The Impact of Contention Resolution verses a priori Channel Allocation on Latency in a Delay Constrained Network

The predominant mechanism used to control access to the underwater acoustic channel is a contention-based collision-avoidance scheme to reserve the shared media, on-demand, before sending data to avoid retransmission costs incurred with collisions. This paper reports analysis of such a mechanism for a simple linear, or backbone, type topology as compared to the use of an a priori allocation of capacity to each host, thereby eliminating the need for such access coordination. The former incurs RTS/CTS propagation overhead while the latter incurs a transmission penalty due to the reduced transmission rate of each allocated channel. Therefore, the selection of an appropriate access mechanism, given consideration of the message size and the propagation distance, may have a significant impact on the overall latency of a message. In particular, given the topology modeled for the study, this paper suggests that relatively small messages or long propagation distances benefit from an a priori channel allocation, while large messages or short distances are more effectively served by an access coordination method, such as RTS-CTS exchanges. Further, as the number of frames in a message or the number of hops over which the message is relayed increases the relative latency performance of the a priori allocation improves over that of the coordinated access. The results establish a prior channel allocation as an appealing media access scheme in delay constrained networks and should provide an impetus for further research so as to maximize the performance potential of such schemes.