type = 01 and Subtype = 0011 as indicated in the frame controlfield to represent the MRTS control frame. The original duration field iseliminated since the channel C0 is for contention and broadcast use only.

Thereforethe NAV will not be used in C0 when contending for the channel access. Theadditional fields selected channel (SC), channel usage indication (CUI) and thenth used channel’s offset are described as follows. The SC field indicateswhich channel that the sender prefers to transmit data with the receiver.      The preferred channel (selected) is notcompulsory for the receiver depending on the availability of the channel on thereceiver’s side.The CUI field length is one octet long and the content of CUI indicates thestatus of the usage in each channel.  Thebit will be set to 0 if the corresponding data channel is not in use; the bitwill be set to 1, if the corresponding data channel is in use.     When a node has received a MRTS frame, it will compare the SC field ofthe MRTS with its channel status and then check whether it can satisfy therequest.

If the preselected channel is also available in receiver’s side, thereceiver will grant the transmission request and reply the MCTS frame back to thesender immediately. Otherwise, the preselected channel cannot be granted to usesince the preselected data channel in receiver’s side is not free. The receiverthen reselects another available channel according to comparing with the statusof channel usage of the sender. The reselection rules are as follows:1)  If the sender has another free data channeland the channel is also available in receiver’s side. The receiver will selectthe common available channel to receive data frames.2)  If there is no available free channel in theside of the sender or receiver now, the receiver will compare all data channelsof both sender and receiver and then select a common channel which will beearliest released.      Channel information from both sides aretaken in order to prevent the hidden node problem.

After the checking process,the receiver will reply a MCTS frame back to the sender to make the finaldecision. The MCTS frame contains the current the usage status of datachannels.      Taking Fig. 3 for example, assuming thereare 5 mobile nodes in the ad hoc network. Node c and d are the exposedterminal of node a and b, and node e is the hidden terminal of node b. Initially node efinishes its backoff count down and then sends an MRTS frame to request the channel1 for transmitting data. The receiver node dapproves the request since the channel 1 is also available in side of d. After the negotiation of node d and e, node a finishes itsbackoff count down and sends an MRTS to node b to ask channel 1 for transmitting data.

Since channel 1 has beenreserved by node d and e, the request could not be accepted.Node b compares channel statuses ofnode a with node b and then selects an available channel2 in this example and sends MCTS back to node a. After receiving an MCTS from node b, node a is notifiedthat channel 1 would not be accepted and the agreed channel is channel 2. Node a will resend an MRTS to refresh thereservation information (to node c inthis example).Fig.

3 Transmission of MRTS/MCTS frames to select a channelD. Broadcastcasting in AMNP                   The broadcastoperation is an important activity in ad-hoc networks. Broadcasting is done toachieve routing information exchanges, address resolution protocol and messageadvertisement etc. Broadcasting can be done easily when there is a singlechannel but in multichannel environment, a node may miss the broadcast frame whenis currently transmitting or receiving data from other nodes. Here a singletransceiver constraint is chosen. To solve this problem, AMNP uses a designatedcontrol frame named broadcast beacon (BB) to announce to its neighbouring nodesof an upcoming broadcast transmission.Fig. 4 Frameformat of Broadcast Beacon    All nodes which received the BB will stay in the contention channel andwait a broadcast waiting time (BWT)to receive this frame even though it has made a successful reservation.

All thescheduled reservations will be delayed a SIFS + BWT + SIFS + broadcast framelength + SIFS period.    Several problems remain by adopting thistransmission of the broadcast frame after a SIFS interval. The following fourcases are considered as shown in Fig 5, to describe the broadcast problemsoccurred in the multichannel environment.Case1: After finishing the transmission where the sender and the receiver willreturn to the contention channel during the time period of the beginning of theBB and before the broadcast frame.Fig. 5 Broadcastproblems in Multichannel environmentCase3: At a finished transmission where the sender and the receiver will return tothe contention channel in the broadcast frame.

Case4: At a finished transmission where the sender and the receiver will return tothe contention channel after the broadcast frame.   Incase1 the nodes will receive the broadcast frame because it stays connected incontention channel after the transmission so it receives the broadcast frame.In case 2 it is not sure the nodes will receive the broadcast frame dependingupon the physical response time and ready time.    Case 3 and case 4 will definitelymiss the broadcast frame, to solve this problem we prefer reliable broadcastalgorithm.E.  Reliablebroadcast Algorithm        In reliable broadcast algorithm itrequires only selected forward nodes among the 1-hop neighbours to send ACKs toconfirm their receipt of the packet. Forward nodes are selected in such a waythat all senders’ 2-hop neighbour nodes are covered.

