Zhang Ping Kang Guixia Tian Hui

（Wireless Technology Innovation Institute， Beijing University of Posts and Telecommunications，Beijing 100876， China）

AbstractThe Zig Bee as the main supporting technology for the wireless sensor network has received extensive attention.Completed Zig Bee protocol kits consist of the high-level application specification，the application convergence layer，the network layer，the data link layer，and the Physical（PHY）layer.The protocols of the network layer and higher ones are set up by the Zig Bee Alliance，the physical layer and the Media Access Control（MAC）layer are defined by the IEEE 8O2.15.4 standard.The IEEE 8O2.15.4 physical layer simply adopts technologies of mapping from bit to symbol，conversion from symbol to chip sequence，and Offset Quadrature Phase Shift Keying（OQPSK）modulation，avoiding complicated arithmetic such as channel coding.The IEEE 8O2.15.4 MAC layer adopts Carrier Sense Multiple Access with Collision Avoidance（CSMA/CA）technology and supports the sleeping mode.The Zig Bee enables low data transmission rate，low power consumption at low cost，and is more suitable for industry monitoring systems，sensor networks，home monitoring systems，and security systems.

Foundation Sponsored Program:Beijing Natural Science Foundation（No.4062023）

I n the recent decade，the wireless and mobile communication has been developing at an unprecedented speed.With the increasing number of various portable personal communication equipment and consumer electronics，people place a new demand for short distance wireless and mobile communication system.Due to the demand，short distance wireless communications，including Wireless Local Area Network（WLAN），Bluetooth，mobile Ad Hoc，Ultra Wideband（UWB），and ZigBee，appeared quite unexpectedly and show great potential in applications.The Zig Bee technology，characterized by low rate，low power consumption and low cost，has received extensive attention as the main supporting technology in wireless sensor networks.[1-4]

The Zig Bee is a new technology in short distance communication.It uses 2.4 GHz frequency band and adopts the frequency-hopping technology.Compared to Bluetooth，the Zig Bee is simpler，and has lower power consumption and cost.[5]

Followings are the technical characteristics of Zig Bee.·Low data transmission rate.It ranges only from 1Okbps to 25Okbps.

·Low power consumption.With the standby mode of low power consumption，two AA batteries can last for six months to two years.

·Low cost.The Zig Bee data transmission rate is low，the protocolis simple and the patent is free.

·Huge network volume.Each ZigBee network can maximally support 255 devices.

·Short time delay.It's usually 15 ms to 3Oms.

·Safty.The ZigBee provides functions such as data integrity checking and authentication，and adopts Advanced Encrypting Standard 128（AES-128）for encryption arithmetic.

·Small effective range.The effective coverage is 1Om to 75 m，which is varying depending on the actual transmitted power and different application modes.

·Flexible working frequency bands.Working frequency bands are 2.4 GHz，868 MHz（in Europe）and 915 MHz（in USA），which are all license-free frequency bands.

1 The ZigBee Protocol

1.1 The ZigBee Protocol Framework

The Zig Bee protocol stack consists of a set of sublayers.Each layer provides certain services to its upper layer:the data entity provides the data transmission service；the managing entityprovides all other services.Each service entity provides the service interface for its upper level through Service Access Point（SAP）and each SAPprovides a serial of basic service instructions to complete certain functions.band.It defines two working frequencies，which are 2.4 GHz and 868/915 MHz.

▲Figure 1. The ZigBee protocol framework.

In IEEE 8O2.15.4，27 channels with three rates are distributed:16 channels with rate of 25Okbps at 2.4 GHz frequency band，1Ochannels with rate of 4Okbps at 915 MHz frequency band，one channelwith rate of 2Okbps at 868 MHz frequency band.

The central frequency of those channels is defined as follows:

Fc=868.3 MHz，k=O；

Fc=9O6 MHz+2（k-1）MHz，k=1，2…1O；

Fc=2 4O5 MHz+5（k-11）MHz，k=11，12…26.

