Coax For Cable Forms A Super Home Network

electronic design
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12-03-07

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.

The Home Phone Networking Alliance (HPNA) standard uses the existing built-in twisted-pair phone lines and cable TV coax together to carry the video. It utilizes the bandwidth well above the voice frequencies used by the phone and above the bands used for DSL if it�s used as the broadband connection.

HPNA appears to work well, with claims of data rates to 240 Mbits/s. But can it sustain multiple uncompressed HD streams reliably? Some carriers who plan to offer in-home video on demand (VoD) and Internet Protocol TV (IPTV) have already adopted it. For example, AT&T selected the HPNA solution for its U-verse IPTV system, which is now being deployed.

The HomePlug Alliance also has a standard that permits high data rates up to about 200 Mbits/s using the standard home ac power wiring. That is certainly an option, but like HPNA, how well it can handle multiple HD streams remains to be seen.

MOCA ON TOP
On the other hand, MoCA uses coax, which was selected to deliver video in the first place. The cable has the bandwidth and it already exists in most places, making it a truly desirable option for home networking.

The MoCA organization, which now includes more than 50 members, has certified over a dozen bridges, routers, gateways, and modems. Furthermore, MoCA is incorporated in more than 700,000 of the home systems in Verizon�s FiOS passive optical fiber network for delivering video and other broadband services. That is expected to grow to 18 million homes by 2010.

Also, the c.LINK EN2210 chip set from MoCA founder and member Entropic Communications implements the MoCA 1.1 standard. With Broadcom and Conexant recently joining MoCA, we might expect to see some additional second-source silicon in the near future.