Medium access algorithm for tactical networks

Abstract

In this era the wireless networks are getting important everywhere as it doesn’t require any physical medium or to lay down any medium like wires etc. it is more important in tactical/combat networks where it is impossible to lay down any other medium for the communication. The medium in the wireless network is air, which is a broadcast medium, means anyone can transmit a message in the form of signals at any time. This nature of the medium make it impossible to use without some management as if more than one node transmits at the same time the receiver of the signal will receive a garbage and no useful information. So this problem is to be handled by the system and the transmitting node must be capable of finding out when it should transmit and when it should not. This task is performed by the MAC (Medium Access Control) which is a sub-layer of the Data Link Layer. MAC layer basically deals with the access to the medium and based on the physical medium it uses different protocols to for the access of medium. For wireless networks especially of ad-hoc type collision is the main issue due to which the throughput decreases. Different protocols like 802.11, MACA and MACAW etc uses carrier sense to avoid collision but it also does not get rid of collisions. Other technique is time division multiple access (TDMA) in which every node has the information about when to transmit. This is made possible by dividing time into slots and a node is given access to the medium during the time slot which is assigned to it. This way the collision is avoided as every node has a pre-defined slot in which it can transmit. But this technique has a flaw of wastage of bandwidth when only few nodes are transmitting. Also there are special routing protocols on the network layer for the wireless ad-hoc networks so that the routes are easy to find and maintain. We have proposed a new algorithm to make maximum use of the bandwidth with avoiding the collision. We have proposed a new idea of merging two layers to avoid overheads of layers and merged network and data link layer into one and this new layer will now perform the task of two layers. We have made this design for the tactical networks that uses an AODV protocol for the routing and TDMA protocol for the medium access. Now instead of generating a message on the network layer and that message is then transmitted according to a new protocol on the MAC 7 layer now in a single layer a message is just mapped onto its respective time slot saving overhead. We have divided this task into two phases first is the finding of the route through the exchange of AODV control messages and then the actual communication. We have also made a node capable of sending more than one control messages at a time in a single time slot which saved a lot of time. Then we designed a slot allocation algorithm which made it possible to utilize the maximum bandwidth by finding out which slots are free and make used of them. The algorithm exploits the information exchanged in AODV control messages and in routing table and find out the free slots and allocates them to the active nodes. This way we have increased the throughput and also multiple slots are allocated to a node without any collision as the algorithm run on all the active nodes and every node has the information about which slot is allocated to it and which are to be left. The cross layer designs that exists are not exactly based on merging of layers rather they use the information of another layer to decide something and improve the communication but we have merged two layers into one and saved overhead. We have tested our algorithm on Matlab and it is very obvious from the results that sending multiple control messages in one slot saved a lot of frames and decreased call setup time and then the allocation algorithm proved to make maximum utilization of the bandwidth by allocating as much free slots to active nodes as possible.