Wireless connectivity of a vast number of industrial and home applications has modest transmission data requirements, but demands reliable and secure communication using simple low-cost and low-power radio systems. In the quest for high-bandwidth, multimedia-capable wireless networks, the need for cost and power-effective radio solutions for this vast number of fairly simple applications was only recently addressed by a standardized technology.
The IEEE 802.15.4 standard and ZigBee wireless technology are designed to satisfy the market's need for a low-cost, standard-based and flexible wireless network technology, which offers low power consumption, reliability, interoperability and security for control and monitoring applications with low to moderate data rates.
The complexity and cost of the IEEE802.15.4/Zigbee-compliant devices are intended to be low and scalable (application dependent) in order to enable broad commercial adaptation in cost-sensitive applications. In addition, the compliant system implementations will enable long battery life by using the power-saving features at the physical, MAC and network layers specified by this standard.
In this respect, the implementation of the physical layer of the IEEE 802.15.4 standard, including the RF, IF and de-modulation must be optimized to meet the challenging low-cost and low-power targets.
1.1 Evolution of LR-WPAN Standardization
The cellular network was a natural extension of the wired telephony network that became pervasive during the mid-20th century. As the need for mobility and the cost of laying new wires increased, the motivation for a personal connection independent of location to that network also increased. Coverage of large area is provided through (1-2km) cells that cooperate with their neighbors to create a seemingly seamless network. Examples of standards are GSM, IS-136, IS-95. Cellular standards basically aimed at facilitating voice communications throughout a metropolitan area.
During the mid-1980s, it turned out that an even smaller coverage area is needed for higher user densities and the emergent data traffic. The IEEE 802.11 working group for WLANs is formed to create a wireless local area network standard.
Whereas IEEE 802.11 was concerned with features such as Ethernet matching speed, long-range(100m), complexity to handle seamless roaming, message forwarding, and data throughput of 2-11Mbps, WPANs are focused on a space around a person or object that typically extends up to 10m in all directions. The focus of WPANs is low-cost, low power, short range and very small size. The IEEE802.15 working group is formed to create WPAN standard. This group has currently defined three classes of WPANs that are differentiated by data rate, battery drain and quality of service (QoS). The high data rate WPAN (IEEE 802.15.3) is suitable for multi-media applications that require very high QoS. Medium rate WPANs (IEEE802.15.1/Blueetooth) will handle a variety of tasks ranging from cell phones to PDA communications and have QoS suitable for voice communications. The low rate WPANs (IEEE 802.15.4/LR-WPAN) is intended to serve a set of industrial, residential and medical applications with very low power consumption and cost requirement not considered by the above WPANs and with relaxed needs for data rate and QoS. The low data rate enables the LR-WPAN to consume very little power.
1.2 ZigBee and IEEE 802.15.4
ZigBee technology is a low data rate, low power consumption, low cost; wireless networking protocol targeted towards automation and remote control applications. IEEE 802.15.4 committee started working on a low data rate standard a short while later. Then the ZigBee Alliance and the IEEE decided to join forces and ZigBee is the commercial name for this technology.
ZigBee is expected to provide low cost and low power connectivity for equipment that needs battery life as long as several months to several years but does not require data transfer rates as high as those enabled by Bluetooth. In addition, ZigBee can be implemented in mesh networks larger than is possible with Bluetooth. ZigBee compliant wireless devices are expected to transmit 10-75 meters, depending on the RF environment and the power output consumption required for a given application, and will operate in the unlicensed RF worldwide (2.4GHz global, 915MHz in USA OR 868MHz in Europe). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz.
IEEE and ZigBee Alliance have been working closely to specify the entire protocol stack. IEEE 802.15.4 focuses on the specification of the lower two layers o f the protocol (physical and data link layer). On the other hand, ZigBee Alliance aims to provide the upper layers of the protocol stack (from network to the application layer) for interoperable data networking, security services and a range of wireless home and building control solutions, provide interoperability compliance testing, marketing of the standard, advanced engineering for the evolution of the standard. This will assure consumers to buy products from different manufacturers with confidence that the products will work together.
IEEE 802.15.4 is now detailing the specification of PHY and MAC by offering building blocks for different types of networking known as star, mesh, and cluster tree. Network routing schemes are designed to ensure power conservation, and low latency through guaranteed time slots. A unique feature of ZigBee network layer is communication redundancy eliminating single point of failure in mesh networks. Key features of PHY include energy and link quality detection, clear channel assessment for improved coexistence with other wireless networks.
1.3 Why is ZigBee Needed
There are a multitude of standards like Bluetooth and WiFi that address mid to high data rates for voice, PC LANs, video, etc. However, up till now there hasn't been a wireless network standard that meets the unique needs of sensors and control devices. Sensors and controls don't need high bandwidth but they do need low latency and very low energy consumption for long battery lives and for large device arrays.
There are a multitude of proprietary wireless systems manufactured today to solve a multitude of problems that don't require high data rates but do require low cost and very low current drain. These proprietary systems were designed because there were no standards that met their application requirements. These legacy systems are creating significant interoperability problems with each other and with newer technologies.
ZigBee is poised to become the global control/sensor network standard. It has been designed to provide the following features:
Low power consumption, simply implemented
Users expect batteries to last many months to years! Consider that a typical single family house has about 6 smoke/CO detectors. If the batteries for each one only lasted six months, the home owner would be replacing batteries every month!
In contrast to Bluetooth, which has many different modes and states depending upon your latency and power requirements, ZigBee/IEEE 802.15.4 has two major states: active (transmit/receive) or sleep. The application software needs to focus on the application, not on which power mode is optimum for each aspect of operation.
Even mains powered equipment needs to be conscious of energy. ZigBee devices will be more ecological than their predecessors saving megawatts at it full deployment. Consider a future home that has 100 wireless control/sensor devices,
o Case 1: 802.11 Rx power is 667 mW (always on)@ 100
Devices/home & 50,000 homes/city = 3.33 megawatts
o Case 2: 802.15.4 Rx power is 30 mW (always on)@ 100
Devices/home & 50,000 homes/city = 150 kilowatts
o Case 3: 802.15.4 power cycled at .1% (typical duty cycle) = 150 watts
Low cost to the users means low device cost, low installation cost and low maintenance.
o ZigBee devices allow batteries to last up to years using primary cells
(low cost) without any chargers (low cost and easy installation). ZigBee's simplicity allows for inherent configuration and redundancy of network devices provides low maintenance.
