The range of radio frequencies used to convey telecommunications services. In cable TV networks, bandwidth refers to the sizeable spectral region between 5 MHz, and as high as 1 GHz (although most systems top out at 750 MHz, and a handful top out at 870 MHz.)
Lately, “bandwidth” seems to be gaining favor, conversationally, as a synonym for “capacity” and “throughput.” People talk about “high bandwidth” networks, to mean fat pipes. Or they’ll say “high bandwidth” to describe the speeds of their cable or DSL modems, versus “low bandwidth” to describe dial-up telephone connections. This isn’t incorrect, except to be the conversational equivalent of making an end from a means.
How is bandwidth related to spectrum? In a sense, “spectrum” is what you’re born with (5-750 MHz, in cable networks). “Bandwidth” is what you make of it: how you partition it, how you architect networks around it and how you compress the material riding over it.
Understanding cable bandwidth is like looking at a bookshelf. Let’s say it’s an organized shelf, with room for 750 slots – where the 750 correlates to the total bandwidth (in MegaHertz, or MHz) of most U.S. cable systems.
On the far left sit 18 skinny pamphlets. In bandwidth terms, each is about 2 MHz wide. That’s the upstream path, used to convey information from homes back up the network, to the headend. The contents of the upstream path use about 5 percent of the total shelf, specifically located between 5-40 MHz.
To the right of the pamphlets sit 83 hardbacks. Those are the analog channels — basic and expanded basic service. In bandwidth terms, each is 6 MHz wide. In total, the hardbacks use about two-thirds of the shelf, from 52-550 MHz.
(The gap between the upper edge of the upstream, at 40 MHz, and the lower edge of the analog zone, at 52 MHz, is a necessary cushion to prevent upstream and downstream channels from mixing with one another. In our bookshelf model, imagine a volume had been removed and never returned.)
To the right of the hardbacks is a mixture of books. Several are paperbacks. Some are those big, over-sized art books, with the striking photographs, that people always seem to give you when you’re out of town and don’t have time to get to a shipping store.
Together, the paperbacks and the art books are the digital channels, both standard definition and high-definition; broadcast-style and on-demand. Digital channels use about a quarter of the shelf, from 550-750 MHz. (Broadband Internet and voice-over-IP traffic, which is digital, rides here too, but well set them aside for this discussion.)
The “digital paperbacks” are the stuff of today’s digital tiers: Regular TV channels that are digitized and compressed, plus the goods of video-on-demand, subscription-on-demand, and all the other on-demands.
The big books, naturally, are the HDTV channels — gorgeous but, as airline attendants say, heavy and awkward.
The ration of paperbacks to art books depends on how many of each is on the lineup. The 200 MHz of bandwidth that resides between 550-750 MHz translates into about 33 channels in traditional widths of 6 MHz. That means there’s room for 330 or so digital channels (the paperbacks) and no HDTV (the art books). Or, there’s room for 99 HDTV channels, and no “standard definition” digital channels (the paperbacks).
The ultimate mix is usually some combination of both.
Usage: One of the primary concerns of any cable operator is whether there’s enough bandwidth for all the services that could be offered