RFID technology provides the main function of reader and tag on the error-free communication between the (two-way), to identify a variety of labels and communication,

select the intended target label, tag write operation and coverage。 Between reader and tag communications, according to scattering techniques, by asking the amplitude modulation achieved。 On the other hand, signal transmission in the inquiry, the reader sends a constant signal power, so that label to respond to the previous signal, the load modulation technique。 Reader signal change from the constant reflection of the power to receive tag data。

When the reader asked a group of labels, which may respond immediately, resulting in the collision。 RFID technology is the most important standard ISO / IEC 18000-6 standard。 This standard defines a standard identification system, working in  the ultra-high frequency (UHF) frequency range。 In particular, the provisions of this standard RFID systems in Europe in the 868MHz band is about。 18000-6 standard defines the transmission of two different types: (i) A  type

(ii) B-type of two types。 40kbps transmission rate of the two types of use, and binary phase modulation。 Two different types of transmission exists in the media access control (MAC) protocol。 If the reader found a collision, according to Aloha MAC protocol A type of label forwarding inquiry  request。 Under the agreement, the label  produced in the packet sent it immediately, within a specified time when there is no confirmed information received, he said that the collision occurred。 In contrast, B-type labels, use the binary  tree   protocol   (BTP)   to   prevent the

collision mechanism。 BTP mechanism is the case, when the collision came, tag class is pided into two sub-categories: immediate attempt to resend their information and the other readers have to wait until a new inquiry。 Ultimately, only one label re-issued, and readers will successfully obtain the  identity (or other stored information。) This recursive process is repeated until all the tags no longer respond to inquiries signals。

4ZigBee-RFID hybrid networks

4。1System Structure

Our system model consists of an N-node networks, nodes are used to monitor the phenomenon of particular interest。 To assume a minimum density of space observatories, such as the need for monitoring the surface of a given interest (in the region A), RFD minimum number Nmin, and  set  the number of N RFD larger than Nmin, we RFD methods include the selective implementation of a wake-up。 More precisely our  strategy including selective switching RFD networks nodes to balance energy consumption。 In order to maximize the life of the networks, as  long as the remainder of an active RFD below a fixed threshold energy Eth will be the switch to the sleep state。 At the same time, one of the nodes in the remaining N-Nmin (previously closed) has been awakened, thus ensuring the whole phenomenon is used to detect specific space density of the smallest stations。 This process is known as deep sleep algorithm。 A description of the example, in a given time the algorithm uses the networks of deep sleep。 We

point out that, RFD only switch to sleep state, rather than being closed, thus preventing its loss in the networks coordinator and  other RFD with synchronization。 In fact,  according to ZigBee standard, the networks connection symbolic execution, an RFD may be opened and the introduction of a wake-up symbol longer than the delay, because the RFD must wait before starting to transmit a new  beacon to the same networks synchronization。 On the contrary, the RFD in the sleep state into active only in a beacon transmission is planned and then, if not necessary, return to sleep without losing their synchronization with the networks。 Deep sleep algorithm goal is to achieve energy balance in the RFD, and then to the residual energy of each RFD can be balanced。 When the RFD to sleep, in fact, its energy consumption is lower than the active 4 orders of magnitude。