Nuts & Volts - April 2005

The Design Cycle

results in putting the TRENDnet TEW-222CF wireless LAN CompactFlash card on the air.

We’ve already discussed the init_USART and init_cf_card functions. The fun begins with the call to airdrop_config(BSS). This tells the TEW-222CF that we want to participate in an infrastructure network (BSS) and ultimately associate with an access point. The next call to airdrop_config(SSID) is optional. A string can be defined with the name of the particular network (BSS) you would like to join. The service set identifier (SSID) is a 32-character identifier that you assign to your access point/router and is used to differentiate one wireless LAN from another. For instance, my lab wireless LAN has an SSID of “EDTP.” To join the EDTP BSS, I must code the AirDrop-P 802.11b driver in the appropriate location as follows:

The TRENDnet TEW-222CF Link LED is still flashing at this point, indicating that we are still in the setup process and haven’t made any real attempts to get on the air. The next items we need to check off of our shopping list are the MAC and IP addresses for the wireless NIC. The TRENDnet TEW-222CF’s MAC address is garnered electronically via an 802.11b driver call, and the IP address is defined inside the 802.11b driver code as follows:

char ipaddrc[ 4] = { 192,168,0,151 };

The downside of just copying the CompactFlash card’s MAC address and hand coding it in is that, if you change CompactFlash cards, you’ll have to reenter the new MAC address manually.

We’re just about to enter the EDTP access point’s air space. The next set of function calls in Listing 3 turn on the 802.11b radio and send probe messages out into the ether. The TEW-222CF is actually “fishing” at this point. The TEW-222CF’s Activity LED is flashing as data is passed from the now active TEW- 222CF NIC to any access point in the range of the TEW-222CF’s probing

radio signal. If everything we’ve done this far was correct, in less than a second the TEW-222CF’s Activity LED will flicker and extinguish and the TEW-222CF’s Link LED will glow steadily. This indicates that the AirDrop-P has contacted an access point and gotten a response from the access point, telling it who the access point is and what capabilities the access point has.

The AirDrop-P then associates with the selected access point (in our case, the “EDTP” access point), authenticates itself with the access point — if necessary — and joins the EDTP access point’s BSS. The AirDrop-P can now send and receive meaningful data on the wireless LAN with the help of the EDTP access point.

Leaving the (Air)Drop Zone

char tempstring[] = “EDTP”; //Your desired SSID here

By leaving the string null (“” instead of “EDTP”), the first available BSS will be selected by the TEW- 222CF CompactFlash NIC for association and connection.

E l e c t r o n i c s NUTS & VOLTS

F o r

E v e r y t h i n g

Now that a wireless communications channel has been established, the AirDrop-P can communicate with other Ethernet devices on the wireless LAN and wired LAN segment, using standard Internet protocols.

The wireless LAN world is very secretive on the outside; however, you can use the basic 802.11b driver code I’ve provided in conjunction with open source 802.11b Linux examples from the Internet to gain enough knowledge (and operational code) to easily put an AirDrop-P on your local Ethernet LAN. There’s also a dedicated Internet user group for those of you that are interested in sharing AirDrop-P experiences and source code. If you’re the least bit interested in wireless LAN communications with microcontrollers, I highly suggest monitoring the AirDrop-P user group’s chatter. NV