With so many standards and applications, interoperability is a challenge, especially when different protocols share the same frequency bands. RF performance and range issues play into this as well since minimum power use is desired.
Wi-Fi is the most popular and widely used and accepted high bandwidth wireless transport medium. Not every application needs and can afford Wi-Fi from a cost, power, and processor resource point of view. For example, a lower bandwidth wireless headphone link may be better off using Bluetooth as part of the personal area network. However, Wi-Fi is the connect medium of choice and acceptance, especially with public area networked Wi-Fi hot zones opening up everywhere.
Third parties and chip makers offer modular solutions which can help get a project working when time to market is tight. Third party manufacturers want you to OEM their modular solutions and chip makers want you to use their chips. Both are valid approaches depending on budget, size, time, and form factor constraints.
Modular solutions can also be used in a middle-of-the-road approach where quick path to feasibility studies and functional prototyping can be achieved using modules, while a concurrent design project rolls your own solution.
This article discusses several off-the-shelf Wi-Fi modules that can be integrated into your applications. Modules from third party companies, as well as development/evaluation kits and public domain reference designs from semiconductor manufacturers, are included. All parts, modules, and development kits as well as training and documentation modules are available on the Digi-Key website and links are provided.
Like many things these days, Wi-Fi comes in different forms. A common element, however, is that all Wi-Fi operational specifications are dictated by the Wi-Fi Alliance, which enforces the Wi-Fi ‘brand’ based on the IEEE 802.11 specifications. The Alliance specifies the data structures, encryption techniques, frequencies, packet configurations, and sub protocols used by wireless Wi-Fi LANs.
For the most part, power ratings limit interconnectivity range to around 20 to 32 meters. This will vary depending on structure and non-line-of-sight situations. It should be noted there is also a long range Wi-Fi standard beginning to emerge.
The most commonly used Wi-Fi standards involve implementation of 802.11b, 802.11g, and 802.11n. The 802.11b version has a raw data rate of 11 Mbits/sec and can be stepped on by baby monitors, microwaves, and Bluetooth headsets (Fig. 1A).
Figure 1A: 802.11 b and g standards use 22 MHz channel spacing with Orthogonal Frequency Division Multiplexing to achieve up to 54 Mbits/sec raw data rates.
The faster 802.11g uses the same Orthogonal Frequency Division Multiplexing (OFDM) method of encoding digital data on multiple carrier frequencies as 802.11b, but ups the data rate to 54 Mbits/sec while retaining backward compatibility with 802.11b. In practical application, though, after all the forward error correction codes and exchanges, the usable data rate is more along the lines of 22 Mbits/sec.
Wi-Fi uses five channels in the 2.4 GHz band. It is important to know that Europe and Japan have additional channels that the U.S. does not allow. The U.S. supports channels 1-11 while Europe has 1-13. Japan supports channels 1-14. This means manufacturing international units may have slightly different RF sections and antennas, as well as code. Different countries also define different levels of allowable transmitter power and time that a channel can be occupied.
The fastest variant, 802.11n, is capable of a whopping 600 Mbit/sec raw data rate using four streams at 40 MHz channel widths (20 MHz ins b and g). (Figure 1B).
Figure 1B: 802.11n uses 40 MHz wide channels providing double the sub carrier bandwidth to achieve faster raw data rates.
802.11n adds a Multiple Input Multiple Output antenna scheme (MIMO) to the physical layer and through a technique called Spatial Division Multiplexing, independent data streams are decoded and processed in parallel within one spectral channel of bandwidth. Each stream requires its own antenna at both the transmit and receive side, and as a result can add more cost and complexity to the PCB design. More processing power is needed as well since pre-encoding and post-encoding techniques are needed to achieve the desired data rates.
802.11n can also operate in the 5 GHz band which is recommended to achieve full data rates. Range and penetration are not as good with 5 GHz, but there are fewer devices sharing the frequency bands and this will allow more interoperability. This factor is especially important since 802.11n uses double the radio spectrum compared to the older 802.11b and g versions. Consequently, only one 802.11 network can operate on the 2.4 GHz band without interference from other WLAN devices.
Module makers have the same raw parts at their disposal that we would use to create a discrete Wi-Fi solution. They depend on chip makers to provide the ICs and the IP. Often, chip makers will provide some of the IP, especially the protocol stacks, either as a freebee when you buy their chips, or as part of a licensed arrangement.
Chip suppliers will also often provide reference designs that can be cut and pasted into your design. These are good starting points, but beware that these may not flawlessly integrate into your application. Slight variations in magnetics, PCB layouts, and material characteristics will affect performance.
Frequently, chip makers will partner with a third party company to create modules that benefit both companies. The semi companies have access to a refined and finished implementation using their chips and can offer it to customers as a development and evaluation tool. The third party company uses the chip company as a marketing arm, providing credibility and exposure for their products.
Another benefit of the modular approach is that when you attempt your own implementation you must then obtain the proper certifications, confirmations, and interoperability blessings from a testing agency and the Wi-Fi Alliance. This usually has already been done for you when you select a module.
Where to start
The most cost effective and performance-rich modules for immediate use are the 802.11 b/g modules. Since g is backward compatible with b, a low cost and fairly high speed offering can be obtained from multiple suppliers.
As we indicated earlier, with Wi-Fi the data rate isn’t always what the theoretical spec promises. For example, Roving Networks’ excellent RN-131G supports 802.11 b/g, but with a max user data rate of 1 Mbits/sec (Figure 2).
Figure 2: In addition to providing Wi-Fi 802.11 b/g connectivity, the Roving Networks module includes sensors, I/O, Real Time Clock, UART, and FLASH/RAM that can hold user data and code.