One IEEE 8O2.15.4 network can choose one working channel from the 27 channels based on availability，congestion，and data rate.Seen from the perspectives of energy and cost efficiency，different data rates can provide better choices to different applications.For instance，for some computer periphery and interactive toy 25Okbps rate might be necessary.For other applications，for instance，sensors，intelligent marks and consumer electronics，low rate as 2Okbps can meet the requirements.

2 The IEEE 802.15.4 Physical Layer

The system architecture of ZigBee protocol stack[6]is shown in Figure 1.The IEEE 8O2.15.4-2OO3 standard[7]defines the lowest two layers:the Physical layer（PHY）and the Media Access Control layer（MAC）.The Zig Bee alliance provides designs for the network layer and the Application Layer（APL）framework.The framework of APL includes the Application Support Sublayer（APS），Zig Bee Device Object（ZDO），and application objects set up by manufacturers.

The following is the estimated requirement for ZigBee protocol kits.It needs an 8-bit processor for hardware such as 8Oc51，and a 32 kB ROM for software.The smallest software needs 4 kBROM.The main nodes need more RAM to store the information of all the nodes such as equipment information，data package forwarding list，equipment correlation list，and encryption key storage related to security.

1.2 The Integration of IPv6 and IEEE 802.15.4

The Zig Bee alliance hopes to establish a wireless network，which can connect every electronic device.Networks with several billion nodes will soon use up the compressed IPv4 addresses.Therefore，the integration of IPv6 and IEEE 8O2.15.4 is the developing trend of the sensor network.The IPv6 adopts 128-bit length of address and can almost provide addresses without limitation.Conservative estimation shows that，IPv6 can actually distribute 1 OOOaddresses to every square meter area on the earth.

1.3 Working Frequency Band and Rate

The IEEE 8O2.15.4 works at the Industrial，Scientific，and Medical（ISM）frequency

The binary data from the IEEE 8O2.15.4 PHYProtocol Data Unit（PPDU）are combined into four-bit binary data symbol serially（based on number of byte，from low to high）.Each data symbol（corresponding to one of the 16 status groups）is mapped into a 32-bit pseudonoise chip for the convenience of transmission.Then the continuous chip sequence is modulated（adopting minimum key control method）to the carrier，that is，to adopt the Offset Quadrature Phase Shift Keying（OQPSK）modulation method.The IEEE 8O2.15.4 physical layer transmission is shown in Figure 2.

The physical layer on 868/915 MHz frequency band uses simple Direct Sequence Spread Spectrum（DSSS）method.Each PPDUdata transmission bit is spread by the chip sequence with the maximum length of 15（coded by m-sequence made of multiple groups of+1 and-1），then use binary phasekeying technology to modulate this extended bit sequence.Different data transmission rate applies to different occasions.

▲Figure 2. The IEEE 802.15.4 physical layer transmission.

3 The IEEE 802.15.4 MAC Layer

The IEEE 8O2.15.4 MAC layer provides:the MAC layer data service and the MAC layer management service.The former one enables the MAC Protocol Data Unit（MPDU）to receive and send through the physical layer data service.The latter one accesses the higher layer through MAC Layer Management Entity（MLME）Service Access Point（SAP）of the MAC layer.Beacon management，Guaranteed Time Slot（GTS）management，frame Acknowledgement（ACK），ACK frame transmission，and nodes access and separation，characterize the IEEE 8O2.15.4 MAClayer.

3.1 The Super Frame Structure

The low rate wireless personal area network is allowed to use super frame structure.Coordinator on the sensor network defines the format of super frame.A super frame is divided into 16 equal time slots，transmitted by coordinator，as shown in Figure 3.The network beacon separates each super frame.The beacon frame is transmitted in the first time slot of the super frame.In case the coordinator doesn't want to use the super frame structure，it will terminate the beacon transmission.The beacon is used to synchronize the accessed device，differentiate personal area networks，and describe the super frame structure.Any device wishing to communicate at the Contention Access Period（CAP）needs to use Carrier Sense Multiple Access with Collision Avoidance（CSMA-CA）.

