Charter won $1.22 billion in the RDOF grant auction and promised on the short-form to build gigabit broadband. Charter won grant areas in 24 states, including being the largest winner in my state of North Carolina. I’ve had several people ask me if it’s possible to build rural coaxial networks, and the answer is yes, but with some caveats.

Charter and other cable companies use hybrid fiber-coaxial (HFC) technology to deliver service to customers. This technology builds fiber to neighborhood nodes and then delivers services from the nodes using coaxial copper cables. HFC networks follows a standard called DOCSIS (Data Over Cable Interface Specification) that was created by CableLabs. Charter currently uses the latest standard of DOCSIS 3.1 that easily allows for the delivery of gigabit download speeds, but something far slower for upload.

There are several distance limitations of an HFC network that come into play when deploying the technology in rural areas. First, there is a limitation of roughly 30 miles between the network core and a neighborhood node. The network core in an HFC system is called a CMTS (cable modem terminating system). In urban markets, a cable company will usually have only one core, and there are not many urban markets where 30 miles is a limiting factor. But 30 miles becomes a limitation if Charter wants to serve the new rural areas from an existing CMTS hub that would normally be located in larger towns or county seats. In glancing through the rural locations that Charter won, I see places that are likely going to force Charter to establish a new rural hub and CMTS. There is new technology available that allows a small CMTS to be migrated to the field, and so perhaps Charter is looking at this technology. It’s not a technology that I’ve seen used in the US, and the leading manufacturers of small CMTs technology are the Chinese electronics companies that are banned from selling in the US. If Charter is going to reach rural neighborhoods, in many cases they’ll have to deploy a rural CMTS in some manner.

The more important distance limitation is in the last mile of the coaxial network. Transmissions over an HFC network can travel about 2.5 miles without needed an amplifier. 2.5 miles isn’t very far, and amplifiers are routinely deployed to boost the signals in urban HFC networks. Engineers tell me that the maximum number of amplifiers that can be deployed is 5, and beyond that number, the broadband signal strength quickly dies. This limitation means that the longest run of coaxial cable to reach homes is about 12.5 miles. That’s 12.5 miles of cable, not 12.5 miles as the crow flies.

To stay within the 12.5-mile limit, Charter will have to deploy a lot of fiber and create rural nodes that might serve only a few homes. This was the same dilemma faces by the big telcos when they were supposed to upgrade DSL with CAF II money – the telcos needed to build fiber deep into rural areas to make it work. The telcos punted on the idea, and we now know that a lot of the CAF II upgrades were never made.

Charter faces another interesting dilemma in building a HFC network. The price of copper has steady grown over the last few decades and copper now costs four times more than in 2000. This means that the cost of buying coaxial cable in relatively expensive (a phenomenon that anybody building a new house knows when they hear the price of new electrical wires). It might make sense in a rural area to build more fiber to reduce the miles of coaxial cable.

Building rural HFC makes for an interesting design. There were a number of rural cable systems built sixty years ago at the start of the cable industry, because these were the areas in places like Appalachia that had no over-the-air TV reception. But these early networks carried only a few channels of TV, meaning that the distance limitations were a lot less critical. But there have been few rural cable networks built in more recent times. Most cable companies have a metric where they won’t build coaxial cable plant anywhere with fewer than 20 homes per road mile. The RDOF grant areas are far below that metric, and one has to suppose that Charter thinks that the grants make the math work.

To answer the original question – it is possible to build rural coaxial networks that can deliver gigabit download speeds. But it’s also possible to take some shortcuts and overextend the amplifier budget and curtail the amount of bandwidth that can be delivered. I guess we’ll have to wait a few years to see what Charter and others will do with the RDOF funding.