I completed a project in late 2005 involving the testing of a basic ZigBee network - 2 modules. That being said, don’t be surprised if the information is a bit old. I will be publishing my complete findings once a week from the start of the project to the end. On other days I will continue to post news and events relating to ZigBee as they arise.

This project used the IEEE 802.15.4-2003 standard which has now been superceded by the publication of IEEE 802.15.4-2006. Next post will give a summary of the 2003 standard and the difference between it and the 2006 standard.

A list of References has been posted. Please see link on right.

IEEE 802.15.4

The Institute of Electrical and Electronics Engineers (IEEE) 802.15 is a working group for Wireless Personal Area Networks (WPAN). Specifically, Task Group 4 (TG4) investigates low data rate solutions with multi-month to multi-year battery life and very low complexity. These solutions operate in an unlicensed, international frequency band. The two working groups combine to make up IEEE 802.15.4. This is a standard that was approved in May 2003 (IEEE, 2005).

The IEEE 802.15.4 features(IEEE, 2005) are:

• Data rates of 250 kilobits per second (kbps), 40kbps, and 20kbps.
• Two addressing modes; 16-bit short and 64-bit IEEE addressing
• Support for critical latency devices, such as joysticks
• Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) channel access
• Automatic network establishment by the coordinator
• Fully handshaked protocol for transfer reliability
• Power management to ensure low power consumption
• 16 channels in the 2.4GHz ISM band, 10 channels in the 915MHz and one channel in the 868MHz band

The full name of the standard is Part15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs) (IEEE, 2003). It is an open standard and is available free of charge.

ZigBee

ZigBee is a wireless standard that takes advantage of everything the IEEE 802.15.4 standard has to offer. It is a protocol that defines:

• mesh, star and cluster tree network topologies
• data security features, and
• interoperable application profiles

The ZigBee standard has gone through many forms before reaching the stage it is at now. It was formerly known as PURLnet, RF-Lite, Firefly, and HomeRF Lite. The ZigBee slogan is “Wireless Control That Simply Works”.

IEEE 802.15.4 and ZigBee Protocol Stack

The ZigBee Stack and 802.15.4 Stack

From the above figure we can see that the ZigBee standard defines the Data Link Controller (DLC) layer, Network layer (NWK) and ZigBee profiles. Profiles are an agreement on messages, message formats and processing actions.

There are two types of ZigBee Profiles (Lee, 2005):

• Device Profile: This profile describes how general ZigBee device features are implemented, such as Binding, Device Discovery and Service Discovery
• Application Profile: This profile is application specific and consists of a list of ZigBee Device Descriptions. Each of the descriptions work together to form an application

ZigBee Alliance

According to the ZigBee alliance website (http://www.zigbee.org), their goal is to provide the consumer with ultimate flexibility, mobility, and ease of use by building wireless intelligence and capabilities into every day devices. They focus on:

• Defining the network, security and application software layers
• Providing interoperability and conformance testing specifications
• Promoting the ZigBee brand globally to build market awareness
• Managing the evolution of the technology

The initial markets for the ZigBee Alliance include:

• Home Control
• Building Automation
• Industrial Automation

Cost

The costs involved with joining the ZigBee alliance are listed below. Figures are expressed in US dollars:

• Promoter: $40,000 per year
• Participant: $9,500 per year
• Adopter: $3,500 per year

ZigBee offers Original Equipment Manufacturers (OEM) multiple silicon sources and ZigBee networking layer suppliers. Testing products for compliance has additional costs associated with it as does using the ZigBee logo.

ZigBee Market

In September 2004, ABI Research released a report forecasting the deployment of 80 million ZigBee devices by the end of 2006. (Gosling, 2004)

In addition to the ZigBee 1.0 specification being finalised in December 2004, the first ZigBee compliant platform certificates were awarded to Chipcon, CompXs, Ember, and Freescale Semiconductor. This will help push end product development forward.

Performance

A number of performance metrics are used in this project that is standard for any communications network. The following lists the performance metrics used and the reasons for using them.

Packet Throughput

Packet throughput can be defined as the ratio of the number of packets received by the network to the number of packets transmitted. This can also be expressed in the reverse way – packet loss rate. In this project, packet loss and packet throughput will be expressed as a percentage. Packet throughput will also be talked about in regards to the number of packets transmitted per second.

Latency

This can also be referred to as delay. Latency, in this context, is defined as one way delay, or half the round trip time (RTT). In a larger network, especially those using a mesh topology, the one way delay is not necessarily half the RTT. This is due to the fact that the route taken from source to destination may not be the same as the route taken from destination back to source. (Received time stamp – Transmitted time stamp)/ 2

Data Throughput

This will be expressed in both kbps and bps, the equation used to define this is: bits transmitted/latency. This value will be compared to the nominal bandwidth of 250kbps. It is only concerned with the actual data being sent and not any headers from the PHY or MAC layers. The PHY and MAC layers account for 25 Bytes, or 400 bits. The equation used to define this is: (bits transmitted – 400)/latency. This value will be compared to the nominal bandwidth of 250kbps. Data throughput (real time) will also be used and this is referring to the number of data bits transmitted divided by second. It takes into account all delays in the network.

Purpose

The purpose of this project is to provide a way to determine if ZigBee is a suitable wireless technology for use in the home automation industry. The performance of the network will give a clear indication if it is suitable or not.

Scope

The ZigBee standard is currently an ‘open’ standard only to those that are a part of the ZigBee Alliance. For this reason, the ZigBee standard was not used to implement the application layer.

The network application was written based upon the IEEE 802.15.4 standard. The network under test is limited to one personal area network (PAN) coordinator and one device. The 2.4 GHz Industrial Scientific Medical (ISM) frequency band was used.

Guaranteed Time Slot (GTS) and beaconing is not used in the network application nor is security. Power consumption and antenna design are not tested and their performance is not taken into account.

Objectives

The main objective is to generate data from a network and then analyse this data. In order to do this the following needs to be accomplished:

• Create a network with a home automation application and obtain data
• Create a Personal Computer (PC) application that collects data from the network and presents it to the user

Until next time….

Tags: zigbee, ZigBee Project, IEEE 802.15.4, Mesh Networks, Home Automation, Wireless Home Automation