Each channel is 6 MHz. 2 says early systems 200 MHz (33 channels), now up to 550 MHz (91 channels). But 2 seems old (maybe year 2000?), and 5 shows up to 1000 MHz bandwidth. 2 says post-1989, digital cable and mpeg compression allowed 10 TV channels on a single 6 MHz bandwidth analog channel. Digital Cable also allowed them to use encryption (instead of just simple scrambling) (also allows more data). Of course in 2011, we have HDTV and also better video compression.
1 says we share a single CMTS with our neighbors, possibly 4,000 to 150,000+ cable modems. The CMTS (cable modem termination system) connects coaxial connections to the internet via ethernet interfaces, which is HFC (hybrid fiber coax).
1 claims our internet upstream is a single 2 MHz channel, and our download goes over a single 6 MHz channel… Shared with hundreds of neighbors! The upstream is time-shared. While the downstream is just shared. My interpretation is, we share a single wire with hundreds of neighbors, with a bandwidth of 0 to 1000 MHz. Most of this wire is used to send everyone the same TV channels (ie, we get every channel, but to view it we need a TV box that decrypts the channels that we subscribe to). We time-share (with hundreds of neighbors) a single 2 MHz band for uploads. We use a single 6 MHz channel for download, shared with hundreds of neighbors. So everything our neighbors download, we also download. The only thing that stops us from seeing our neighbors’ downloads is encryption, and the fact that our cable modem filters out the packets that don’t have our cable modem’s address on it.
1 seems to be from 2000 (ie, 11 years old). It does mention that the cable provider can decide to add a new channel for internet, and split the base of users that share that channel. So this could mean that even if our shared wire covers 1000 people, that could be split into 100 people over 60 MHz. So, our internet might be on the 900 MHz band, shared with 100 neighbors. Plus, on the 906 MHz band, is another different 100 neighbors’ internet connection. etc
Because our connection is shared with lots of neighbors, they throttle our connection based on usage, and in some locations are considering per-user bandwidth caps.
If it were up to me, I would say 1000 / 6 = 166 channels, so we should just ditch the cable TV channels, and dedicate them all to internet downloads! However, there’s probably other factors to consider beyond my simplistic internet-greedy preferences O:-)
Economics: sending everyone the same TV channels is an efficient way to send lots of video data. They can also keep the price of internet lower, by separately selling the cable TV bandwidth (and phone line bandwidth).
Bottleneck: There might be a bottleneck somewhere else, such as the CMTS. I’m totally speculating on this.
Finally, keep in mind that I think the articles I read (1, 2) were from 2000, and a lot might have changed since then. So maybe in 2011, more of the 6 MHz channels are dedicated to internet? I’d be curious to find out more.
Links:
1. http://www.howstuffworks.com/cable-modem.htm/printable
2. http://electronics.howstuffworks.com/cable-tv.htm
3. http://www.linktionary.com/c/cabledata.html
4. http://en.wikipedia.org/wiki/Cable_modem_termination_system
5. http://en.wikipedia.org/wiki/North_American_cable_television_frequencies
6. http://en.wikipedia.org/wiki/Bandwidth_throttling
7. http://en.wikipedia.org/wiki/Bandwidth_cap
8. http://lifehacker.com/5215011/which-is-worse-bandwidth-caps-or-throttling
Pem (Admin) :: 2011/01/23 (Sunday, January 23, 2011) ::
Computers / Tech, Internet / Web ::
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