High density of nodes per network
o ZigBee's use of the IEEE 802.15.4 PHY and MAC allows networks to handle any number of devices. This attribute is critical for massive sensor arrays and control networks.
Simple protocol, global implementation
o ZigBee's protocol code stack is estimated to be about 1/4th of Bluetooth's or 802.11's. Simplicity is essential to cost, interoperability, and maintenance. The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868 MHz band in Europe, the 915 MHz band in N America, Australia, etc; and the 2.4 GHz band is now recognized to be a global band accepted in almost all countries.
1.4 ZigBee vs. Bluetooth
ZigBee looks rather like Bluetooth but is simpler, has a lower data rate and spends most of its time snoozing. This characteristic means that a node on a ZigBee network should be able to run for six months to two years on just two AA batteries.
The operational range of ZigBee is 10-75m compared to 10m for Bluetooth
(without a power amplifier).
ZigBee sits below Bluetooth in terms of data rate.
The data rate of ZigBee is
250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz whereas that of Bluetooth is 1Mbps.
ZigBee uses a basic master-slave configuration suited to static star networks of many infrequently used devices that talk via small data packets. It allows up to 254 nodes. Bluetoothâ„¢s protocol is more complex since it is geared towards handling voice, images and file transfers in ad hoc networks. Bluetooth devices can support scatter nets of multiple smaller non-synchronized networks (piconets). It only allows up to 8 slave nodes in a basic master-slave piconet set-up.
When ZigBee node is powered down, it can wake up and get a packet in around 15msec whereas a Bluetooth device would take around 3sec to wake up and respond.
ZigBee and Bluetooth are two solutions for two different application areas. Bluetooth has addressed a voice application by embodying a fast frequency hopping system with a master slave protocol. ZigBee has addressed sensors, controls, and other short message applications by embodying a direct sequence system with a star or peer to peer protocols.
1.5 Wireless technology comparison chart
Wi-Fi Bluetooth WiMAX WiMedia ZigBee
Primary Use Laptop networking Cable replacement, cellphones Wireless broadband Internet access Multimedia consumer electronics Sensor networks, industrial control
LAN type WLAN WPAN WMAN WPAN WPAN
IEEE 802.11n 802.15.1 802.16 802.15.3 802.15.4
Standards Wi-Fi Alliance Bluetooth SIG WiMAX Forum WiMedia Alliance ZigBee Alliance
URL wi-fi.org bluetooth.org wimaxforum.org wimedia.org zigbee.org
Range(m) 100m 10-100m 50km 4-10m 30-70m
Bands 2.4 GHz 2.4 GHz 2.5 GHz, 3.5 GHz 3.1-10.6 GHz 2.4 GHz, 866/900 MHz
Data Speeds 11-54 Mbps 1 Mbps 280 Mbps 110-480 Mbps 20-250Kbps
BOM (US$) 9 6 150 20 3
Battery Life Hours Days N/A Days-weeks Months-years
ZigBee / IEEE 802.15.4 WPLAN
2.0 ZigBee / IEEE 802.15.4 WPAN
The main features of this standard are network flexibility, low cost, very low power consumption, and low data rate in an adhoc self-organizing network among inexpensive fixed, portable and moving devices. It is developed for applications with relaxed throughput requirements which cannot handle the power consumption of heavy protocol stacks.
2.1 Components of WPAN
A ZigBee system consists of several components. The most basic is the device. A device can be a full-function device (FFD) or reduced-function device (RFD). A network shall include at least one FFD, operating as the PAN coordinator.
The FFD can operate in three modes: a personal area network (PAN) coordinator, a coordinator or a device. An RFD is intended for applications that are extremely simple and do not need to send large amounts of data. An FFD can talk to RFDs or FFDs while an RFD can only talk to an FFD.
2.2 Network Topologies
ZigBee supports 3 types of topologies - star topology, peer-to-peer topology and cluster tree topology.
2.2.1 Star Topology
In the star topology, the communication is established between devices and a single central controller, called the PAN coordinator. The PAN coordinator may be mains powered while the devices will most likely be battery powered. Applications that benefit from this topology include home automation, personal computer (PC) peripherals, toys and games.
After an FFD is activated for the first time, it may establish its own network and become the PAN coordinator. Each start network chooses a PAN identifier, which is not currently used by any other network within the radio sphere of influence. This allows each star network to operate independently.
2.2.2 Peer-to-peer Topology
In peer-to-peer topology, there is also one PAN coordinator. In contrast to star topology, any device can communicate with any other device as long as they
are in range of one another. A peer-to-peer network can be ad hoc, self-organizing and self-healing. Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory tracking would benefit from such a topology. It also allows multiple hops to route messages from any device to any other device in the network. It can provide reliability by multi path routing.
2.2.3 Cluster-tree Topology
Cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs and an RFD may connect to a cluster-tree network as a leave node at the end of a branch. Any of the FFD can act as a coordinator and provide synchronization services to other devices and coordinators. Only one of these coordinators however is the PAN coordinator.
The PAN coordinator forms the first cluster by establishing itself as the cluster head
(CLH) with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and broadcasting beacon frames to neighboring devices. A candidate device receiving a beacon frame may request to join the network at the CLH. If the PAN coordinator permits the device to join, it will add this new device as a child device in its neighbor list. The newly joined device will add the CLH as its parent in its neighbor list and begin transmitting periodic beacons such that other candidate devices may then join the network at that device. Once application or network requirements are met, the PAN coordinator may instruct a device to become the CLH of a new cluster adjacent to the first one. The advantage of this clustered structure is the increased coverage area at the cost of increased message latency.
