The use of multiple channels in wireless sensor networks helps avoid interference and increases the overall network throughput. In addition, introducing multi-interface nodes further helps in increasing the packet delivery rate for those specific nodes.
In this paper, we propose a channel allocation method that takes into consideration the existence of multi-interface nodes in a multi-hop wireless sensor network. It is based on HMC-MAC, a hybrid MAC protocol that uses CSMA/CA, for data exchange, TDMA sequencing nodes activity and FDMA for allowing simultaneous collision free exchange.
We evaluated our method using NS2 simulator and results show that our MAC protocol improves the overall network performance compared to other protocols with periodic high data rate traffic and burst traffic.
In this section we recall how the network is created and neighbor discovery is done according to HMC-MAC.
A. Network Creation, Beacon Propagation and Neighbor Discovery:
The first node to be activated in the network is the Network Coordinator (NC) and is on depth O. The NC broadcasts a beacon that is propagated in a multi-hop manner to reach all the nodes of the network based on the MaCARI protocol. Please refer for more information on the time segmentation and beacon propagation.
In order to avoid overloading the network with long control messages to exchange neighborhood information, we use bitmaps to represent neighbors. Using the local propagation order (which is a list of all the node addresses of the network), every node is able to build and manage a bitmap that represents all the nodes in the network. Each index of the bitmap corresponds to the node address with the same index in the propagation order.
B.Channel allocation scheme and node activity:
The role of the NC is to divide time into intervals and inform all nodes of this time segmentation using the beacon frame.
A. Comparison with Other Methods:
Most protocols in the literature use 2-hop information to allocate channels and some of them use random allocation. Others divide the network into clusters. Accordingly, we chose to compare our protocol with four different allocation methods: HMC without segmentation method, 2-hop method, random method and cluster method.
B. Aggregate Throughput:
Figures 2 and 3 present the results in terms of aggregate throughput. For HMC-MAC protocol, it is clear that when the number of generated packets increases the aggregate throughput increases.
All other protocols reach saturation(figure 2) or rise slowly (figure 3) when the number of generated packets becomes greater than 8 pkts/sec/node due to the high number of collisions and frame repetitions. For the cluster method, the number of packets collected by the NC does not exceed 100 packets/sec and reaches saturation when the number of generated packets exceeds 4 pkts/sec/node.
C. Number of Dropped Packets:
Figures 4 and 5 present the results in terms of number of dropped packets. According to the CSMAICA algorithm of the IEEE 802.15.4 standard, the frame is repeated four times and is dropped if it is not acknowledged or not sent after the fourth repetition.
CONCLUSION AND PERSPECTIVES:
In this paper, we evaluated under high data rates the performance of HMC-MAC, a hybrid multi-channel MAC protocol for WSNs. It is based on 3-hop neighborhood channel allocation to avoid interference when data frame acknowledgments are used. We evaluated by simulation the efficiency of HMC-MAC and compared it to other MAC protocols.
Results showed that HMC-MAC considerably reduces the number of collisions and frame repetitions compared to the other methods, and this in two different traffic scenarios and different random network topologies. HMC-MAC copes much better than the other protocols when we increase the offered load, this is even more obvious when we use a burst traffic generation.
Source: Clermont University
Authors: Rana Diab | Gerard Chalhoub | Michel Misson