MULTIHOP FORWARDING THROUGHPUT ANALYSIS IN PRESENCE OF HIDDEN / EXPOSED TERMINALS

A wireless network that uses intermediate nodes to transmit data to the destination suffers from the hidden/exposed ter minals problem unless specific steps are taken by the protocol i mplementation to overcome this. The IEEE 802.15. 4 standard specifies no such measure. At the same time packet transmission through multiple hops can be more energy efficient than direct packet transmission, and at times it is the only solution. In this paper, a multihop wireless sensor network with hidden/exposed terminals is considered and its throughput performance is analyzed. The analysis is verified by extensive ns-2 simulations. I I NTRODUCTION AND MOTIVATION In a distributed networked sensing task, collected data at t he field sensors are forwarded to a predetermined sink node – which could be a remote monitoring and control center, or a clusterhead in a hierarchical sensor network. Limited noda l energy and high premium on network lifetime in such a network warrants that the remotely located field nodes communicate t o the data sink via multiple hops. Multihop nature of forwarding implies that, in addition to forwarding their own data, t he intermediate nodes will be responsible for relaying data of the peripheral nodes. Therefore, field data from multiple nodes ar accumulated (in uncompressed or compressed form) as the sin k node is approached, and as a result the traffic flow intensity i ncreases in the same direction. As multiple nodes try to forwa rd data to one node at an intermediate stage, contention to acce ss the wireless channel occurs among them. In addition, the hid den/exposed nodes problem in a multihop forwarding leads to queueing/dropping of packets at the intermediate nodes. Many-to-one forwarding in IEEE 802.15. 4 sensor networks has been studied from congestion control fairness perspect ives [4], where transport layer scheduling is suitably controll ed to avoid packet dropping as they propagate toward the sink. To enable successful many-to-one forwarding, access schedul ing mechanisms and buffer requirements were investigated in [1 3]. Another set of work proposed medium access control (MAC) level solutions to resource constrained multihop sensor ne tworks (e.g., [5, 10, 14]). While the prior works addressed th e important issue of data forwarding constraints, to the best of our knowledge, an analytic measure of network throughput in a many-to-one multihop sensor network is still missing. Systems implementation level many-to-one multihop forwarding constraints, such as message length and buffer size , were studied in [7], where the number of message transmissions was reduced by in-network aggregation. It also showed how the data forwarding performance drastically decreases with increase in hop length, but did not address the effect of multiaccess constraints. A few recent MAC level analytic performance models of CSMA/CA (carrier sense multiple access with collision avoidance) sensor networks inclu de [6,8,9,11]. [8] considered star network topology with beac onenabled nodes and two-way communication. [11] investigate d multiaccess contention in a many-to-one data aggregation s cenario with beacon-enabled nodes, where the nodes were modeled such that the field nodes are always in the transmit state and the sink is in the receive state. All of these analytic wor ks addressed the single-hop aggregation problem. In the current work, we present a stochastic model to characterize the throughput of a multihop many-to-one ad hoc sen sor network by accounting the multiaccess collisions and hi dden/exposed nodes. The analysis is verified by extensive net work simulations usingns-2. The developed model can be used to study the effect of network traffic generation rate on the allowable number of forwarding hops, or conversely, for a given maximum number of hops to the sink, maximum allowable traffic generation rate at the nodes. II CSMA/CA ALGORITHM DESCRIPTION A detailed explanation of the IEEE 802.15. 4 specification ca be found in [2]. For the analysis of a multihop network we consider the unslotted CSMA/CA algorithm. Until now, to our knowledge, only the slotted 802.15. 4 CSMA/CA analysis has been carried out [8,11]. While a slotted CSMA/CA is expected to show better performance in comparison to unslotted one, 802.15. 4 achieves synchronization amongst the nodes by the transmission of beacon packets by the coordinators. The bea con packets help in setting the internal clocks of all reduce function devices (RFDs). As explained in [1,2], the nodes wa it for periodic reception of beacons. The loss of beacon reception renders a RFD useless (or orphaned) until the next beaco n i received and resynchronization can take place. Due to the importance of beacon packets, nodes take special care not to transmit any other packet during the expected arrival of a be ac n packet. The technique of collision avoidance during beacon packet transmission works only if all nodes can hear each other, i.e , when there are no hidden terminals, or there is only one PAN coordinator. In a cluster tree topology which has one PAN coordinator and several coordinators each transmitting beac ons, if the coordinator is hidden from a node, then collision avoi dance will not work. This situation is depicted in figure 1. In