Moreover, no ACK is neededfor non-forward 1-hop neighbours, each of which is covered by at least two forwardneighbours, one by the sender itself and one by one of the selected forwardnodes. The sender waits for the ACKs from all of its forward nodes. If not allACKs are received, it will resend the packet until the maximum times of retryis reached. If the sender fails to receive all ACKs from the forward nodes, itassumes that the non-replied forward nodes are out of its range and choosesother nodes to take their roles as forward nodes.The forwardnodes are selected based on the following greedy algorithm:Inthe sample network shown in Figure 6, N(1)={1,2,3,4,6} and N2(1)={1,2,3,4,5,6,7}.

When using theFNSSP, sender node 1 selects nodes 2, 3 and 4 as its forward nodes. Node3 is selected because there is no node in N(1) to cover it.Algorithm: Forward Node Set Selection Process (FNSSP)Step1: The forward node set F is initialized to be empty.Step 2: Add in F thenode that covers the largest number of 2-   hop neighbours that are not yet covered by current F. A tie is broken bynode ID.Step 3: Repeat step 2 untilall 2-hop neighbours are covered.Fig .6 A sample network where the sender 1uses the FNSSP to select its forward nodes.

iv Simulation results            The network is varied from 54 to 108nodes. The mobility model uses the randomwaypoint model in a rectangular field. Here each mobile nodestarts its journey from a random location to a random destination with arandomly chosen speed (uniformly distributed between 0–94 m/s). Simulation Parameters                      Value Simulation Area 300m*300m Transmission range 100 m Transmission rate 2 Mb/sec SIFS 10µs DIFS 50µs MRTS frame length variable 160 bits MCTS frame length 112 bits ACK frame length 112 bits MAC header length 34 octets broadcast frame length 128 octets            Table 1. SimulationConfiguration Parameter     In all simulation analysis, one contention channel and 11 data channelsare considered. Simulation area is 300m x 300m, transmission range is about100m and transmission rate is about 2Mb/sec.     In Fig 7 When 54 nodes are considered AMNP performs better than IEEE802.

11. When frame arrival rate increases to 20, a significant amount ofincrease of throughput can be noted.Fig. 7 Comparison of Throughput derived by IEEE802.11 and AMNPWhen 108 nodes areconsidered AMNP performs better than IEEE 802.11.

When frame arrival rateincreases to 20, a considerable amount of increase of throughput can be notedwhen compared to IEEE 802.11.Fig.8 Comparison of Throughput derived by IEEE 802.

11 and AMNPMACdelay is the sum of MAC operations including back-off countdown, channelnegotiation and transmission delay. Fig. 9 Comparison of Mac delay derived by IEEE802.11 and AMNPFig.

10 Comparison of Mac delay derived by IEEE802.11 and AMNPThe Fig. 9 and Fig. 10 show comparativedelay analysis for IEEE 802.11 and AMNP protocol with 54,108 nodes.

It is seenthat AMNP protocol has lower MAC delay compared to IEEE 802.11 protocol.Fig.

11 Comparison of Mac delay derived by RBA andAMNP-RBA End- to – End delay is the total delay in the network; AMNP-RBA has higher delaybecause it is sum of back-off countdown, channel negotiation, transmissiondelay and the delay in broadcasting.Broadcastdelivery ratio is the ratio of the nodes that received the broadcast packets tothe number of the network. AMNP-RBA has lesser BDR compared to RBA because itis used in multichannel environment.

Fig.12 Comparison of Broadcast delivery ratio derived by RBA and    AMNP-RBAv Conclusion            The multi-hopMANET transmission capacity can be improved by adopting parallel multichannelaccess schemes. AMNP protocol addresses the problems like multichannel hiddenterminal problem and the multichannel broadcast problem. This is due to thosemobile nodes that cannot listen to all channels simultaneously.

The proposednew MRTS and MCTS handshaking message conquers the multichannel hidden terminalproblem.  The BB control frame to conquerthe multichannel broadcast problem. The performance analysis shows that thereis an encouraging result. The parameters are compared for 802.11 and AMNP.

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