2.3 ZigBee Architecture
ZigBee architecture comprises a PHY, which contains the radio frequency (RF) transceiver along with its low-level control mechanism, and a MAC sublayer that provides access to the physical channel for all types of transfer. The upper layers consists of a network layer, which provides network configuration, manipulation, and message routing, and application layer, which provides the intended function of a device. An IEEE 802.2 logical link control (LLC) can access the MAC sublayer through the service specific convergence sublayer (SSCS). Chapter 3 describes the physical layer of IEEE 802.15.4. Chapter 4 explains the MAC layer of IEEE 802.15.4. Chapter 6 gives the routing mechanisms that are going to be used in the ZigBee.
PHYSICAL LAYER 3
3.0 IEEE 802.15.4 PHY
The PHY provides two services: the PHY data service and PHY management service interfacing to the physical layer management entity (PLME). The PHY data service enables the transmission and reception of PHY protocol data units (PPDU) across the physical radio channel.
The features of the PHY are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium.
The standard offers two PHY options based on the frequency band. Both are based on direct sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz. The higher data rate at 2.4GHz is attributed to a higher-order modulation scheme. Lower frequencies provide longer range due to lower propagation losses. Low rate can be translated into better sensitivity and larger coverage area. Higher rate means higher throughput, lower latency or lower duty cycle. This information is summarized in the table below.
There is a single channel between 868 and 868.6MHz, 10 channels between 902.0
and 928.0MHz, and 16 channels between 2.4 and 2.4835GHz as shown in
Several channels in different frequency bands enables the ability to relocate within spectrum. The standard also allows dynamic channel selection, a scan function that steps through a list of supported channels in search of beacon, receiver energy detection, link quality indication, channel switching.
Receiver sensitivities are -85dBm for 2.4GHz and -92dBm for 868/915MHz. The advantage of 6-8dB comes from the advantage of lower rate. The achievable range is a function of receiver sensitivity and transmits power.
The maximum transmit power shall conform with local regulations. A compliant device shall have its nominal transmit power level indicated by the PHY parameter, phyTransmitPower.
Figure 3.2: Operating frequency bands.
3.1 Receiver Energy Detection (ED)
The receiver energy detection (ED) measurement is intended for use by a network layer as part of channel selection algorithm. It is an estimate of the received signal power within the bandwidth of an IEEE 802.15.4 channel. No attempt is made to identify or decode signals on the channel. The ED time should be equal to 8 symbol periods.
The ED result shall be reported as an 8-bit integer ranging from 0x00 to 0xff. The minimum ED value (0) shall indicate received power less than 10dB above the specified receiver sensitivity. The range of received power spanned by the ED values shall be at least 40dB. Within this range, the mapping from the received power in decibels to ED values shall be linear with an accuracy of + - 6dB.
3.2 Link Quality Indication (LQI)
Upon reception of a packet, the PHY sends the PSDU length, PSDU itself and link quality (LQ) in the PD-DATA. indication primitive. The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using receiver ED, a signal-to-noise estimation or a combination of these methods. The use of LQI result is up to the network or application layers.
The LQI result should be reported as an integer ranging from 0x00 to 0xff. The minimum and maximum LQI values should be associated with the lowest and highest quality IEEE 802.15.4 signals detectable by the receiver and LQ values should be uniformly distributed between these two limits.
3.3 Clear Channel Assessment (CCA)
The clear channel assessment (CCA) is performed according to at least one of the following three methods:
Â¢ Energy above threshold. CCA shall report a busy medium upon detecting any energy above the ED threshold.
Â¢ Carrier sense only. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4. This signal may be above or below the ED threshold.
Â¢ Carrier sense with energy above threshold. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4 with energy above the ED threshold.
3.4 PPDU Format
The PPDU packet structure is illustrated in Figure 3.3. Each PPDU packet consists of the following basic components:
Â¢ SHR, which allows a receiving device to synchronize and lock into the bit stream
Â¢ PHR, which contains frame length information
Â¢ A variable length payload, which carries the MAC sub layer frame.
Figure 3.3: Format of the PPDU.
MEDIA ACCESS CONTROL LAYER
MAC LAYER 4
4.0 IEEE 802.15.4 MAC
The MAC (Media access control) layer sub layer provides two services: the MAC data service and the MAC management service interfacing to the MAC sub layer management entity (MLME) service access point (SAP) (MLMESAP). The MAC data service enables the transmission and reception of MAC protocol data units
(MPDU) across the PHY data service.
The features of MAC sub layer are beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association and disassociation.
4.1 Frame Structure
The frame structures have been designed to keep the complexity to a minimum while at the same time making them sufficiently robust for transmission on a noisy channel. Each successive protocol layer adds to the structure with layer-specific headers and footers.
The IEEE 802.15.4 MAC defines four frame structures
Â¢ A beacon frame, used by a coordinator to transmit beacons.
Â¢ A data frame, used for all transfers of data.
Â¢ An acknowledgment frame, used for confirming successful frame reception.
Â¢ A MAC command frame, used for handling all MAC peer entity control transfers.
The data frame is illustrated below:
The Physical Protocol Data Unit is the total information sent over the air. As shown in the illustration above the Physical layer adds the following overhead:
Preamble Sequence 4 Octets Start of Frame Delimiter 1 Octet Frame Length 1 Octet
The MAC adds the following overhead:
Frame Control 2 Octets Data Sequence Number 1 Octet Address Information 4 â€œ 20 Octets Frame Check Sequence 2 Octets
In summary the total overhead for a single packet is therefore 15 -31 octets (120 bits); depending upon the addressing scheme used (short or 64 bit addresses). Please note that these numbers do not include any security overhead.
4.2 Channel access and Addressing
Two channel-access mechanisms are implemented in 802.15.4. For a non-beacon network, a standard ALOHA CSMA-CA (carrier-sense medium-access with collision avoidance) communicates with positive acknowledgement for successfully received packets. In a beacon-enabled network, a superframe structure is used to control channel access. The superframe is set up by the network coordinator to transmit beacons at predetermined intervals (multiples of 15.38ms, up to 252s) and provides
16 equal-width time slots between beacons for contention-free channel access in each time slot. The structure guarantees dedicated bandwidth and low latency. Channel access in each time slot is contention-based. However, the network coordinator can dedicate up to seven guaranteed time slots per beacon interval for quality of service.
