User's Manual

6 MDS i Net 900 Installation Guide MDS 05-2873A01, Rev. A
DRAFT 6—8/29/01
3.2 Site Selection
For a successful installation, careful thought must be given to selecting
proper sites for the Access Points and Station Adapters. Suitable sites
should provide:
• Protection from direct weather exposure
• A source of adequate and stable primary power
• Suitable entrances for antenna, interface or other required
cabling
• Antenna location that provides an unobstructed transmission
path in the direction of the associated station
( s )
These requirements can be quickly determined in most cases. A possible
exception is the last item—verifying that an unobstructed transmission
path exists. Radio signals travel primarily by line-of-sight, and obstruc-
tions between the sending and receiving stations will affect system per-
formance. If you are not familiar with the effects of terrain and other
obstructions on radio transmission, the discussion below will provide
helpful background.
Terrain and Signal Strength
While the 900 MHz band offers many advantages over VHF and lower
UHF frequencies for data transmission, the band is more prone to signal
attenuation from obstructions such as terrain, foliage or buildings in the
transmission path.
A line-of-sight transmission path between the central Access Point sta-
tion and its associated remote access station site
(
s
)
is highly desirable
and provides the most reliable communications link. A line-of-sight
path can often be achieved by mounting the station antenna on a tower
or other elevated structure that raises it to a level sufficient to clear sur-
rounding terrain and other obstructions.
The importance of a clear transmission path relates closely to the dis-
tance to be covered by the system. If the system is to cover only a limited
geographic area, say up to 3 miles (4.8 km), then some obstructions in
the transmission path can usually be tolerated with minimal impact. For
longer range systems, any substantial obstruction in the transmission
path could compromise the performance of the system, or block trans-
mission entirely.
Much depends on the minimum signal strength that can be tolerated in
a given system. Although the exact figure will differ from one system to
another, a Received Signal Strength Indication (RSSI) of –80 dBm or
stronger will provide acceptable performance in many systems. While
the equipment will work at lower signal strengths, this provides a “fade