Secrets of the Cable Universe #2: Bandwidth, pt 1

As with previous installments (ok, the one :P), this post also isn't so much a "secret" as it is a clearer explanation, with some implications, of widely available information. There's been an increasing fervor building around bandwidth, bandwidth management, and its implications as consumers are now finally beginning to consume rich web content at scale (if "Monkeys kissing women" , "Man in underwear" or crappily digitized stolen content can be considered "rich" :P).

So how much bandwidth does the Cable infrastructure provide? In past lives, I'd always heard that Cable was a "shared" pipe, while technologies like DSL were not - so let's explore what that means.

To go back to first principles, the coaxial cable coming in from the street to your house delivers about 750mhz (or so that's usable) of information spectrum (I'm not going to get into RF modulation and how it works here). In the old days, that 750mhz was split into 6mhz channels, which turned out to be about what you needed to deliver a single ucompressed "standard defintion" (NTSC) audio/video signal - a channel, basically.

And that's why you basically had about 100 or so channels on Cable, and no more, really - there's a limited capacity to what the "cable" from the Cable head-end into your home could deliver. Fundamentally, it was constructed as a multicast technology - broadcast from the Cable company's head-ends, down your street, and split into homes in your neighborhood. Each node from the head end could pump a signal of sufficient strength to service from 200 to 2000 or so homes (really, really, roughly).

And everybody got basically the same content (though so-called "Conditional Access" would encrypt at the head-end and decrypt either at entry to your house, or on your settop box - again, beyond the scope of this discusion).

Then came digital signals, and things got interesting. Turns out, that over these 6mhz bands, you could send, oh, about 40Mbps of information (still multicast, of course - meaning everyone gets the same information). And if you MPEG-2 compressed your video, that worked out to - for Standard-def (SD) video - about 10 or so video streams per channel (figuring 3.5 to 4Mb per video) - or to put it another way, you could pack 10 or more digital "channels" into the bandwidth occupied by a single analog channel.

In part 2, I'll cover how this maps to Internet Connectivity and your Cable modem and how much bandwidth Cable really delivers to the home(its not the number you think - do the math implied above) .