2ZigBee standard overview

ZigBee standard for home wireless personal area networkss  (LR-WPANs),, allows the networks to create, manage, and high energy savings through a wireless channel for data transmission。  ZigBee standard expected of the three different node types: (i) coordinator, (ii) routers, and (iii) the terminal equipment。 In the absence of a direct communication link, the routers used to transmit data packets to the correct address。 Coordinator, in addition to transmission of data packets, can also create a networks of other nodes to change the parameters of communication (such as a networks identifier (ID), a synchronous frame), and transmission networks management commands。  Routers and the coordinator is called full-function devices (FFDs), for example, they can achieve the full functionality of ZigBee standard required to establish and maintain communications。 Terminal Equipment, also known as reduced function devices (RFDs), can only collect data from  sensors, these values into the appropriate packets, and then send them to the target node。

ZigBee standard is based on IEEE802。15。4, in the ISO / OSI protocol stack of the first two layers, using a carrier sense with collision avoidance (CSMA / CA) Media Access Control (MAC) protocol and work in the 2。4 GHz band (similar to IEEE 802。11 Standard [15])。 In addition, IEEE802。15。4 standard provides an optional ACK messages to ensure correct delivery of data packets。 We note that the ZigBee wireless networks  media

access mechanism using an algorithm to reduce the return data packet collisions。 In the transmission before a new packet, the node will wait for a time, the length of time beginning in the networks randomly selected intervals。 After the end of this period, the node will try to send its data packets: if it detects a collision, it will wait twice the previously selected time interval; the contrary if the line is idle, it sends its own Packets。 When the wait interval reaches its maximum value, the process is repeated 5 times。 The backoff algorithm to be considered in the context of the end node sends its data packets will be successful。

ZigBee standard provides a technology that can improve the RFDs in sync。 This technology is based on periodic signal transmission time slot by, also known as beacons (beacon), issued from the networks coordinator。 In particular, the standard defines a specific frame, called super frame, as shown in Figure 37。1。 The frame has two main  parts:

(i)active region and (ii) the passive zone。 Activity area in turn is pided into two cycles:

(1)access for periodic (CAP), and (ii) idle for periods (CFP)。 In the CAP, only the RFD rely MCSA / CA protocol to access  the channel, but only in the CFP time slot previously reserved to ensure that (GTS) of the RFD can access the channel, thus avoiding the collision。 Finally, all of the RFD in the passive zone into sleep state, thereby saving electricity。 In accordance with the ZigBee symbols, two concurrent   beacon   beacon   interval   is also

called sequence (BO), while the activity is called the super-frame duration of the frame duration (SD)。

3RFID technology

RFID technology shows two different devices: (i) readers and (ii) an answering machine or label。 Tag is an electronic data storage device used to identify and determine the tracking object。 On the contrary, the reader used to ask the label。 Tags can not be pided into different categories according to the power of three main categories, (i) active   tag,

(ii)semi-passive tags and (iii) passive tags【12】。

Active tag reader to respond to questions, use the built-in battery operation and signal transmission。 The semi-passive mode  tags only use their batteries on the built-in processor power, do not broadcast the return signal。 Finally, the passive tags, not equipped with batteries, the use of the energy of the received signal reader response (scattering)。 Clearly, passive tags have the longest life Clearly, passive tags have the longest life (basic unlimited), and active tags longest transmission distance。 Once a label reader asked, and answered correctly the  label, not the collision was the other tags (using the appropriate anti-collision mechanism), the reader sends a confirmation message to the label to confirm the correct message is received。