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Wireless networking for embedded systems

PICmciro 802.11b interface
Prototype PICmicro 802.11b interface - ER21 wireless development board

Introduction

The speed and convenience of Ethernet, without the wires!

Wireless networking is gaining widespread acceptance, through the adoption of the IEEE 802.11b (Wi-Fi) standard. Computer stores are selling low-cost equipment to allow desktop and laptop PCs to communicate wirelessly, and the major hardware and software vendors see Wi-Fi compatibility as a major component of their networking strategy.

What about embedded systems? There are still a large number of proprietary radio networking systems on the market, but why use a non-standard network, when it is so cost-effective to use standard network infrastructure? The missing link is the ability to integrate low-cost microcontroller-based embedded systems into a conventional network, and this is provided by the Iosoft ChipWeb Wireless project.

What is IEEE 802.11b?

The original IEEE 802.11 standard describes a radio networking scheme with data rates of 1 or 2 Mbit/s. It can be used as a replacement for Ethernet, and functions in two primary modes:

  • Independent Basic Service Set (IBSS), or ad-hoc mode
  • Extended Service Set (ESS) or infrastructure mode

An IBSS or ad-hoc network is a peer-to-peer system, where each node can transmit when the network is idle, and there is direct communication between nodes on the network. ESS networks are more common, and use radio 'hubs' (known as access points) to coordinate traffic between nodes, so as to maximise reliability and minimise power consumption.

Access points can also act as gateways into conventional Ethernet networks, allowing transparent communication between wired and wireless nodes. This allows existing network software to be used on a wireless LAN (WLAN) with no changes whatsoever; TCP/IP networking proceeds as normal, with all the radio-specific aspects being confined to the low-level device drivers.

The 802.11b standard introduced higher-speed operation (up to 11 Mbit/s), with slower rates being employed as an automatic fall-back in the event of poor reception. Both 802.11 and 802.11b operate in the unlicensed 2.4 GHz Industrial Scientific Medical (ISM) frequency band, so no radio license is required for their use.

What is IEEE 802.15.4?

There are many wireless networking applications that do not require the speed & complexity of 802.11, so the 802.15.4 specification for a 'Low-Rate Wireless Personal Area Network' (LR_WPAN) was created. Operating in the 868/915 MHz or 2.4 GHz bands, it offers:

  • Raw data rate up to 250 kbit/s
  • Simple radio design with low power consumption
  • Minimal software overhead

Single-chip 802.15.4 transceivers are available from many vendors, though they are often marketed under the Zigbee® name (see below). Since its publication in 2003 the standard has gained ready acceptance in cost-sensitive low-rate networking applications such as remote monitoring and home security.

What is Zigbee?

The 802.15.4 specification only covers the lower networking layers (MAC and PHY). To achieve inter-operability over a wide range of applications such as Home, Industrial or Building Automation, the higher layers must be standardised as well.

The Zigbee Alliance has produced such a standard, using 802.15.4 wireless (generally in the 2.4 GHz band) as the low-level transport. Through the use of 'profiles', the specification may customised to suit various application areas.

The first fully released Zigbee specification was v1.0 in 2005; an enhanced version called Zigbee Pro is to be released in 2007, but this is not backwards-compatible due to changes in the wireless data format. There is an interim version, known as Zigbee 2006, which is largely binary-compatible with Zigbee Pro, but retains some features of the older specification.

What are the alternatives to Zigbee?

The 802.15.4 transport is often used with a proprietary application layer, or with IP encapsulation to form a lightweight TCP/IP solution. This offers all the convenience of wireless TCP/IP (usuallly UDP/IP) with full internetworking capability, though the data size may be limited to avoid excessive fragmentation.

Why not Bluetooth?

Bluetooth is an alternative wireless networking standard that uses the same 2.4 GHz radio band as 802.11 and 802.15.4, but employs frequency-hopping technology. Its primary focus is the 'personal connectivity' marketplace, linking mobile phones, palm-tops, laptops, cameras and printers so that they can exchange data wirelessly. It offers the prospect of low-cost, low-power radio networking, though at the time of writing it has failed to break away from its hand-held roots, and has found little acceptance in the wider-scale commercial and industrial networking areas.

Iosoft has chosen IEEE 802.11b and 802.15.4/Zigbee networking because of the compatibility with existing network hardware and software, and flexibility in accommodating large or small numbers of nodes per network.

Wireless development kit

The Iosoft ChipWeb 802.11b Wireless Development Kit is available; click here for more information.

Hardware and software development

With years of experience in embedded wireless development, Iosoft can produce custom RF hardware & software solutions based on 802.11 and 802.15.4 technologies. Contact us for more information.