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

Continuing from Part 1.

To cut to the chase, your coaxial cable (and hence your cable company) is capable of delivering (roughly somewhat less than) 5Gbps into your house.

The math for that number is pretty simple. There's about 750mhz (or so) or RF spectrum avaible, divided into 6mhz "channels". Why 6mhz? Because that's about how much spectrum you need to deliver an uncompressed analog NTSC AV signal - in the digital world, that translates into (about) 40Mbps. So 40*125 = 5 (you know, adjusting for decimal places).

That sounds like a big number, and it is, but there are a few mitigating factors, as discussed last time.

First, it is a "shared" connection. There a certain number of households grouped into a "service group", usually between 200 to 2000 (very broadly), which connect to some physical networking gear at the cable plant (I use the term "plant" very loosely here). Within that service group, DOCSIS (the cable networking data interface protocol) basically works like a form of encrypted ethernet. Everybody in that service group sees all the packets, but cannot decode those packets.

So right off the bat, your effective sustained speed (more on this concept in a bit), is 5Gbps divided by [number of homes in your service group].

Additionally, some portion of that spectrum, those channels, is consumed by television - In fact, almost all of it. Today, only a single 6mhz channel is allocated to your High Speed Data (HSD) connection. That's a big part of what DOCSIS 3.0 promises - the ability to "bond" channels to effectively multiply the available bandwidth by N number of 6mhz channels.

Assuming there's channel spectrum to allocate...

In digital form, some 10 to 16 or so Standard-Def (SD) channels can fit in a single 6mhz channel (multiplexed into a single MPEG container over that channel, for those curious - this is also important because it has implications for compressions; specifically regarding CBR v. VBR). Two to maybe 3 or four High-Def (HD) channels can fit in the same 40Mbps. The range incidentally, is largely a function of compression quality per digital channel; this is probably worth a future post.

Short version: you get either 1 analog channel, 10 to 16 (or so) SD digital channels, or 2 to 4 (or so) HD digital channels.

Thr rub is this: most of the U.S. is still analog. Or at least, enough of it that most cable operators carry around 80 or so analog channels (out of a possible 110 or so). Then they consume another 6 to 8 or so "double carrying" the same channels in digital form, and the rest is allocated to HD channels (triple carrying many channels) and VOD (Video-On-Demand - more on how this works in a future post also).

Which doesn't leave a lot of room for your HSD connection.

Interesting, the core technology and bandwidth available compares reasonably favorably even with newer technologies like optical fiber (remember that we're talking about the connection to the home from the "edge" of the network - "inside" the cable network is often optical fiber, already; the challenge here is the "last mile"). Mostly the advantage of networks like FiOS is smaller service group sizes (owing to larger capital investments and other "late mover" advantages), and less legacy encumberances.