Hack 7 900 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. |
لطفا منتظر باشید ...