The super frame structure consists of two parts as active and inactive.In the inactive part，coordinator doesn't contact with network，entering the low energy mode.For the low time delay applications or applications requiring special bandwidth，the network coordinator sacrifices the active part of the super frame.Such part is called GTS.The GTSconsists of the Contention Free Period（CFP），which always follows CAPat the end of the super frame，as shown in Figure 4.The network coordinator can distribute seven GTS，each of which can occupy more than one time interval.The CAPwill leave sufficient time to the accessed network device based on competition and the device wishing to access the network.All the transmission based on competition will be completed before CFPbegins，meantime，GTStransmission will definitely complete before GTSstarts.

3.2 The CSMA-CA Mechanism

The low rate wireless personal area network uses two different channel access mechanisms according to the different architecture of the network.The network without beacon uses the slotless CSMA-CAchannel access mechanism.Every time the device wants to transmit data frame or MACcommand，it will wait for a random time.Then if channel is detected as idle，the device will transmit the data.Otherwise，the device will wait for another random time before accessing the channel again.The transmission of ACK frame doesn't use CSMA-CAmechanism.

The network with beacon uses the slotted CSMA-CA channel access mechanism.Back off slots queue at the beginning of beacon transmission.Whenever the device wants to transmit data frame at CAP，it needs to confirm the boundary of the next back-off slot and wait for several random randomly.After that，if the channelis detected as busy，the device will transmit data before waiting for several random slots again.Otherwise the device willtransmit at the next slot.The transmitting of ACK frame and beacon frame don't need the CSMA-CA mechanism.

3.3 Safety Mode

In the low rate wireless personal area network，device can choose any safety mode according to its own requirements:the non-safety mode，the Access Control List（ACL）mode and safety mode.

The non-safety mode is the default mode of MAC sublayer.Device under such mode won't carry out any security check on received frames.When a device receives a frame，it only checks the destination address of the frame.If the destination address is the local equipment or broadcast address，the frame will be forwarded to the upper level，ouherwise it will be discarded.

▲Figure 3. Super frame structure without GTS.

▲Figure 4. The super frame structure with GTS.

▲Figure 5. The network topology.

When the equipment is set as Promiscuous mode，it will forward all its received frames to the upper level.

The ACLmode provides access control service for communications.Based on source address，high levels can instruct on MAC sublayers to filter received frames，by setting ACLitems in the MAC sublayers.Therefore，the MAC sublayer under such mode is not provided with encryption protection and it is necessary for the high level to take other mechanisms to ensure the safety of communication.

The safety mode provides four safety modes of services for the received or transmitted frames:access control，data encryption，frame integrity check，and sequence update.

4 The ZigBee Network Layer

The ZigBee network layer willmainly consider adopting network protocols based on the Ad Hoc technology，which should include the following functions.

（1）Universal network layer functions，setting up and maintenance of topology structure，naming and correlated business such as addressing，routing，and security.

（2）Saving power，same as the IEEE 8O2.15.4 standard.

（3）Self organizing and maintenance functions，maximally reducing the expense and maintenance cost of consumers.The ZigBee is for wireless sensor networks.Due to the variety and diversification of network topology，the network protocols appropriate for application occasions will be chosen.

The wireless network topology structure is divided into star network，mesh network，and mixed network（star+mesh network），as shown in Figure 5.A great number of network nodes，complicated dynamic variation of network architecture，characterize the wireless sensor network topology structure.Here we compare three basic network topology structures and choose the topology structure more appropriate for large-scale sensor networks.

The basic star network topology structure（Figure 5（a））is a single-hop system，for instance，the traditional wireless network.Abase station node could be a PC，a Personal Digital Assistant（PDA），special control equipment，an embedded network server，or other gateway communicating with high data rate equipment.Terminal nodes in the network could be different.It can be seen easily that the star network topology is not feasible when the sensor nodes are far from the base station node or distributed in a large scale.That's because the energy of the sensor node is limited.If node need to ensure the Signal to Noise Ratio（SNR）value for correctly receiving data，the sensor node must transmit data with more power to the base station nodes.Therefore，its energy will soon be exhausted.

The mesh network topology is a multi-hop network system，where allthe wireless sensor nodes are same.They can directly communicate with each other.Each time the network will choose one or more routings to carry out multi-hop transmission to transmit data information to base station（Figure 5（b））.

