Someone translated ZigBee into "ZigBee", one of the standards of Wireless Personal Area Networks (Wireless Personal Area Networks, WPAN), which was announced on June 27, 2005. In addition to the logical link control (Logic Link Control, LLC) layer, media access control (Media Access Control, MAC) layer, and physical (Physical, PHY) layer using the IEEE 802.15.4 standard published in October 2003, ZigBee The standards stipulate the standards of the application layer and the network layer, and the security encryption service standards of the MAC, application layer, and network layer.
The transmission range of WPAN is smaller than that of Wireless Local Area Networks (WLAN). Currently, there are three main open standards for WPAN technology that are commonly heard, namely Bluetooth, WiMedia, and ZigBee. Its standards organization and IEEE802 international standards The designated organization cooperates closely. The aforementioned three WPAN standards are equivalent to IEEE802.15.1, IEEE802.15. 3a and IEEE802.15.4. The relationship between the scope set by this public network standard and the scope set by the IEEE Standards Working Group can be illustrated from the hierarchical network structure of ZigBee and IEEE802.15.4 in Figure 1, ie IEEE802.15.4 is responsible for setting WPAN The LLC, MAC and PHY standards in the protocol, and ZigBee uses these three bottom layers to develop the network layer and application layer.
Among the three WPAN standards, Bluetooth is currently mainly used for short-distance wireless voice transmission; WiMedia is mainly used for short-distance wireless multimedia transmission; and ZigBee is more suitable for short-distance wireless transmission with low power consumption and low data volume, ZigBee The main killer applications include wireless sensors and controllers for home automation and building automation, such as wireless meter reading systems and wireless sensor networks. In the following, ZigBee is used to represent the standard generic name from PHY to application layer in Figure 1.
ZigBee network architecture supports multiple types
ZigBee provides three data transmission rates, namely 20kbps on 800MHz channel, 40kbps on 900MHz channel, and 250kbps on 2.4GHz channel. The center frequency and the number of channels of these three ZigBee bands are 868MHz single channel used in Europe, 915MHz used in the Americas, which contains 10 channels, and 2.45GHz, which is commonly used in the world, contains 16 channels. ZigBee uses 16bit short addressing and 64bit extended addressing.
ZigBee can support up to 254 components network communication.
In terms of multiple access, ZigBee uses two modes of carrier frequency detection (Carrier Sense MulTIple Access-Collision Avoidance, CSMA-CA) and guaranteed time slot (Guaranteed TIme Slots, GTS), of which GTS is similarly reserved. Time domain multiple access (TIme Division MulTIple Access, TDMA). In the GTS mode, the ZigBee components or devices to be communicated can obtain the access right to the channel without going through the random competition mechanism in CSMA-CA, which is particularly useful for the situation of emergency message transmission, such as the alarm with ZigBee Sensing element.
In order to consider the diversification and price of ZigBee products, in the ZigBeee network architecture, there can be two kinds of devices, namely full-function devices (Full-Function Device, FDD) and reduced-function devices (Reduced-Function Device, RFD ). The software and hardware of FFD can be used as a network coordinator (Coordinator) of the entire WPAN, or a network coordinator of ZigBee cluster (Cluster), or a general component. FFD can communicate with RFD grasshopper FD, and RFD can only communicate with FFD, so RFD can be realized with the least hardware and software resources and memory.
ZigBee network architecture supports point-to-point and star-shaped, star-shaped can also be tree-shaped. In the star WPAN type, the FFD network coordinator acts as a controller, which is responsible for starting or terminating the communication between ZigBee components, and is responsible for selecting certain main WPAN parameters, and also acts as a route between components within the network. Function; star type is especially suitable for home or building automation network applications with a central controller. The point-to-point type may be a mesh type. In the point-to-point WPAN type, any ZigBee component can directly communicate with any ZigBee component in the transmission range without going through the network coordinator. The point-to-point type is particularly suitable for wireless network sensor applications, and can realize AdHoc Networks or Mesh Networks with Multihop. In a tree or mesh configuration, you can use the ZigBee router to expand the network.
