7 A] Explain IEEE 802.15.4.
• The IEEE 802.15.4 standard represents the most popular standard for low data rate wireless personal area networks (WPAN).
• This standard was developed to enable monitoring and control applications with lower data rate and extend the operational life for uses with low-power consumption.
• This standard uses only the first two layers—physical and data link—for operation along with two new layers above it: 1) logical link control (LLC) and 2) service specific convergence sublayer (SSCS). The additional layers help in the communication of the lower layers with the upper layers. Figure 4.1 shows the IEEE 802.15.4 operational layers.
• The IEEE 802.15.4 standard was curated to operate in the ISM (industrial, scientific, and medical) band.
• The direct sequence spread spectrum (DSSS) modulation technique is used for communication purposes, enabling a wider bandwidth of operation with enhanced security by the modulating pseudo-random noise signal. This standard exhibit high tolerance to noise and interference and offers better measures for improving link reliability.
• Typically, the low-speed versions of the IEEE 802.15.4 standard use binary phase shift keying (BPSK), whereas the versions with high data rate implement offset quadrature phase shift keying (O-QPSK) for encoding the message to be communicated.
• Carrier sense multiple access with collision avoidance (CSMA-CA) is the channel access method used for maintaining the sequence of transmitted signals and preventing deadlocks due to multiple sources trying to access the same channel.
• The transmission, for most cases, is line of sight (LOS), with the standard transmission range varying between 10 m to 75 m. The best-case transmission range achieved outdoors can be up to 1000 m.
• This standard typically defines two networking topologies: 1) Star and 2) mesh.
• There are seven variants identified with in IEEE 802.15.4—A, B. C, D, E, F, and G. Variants A/B are the base versions, C is assigned for China, and D for Japan. Variants E, F, and G are assigned respectively for industrial applications, active RFID (radio frequency identification) uses, and smart utility systems.
• The IEEE 802.15.4 standard supports two types of devices: 1) reduced function device (RFD) and 2) full function devices (FFD). FFDs can talk to all types of devices and support full protocol stacks. However, these devices are costly and energy consuming due to increased requirements for support of full stacks. In contrast, RFDs can only talk to an FFD and have lower power consumption requirements due to minimal CPU/RAM requirements.
• The IEEE 802.15.4 standard supports two network types: 1) Beacon-enabled networks and 2) non-beacon-enabled networks. The periodic transmission of beacon messages characterizes beacon-enabled networks. Here, the data frames sent via slotted CSMA/CA with a superframe structure. These beacons are used for synchronization and association of other nodes with the coordinator. The scope of operation of this network type spans the whole network.