There’s a growing need to
transport video and audio
around the home between
HDTV sets, DVD players,
PVRs, and even PCs.
Now, a technology sponsored
by the Multimedia over Coax
Alliance (MoCA) ensures the quality
of service (QoS) required for tripleplay
delivery of voice, video, and data
via the coax wiring that runs
throughout most homes.
USING F-CONNECTORS
MoCA’s proprietary design uses coax
cabling that’s already installed in over
90% of the estimated 110 million
homes in the U.S. That cabling delivers
video and high-speed Internet
service from the local cable supplier in
the 5- to 860-MHz range. Satellite
receivers may also use the cable bandwidth
from 950 to 2450 MHz. MoCA
uses the cable bandwidth from 860 to
950 MHz for its broadband networking
functions.
The MoCA spec divides that bandwidth
into 50-MHz channels and modulates the
digital video on them using 256-subcarrier
orthogonal frequency-division multiplexing
(OFDM) in each channel. Adaptive
modulation using binary phase-shift
keying (BPSK) to 256-QAM (quadrature
amplitude modulation) adjusts to the specific
characteristics of the network. A
time-division multiple-access (TDMA)
method with time-division duplexing
(TDD) is used to assign time slots.
Most coax wiring is RG-6 or RG-
59/U, which is the 75-O low-loss cable
that’s more than able to handle signals
in that frequency range over short distances
within a home. The standard
relies on the ability of the splitters normally
used in home cable installations
to work in both directions for twoway
communications between any
nodes on the cable.
Contrary to popular belief, the splitters
(passive devices designed for oneway
transmission) easily work in both
directions. Any room with an F-connector jack will automatically be able to
speak to any central video source or any
other node (see the figure).
Providers of high-definition video find
the MoCA solution so desirable because
the cable can reliably deliver up to 175
Mbits/s of gross data speed and easily
more than 100 Mbits/s of net speed,
which is enough to handle multiple
SDTV or HDTV streams uncompressed.
Name one other home-networking technology
that can do that!
Version 1.1 of the MoCA standard
incorporates parametrized QoS (PQoS).
This feature manages the cable bandwidth
and the prioritization of multiple
streams of HD content and packet
aggregation, achieving a net throughput
of 175 Mbits/s. MoCA 1.1 also increases
the number of network nodes from
eight to 16.
THE COMPETITION
Other contenders
for the home-networking
space also can deliver the triple play,
with 802.11 Wi-Fi being the most
widely used method. But even in its
fastest current form, 802.11g, Wi-Fi
can only deliver up to 54 Mbits/s peak,
and that’s rarely achieved. More common
is just less than half that, even
under the best conditions.
When walls, ceilings, floors, and other
obstacles interfere, the signal level drops
off and the data rate follows to maintain
a link. Such speeds are more than adequate
for Internet access but not good
for quality video.
The latest Wi-Fi standard, 802.11n,
helps in delivering video. The new Wi-
Fi routers and gateways use multipleinput/
multiple-output (MIMO) to
achieve higher speeds of up to about
100 Mbits/s. But with widely varying
propagation paths in a home environment,
that may be marginal at best.
Wireless is tempting, since it is so
easy to set up and no new wires are
needed. Yet it remains to be seen
whether 802.11n will deliver the kind
of performance and QoS that consumers
expect for their video. Still, it’s
a prime contender.
Ultra-Wideband (UWB) easily provides
up to 480 Mbits/s, but over a
range of less than 10 m. It’s not the
best option for whole-house or roomto-
room links. For short-range interconnections
between video boxes,
though, it’s a great low-cost winner.
Some better longer-range solution is
still needed. Still in development,
Wireless HD uses the 60-GHz band to
deliver the fastest speeds so far, but
range remains an issue.
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