A DOCSIS cable network is effectively a radio network that operates only inside the coaxial cable. This is why you will hear cable network capacity described using megahertz, which is a measure of the frequency of a radio transmission. Historically cable networks came in various frequency sizes such as 350 MHz, 650 MHz or 1,000 MHz.
The size of the available frequency, in megahertz, describes the capacity of the network to carry cable TV channels or broadband. Historically one analog TV channel uses about 6 MHz of frequency – meaning that a 1,000 MHz system can transmit roughly 167 channels of traditional analog TV.
Obviously cable networks carry more channels than this, which is why you’ve seen cable companies upgrade to digital system. The most commonly used digital compression scheme can squeeze six digital channels into the same frequency that carries one analog channel. There are new compression techniques that can squeeze in even more digital channels into one slot.
In a cable network each slice of available frequency can be used to either transmit either TV channels or else be used for broadband. If a cable companies wants more broadband capacity they must create room for the broadband by reducing the number of slots used for TV.
It is the overall capacity of the cable network along with the number of ‘empty’ channel slots that determine how much broadband the network can deliver to customers. A cable system needs roughly 24 empty channel slots to offer gigabit broadband download speeds. It’s a lot harder to carve out enough empty channels on smaller capacity networks. An older cable system operating at 650 MHz has significantly less capacity for broadband than a newer urban system operating at 1,000 MHZ or greater capacity.
One of the primary benefits of DOCSIS 3.1 is the ability to combine any number of empty channels into a signal broadband stream. But the task of upgrading many older networks to DOCSIS 3.1 is not just a simple issue of upgrading the electronics. If a cable company wants the faster broadband speeds they need to also upgrade the overall capacity of the network. And the upgrade from 350 MHz or 650 MHz to 1,000 MHz is often expensive.
The higher capacity network has different operating characteristics that affect the outside cable plant. For example, the placement and spacing of cable repeaters and power taps is different in a higher frequency network. In some cases the coaxial cable used in an older cable networks can’t handle the higher frequency and must be replaced. So upgrading an older cable network to get faster speeds often means making a lot of changes in the physical cable plant. To add to the cost, this kind of upgrade also usually means having to change out most or all of the cable settop boxes and cable modems – an expensive undertaking when every customer has multiple devices.
The bottom line of all of this is that it’s not necessarily cheap or easy to upgrade older or lower-capacity cable networks to provide faster broadband. It takes a lot more than upgrading the electronics to get faster speeds and often means upgrades the physical cable plant and replacement of settop boxes and cable modems. Cable operators with older networks have to do a cost/benefit analysis to see if it’s worth the upgrade cost to get faster broadband. Since most older cable systems are in rural small towns, this is one more hurdle that must be overcome to provide faster broadband in rural America.