1). Further, it might be pointed out that a multi-hop scenario may thenthereby be too energy-hungry for some specific Inhibitors,Modulators,Libraries body sensors (e.g., the ones close to the care unit) and thus jeopardize their own medical data transmissions. It is therefore assumed that the single-hop star-based topology is the most energy-efficient, because this does not put body sensor��s own medical data transmission at risk, avoiding unnecessary battery replacements.Figure 1.A star-based BSN in a potential medical scenario.In a IEEE 802.15.4 star-based BSN, the beacon-enabled mode appears to allow for the greatest energy efficiency. Indeed, it allows the transceiver to be completely switched off up to 15/16 of the time when nothing is transmitted/received, while still allowing the transceiver to be associated to the network and able to transmit or receive a packet at any time [2].
The beacon mode introduces Inhibitors,Modulators,Libraries the so-called superframe structure (Figure 2).Figure 2.IEEE 802.15.4 MAC superframe structure in beacon-enabled mode (active).As previously mentioned, the superframe structure starts with the beacon, which is a small synchronization packet sent by the BAN coordinator, carrying service information for the BSN maintenance and notifying body sensors about pending data in the downlink. The inter-beacon period is partially or entirely occupied by the superframe, divided into 16 slots. The number of slots at the tail of the superframe may be used as GTS, i.e., they can be dedicated to specific body sensors with no contention (see Figure 2, CFP). This functionality targets very low latency applications, but does not scale properly to highly dense BSNs (i.
e., saturation conditions), since the number of dedicated Inhibitors,Modulators,Libraries Inhibitors,Modulators,Libraries slots would not be sufficient to accommodate more than seven body sensors at a time. In such conditions, it is better to use the contention access mode, where the sparse data is statistically multiplexed. Batimastat In the contention access period, distributed channel accesses in the uplink are coordinated by a slotted CSMA/CA mechanism, while indirect transmission is used in the downlink. As we will see later, the CSMA/CA mechanism has a significant impact on the overall energy and performance of the uplink. According to the slotted CSMA/CA algorithm in [2], a node must sense the channel free at least twice before being able to transmit, this corresponds to the decrement of the so-called contention windows.
The first sense must be delayed by a random delay chosen between 0 and 2BE�C1, where BE is the back-off exponent. This randomness serves to reduce the probability of collision when two nodes simultaneously sense the channel, assess it free and decide to transmit at the same time. When the channel is selleck sensed busy, transmission may not occur and the next channel sense is scheduled after a new random delay computed with an incremented back-off exponent.