Device addresses employ 64-bit IEEE and optional 16-bit short addressing. The address field within the MAC can contain both source and destination address information (needed for peer-to-peer operation). This dual address information is used in mesh networks to prevent a single point of failure within the network.
4.3 Super Frame Structure
The LR-WPAN standard allows the optional use of a superframe structure. The format of the superframe is defined by the coordinator. The superframe is bounded by network beacons, is sent by the coordinator (See Figure 4) and is divided into 16 equally sized slots. The beacon frame is transmitted in the first slot of each superframe. If a coordinator does not wish to use a superframe structure it may turn off the beacon transmissions. The beacons are used to synchronize the attached devices, to identify the PAN, and to describe the structure of the superframes. Any device wishing to communicate during the contention access period (CAP) between two beacons shall compete with other devices using a slotted CSMA-CA mechanism. All transactions shall be completed by the time of the next network beacon.
For low latency applications or applications requiring specific data bandwidth, the PAN coordinator may dedicate portions of the active superframe to that application. These portions are called guaranteed time slots (GTSs). The guaranteed time slots comprise the contention free period (CFP), which always appears at the end of the active superframe starting at a slot boundary immediately following the CAP, as shown in Figure 5. The PAN coordinator may allocate up to seven of these GTSs and a GTS may occupy more than one slot period. However, a sufficient portion of the CAP shall remain for contention based access of other networked devices or new devices wishing to join the network. All contention based transactions shall be complete before the CFP begins. Also each device transmitting in a GTS shall ensure that its transaction is complete before the time of the next GTS or the end of the CFP.
4.4 CSMA-CA Algorithm
If superframe structure is used in the PAN, then slotted CSMA-CA shall be used. If beacons are not being used in the PAN or a beacon cannot be located in a beacon- enabled network, unslotted CSMA-CA algorithm is used. In both cases, the algorithm is implemented using units of time called backoff periods, which is equal to aUnitBackoffPeriod symbols.
In slotted CSMA-CA channel access mechanism, the backoff period boundaries of every device in the PAN are aligned with the superframe slot boundaries of the PAN coordinator. In slotted CSMA-CA, each time a device wishes to transmit data frames during the CAP, it shall locate the boundary of the next backoff period. In unslotted CSMA-CA, the backoff periods of one device do not need to be synchronized to the backoff periods of another device.
Each device has 3 variables: NB, CW and BE. NB is the number of times the CSMA-CA algorithm was required to backoff while attempting the current transmission. It is initialized to 0 before every new transmission. CW is the contention window length, which defines the number of backoff periods that need to be clear of activity before the transmission can start. It is initialized to 2 before each transmission attempt and reset to 2 each time the channel is assessed to be busy. CW is only used for slotted CSMA-CA. BE is the backoff exponent, which is related to how many backoff periods a device shall wait before attempting to assess the channel. Although the receiver of the device is enabled during the channel assessment portion of this algorithm, the device shall discard any frames received during this time.
In slotted CSMA-CA, NB, CW and BE are initialized and the boundary of the next backoff period is located. In unslotted CSMA-CA, NB and BE are initialized (step1). The MAC layer shall delay for a random number of complete backoff periods in the range 0 to 2BE - 1 (step 2) then request that PHY performs a CCA (clear channel assessment) (step 3). The MAC sublayer shall then proceed if the remaining CSMA-CA algorithm steps, the frame transmission, and any acknowledgement can be completed before the end of the CAP. If the MAC sublayer cannot proceed, it shall wait until the start of the CAP in the next
superframe and repeat the evaluation.
If the channel is assessed to be busy (step 4), the MAC sublayer shall increment both NB and BE by one, ensuring that BE shall be no more than aMaxBE. In slotted CSMA-CA, CWcan also be reset to 2. If the value of NB is less than or equal to macMaxCSMABackoffs, the CSMA-CA shall return to step 2, else the CSMA-CA shall terminate with a Channel Access Failure status.
If the channel is assessed to be idle (step 5), in a slotted CSMA-CA, the MAC sublayer shall ensure that contention window is expired before starting transmission. For this, the MAC sublayer first decrements CW by one. If CW is not equal to 0, go to step 3 else start transmission on the boundary of the next backoff period. In the unslotted CSMA-CA, the MAC sublayer start transmission immediately if the channel is assessed to be idle.
4.5 Data Transfer model
Three types of data transfer transactions exist: from a coordinator to a device, from a device to a coordinator and between two peer devices. The mechanism for each of these transfers depend on whether the network supports the transmission of beacons. When a device wishes to transfer data in a nonbeacon-enabled network, it simply transmits its data frame, using the unslotted CSMA-CA, to the coordinator. There is also an optional acknowledgement at the end as shown in Figure 4.3.
Figure 4.3: Communication to a coordinator in a beacon-enabled network.
When a device wishes to transfer data to a coordinator in a beacon-enabled network, it first listens for the network beacon. When the beacon is found, it synchronizes to the superframe structure. At the right time, it transmits its data frame, using slotted CSMA-CA, to the coordinator. There is an optional acknowledgement at the end as shown in Figure 4.4.
Figure 4.4: Communication to a coordinator in a non beacon-enabled network.
The applications transfers are completely controlled by the devices on a PAN rather than by the coordinator. This provides the energy-conservation feature of the ZigBee network. When a coordinator wishes to transfer data to a device in a beacon-enabled network, it indicates in the network beacon that the data message is pending. The device periodically listens to the network beacon, and if a message is pending, transmits a MAC command requesting this data, using slotted CSMA- CA. The coordinator optionally acknowledges the successful transmission of this packet. The pending data frame is then sent using slotted CSMA-CA. The device acknowledged the successful reception of the data by transmitting an acknowledgement frame. Upon receiving the acknowledgement, the message is removed from the list of pending messages in the beacon as shown in Figure 4.5.
Figure 4.5: Communication from a coordinator in a beacon-enabled network.
