IEEE 802.15.4

The IEEE 802.15.4 standard describes a communication protocol for wireless personal area networks ( WPAN). It defines the two lowest layers of the OSI model, the physical and data link to the MAC layer. Higher log levels with functions for routing and application interface is responsible for other standards for wireless networks such as ZigBee. Significant development goals for the protocol are low power consumption for prolonged operation on battery power, low-cost hardware, secure transmission, use the license-free ISM bands and parallel operation with other stations on these frequencies, especially WLAN and Bluetooth. These features make the standard IEEE 802.15.4 is suitable especially for wireless sensor networks ( WSN) and for direct body-worn sensors and actuators ( WBAN, Wireless Body Area Network).

  • 2.1 Frequency bands and data rates
  • 2.2 Transmission power
  • 2.3 Hardware
  • 3.1 CSMA / CA
  • 3.2 Transmission method 3.2.1 Unslotted Fashion ( nonbeacon -enabled )
  • 3.2.2 slotted mode ( beacon -enabled )

Basic

In the late 1990s a need for a simple standard for wireless data transfer for devices with low power consumption and low data transmission rate was seen. The then-available IEEE 802.11 and Bluetooth were too complex and had too much energy requirements, they can be implemented with inexpensive components can. In the development of IEEE 802.15.4 consequently did not have a high data transfer rate, but the energy management and the simplicity of the standardized protocol highest priority.

Characteristic of the nodes of an IEEE 802.15.4 network are the long periods of rest, which one node can linger most of the time in an energy -saving standby mode. As soon as he wants to send or receive data, it can wake up in only 15 ms, then handle communication and go back to sleep. This allows battery-powered network nodes achieve typical maturities of six months to two years.

Topologies

The IEEE 802.15.4 provides two types of nodes with different functions before, the reduced function devices ( RFD) and Full Function Devices ( FFD). RFD have only a subset of the standardized functionality, whereby the communication with him FFDs is only possible, however, it can be also developed a simple and cost -effective. RFDs are typical sensors or actuators in the network, which only very rarely send or receive, take any administrative tasks and so spend most of their time in a power-saving state data. FFDs, however, feature the full functionality and can communicate both with RFDs and other FFDs. A FFD per network takes on the special function of the PAN coordinator. He sets the PAN identifier which defines the network of other IEEE 802.15.4 networks in range. Furthermore, he has the slotted mode, the synchronization of all network nodes. A network may have up to 254 nodes.

The standard defines Based on the above node types, three different network topologies:

  • Star. In a star all nodes communicate directly with the coordinator. The coordinator is in such a constellation usually a powerful device with connection to the grid, while the other nodes are battery powered.
  • Peer-to- peer. In this network, although there is also a coordinator, but the nodes can directly communicate with each other if they are in range of each other.
  • Tree (cluster tree). RFDs here represent the leaves of a tree. You are connected to FFDs, which take over the function of a coordinator for a portion of the network. The FFDs themselves are in turn connected directly or indirectly to the PAN coordinator of the whole network over other FFDs with coordinator function. The resulting tree structure with the PAN coordinator as root represents a mixture of the first two topologies with the definitions possibilities of standards alone, however, is not a complete meshing and routing of messages (routing) possible.

Since the standard does not define a network layer (network layer), functions such as routing must be implemented by higher layers of other protocols that are based on IEEE 802.15.4. So true mesh networking is possible in which the FFDs serve as a repeater and each node can communicate via intermediate stations with another, even RFDs with other RFDs.

Standardized interfaces

IEEE 802.15.4 defines the two lowest layers of the OSI reference model. Higher layers are realized by other standards such as ZigBee. The individual layers carry out specific tasks for which they take services of the next lower layer to complete. The treasury functions they in turn provide a service the overlying layers. The interfaces between the layers are called Service Access Point. The exact implementation of the functions can be carried out differently by each manufacturer, but the available functional and service level is statically defined. Thus, the standard ensures that the communications between devices running smoothly with implementations from different manufacturers.

Physical layer

Frequency bands and data rates

For radio transmission, the ISM bands 868/915 MHz (Europe and USA) and 2.45 GHz are ( almost worldwide ) are available. Due to the different frequencies only a band can be used by the hardware at a time. The distribution of the radio modules for the 2.45 GHz band is very large, only a few use the 868/915-MHz-Band.

