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, lower frequency signals travel further than higher frequency signals. You can make higher frequency signals appear to travel further, but to do so you need to increase the power, antenna gain, or both.

Another curious property of radio is that the requirement of having line of sight between the devices becomes more important at higher frequencies, but is less critical at lower frequencies. Higher frequencies don’t fare so well when there are obstacles between the ends of the radio link (particularly in urban and indoor settings). This property, combined with the advantage of greater range, means that 900 MHz equipment can be used in a variety of situations where 802.11b/g or 802.11a don’t fare as well. It can penetrate foliage, buildings, and other obstacles better than its 802.11 counterparts. Of course, the big trade-off is throughput.

Pros

• Higher power and superior range.

• Equipment doesn’t compete with the increasingly crowded 2.4 GHz ISM band, but must still tolerate 900 MHz phones, video cameras, baby monitors, and other devices.

Cons

• Low data throughput, from serial speeds of 9,600 bps up to 2 Mbps or so.

• Very little vendor interoperability.

• With the advent of 802.11 networking, 900 MHz gear has increasingly limited availability.

• Equipment can be quite expensive compared to 802.11 gear.

Recommendation

A number of manufacturers offer serial or Ethernet to 900 MHz bridges. While Ethernet is generally preferable, the serial devices are perfectly capable of supporting a PPP connection between two sites. If you need to create a long distance point-to-point link (particularly where clean line of sight just isn’t possible) and can cope with limited data rates, then this equipment might be right for your project. Expect the hardware to be difficult to locate and a bit more expensive than the typical consumer grade 802.11b equivalent.