Each sensor node on the mesh network has more than one route to reach base station node，therefore，it has better fault tolerance capacity.Besides，such multiple-hop system replaces single-hop distance with multi-hop distance and decreases the required source sensor transmitting power.On the other hand，due to the huge number of sensor network nodes and relay nodes，which are distributed randomly，it is very difficult to search multi-hop router and carry out router maintenance and repair.Meantime，sensor nodes must listen the information and variation on some route in the network to effectively transmit data.Therefore，it increases the energy consumption and decreases the network life.

The mixed network topology（Figure 5（c））intends to combine the advantages including the simplicity of the star network and the multi-hop transmission and self-healing from the mesh network.The hierarchical network structure is typical in the mixed network and more appropriate for wireless sensor network applications with many nodes.In the hierarchical network the nearby sensors form independent clusters through designation by base station nodes or self-organizing.Each cluster chooses one cluster head node.Sensor nodes，included in the cluster through mechanism arranged by cluster head，transmit the collected information to cluster head.The cluster head will converge received information and forward it to base station nodes.In such hierarchical network it is unequalamong sensor nodes.On one hand，such method reduces the information redundancy through data convergence，decreases the transmitting power of source nodes through multi-hop，and reduces network energy consumption.On the other hand，the method decreases the transmission range of most of the nodes on the network through control of the cluster head，enhances the bandwidth reuse rate，free networknodes from keeping listening status，decreases expense on route and data process，and extends the life of network.

Though such hierarchical cluster-structure topology protocolis studied heavily in traditional Ad Hoc networks，the hierarchical protocol in Ad Hoc network topology structure doesn't consider such characteristics as huge number of nodes，limited nodes energy，arithmetic capacity，and more.Therefore，such protocol cannot be directly applied to wireless sensor networks.Based on the conventional Ad Hoc network protocol，researchers formulate the network topology protocol aiming to all sorts of problems in sensor networks，and the topology protocol is more practical and target-oriented.Typical protocols adopting such mixed network layer topology structure are Low Energy Adaptive Clustering Hierarchy（LEACH），Power-efficient Gathering in Sensor Information System（PEGAGIS），and Threshold Sensitive Energy Efficient Sensor Network Protocol（TEEN）[8，9].

In one word，the network topology structure is decided by the location of network nodes to base station，and the different wireless parameters in the network.The star network has the advantage when the base station is close to the sensor nodes，or even located in between while the distribution of network is rather small.At the same time，the mesh and mixed layer networks structures are more suitable choices if the base station is located far and the sensor nodes are scattered.

It can be seen that though the mesh topology structure has strong robustness，the nodes architecture and network realization become complicated especially for the sensor network with mobile nodes，because of the clear location requirement of sensor nodes and high route expenses.Therefore，in order to save power，simplify sensor network setting up and sensor nodes，most of the sensor routing study is based on the hierarchical network structure.

5 ZigBee Application Presentation Examples

Utilizing Zig Bee wireless sensor networks，Beijing University of Post and Telecommunications has set up an environment monitoring system.

The system takes the environment on campus as the monitoring subject，randomly arranges sensor data collection nodes to sense and collect live changes of environment parameters as temperature and light strength.

The collecting nodes through microprocessor collect data information，and transmit data to sensor gateway convergence nodes through antenna transmission module.The gateway nodes communicate with computers and the communication software enables live display of environment information.The hardware adopts sensor data collection nodes and gateway convergence nodes from Crossbow Company，and complies with the Zig Bee industry standard.The software platform adopts TinyOS software programming platform and integrates hardware to demonstrate the development.

6 Conclusion

The IEEE 8O2.15.4 protocol standard is mainly for low rate wireless personal area network and wireless sensor network applications.It is the main supportive technology for the Zig Bee physicallayer and the MAC layer.Because such standard adopts simple physical and MAC layer protocols，it is characterized by low power consumption and low cost.Therefore，the Zig Bee is more suitable for applications such as industry monitoring，sensor networks，and home monitoring.Meantime，the layered network structure can further save power in wireless sensor networks when the base station is far and sensor nodes are scattered.