The physical layer works around the transceiver
The frequency bands used by the ZigBee physical layer are 868-868.6MHz, 902-928MHz, and 2400-2483.5MHz. No license is required. There are 27 channels in these three frequency bands. The main tasks of the physical layer are to activate and deactivate the wireless transmission receiver, transmit and receive data, use channel selection, do signal energy detection on the current channel, data modulation transmission and reception demodulation, and channel occupancy assessment (CCA ), And perform link quality indication (LQI) on the received packets. ZigBee physical layer frame format, where SHR stands for Synchronization Header, SFD stands for Start-of-frame Delimiter, PHR stands for PHY Header, and PSDU stands for The physical layer service protocol data unit (PHY Service Data Unit), and the preamble is used for synchronization. In addition to the standardized protocol structure in the physical layer, the digital modulation and spread spectrum methods are as follows.
For the 2.4GHz frequency band, Offset QPSK (Pulse-Shaped O-QPSK) and orthogonal spread spectrum coding after pulse shaping are used for data modulation. The spread chip rate (Chip) rate is 2Mchips / s, with a total of 16 positive Crossover frequency sequence. The two-bit data is first converted into bit-to-symbol (Bit-to-Symbol), and then converted from symbol to chip, and then input to Pulse-Shaped O-QPSK modulator (O-QPSK Mod) Complete the action of spread spectrum and modulation.
For the 868 / 915MHz band, BPSK and 15-Chip m-Sequence spread-spectrum differential coding are used for data modulation. The chip rate is 0.3 Mchips / s and there are 2 spread-spectrum sequences. The relationship between modulation and spread spectrum in this 868 / 915MHz frequency band, where the two-bit data is first encoded by a differential encoder (Differential Encoder), after bit-to-chip conversion, and then input to the BPSK Modulator (BPSK Mod ), Complete the spread frequency and modulation actions. The differential encoder can be implemented by an exclusive-or circuit.
Summarizing the data modulation and transmission rate of ZigBee in different frequency bands, it can be seen that the transmission bit rate is the highest in the 2.4GHz band ZigBee. However, due to the characteristics of electromagnetic waves, the signal in the 2.4GHz band has a shorter transmission distance than the signal in the 868 / 915MHz band.
In the above 2.4GHz frequency band data modulation, Pulse-Shaped O-QPSK is equivalent to Minimum-Shift Keying (MSK), and the pulse used by it is as follows: where is one of the spread spectrum The chip time is long.
In the above 868 / 915M Hz band data modulation, the following Raised Cosine Pulse is used to represent each chip bit on the fundamental frequency:
The accuracy of ZigBee PHY layer modulation is determined by the error vector size (EVM). In 1000 chips, EVM must be less than 35%. In addition, the ZigBee transmitter must be able to transmit energy as small as -3dBm. In order to reduce the interference between ZigBee components, ZigBee transmitters must transmit less energy under acceptable conditions.
ZigBee MAC provides a reliable link. The function of this MAC layer is to provide a reliable link between two MAC peers. The main work of ZigBee MAC is to support the connection and disconnection of communication links, generate the indicator signal of the network coordinator component (Beacon), synchronize the indicator signal, use CSMA-CA, and process and maintain GTS. In addition, the encryption sublayer of MAC can support communication encryption.
There are three different modes of MAC's multiple slave access protocol, namely slotted CSMA-CA mode, unslotted CSMA-CA mode, and GTS mode, where GTS is mainly used in emergency situations or The transmission of periodic signals. MAC uses four types of time frame formats, namely index time frame, data time frame, acknowledgment (ACK) time frame, and command (Command) time frame. CSMA-CA is not used when the indicator time frame and ACK time frame are used.
In a star-shaped WPAN network with usage indicators, when a ZigBee component wants to transmit data to the network coordinator, it will first detect the network indicators. If the device detects the indicator, it will act in synchronization with the timeout box. After synchronization is reached, the ZigBee device will activate the time slot to transmit data to the network coordinator. In a network that does not use indicators, when a ZigBee component wants to transmit data to the network coordinator, it will start a non-time slot to transmit data to the network coordinator. After the above data transmission, when the network coordinator receives the data packet, it will return an ACK to the ZigBee component. When the network coordinator has data to transmit to the ZigBee component, it is also similar to the aforementioned protocol procedure.
In point-to-point WPAN, if a ZigBee device wants to transmit data to other ZigBee devices, it must periodically detect the received signal and use the non-time slot method to obtain channel usage rights or to obtain synchronization between the devices.
The general MAC time frame format, where MHR represents the MAC head end and MFR represents the MAC tail end. MHR contains the frame control bar (Frame Control), sequence number (Sequence Number), target ZigBee component ID (Destination ID), target ZigBee component address (Destination Address), transmission side ZigBee component ID (Source ID), and transmission ZigBee component address (Source Address).
