802.15.4
The 802.15.4 standard is a low power, wireless networking standard
developed by the IEEE 802.15 Task Group 4. The standard was released in
2003 and has since been revised and superseded by the 2006 version, but
originated as a response to the growing number of electronic designers
that voiced the need for a standard addressing low-complexity, low data
rate, and (most commonly) battery-operated implementations.
Specifically, development of the standard targeted applications in
the home automation, industrial controls, agriculture, and security
market sectors. There are several other protocols including ZigBee and
ZigBee Pro that incorporate 802.15.4 as the physical and data link
layer.
One may hear the 802.15.4 standard referred to as the MAC, or Medium
Access Control standard, as it defines the communication protocol
between any two peer devices in a network. A device on an 802.15.4
personal area network (PAN) can be implemented conceptually as either a
Full Function Device (FFD) or a Reduced Function Device (RFD).
An FFD node maintains the capabilities of a network coordinator and
is more likely to be mains powered, although the device may not always
be used as such due to the fact that each PAN in a star configuration
can only have a single coordinator node.
An FFD could function as a common node. An RFD node is designedly
less complex, meaning it cannot assume the responsibilities of a
network coordinator and can only speak to an FFD node.
An RFD node will carry a minimal implementation of the application,
hence reducing the cost of the IC, is likely to be the node enabling
the sensor or the actuator for the application, and is also more likely
to be battery-powered, requiring extremely low duty cycles of
operation.
As Figure 7 below shows,
the star topology of an 802.15.4 network can be extended when an FFD
coordinator assigns a new PAN identifier (PAN ID) to one of its
children nodes (that must also be an FFD), creating a cluster of PANs
where only the coordinator nodes can exchange information. Note that
routing is not directly supported by the standard.
 |
| Figure
7 -- network configurations for 802.15.4 |
By defining the communication between two nodes, including a basis
for network management, the 802.15.4 standard presents a flexible
foundation for the development of higher-level network implementations
such as ZigBee.
Although it is a good foundation for higher-level network
development, the 802.15.4 standard maintains a certain level of
complexity that allows it to be employed "as-is" as a reliable method
of communication given the right application considerations. Generally,
a designer should consider the 802.15.4 standard as a fit for his or
her application if it requires the following:
Faith in an industry-standardized physical layer and lower layer
protocol
Freedom to design own higher layer protocol
Free choice of different HW and lower-layer SW vendors
Interoperability on the physical and lower protocol layer
Lower cost on design and development
Support and maintenance by other vendors/providers
and can accept drawbacks such as:
The design and development of higher layer protocol and
application
Radio channel restrictions according to the standard.