When a coordinator wishes to transfer data to a device in a nonbeacon-enabled network, it stores the data for the appropriate device to make contact and request data. A device may make contact by transmitting a MAC command requesting the data, using unslotted CSMA-CA, to its coordinator at an application-defined rate. The coordinator acknowledges this packet. If data are pending, the coordinator transmits the data frame using unslotted CSMA-CA. If data are not pending, the coordinator transmits a data frame with a zero-length payload to indicate that no data were pending. The device acknowledges this packet as shown in Figure 4.6.
Figure 4.6: Communication from a coordinator in a non beacon-enabled network.
In a peer-to-peer network, every device can communicate with any other device in its transmission radius. There are two options for this. In the first case, the node will listen constantly and transmit its data using unslotted CSMA-CA. In the second case, the nodes synchronize with each other so that they can save power
4.6 MAC Layer Security
When security of MAC layer frames is desired, ZigBee uses MAC layer security to secure MAC command, beacon, and acknowledgement frames. ZigBee may secure messages transmitted over a single hop using secured MAC data frames, but for multi-hop messaging ZigBee relies upon upper layers (such as the NWK layer) for security. The MAC layer uses the Advanced Encryption Standard (AES) as its core cryptographic algorithm and describes a variety of security suites that use the AES algorithm. These suites can protect the confidentiality, integrity, and authenticity of MAC frames. The MAC layer does the security processing, but the upper layers, which set up the keys and determine the security levels to use, control this processing. When the MAC
layer transmits (receives) a frame with security enabled, it looks at the destination (source) of the frame, retrieves the key associated with that destination (source), and then uses this key to process the frame according to the security suite designated for the key being used. Each key is associated with a single security suite and the MAC frame header has a bit that specifies whether security for a frame is enabled or disabled.
When transmitting a frame, if integrity is required, the MAC header and payload data are used in calculations to create a Message Integrity Code (MIC) consisting of 4, 8, or 16 octets. The MIC is right appended to the MAC payload. If confidentiality is required, the MAC frame payload is also left appended with frame and sequence counts (data used to form a nonce). The nonce is used when encrypting the payload and also ensures freshness to prevent replay attacks. Upon receipt of a frame, if a MIC is present, it is verified and if the payload is encrypted, it is decrypted. Sending devices will increase the frame count with every message sent and receiving devices will keep track of the last received count from each sending device. If a message with an old count is detected, it is flagged with a security error. The MAC layer security suites are based on three modes of operation. Encryption at the MAC layer is done using AES in Counter (CTR) mode and integrity is done using AES in Cipher Block Chaining (CBC- MAC) mode . A combination of encryption and integrity is done using a mixture of CTR and CBC- MAC modes called the CCM mode.
NETWORK LAYER 5
5.0 NWK LAYER
The NWK layer associates or dissociates devices using the network coordinator, implements security, and routes frames to their intended destination. In addition, the NWK layer of the network coordinator is responsible for starting a new network and assigning an address to newly associated devices.
The NWK layer supports multiple network topologies including star, cluster tree, and mesh, all of which are shown in Figure 5.1
Figure 5.1: Network topologies
In a star topology, one of the FFD-type devices assumes the role of network coordinator and is responsible for initiating and maintaining the devices on the network. All other devices, known as end devices, directly communicate with the coordinator.
In a mesh topology, the ZigBee coordinator is responsible for starting the network and for choosing key network parameters, but the network may be extended through the use of ZigBee routers. The routing algorithm uses a request-response protocol to eliminate sub-optimal routing. Ultimate network size can reach 264 nodes (more than we'll probably need). Using local
addressing, you can configure simple networks of more than 65,000 (216) nodes, thereby reducing address overhead.
5.1 ZigBee Network Node
Â¢ Designed for battery powered or high energy savings
Â¢ Searches for available networks
Â¢ Transfers data from its application as necessary
Â¢ Determines whether data is pending
Â¢ Requests data from the network coordinator
Â¢ Can sleep for extended periods
5.2 Responsibilities of the ZigBee NWK layer
Â¢ Starting a network : The ability to successfully establish a new network.
Â¢ Joining and leaving a network: The ability to gain membership (join) or relinquish membership (leave) a network.
Â¢ Configuring a new device: The ability to sufficiently configure the stack for operation as required.
Â¢ Addressing: The ability of a ZigBee coordinator to assign addresses to devices joining the network.
Â¢ Synchronization within a network: The ability for a device to achieve synchronization with another device either through tracking beacons or by polling.
Â¢ Security: applying security to outgoing frames and removing security to terminating frames
Â¢ Routing: routing frames to their intended destinations.
The network layer builds upon the IEEE 802.15.4 MACâ„¢s features to allow extensibility of coverage. Additional clusters can be added; networks can be consolidated or split up.
5.3 Network Layer Security
The NWK layer also makes use of the Advanced Encryption Standard (AES). However, unlike the MAC layer, the security suites are all based on the CCM mode of operation. The CCM mode of operation is a minor modification of the CCM mode used by the MAC layer. It includes all of the capabilities of CCM and additionally offers encryption-only and integrity-only capabilities. These extra capabilities simplify the NWK layer security by eliminating the need for CTR and CBC-MAC modes. Also, the use of CCM in all security suites allows a single key to be used for different suites. Since a key is not strictly bound to a single security suite, an application has the flexibility to specify the actual security suite to apply to each NWK frame, not just whether security is enabled or disabled
When the NWK layer transmits (receives) a frame using a particular security suite it uses the Security Services Provider (SSP) to process the frame. The SSP looks at the destination (source) of the frame, retrieves the key associated with that destination (source), and then applies the security suite to the frame. The SSP provides the NWK layer with a primitive to apply security to outgoing frames and a primitive to verify and remove security from incoming frames. The NWK layer is responsible for the security processing, but the upper layers control the processing by setting up the keys and determining which CCM security suite to use for each frame. Similar to the MAC layer frame format, a frame sequence count and MIC may be added to secure a NWK frame.
ZigBee ROUTING MECHANISMS
ZigBee ROUTING MECHANISMS 6
6.0 ZigBee routing algorithm
ZigBee routing algorithm can be thought of a hierarchical routing strategy with table-driven optimizations applied where possible. The routing layer is said to start with the well-studied public domain algorithm Ad hoc On Demand Distance Vector (AODV) and Motorolaâ„¢s Cluster-Tree algorithm.
