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.

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) .

Secrets of the Cable Universe #1: VOD ffwd/rwd

I've now been working at Comcast for about 6 weeks-ish, and so far, I'm having a blast. Both culturally and technically its a fairly dramatically different environment (along a different axis than the small-to-big transition that AOL was... more on that another time) - which is always fun.

Although I'm finding my skills, talents, and experience are useful (thankfully), the whole Cable/Telecom universe is completely new to me, so the learning curve is both vast and interesting.

In that vein, I though I might share some of the random but interesting technical tidbits that manifest themselves in odd ways, whether operationally or in terms of the consumer experience. Nothing I'll share is (obviously) actually a secret - its either public information and/or subject to trivial induction from public information.

For example, one of the significant features all the Cable operators (MSOs) have introduced over the last few years is Video-on-Demand (VOD). Unlike the "your-movie-starts-every-15-minutes -on-4-different-channels" model that the satellite providers started with, the new VOD systems actually dynamically allocate a unique "physical" channel from your local cable head end when you select a movie. The video asset is then played over that channel which your set top box (STB) is then tuned to automatically, as if it were any other channel in your channel line up.

So the interesting "secret" is that in order to enable fast forwarding (and rewinding) of the assets, each media file actually has additionaly "trick files": copies of the asset at +/- 2X (or whatever the speed multiplier is). When you press the ffwd or rwd button on your remote control while watching a VOD asset, it's actually dynamically switching to another asset at the right time code, and playing from there.

And that is why you (currently) only have one speed for fast forwarding or rewinding: more would have required many more multiple media assets (one at each speed) in the VOD storage systems.

Clever, but strange...