Wireless Hacks

900 MHz: Low Speed, Better Coverage

Ubiquity is sometimes more important than speed. If you absolutely need to make a link that isn’t possible with 802.11, then this older gear might be for you.

In the days before 802.11, a number of FCC Part 15 wireless networking products were competing in the marketplace. For example, Aironet, Inc. (before it was bought by Cisco) produced the Arlan networking series. The Arlan APs and bridges use 10baseT Ethernet, operate at 900 MHz, and have a data rate of 215 Kbps or 860 Kbps. They also made a number of complementary PCMCIA radio cards (the 655-900, 690-900, and PC1000, for example). These devices put out up to a whopping 1 Watt at 900 MHz. NCR had the WaveLAN 900 MHz line that included an ISA and PCMCIA card that would push 2 Mbps at 250mW. While the data rate can’t compare to modern wireless networking gear, the higher power and lower frequency of this equipment offers significant advantages.

As the frequency of a signal increases, the apparent range it can cover at the same power and gain decreases. For example, a 100mW signal at 5.8 GHz appears to travel less than half the distance of a 100mW signal at 2.4 GHz, which appears to travel less than half that of a 100mW signal at 900 MHz. There is no limit to how far a signal can actually go, but its ability to rise above the background noise and be detected at a usable level is bounded by its power, frequency, and antenna gain. So to put it simply, all other variables being equal, ...