To reduce the noise sensitivity, the spread spectrum DSSS is used. In the 868 MHz frequency band, each symbol on the basis of the modulation method used is a bit and is being implemented on a 15 chip long code. At a chip rate of 300 kchips / s this results in a data rate of 20 kbit / s In the 915 MHz frequency band, the chip rate of 600 kchips / s is twice as high, thus resulting in a data rate of 40 kbit / s. In the 2.45 GHz frequency band modulation with 4 bits / symbol, a symbol spread to 32 chips and a chip rate of 2 Mchips / s is used, which / s leads to a data rate of 250 kbps. The modulation methods used were chosen because they have a good balance between simplicity and robustness.

Since the length field is 7 bits in size, a data packet may comprise 0 to 127 bytes. For data types include more than one byte, the byte order is carried out according to the principle " least significant byte ( little-endian ) first". The range is up to 100 meters outdoors and 30 meters inside.

Development:

  • In 2006, a revised version of the standard raised the data rate of 868 MHz band exclusively with the parallel spread spectrum technology ( PSSS ) to 250 kbit / s and the 915 MHz band, inter alia, with PSSS also at 250 kbit / s.
  • In August 2007, 802.15.4a two other transmission method for the physical layer have been added with the addition of IEEE. The first is a method of ultra-wideband (UWB), which transmits at frequencies below 1 GHz, 3-5 GHz and 6-10 GHz. The other is the chirp spread spectrum method (CSS) that operates in the 2.45 GHz band, and a way to locate the network provides.

Transmission power

The typical output power of a transceiver is 0 dBm (1 mW) and the sensitivity is below -90 dBm. Even if the given model of the path losses for buildings applies, the practice shows that often only one-third of the specified range is possible.

IEEE 802.15.4 was designed for parallel operation with Wi-Fi and Bluetooth. Practice Tests showed problems with the coexistence of IEEE 802.15.4 with Wi-Fi and Bluetooth in the 2.45 GHz band. With Bluetooth, the introduced since version 1.2 adaptive frequency hopping proves that wireless evades but for the remaining frequencies is more common than troublemakers. Wi-Fi makes by the strong growth in traffic problems. The significance of these results is questioned by the ZigBee Alliance and started by a compatibility study a rebuttal.

Hardware

The size of the antenna in the 2.45 GHz band, may be very small, a module with antenna, transceiver and microprocessor has about the dimensions of a two - euro coins. The use of the developed antennas for WLAN kink is also possible.

The graph below shows the block diagram of a transceiver chips, the corresponding hardware from most manufacturers. Only the stepping down to the intermediate frequency is done with analog components, the De-/Modulation done digitally. Transmission occurs in packets and a buffer stores incoming or transmitting data. There are almost no external components required apart from a quartz crystal and decoupling capacitors.

MAC Layer

From the data link layer according to the OSI reference model defines the IEEE 802.15.4 standard with the MAC layer, only the lower sublayer. The implementation of the LLC layer is omitted at this point. Since the MAC layer is implemented by software, thus reducing the scope of the protocol stack. Thus, the MAC layer is the upper end of the IEEE 802.15.4. The ZigBee standard is based directly on this layer. ( The " symbol period" is the basis for all times dar. In the 2.45 GHz band is this 16 microseconds. The values ​​for the MAC layer are all related to the specifications for the 2.45 GHz band. )

CSMA / CA

For collision avoidance when accessing the medium, a CSMA / CA algorithm is used. Prior to transmission, the transmitter checks whether another device is transmitting on the channel in which the signal strength is measured from the antenna. If the channel is free starts the data transfer, otherwise it waits a random period of time and performs a new channel free - test. Should the canal free test repeatedly fails, the algorithm breaks the transmission with the error message " channel busy " from. To transfer backup an ACK packet can be recovered from the recipient. If the acknowledgment of a message, that means a transmission error, the packet is retransmitted. If this happens several times the transmission attempt ends with the error " NOACK ", the recipient is not available. For sending ACK packets no CSMA / CA algorithm is required, but it is sent immediately after receiving a message.

From the error messages after an unsuccessful transmission attempt the overlying layer can draw several conclusions. "NOACK " means the other party is no longer in range. "Busy channel " follows from the channel used is overloaded and should possibly be changed.

The CSMA / CA algorithm specifies that the random backoff is performed prior to the first channel free test, so the effective data rate drops significantly. The waiting time before the first transmission attempt in the 2.45 GHz band is from 0 to 2.24 ms. Should the canal free test fails, it can grow to 9.92 ms. In addition, further delays if the recipient is no ACK. The following formula is based on the requirements of the standards and calculates the backoff time.

