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The key technical difference between MMDS and DBS is the use of ground-based, or terrestrial microwave, rather than geosynchronous satellites. The service difference is delivery of local content, which MMDS achieves by having local production facilities that can insert local over-the-air channels into the national feeds. The Figure shows a schematic of MMDS service.
National television feeds are delivered by the programmer to a production facility.
The feeds can come from geosynchronous satellite transmission or
high-speed wired services, such as fiber-optic networks. Despite what appears to be a good technical fit, there is little current movement to link MMDS with DBS. DBS could provide economic national distribution of programming for resale by
MMDS.
Local content and advertising are acquired over the air, encoded
into MPEG, and multiplexed with the national programming for local
distribution to the viewers. MPEG enables digital multiplexing and
thus is a key facilitator of MMDS. Data services may also be
received from Web content providers. In this case, the information
is in digital format but requires additional processing, such as
encapsulation into MPEG and address resolution, before being
transmitted.
After the programming mix is
determined, composite programming is delivered by satellite or fiber
to the MMDS broadcast tower. Generally, the MMDS headend and the
MMDS broadcast tower are not co-located because the tower should be
placed at a high elevation. At the receiving site, a small microwave
receiving dish a little larger than a DBS dish is mounted outside
the home to receive the signals. A decoder presents the TV images to
the TV set. Other units are capable of decoding data for PC users.
Return-path data is transmitted on another access network; telephone
networks commonly are used for this purpose. For example, it is
possible to have an RJ-11 telephone jack on the set-top box.
Consideration is also being given to other wireless networks, such
as digital PCS and paging networks, for return-path purposes.
The range of MMDS is limited
primarily by line-of-sight. In relatively flat areas,
if the transmitter can be located high enough, the signal can reach
over 50 miles. Pacific Bell Video Services (PBVS), for example,
currently is rolling out MMDS in Los Angeles and Orange counties in
southern California using only two towers. About 75 percent of homes
will be able to receive MMDS signals reliably. The remaining 25
percent are limited by line-of-sight problems.
Because of the availability of
telephone return path, MMDS operators are capable of providing data
service very similar to that of cable. Zenith, Hybrid, and General
Instruments are taking advantage of their data and cable TV
experience to provide data and MMDS modems using telephone return.
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