The network layer includes NLDE and NLMEZigBee. The network layer is divided into two parts, namely the Network Layer Data Entity (NLDE) and the Network Layer Management Entity (NLME). NLDE is responsible for generating network layer protocol data (Network Level PDU) and routing required for transmission. The NLME work includes being responsible for setting the ZigBee component as a network coordinator, or joining or leaving an existing PAN, starting WPAN addressing, finding nearby ZigBee components, finding a routing path, and MAC receiving start control.
The time frame format of the ZigBee network layer mainly includes the network head (NWK Header) and the network load (NWK Payload). The radius column in the network head defines the range of data transmission allowed in the network load. If a ZigBee component is connected to this frame, the Radius value is reduced by one. If the Radius value is reduced to zero, the maximum transmission range is reached. All ZigBee component network layers must provide functions to join and leave WPAN. The ZigBee network coordinator and router must provide the following functions: accept commands from the MAC or application layer to allow other components to join and leave WPAN, specify network logical addresses, and maintain a list of nearby ZigBee components.
In addition, the network layer is also responsible for the path and route finding function between the ZigBee transmission element and the target element. The application layer initiates the discovery process by searching to find peripheral components. The ZigBee application layer currently only defines 240 components with numbers 1 to 240, while 241 to 254 are reserved for future use. In addition, No. 0 and No. 255 are given to other interfaces. The communication foundation of the ZigBee application layer is composed of the categories (Profile) developed by ZigBee product suppliers. A certain category provides solutions to ZigBee's specific application technology needs. The categories are related to the communication information, message format, and processing actions of the ZigBee component application, which are used to generate interoperable decentralized applications, including command transmission between different components, data requests, and processing commands and requests. The cluster of ZigBee components is identified by 8bit Cluster ID. In a category, the cluster ID is unique.
The ZigBee application layer mainly communicates with the network layer through the application layer support sublayer (APS). In APS, there are two software modules: APS data entity (APSDE) and APS control entity (APSME). Among them, APSDE provides data transmission services between different components in the same WPAN, while APSME provides inter-component discovery (Discovery) and connection (Binding) services, and APSME also maintains an APS Information Base (AIB Information Base, AIB). Object database.
The ZigBee component finds other ZigBee components (Device Discovery) and services provided by these components (Service Discovery) by sending a Query to start the discovery process (Discovery Process). The ZigBee application layer uses two Device Discovery requirements formats: 16-bit IEEE address requirements and 64-bitI NWK address requirements. Among them, the IEEE address request is sent under the known NWK address by Unicast, and the NWK address requirement is to send the IEEE address in the data load of the packet by broadcast. Corresponding to the discovery request message, there may be different responses depending on the request message and the characteristics of the requested component. If it is a general ZigBee component, respond to the relevant IEEE or NWK address; if it is a ZigBee network coordinator or router, it responds to the IEEE or NWK address of the network coordinator or router itself, and others connected to the network coordinator or router The IEEE or NWK address of the component. Service Discovery provides ZigBee components to determine the services provided by other ZigBee components. The corresponding response types and methods are similar to Device Discovery. They are also divided into single transmission and broadcast, but the query form and response content are different.
The ZigBee application layer also provides a connection function between components, that is, a logical connection is generated between complementary application components. This connection is recorded in a connection table, which is particularly useful for related control elements or sensors.
After completing Device Discovery and Service Discovery, ZigBee components can establish an association and begin to send commands to a specific ZigBee address. There are three ways in addressing, namely direct addressing, indirect addressing, and broadcast addressing. The ZigBee main control component obtains the address and Cluster ID of the destination component from Device Discovery and Service Discovery, and can use direct addressing. There is a record of direct addressing in the connection table of the ZigBee network coordinator. APSDE can be used for indirect addressing through connection. This is particularly useful for simple sensing elements that want to save energy and memory. In addition, you can use broadcast addressing to broadcast the application to all components. In this case, the destination address is a 16-bit network broadcast address and the broadcast flag in the APS time frame must be set.
There is also a software module called ZigBee Device Object (ZDO) in the application layer, which serves as the interface between application objects, component application classes and APS. ZDO is located between the application architecture and the APS, and is responsible for starting the APS, network layer, and security services, and is responsible for combining the settings from the user, setting information to determine and execute discovery, security control, network control, and connection. This interface also provides address management for component discovery, connection and security functions in the application layer.
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