Wireless Ad Hoc and Sensor Networks

Background on Networking 31reception at the source node, when nodes detect a transmission and cannotdecode it, they set their NAVs for the EIFS duration. The main purposeof the EIFS is to provide enough time for a transmitting user to receivethe ACK frame, so that the duration of EIFS is longer than that of an ACKtransmission. As per the standard IEEE 802.11, the EIFS is obtained usingthe SIFS, the DIFS, and the length of time to transmit an ACK frame atthe physical layer’s lowest mandatory rate, as in the following equationEIFS = SIFS + DIFS + [(8 × ACK size) + Preamble length + PLCP headerlength]/Bit rate, where ACK size is the length (in bytes) of an ACK frame,and Bit rate is the physical layer’s lowest mandatory rate. Preamble lengthis 144 b and PCLP header length is 48 b.1.4.6 Need for Power Sensitive Schemes and ProtocolsWireless cellular, ad hoc, and sensor networks operate with limitedbandwidth and battery resources. Unlike computer networks, wirelessnetworks are bandwidth and interference limited. Interference due to theusers in the network decreases the network’s capacity. Hence, efficientways to combat interference to increase the capacity and spatial reuse ofthe network are of great importance. A solution to this problem is toefficiently control signals from randomly distributed users in the network,which would increase the network capacity and battery lifetime. Therefore,centralized power control schemes (Dontula and Jagannathan 2004)were proposed in the case of cellular networks. A major disadvantage ofthese schemes is that they are computationally expensive, consuminglarge base station computation resources. On the other hand, DPCschemes are a more realistic approach for minimizing the interference.DPC schemes adjust the power levels of the users in the network just bysensing the local environment. These DPC schemes (Dontula and Jagannathan2004) are straightforward, consume minimal mobile userresources, and minimize the transmission of control packet information.In ad hoc wireless networks, where there is no centralized governingagent, a DPC scheme is a natural choice. Determining the power withwhich a mobile user in the network should transmit, by keeping theinterference in check, in a highly dynamic environment is a challengingproblem. In a military environment, where the networks are required tomaintain a low probability of intercept and a low probability of detection,as they operate in remote and unstructured environments, nodes need toradiate as little power as necessary and transmit as infrequently as possible,thus decreasing the probability of detection or interception. A lapsein any of these requirements may degrade the performance and dependabilityof the network.With the advent of ad hoc networks of geographically distributedsensors in remote site environments (e.g., sensors dropped from aircraft