The timeout for the ACK packet is 864 microseconds (symbol period · AckWaitDuration ) and may be renewed only 192 microseconds after receipt of the complete message.

Transmission method

The standard defines two transmission methods. In the so-called fashion Unslotted send the network nodes asynchronously their data. In slotted mode of the PAN coordinator synchronizes the requests by dividing the transmission time periods in so-called super frames.

Unslotted Fashion ( nonbeacon -enabled )

Before each transmission a participant checked on the CSMA / CA, if the channel is busy and sends its data once it is released. Optionally, it may specify in the package sent, whether he wants a response ( ACK), which enable the user to check whether the parcel arrived correctly or the transfer was interrupted. This mode requires no administrative burdens by the PAN coordinator.

A distinction is made between three communication scenarios:

Based on the energy needs in this mode, thus a node, usually the PAN coordinator, always be ready to receive, while the other nodes most of the time to save energy.

Slotted mode ( beacon -enabled )

In slotted mode of the PAN coordinator divides the transmission periods in a so-called super frames. Their structure is visible in the right image. A superframe is bounded by two beacons ( dt signal fire). The coordinator sends beacons at fixed intervals (without CSMA / CA ) so that the participants can synchronize to the beginning of the superframe. The length of the superframe is specified with the parameter BI ( Beacon Interval). This is calculated based on the parameters BO ( Beacon offset) as follows:

BO can take values ​​between 0 and 14. A value of 15 indicates that the super frame is to be ignored. The length of a superframe can therefore between 15 ms and 246 s. The parameter SO ( Superframe Order) of the superframe into an active and an inactive phase is divided. The active period (SD = superframe duration) is calculated using the same formula as before, but BO is replaced by SO:

SO can assume values ​​between 0 and 14, but must be ≤ BO. A value of 15 indicates that the super frame after the beacon has no active phase. The active phase consists of 16 equal-length time slots ( time slots ). The first is the beacon, the remaining share of the Contention Access Period to (CAP ) and Contention Free Period ( CFP). In the CAP, all wishing to send participants compete in each slot by CSMA / CA for their data to send. The timeslots of the CFP are combined to Guaranteed Time Slots ( GTS) of the PAN coordinator and assigned participants firmly so that in this period no competition by CSMA / CA takes place. A participant who receives a GTS, has a guaranteed period in which he is allowed to send only member of the network. GTS does not mean, however, that a certain throughput is guaranteed or real-time conditions are met. GTS only regulates the distribution of transmission capacity within a network. A network of foreign participants who do not adhere to the time slots of the superframe can interfere with the transmission continues. In addition, the allocated transmission capacity can be lower than the required if participants ask more than the coordinator can distribute.

In the inactive phase of the PAN coordinator may enter a power-saving mode and save battery. This is an improvement compared to Unslotted mode, which is achieved at the expense of a higher overhead for synchronization. For participants wishing to transmit the effort increased only minimally by the synchronization and they can continue to spend most of the time to save energy.

In here there are three communication scenarios:

  • For data transmission from the participant to the coordinator is trying to get a free slot in the CAP.
  • Does the coordinator data for the subscriber, he shows this in the transmitted beacon. The participant listens periodically according to the beacons and finally responds in the CAP to collect the data. See the figure.
  • For peer-to -peer, participants then synchronize with each other at first and transfer the data.

Connection

Before a device can communicate with another, a channel for it must be selected. The Coordinator selects possible a channel without competing channels, this, the ED scan (Energy- Detect), in which the signal strength is measured or Active scan is used, in which the active devices on all others as Coordinator prompted by a Beacon Request a beacon to ship. Searches a device connected to a coordinator, this is done with an Active Scan ( only FFD) or a passive scan ( channels to listen to beacons ).

In order to build a network go smoothly, to terminals Report with a request to associate with a Coordinator and confirmed with the assignment of a short address. The choice of short -Address is the responsibility of higher layers.

Encryption and Security

The IEEE 802.15.4 standard provides security measures at the MAC level through message integrity check and symmetric encryption. You can choose between several methods based on CCM and AES. The keys are specified by the overlying layer and subsequently managed by the MAC layer. The encryption is set separately for each communication partner and applied automatically by the MAC layer. A received packet was encrypted, this indicates a parameter in the indication primitive.

The implementation of encryption varies slightly between the 2003 version and 2006.

ISM-Band Advanced Encryption Standard SmartLink
15804
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