However, this module is more than just an OEM radio. Instead of aiming at streaming or data-centric wireless networked applications, this module targets mixed signal and wireless sensors, telemetry, remote equipment monitoring, and other applications that require small size and low power, but not necessarily high speed data.
The surface mount module has an embedded processor that handles all the TCP/IP stack communications as well as power management. It includes an 8 channel analog sensor interface, a real time clock for wakeup and time stamping, 8 Mbits of Flash and 128 Kbytes of RAM, 10 general purpose I/O, and a UART for local host or slave processor interfacing.
At 3.3 V, it consumes only 4 µA while asleep, 40 mA in receive mode, and 210 mA in transmit mode. It also has both an on-board ceramic antenna as well as a U.FL connector for an external antenna if desired.
H&D Wireless offers a suitable module for higher speed data with its up to 54 Mbits/sec HDG104-DN-2 802.11 b/g Wi-Fi module. Claimed to be the smallest Wi-Fi component on the market at 55 square mm (7.7 mm x 7.1 mm), the module is designed to bolt on to an Atmel AVR32 MCU as a host processor.
The 2.7 to 3.3 V H&D module interfaces to the host micro via SPI and provides some digital I/O. It supplies a 40 MHz clock to the processor, but needs a 32 KHz clock back from it. The module incorporates some nice features such as a powerful DMA engine, which minimizes the amount of time a processor needs to stay awake. It also provides programmers with an API-like interface for quick and easy cut and paste software support.
Atmel offers a development kit supporting the H&D radios with its ATEXTWI-FI module (Figure 3). This uses the HGD104 radio on a module that works with the company’s EVK development boards.
Figure 3: Joining forces, Atmel and H&D wireless provide the ATEXTWI-FI module as a test and development platform. Designers can quickly test and evaluate the technology before committing to PCB spins and code development.
Note that this is an example of a module that acts as a peripheral, so it will still require your own embedded host processor design. However, as a modular radio, it is pre certified; CE and FCC compliant, and pre-calibrated, requiring no RF tuning or adjustments. Additionally, these parts come with a programmed MAC address that can also save time and money applying and receiving a block of addresses.
A similar part from Roving Networks is the RN-171, which incorporates an internal 32 bit SPARC processor to seamlessly handle TCP/IP (Figure 4). It comes preloaded with firmware that allows serial communications to a local processor via SPI or via UART. Like the RN131, the RN171 has 8 channels of analog input, a real time clock for wakeup and time stamping, and power management.
Figure 4: Up to 2 Mbits/Sec usable data rate can be obtained from the 802.11 b/g module by using the SPI port. Be aware that the raw host data rate of 54 Mbits/sec is not the usable data rate for the application.
Again, while the maximum data rate the Wi-Fi 802.11 g interface can support is 54 Mbits/sec, these modules do not provide those rates. Instead, they provide the maximum data rates that their serial interfaces can handle. In the case of RN-171, this is 921 Kbits/sec with 500 Kbits/sec for the UART interface, and 2 Mbits/sec for the SPI interface. As such be careful to not confuse the Wi-Fi data rates with usable data rates.
Higher usable speeds
To get closer to the 600 Mbits/sec theoretical data rate promised by 802.11n, you will either have to roll your own design, or look to module suppliers such as Sagrad Inc., to provide OEM modules that achieve 150 Mbit/sec throughputs. For example, the 5 V SG901-1059-5H provides a maximum peak rate of 150 Mbit/sec when taking advantage of the 40 MHz channels of 802.11n (Figure 5). It uses a USB interface to allow the transport of the higher data rates. As such, it is suited for higher end applications including embedded Linux, Windows, and Windows CE.
The self contained SG901 radio module uses a 6 pin 2 mm SIP interface for easy prototyping and testing. The 150 Mbit/sec rate makes this usable for audio and video streaming with more bandwidth left over for other simultaneous COM channels running.
Figure 5: The 802.11n module from Sagrad provides up to 150 Mbits/sec usable data.
For less demanding designs, other selectable data rates can be used including 6, 9, 12, 24, 36, 48, and 54 Mbits/sec.
Dev kits can help
If the offerings of module makers do not meet your design needs you’ll have to design your own modules, or incorporate the module’s functionality onto your PCB. Here is where the Semiconductor Development kits can give you a quick start.
A case in point is the Freescale TWR-WI-FI-RS2101 development system for Wi-Fi designs. The TWR-WI-FI module is part of Freescale’s modular development system based on its Tower architecture. The reconfigurable hardware creates a platform for rapid prototyping, and 802.11n is one of its targets.
TI also has a comprehensive development environment that includes support for wireless LAN and Wi-Fi. The TMDXEVMWI-FI1808L supports the company’s Sitara ARM9 AM1808 applications processor. This is a high end 450 MHz processor for those screaming and demanding applications of which Wi-Fi is just a small percentage of resources.
The daughter card featuring the AM1808 is the TMDXWL1271DC18XL1X; it supports Bluetooth as well as Wi-Fi designs (See Figure 6). A training module from TI is also available on Digi-Key’s website, which describes TI’s Bluetooth offerings.
Figure 6: TI’s development platform for wireless includes the module adapters for the 802.11 development, test and evaluation. An advantage is that the main board can be used to develop other wireless applications like Bluetooth.
Finally, when planning your design remember that Wi-Fi may not be well-suited for all projects. Wireless lighting, for example, could use Wi-Fi in each lamp, but that may not be the most cost and performance effective solution for this application. Power usage, wake up time, communications setup time, as well as cost and size may make other wireless protocols more desirable. (In the example presented a Wi-Fi to ZigBee gateway could be a better solution.)
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