Wireless Ad Hoc and Sensor Networks
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Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks Wireless Ad Hoc and Sensor Networks

tfb.edu.mk
12.07.2015 Views

Distributed Power Control and Rate Adaptation 2476.5.4 Contention TimeThe change in contention time for the proposed DPC scheme is due totwo major factors: (1) more retransmissions during fading channel conditionsand (2) improved channel utilization. During fading channel conditions,retransmissions will increase with the proposed DPC because ofthe possibility of insufficient power for the reception of a packet. As aresult, the average contention time increases. Additionally, higher utilizationdue to the proposed DPC will cause an increase in the throughputcausing congestion. Under these conditions, the proposed protocol willcause certain frames to be delayed longer compared to the 802.11 standard.Therefore, the contention time will increase with the DPC from Zawodniokand Jagannathan (2004).6.5.5 Overhead AnalysisThe proposed MAC protocol requires additional data to be incorporatedinto the 802.11 frames for transmission. This additional information willinclude the current and the new transmitter power value to be used forthe response. All RTS, CTS, DATA, and ACK frames will embed thisinformation. The following analysis is used to evaluate the efficiency ofthe proposed protocol and to compare it with 802.11. In particular, wehave analyzed the case where the RTS/CTS messages are followed by asingle DATA/ACK exchange.6.5.5.1 RTS/CTS Followed by a Single DATA/ACK Frame ScenarioIn this scenario, there will be a total of four frames transmitted: RTS, CTS,DATA, and ACK. This is a typical sequence used for an Ethernet/IP basedpackets (length up to 2500 octets). Each frame includes two power values;thus overhead per data packet will include a total of eight power values.Let the size of power value in octets be expressed by S power ; the overhead(OH) size in octets per data packet is equal to:OH = 4frames × ( S * 2) = 8*Spowerpower(6.10)6.5.5.2 Minimizing Overhead ImpactIn the simulations, the power values are stored as real numbers and theyare sent in the MAC frame. However, in actual implementation, the overheadcan be minimized by allowing discrete values for power levels andlowering the OH in terms of number of bits used for power. Second, thepower values can be embedded in the frame only when the transmitterpower changes between the power levels. This can be accomplished byusing a one-bit flag to indicate whether the power values are added to

248 Wireless Ad Hoc and Sensor Networksthe header or not. The one-bit flag field will be included in all the frames.If the power value does not change from its previous value, the flag iscleared, and no additional data is sent. Otherwise, the bit is set, and thenew power value is included in the header.Let us assume that the power value will change between frames witha probability p. Then the OH per data packet — in case of RTS/CTSfollowed by a single DATA/ACK — will be expressed as:OHsave= 4frames × ( 2× 1bit _ flag + p × 2× Spower) = 8*(1bit _ flag + p * S power )(6.11)where p is the probability with which the change in power level will occurfor a frame, a one-bit flag is used to indicate whether the power value isincluded in the header or not.6.5.5.3 Protocol Efficiency for RTS/CTS/DATA/ACK SequenceThe efficiency of the protocol in terms of OH size can be evaluated as theratio of user data portion to total data transmitted (data + frame headers+ backoff) (Wei et al. 2002). The efficiency can be expressed asSpacketη =S + S + S + S + S + Spacket RTS CTS DATA ACK BACKOFF(6.12)where S packet is the size of data packet in octets; S RTS , S CTS , and S ACK representthe size of RTS, CTS, and ACK frames, respectively, S DATA denotes the size ofDATA frame header (without data packet), and S BACKOFF represents the backofftime given in octets.Because of the implementation of the DPC in the MAC protocol, theframe size of RTS/CTS/DATA/ACK will increase by an amount equal toOH from Equation 6.11 and Equation 6.12, respectively. To understandthe OH, the efficiency of the proposed implementation has been comparedwith that of the standard 802.11 protocol. Different size fields used for thepower levels have been compared: 4-bit, allows 32 different power levels,and 8-bit (one octet) allows 255 power levels and so on. Also, probabilitylevels are used to assess the power change between frames: p = 0.5 impliesthat the change occurs at every second frame and p = 0.1 represents thechange occurring at every tenth frame.In the worst-case scenario all the frames will contain the power fields.Because of the additional OH resulting from the incorporation of powerlevels for the proposed DPC, a 2.5% decrease in efficiency calculatedusing Equation 6.12 is observed when compared to 802.11. Thus the

Distributed Power Control <strong>and</strong> Rate <strong>Ad</strong>aptation 2476.5.4 Contention TimeThe change in contention time for the proposed DPC scheme is due totwo major factors: (1) more retransmissions during fading channel conditions<strong>and</strong> (2) improved channel utilization. During fading channel conditions,retransmissions will increase with the proposed DPC because ofthe possibility of insufficient power for the reception of a packet. As aresult, the average contention time increases. <strong>Ad</strong>ditionally, higher utilizationdue to the proposed DPC will cause an increase in the throughputcausing congestion. Under these conditions, the proposed protocol willcause certain frames to be delayed longer compared to the 802.11 st<strong>and</strong>ard.Therefore, the contention time will increase with the DPC from Zawodniok<strong>and</strong> Jagannathan (2004).6.5.5 Overhead AnalysisThe proposed MAC protocol requires additional data to be incorporatedinto the 802.11 frames for transmission. This additional information willinclude the current <strong>and</strong> the new transmitter power value to be used forthe response. All RTS, CTS, DATA, <strong>and</strong> ACK frames will embed thisinformation. The following analysis is used to evaluate the efficiency ofthe proposed protocol <strong>and</strong> to compare it with 802.11. In particular, wehave analyzed the case where the RTS/CTS messages are followed by asingle DATA/ACK exchange.6.5.5.1 RTS/CTS Followed by a Single DATA/ACK Frame ScenarioIn this scenario, there will be a total of four frames transmitted: RTS, CTS,DATA, <strong>and</strong> ACK. This is a typical sequence used for an Ethernet/IP basedpackets (length up to 2500 octets). Each frame includes two power values;thus overhead per data packet will include a total of eight power values.Let the size of power value in octets be expressed by S power ; the overhead(OH) size in octets per data packet is equal to:OH = 4frames × ( S * 2) = 8*Spowerpower(6.10)6.5.5.2 Minimizing Overhead ImpactIn the simulations, the power values are stored as real numbers <strong>and</strong> theyare sent in the MAC frame. However, in actual implementation, the overheadcan be minimized by allowing discrete values for power levels <strong>and</strong>lowering the OH in terms of number of bits used for power. Second, thepower values can be embedded in the frame only when the transmitterpower changes between the power levels. This can be accomplished byusing a one-bit flag to indicate whether the power values are added to

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