6.1 AODV: Ad hoc On Demand Distance Vector
AODV is a pure on-demand route acquisition algorithm: nodes that do not lie on active paths neither maintain any routing information nor participate in any periodic routing table exchanges. Further, a node does not have to discover and maintain a route to another node until the two need to communicate, unless the former node is offering services as an intermediate forwarding station to maintain connectivity between two other nodes.
The primary objectives of the algorithm are to broadcast discovery packets only when necessary, to distinguish between local connectivity management and general topology maintenance and to disseminate information about changes in local connectivity to those neighboring mobile nodes that are likely to need the information.
When a source node needs to communicate with another node for which it has no routing information in its table, the Path Discovery process is initiated. Every node maintains two separate counters: sequence number and broadcast id. The source node initiates path discovery by broadcasting a route request (RREQ) packet to its neighbors, which includes source address, source sequence number, broadcast id, destination address, destination sequence number, hop cnt. (Source sequence number is for maintaining freshness information about the reverse route whereas the destination sequence number is for maintaining freshness of the route to the destination before it can be accepted by the source.)
The pair source address, broadcast id uniquely identifies a RREQ, where broadcast id is incremented whenever the source issues a new RREQ. When an intermediate node receives a RREQ, if it has already received a RREQ with the same broadcast id and source address, it drops the redundant RREQ and does not rebroadcast it.
Otherwise, it rebroadcasts it to its own neighbors after increasing hop cnt. Each node keeps the following information: destination IP address, source IP address, broadcast id, expiration time for reverse path route entry and source nodeâ„¢s sequence number.
As the RREQ travels from a source to destinations, it automatically sets up the reverse path from all nodes back to the source. To set up a reverse path, a node records the address of the neighbor from which it received the first copy of RREQ. These reverse path route entries are maintained for at least enough time for the RREQ to traverse the network and produce a reply to the sender.
Figure 6.1: Reverse and forward path formation in AODV protocol.
When the RREQ arrives at a node, possibly the destination itself, that possesses a current route to the destination, the receiving node first checks that the RREQ was received over a bi-directional link. If this node is not destination but has route to the destination, it determines whether the route is current by comparing the destination sequence number in its own route entry to the destination sequence number in the RREQ. If RREQâ„¢s sequence number for the destination is greater than that recorded by the intermediate node, the intermediate node must not use this route to respond to the RREQ, instead rebroadcasts the RREQ. If the route has a destination sequence number that is greater than that contained in the RREQ or equal to that contained in the RREQ but a smaller hop count, it can
unicasts a route reply packet (RREP) back to its neighbor from which it received
the RREQ. A RREP contains the following information: source address, dest addr, dest sequence number, hop cnt and lifetime. As the RREP travels back to the source, each node along the path sets up a forward pointer to the node from which the RREP came, updates its timeout information for route entries to the source and destination, and records the latest destination sequence number for the requested destination.
Nodes that are along the path determined by the RREP will timeout after route request expiration timer and will delete the reverse pointers since they are not on the path from source to destination as shown in Figure 6.1. The value of this timeout time depends on the size of the ad hoc network. Also, there is the routing caching timeout that is associated with each routing entry to show the time after which the route is considered to be invalid. Each time a route entry is used to transmit data from a source toward a destination, the timeout for the entry is reset to the current time plus active-route-timeout.
The source node can begin data transmission as soon as the first RREP is received, and can later update its routing information if it learns of a better route.
Each routing table entry includes the following fields: destination, next hop, number of hops (metric), sequence number for the destination, active neighbors for this route, and expiration time for the route table entry.
For path maintenance, each node keeps the address of active neighbors through which packets for the given destination are received is maintained. This neighbor is considered active if it originates or relays at least one packet for that destination within the last active-timeout period. Once the next hop on the path from source to the destination becomes unreachable (hello messages are not received for a certain time, hello messages also ensures that only nodes with bidirectional connectivity are considered to be neighbors, therefore each hello message included the nodes from which the node has heard), the node upstream of the break propagates an unsolicited RREP with a fresh sequence number and hop count of 1to all active upstream nodes. This process continues until all active source nodes are notified. Upon receiving the notification of a broken link, the source nodes can restart the discovery process if they still require a route to the destination. If it decides that it would like to rebuild the route to the destination, it sends out an RREQ with a destination
sequence number of one greater than the previously known sequence number, to ensure that it builds a new, viable route and that no nodes reply if
they still regard the previous route as valid.
6.2 Cluster-Tree Algorithm
The cluster-tree protocol is a protocol of the logical link and network layers that uses link-state packets to form either a single cluster network or a potentially larger cluster tree network. The network is basically self-organized and supports network redundancy to attain a degree of fault resistance and self-repair.
Nodes select a cluster head and form a cluster according to the self-organized manner. Then self-developed clusters connect to each other using the Designated Device (DD).
6.2.1 Single Cluster Network
The cluster formation process begins with cluster head selection. After a cluster head is selected, the cluster head expands links with other member nodes to form a cluster.
After a node turns on, it scans the channels to search for a HELLO message form other nodes (HELLO messages correspond to beacons in MAC layer of IEEE
802.15.4). If it canâ„¢t get any HELLO messages for a certain time, then it turns to a cluster head as shown in Figure 6.2 and sends out HELLO messages to its neighbours. The new cluster head wait for responses from neighbours for a while. If it hasnâ„¢t received any connection requests, it turns back to a regular node and listens again. The cluster head can also be selected based on stored parameters of each node, like transmission range, power capacity, computing ability or location information.
Figure 6.2: Cluster head selection process.
After becoming the cluster head (CH), the node broadcasts a periodic HELLO message that contains a part of the cluster head MAC address and node ID 0 that indicates the cluster head. The nodes that receive this message send a CONNECTION REQUEST message to the cluster head. When the CH receives it, it responds to the node with a CONNECTION RESPONSE message that contains a node ID for the node (node ID corresponds to the short address at the MAC layer). The node that is assigned a node ID replies with an ACK message to the cluster head. The message exchange is shown in Figure 6.3.
Figure 6.3: Link setup between CH and member node.
