About IEEE Standard 802.15.4

The IEEE 802.15.4 is a standard that defines the protocol and interconnection of devices via radio communication in a personal area network (PAN). It uses a "collision avoidance" technique to support multi-access. In contrast, IEEE 802.3 - the Ethernet standard - utilizes collision detection with backoff technique. Although media access is contention based, it supports "time slots" for time critical data.

The standard is defined for low-rate, wireless personal area networks (LR_PAN) – a low-cost communication network with limited power and relaxed throughput requirements. The core goals of the standard are ease of installation, reliable data transfer, short-range operation, extremely low cost, and long battery life.

Two types of devices are defined by the standard: a full-function-device (FFD) and a reduced-function-device (RFD). FFD devices typically perform network management functions such as routing, coordination, networking formation, and other management functions. In contrast RFD devices typically interact directly with the application processes. These nodes (often referred to as leaf nodes) contain the firmware and hardware that perform data capture, control functions, and other application specific functions. They can also be mobile, depending on application, and therefore have stringent requirements for low power and memory space. FFD devices usually have higher power requirements to permit “always on” operation to facilitate network routing, data analysis/aggregation, and other more demanding operations. An FFD can talk to RFDs and other FFDs, while an RFD can only talk to an FFD.

The standard defines both the "physical layer" and the "medium access layer". The physical layer specifications (PHY) define three low-power unlicensed radios. These include a spread spectrum radio operating at 2.4 GHz with a basic bit rate of 250 kilobits per second, a radio for the US market at 915 MHz and for Europe and Japan at 868 MHz. These latter frequencies have data rates of 40 kb/s and 20 kb/s, respectively.

The medium access layer (MAC) spec defines how multiple 802.15.4 radios operating in the same area will share the airwaves. The MAC supports 2 basic network topologies:

 

• Star: one FFD coordinates and other FFDs and RFDs join its network and communicate directly with the coordinating FFD

  

 

 

 

 

 

• Peer-to-Peer: devices can communicate with one another freely

 

From the latter, more complex network topologies can be implemented:

 

• Tree: a hierarchical structure allowing data to be relayed to a root

  

 

 

 

 

 

• Mesh: Nodes share routing responsibilities and there is no master coordinator

 

 

 

 

 

 

 

Defining a PHY and a MAC doesn't guarantee that different devices from different manufacturers will be able to talk to one other. The ZigBee specification under the auspices of the ZigBee alliance builds on the 802.15.4 foundation with additional protocol layers to make interoperability a reality.