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This chapter is an overview of the IEEE 802.15.4 specification. 802.15.4 defines a standard for a low-rate WPAN (LR-WPAN).
3.1 Scope of 802.15.4
802.15.4 is a packet-based radio protocol. It addresses the communication needs of wireless applications that have low data rates and low power consumption requirements. It is the foundation on which ZigBee is built. Figure 4.1 shows a simplified ZigBee stack, which includes the two layers specified by 802.15.4: the physical (PHY) and MAC layers.
3.1.1 PHY Layers
The PHY layer defines the physical and electrical characteristics of the network. The basic task of the PHY layer is data transmission and reception. At the physical/electrical level, this involves modulation and spreading techniques that map bits of information in such a way as to allow them to travel through the air. Specifications for receiver sensitivity and transmit output power are in the PHY layer.
The PHY layer is also responsible for the following tasks:
enable/disable the radio transceiver
link quality indication (LQI) for received packets
energy detection (ED) within the current channel
clear channel assessment (CCA)
3.1.2 MAC Layer
The MAC layer defines how multiple 802.15.4 radios operating in the same area will share the airwaves. This includes coordinating transceiver access to the shared radio link and the scheduling and routing of data frames.
There are network association and disassociation functions embedded in the MAC layer. These functions support the self-configuration and peer-to-peer communication features of a ZigBee network.
The MAC layer is responsible for the following tasks:
beacon generation if device is a coordinator
implementing carrier sense multiple access with collision avoidance (CSMA-CA)
handling guaranteed time slot (GTS) mechanism
data transfer services for upper layers
3.2 Properties of 802.15.4
802.15.4 defines operation in three license-free industrial scientific medical (ISM) frequency bands. Below is a table that summarizes the properties of IEEE 802.15.4 in two of the ISM frequency bands: 915 MHz and 2.4 GHz.
3.2.1 Transmitter and Receiver
The power output of the transmitter and the sensitivity of the receiver are determining factors of the signal strength and its range. Other factors include any obstacles in the communication path that cause interference with the signal.
The higher the transmitter's output power, the longer the range of its signal. On the other side, the receiver's sensitivity determines the minimum power needed for the radio to reliably receive the signal. These values are described using dBm (deciBels below 1 milliwatt), a relative measurement that compares two signals with 1 milliwatt used as the reference signal. A large negative dBm number means higher receiver sensitivity.
3.2.2 Channels
Of the three ISM frequency bands only the 2.4 GHz band operates world-wide. The 868 MHz band only operates in the EU and the 915 MHz band is only for North and South America. However, if global interoperability is not a requirement, the relative emptiness of the 915 MHz band in non-European countries might be an advantage for some applications.
For the 2.4 GHz band, 802.15.4 specifies communication should occur in 5 MHz channels ranging from 2.405 to 2.480 GHz.
3.3 Network Topologies
According to the IEEE 802.15.4 specification, the LR-WPAN may operate in one of two network topologies: star or peer-to-peer. 802.15.4 is designed for networks with low data rates, which is why the acronym "LR" (for "low rate") is prepended to "WPAN."
As shown in Figure 3.1, the star topology has a central node with all other nodes communicating only with the central one. The peer-to-peer topology allows peers to communicate directly with one another. This feature is essential in supporting mesh networks.
3.4 Network Devices and their Operating Modes
Two types of devices can participate in a LR-WPAN: a full function device (FFD) and a reduced function device (RFD).
An RFD does not have routing capabilities. RFDs can be configured as end nodes only. They communicate with their parent, which is the node that allowed the RFD to join the network.
An FFD has routing capabilities and can be configured as the PAN coordinator. In a star network all nodes communicate with the PAN coordinator only so it does not matter if they are FFDs or RFDs. In a peer-to-peer network there is also one PAN coordinator, but there are other FFDs which can communicate with not only the PAN coordinator, but also with other FFDs and RFDs.
There are three operating modes supported by IEEE 802.15.4: PAN coordinator, coordinator, and end device. FFDs can be configured for any of the operating modes. In ZigBee terminology the PAN coordinator is referred to as simply "coordinator." The IEEE term "coordinator" is the ZigBee term for "router."
3.5 Addressing Modes Supported by 802.15.4
802.15.4 supports both short (16-bit) and extended (64-bit) addressing.
An extended address (also called EUI-64) is assigned to every RF module that complies to the 802.15.4 specification.
When a device associates with a WPAN it can receive a 16-bit address from its parent node that is unique in that network.
3.5.1 PAN ID
Each WPAN has a 16-bit number that is used as a network identifier. It is called the PAN ID. The PAN coordinator assigns the PAN ID when it creates the network. A device can try and join any network or it can limit itself to a network with a particular PAN ID.
ZigBee PRO defines an extended PAN ID. It is a 64-bit number that is used as a network identifier in place of its 16-bit predecessor.
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