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In communications networks, nodes share a common medium for transmitting data packets, and the medium access control (MAC) protocol is responsible for regulating access to that common medium.
Medium access solutions generally fall into two categories: contention based and scheduled (contention-free). The collision-free approach of scheduled schemes provides high-throughput in high-demand scenarios at the expense of overhead and packet delay. In comparison, contention-based approaches provide low-delay times at low to moderate network loads, but performance begins to degrade rapidly as the network becomes saturated.
Although initial work has been done in the wireless sensor network field to combine the benefits of both approaches in response to changing network load, no medium access scheme has been proposed that is capable of accommodating effectively and efficiently the multiple flows that exist in a wireless sensor network. While overall network performance may be optimized, individual flows may perform poorly in existing wireless sensor network medium access control solutions. The salient shortfall in these hybrid approaches is that they treat the flows in aggregation rather than individually.
In response to this gap in the technical offering, Navy scientists have devised an energy-efficient, traffic-adaptive cooperative wireless sensor medium access control (CWS-MAC). This flow-specific medium access method for wireless networks is capable of providing contention- or non-contention-based medium access.
The CWS-MAC includes a contention-based medium access mechanism; and a non-contention-based Time Division Multiple Access (TDMA) framing scheme. The former is superimposed on top of the TDMA framing scheme through the use of an interframe space and a contention beacon that gives the contention-based flow global priority over the non-contention-based flow.
In operation, a node, having non-contention packets to transmit in its TDMA slot, waits for the duration of interframe space and then senses the medium. If the medium is free (no contention beacon has been transmitted in its two-hop neighborhood), the node may transmit its non-contention packets. If a contention beacon is detected the space is effectively seized as a contention slot. To ensure the non-contention flow is not choked off, a portion of the original TDMA slot is reserved, for use by non-contention packet transmission.
- Hybrid approach combines the low-demand delay performance of a contention-based scheme with the high-demand throughput performance of a non-contention (scheduled) approach
- Energy-efficient operation is achieved by implementing an adaptive sleep cycle that makes use of a preamble sampling mechanism
- US patent 8,724,598 available for license