Microwave Data Systems iNET 900 Series, Руководство пользователя

Страница: 139 / 180

MDS 05-2806A01, Rev. E.1 MDS iNET 900 Series User’s Guide 131
Invisible place holder
Invisible place holder
.
Figure 5-3. Mounting Bracket Dimensions
5.1.2 Site Selection
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 as unobstructed a transmission
path as possible 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.
5.1.3 Terrain and Signal Strength
While the license-free 900 MHz band offers many advantages for data
transmission services, signal propagation is affected by attenuation from
obstructions such as terrain, foliage or buildings in the transmission
path.
A line-of-sight transmission path between the central transceiver and its
associated remote site
(
s
)
is highly desirable and provides the most reli-
able communications link.
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 –77 dBm (–80
dBm for iNET II) or stronger will provide acceptable performance in
many systems. While the equipment will work at lower-strength signals,
signals stronger than – 77 dBm provide a “fade margin” of 15 dB to
2.75
˝
(7 cm)
7.25 ˝ (18.4 cm)