2. CableLAN Technology

There are actually a couple of competing technologies in the relatively new Ethernet-over-coax arena. Entropic is a chip company whose technology is the basis of the Multimedia Over Coax Alliance's (MoCA) specification. Entropic says their technology has a physical layer rate of 270Mbps and is targeted at supporting multiple high and standard definition digital video and data streams over typical household coax cabling. While Entropic had working demonstrations of its technology in partners Toshiba and Panasonic's booths at January's CES, it's uncertain when real product will hit the shelves.

Coaxsys says its "Pure Speed" system has performance equivalent to 100Mbps Ethernet. The system requires a "Network Hub" to be inserted at the coax distribution point as well as adapters for each coax-to-Ethernet conversion. Though Coaxsys announced its product in May 2003, they still haven't been able to provide product for review.

Corinex, on the other hand, is shipping real product and had a pair of its CableLAN adapters at my door as soon as I was ready for them. Since Corinex is a power line networking company at heart, CableLAN leverages their expertise in HomePlug technology. So CableLAN can actually be thought of as a HomePlug-over-coax product.

Figure 1 shows that CableLAN operates in a 1 -24MHz frequency band, which is below the band used by cable TV signals.

Figure 1: CableLAN and CATV Spectrum use
(from Corinex documentation)

However, since DOCSIS cable modems use a 6 MHz wide channel in the 50 - 750 MHz band for downstream traffic and another 6MHz channel in the 5 - 42 MHz band for upstream data, a passive filter will be typically be required if you have cable modem service and you'll be installing CableLAN (Figure 2).

Figure 2: CableLAN with DOCSIS Cable Modem
(from Corinex documentation)

Corinex is happy to have CableLAN used by home networkers, but their real target is the MDU (Multiple Dwelling Unit) market (apartment houses, hotels / motels, etc.). It turns out that if you have more than 16 nodes on a HomePlug network, performance drops to a crawl. This isn't really a problem for home networks, but can be a killer in MDU applications.

Since MDUs usually have cable TV running to each unit, though, CableLAN can be used in two ways. Small installations (under 15 units - you need to use one node to bridge to the Ethernet network) can be entirely CableLAN based. Larger installations are better served by a mix of CableLAN and straight HomePlug (Figure 3), which allows breaking the LAN into smaller 15 node segments.

Figure 3: Combination CableLAN and HomePlug network
(from Corinex documentation)
(click on the image for a larger view)