If all nodes are located in the range of the cluster head, the topology of connection becomes a star and every member nodes are connected to the cluster head with one hop. A cluster can expand into a multi-hop structure when each node supports multiple connections. The message exchange for the multi hop cluster set up procedure is shown in Figure 6.4.
Figure 6.4: Multi hop cluster setup procedure.
If the cluster head has run out of all node IDs or the cluster has reached some other defined limit, it should reject connection requests from new nodes. The rejection is through the assignment of a special ID to the node.
The entry of the neighbour list and the routes is updated by the periodic HELLO message. If a node entry does not update until a certain timeout limit, it should be eliminated.
A node may receive a HELLO message from a node that belongs to different cluster. In that case, the node adds the cluster ID (CID) of the transmitting node in the neighbour list and then sends it inside a LINK STATE REPORT to the CH so that CH knows which clusters its cluster has intersection.
The LINK STATE REPORT message also contain the neighbors node ID list of the node so that the CH knows the complete topology to make topology optimizations. If the topology change is required, then the CH sends a TOPOLOGY UPDATE message. If a member receives a TOPOLOGY UPDATE message that the different parent node is linked to the node, it changes the parent node as indicated in the message. And it also records its child nodes and the nodes below it in the tree at this time.
If a member node has trouble and becomes unable to communicate, the tree route of the cluster would be reconfigured. The CH knows the presence of a trouble by the periodic LINK STATE REPORT. When the cluster head has trouble, the distribution of HELLO message is stopped and all member nodes know that they have lost the CH. The cluster would then be reconfigured in the same way as the cluster formation process.
6.2.2 Multi-Cluster Network
To form a network, a Designated Device (DD) is needed. The DD has responsibility to assign a unique cluster ID to each cluster head. This cluster ID combined with the node ID that the CH assigns to each node within a cluster forms a logical address and is used to route packets. Another role of the DD is to calculate the shortest route from the cluster to the DD and inform it to all nodes within the network.
When the DD joins the network, it acts as the CH of cluster 0 and starts to send HELLO message to the neighborhood. If a CH has received this message, it sends a CONNECTION REQUEST message and joins the cluster 0. After that, the CH requests a CID to the DD. In this case, the CH is a border node that has two logical addresses. One is for a member of the cluster 0 and the other is for a CH. When the CH gets a new CID, it informs its member nodes by the HELLO message.
If a member has received the HELLO message from the DD, it adds CID 0 in its neighbor list and reports to its CH. The reported CH selects the member node as a border node to its parent cluster and sends a network connection request message to the member node to set up a connection with the DD. The border node requests a connection and joins the cluster 0 as its member node. Then it sends a CID REQUEST message to the DD. After the CID RESPONSE message arrival, the border node sends NETWORK CONNECTION RESPONSE message that contains a new CID to the CH when the CH gets a new CID, it informs to its member nodes by the HELLO message.
The clusters not bordering cluster 0 use intermediate clusters to get a CID. Again, either the CH becomes the border node to its parent cluster or the CH names a member node as the border to its parent cluster. These processes are shown in Figures 6.5,6.6,6.7,6.8.
Figure 6.5: CID assignment 1
Figure 6.6: CID assignment 2.
Figure 6.7: CID assignment 3.
Figure 6.8: CID assignment 4.
Each member node of the cluster has to record its parent cluster, child/lower clusters and the border node IDs associated with both the parent and child clusters. The DD should store the whole tree structure of the clusters.
Like the nodes in the clusters, the CHs report their link state information to the DD. The CH periodically sends a NETWORK LINK STATE REPORT message that contains its neighbor cluster CID list to the DD. Then this information can be used to calculate the optimized route and periodically update the topology for the network redundancy. In the same way, the DD can send TOPOLOGY UPDATE message to inform up-to-date route from the DD to the clusters.
A backup DD (BDD) can be prepared to prevent network down time due to the DD trouble. Inter-cluster communication, which is shown in Figure 6.9, is realized by routing. The border nodes act as routers that connect clusters and relay packets between the clusters. When a border node receives a packet, it examines the destination address, then forwards to the next border node in the adjacent cluster or to the destination node within the cluster.
Figure 6.9: A multi cluster network and the border nodes.
Only the DD can send a message to all the nodes within its network. The message is forwarded along the tree route of clusters. The border node should forward the broadcast packet from the parent cluster to the child cluster.
APPLICATION LAYER 7
7.0 APPLICATION LAYER
The ZigBee application layer consists of the APS sub-layer, the ZDO and the manufacturer-defined application objects. The responsibilities of the APS sub-layer include maintaining tables for binding, which is the ability to match two devices together based on their services and their needs, and forwarding messages between bound devices. Another responsibility of the APS sub-layer is discovery, which is the ability to determine which other devices are operating in the personal operating space of a device. The responsibilities of the ZDO include defining the role of the device within the network (e.g., ZigBee coordinator or end device), initiating and/or responding to binding requests and establishing a secure relationship between network devices. The manufacturer-defined application objects implement the actual applications according to the ZigBee-defined application descriptions
7.1 Application Support Layer
This layer provides the following services:
Â¢ Discovery: The ability to determine which other devices are operating in the personal operating space of a device.
Â¢ Binding: The ability to match two or more devices together based on their services and their needs and forwarding messages between bound devices
7.2 The General Operation Framework (GOF)
The General Operation Framework (GOF) is a glue layer between applications and rest of the protocol stack. The GOF currently covers various elements that are common for all devices. It includes sub addressing and addressing modes and device descriptions, such as type of device, power source, sleep modes, and coordinators. Using an object model, the GOF specifies methods, events, and data formats that are used by application profiles to construct set/get commands and their responses.
Actual application profiles are defined in the individual profiles of the IEEE's working groups. Each ZigBee device can support up to 30 different profiles.
Currently, only one profile, Commercial and Residential Lighting, is defined. It includes switching anddimming load controllers, corresponding remote-control devices, and occupancy and light sensors.
7.3 ZigBee Device
There are two physical device types for the lowest system cost. The IEEE standard defines two types of devices:
Â¢ Full function device (FFD)
o Can function in any topology
o Capable of being the network coordinator
o Capable of being a coordinator
o Can talk to any other device
Â¢ Reduced function device (RFD)
o Limited to star topology
o Cannot become a network coordinator
o Talks only to a network coordinator
o Very simple implementation
An IEEE 802.15.4/ZigBee network requires at least one full function device as a network coordinator, but endpoint devices may be reduced functionality devices to reduce system cost.
Â¢ All devices must have 64 bit IEEE addresses
Â¢ Short (16 bit) addresses can be allocated to reduce packet size
Â¢ Addressing modes:
o Network + device identifier (star)
o Source/destination identifier (peer-peer)
7.4 ZigBee Device Objects
Â¢ Defines the role of the device within the network (e.g., ZigBee coordinator or end device)
Â¢ Initiates and/or responds to binding requests
Â¢ Establishes a secure relationship between network devices selecting one of
ZigBeeâ„¢s security methods such as public key, symmetric key, etc.
ZigBee - APPLICATIONS 8
8.1 Product Examples
Warehouses, Fleet management, Factory, Supermarkets, Office complexes
Â¢ Gas/Water/Electric meter, HVAC
Â¢ Smoke, CO, H2O detector
Â¢ Refrigeration case or appliance
Â¢ Equipment management services & PM
Â¢ Security services
Â¢ Lighting control
Â¢ Assembly line and work flow, Inventory
Â¢ Materials processing systems (heat, gas flow, cooling, chemical)
Energy, diagnostics, e-Business services
Â¢ Gateway or Field Service links to sensors & equipment
â€œ Monitored to suggest PM, product updates, status changes
Â¢ Nodes link to PC for database storage
â€œ PC Modem calls retailer, Service Provider, or Corp headquarters
â€œ Corp headquarters remotely monitors assets, billing, energy management
8.2 Home & Diagnostics Examples
Â¢ Mobile clients link to PC for database storage
â€œ PC links to peripherals, interactive toys
â€œ PC Modem calls retailer, SOHO, Service Provider
Â¢ Gateway links to security system, temperature sensor, AC system, entertainment, health.
Â¢ Gateway links to field sales/service
IEEE 802.15.4 is a new standard that still needs to pass through the circles of rigorous technology critics and establish its own place in the industry. Predictions for the future of ZigBee-enabled devices are a popular topic for numerous market- research firms.
While I intend to stay objective, I believe, based on protocol features implemented in 802.15.4, that ZigBee has a bright future. Backed by IEEE, ZigBee has the potential to unify methods of data communication for sensors, actuators, appliances, and asset-tracking devices. It offers a means to build a reliable but affordable network backbone that takes advantage of battery-operated devices with a low data rate and a low duty cycle. ZigBee can be used in many applications, from industrial automation, utility metering, and building control to even toys. Home automation, however, is the biggest market for ZigBee-enabled devices. This follows from the number of remote controlled devices (or devices that may be connected wirelessly) in the average household. This cost-effective and easy-to- use home network potentially creates a whole new ecosystem of interconnected home appliances, light and climate control systems, and security and sensor sub networks.
On the web
ZigBee Alliance, http://www.caba.org/standard/zigbee.html.
ZigBee Alliance, http://www.zigbee.org
IEEE 802.15.4 web site, http://www.ieee802.org/15/pub/TG4.html
On the press
LAN-MAN Standards Committee of the IEEE Computer Society, Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), IEEE, 2003
IEEE P802.15 Working Group for WPANs, Cluster Tree Network
TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION 06
1.1 EVOLUTION OF LR-WPAN STANDARDIZATION 06
1.2 ZigBee AND IEEE 802.15.4 07
1.3 WHY IS ZigBee NEEDED 08
1.4 ZigBee AND BLUETOOTH 09
1.5 WIRELESS TECHNOLOGY COMPARISON CHART 10
CHAPTER 2: ZIGBEE / IEEE 802.15. 4 WPAN 12
2.1 COMPONENTS OF WPAN 12
2.2 NETWORK TOPOLOGIES 12
2.2.1 STAR TOPOLOGY 12
2.2.2 PEER-TO-PEER TOPOLOGY 13
2.2.3 CLUSTER-TREE TOPOLOGY 14
2.3 ZIGBEE ARCHITECTURE 15
CHAPTER 3: IEEE 802.15. 4 PHY 17
3.1 RECEIVER ENERGY DETECTION (ED) 19
3.2 LINK QUALITY INDICATION (LQI) 19
3.3 CLEAR CHANNEL ASSESSMENT (CCA) 20
3.4 PPDU FORMAT 20
CHAPTER 4: IEEE 802.15. 4 MAC 22
4.1 FRAME STRUCTURE 22
4.2 CHANNEL ACCESS AND ADDRESSING 23
4.3 SUPER FRAME STRUCTURE 24
4.4 CSMA-CA ALGORITHM 25
4.5 DATA TRANSFER MODEL 26
4.6 MAC LAYER SECURITY 28
TABLE OF CONTENTS (2)
CHAPTER 5: NERWORK LAYER 31
5.1 ZIGBEE NETWORK NODE 32
5.2 RESPONSIBILITIES OF THE ZIGBEE NWK LAYER 32
5.3 NETWORK LAYER SECURITY 33
CHAPTER 6: ZIGBEE ROUTING MECHANISM 35
6.1 AODV: AD HOC ON DEMAND DISTANCE VECTOR 35
6.2 CLUSTER-TREE ALGORITHM 38
6.2.1 SINGLE CLUSTER NETWORK 38
6.2.2 MULTI-CLUSTER NETWORK 41
CHAPTER 7: APPLICATION LAYER 46
7.1 APPLICATION SUPPORT LAYER 46
7.2 THE GENERAL OPERATION FRAMEWORK (GOF) 46
7.3 ZIGBEE DEVICE 47
7.4 ZIGBEE DEVICE OBJECTS 47
CHAPTER 8: ZIGBEE - APPLICATIONS 48
8.1 PRODUCT EXAMPLES 49
8.2 HOME & DIAGNOSTICS EXAMPLES 49
ZIGBEE: CONCLUSION 50
ZIGBEE: BIBLIOGRAPHY 51