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MacLeech
The one and only
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join:2001-07-14
SoCal
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2 recommendations

reply to justice1776

Re: How's does it all really work?

Click for full size
said by justice1776:
25 users per node...
The number of users per node depends on subscription rate, number of homes passed per node, and general system design architecture. Homes passed per node can be anywhere from 100-2000 and subscription rate depends on competition, marketing, and system performance. Most cable companies design for 10-25% subscription rates for cable modem customers.

So this means anywhere from 10-500 modem customers per node depending on system architecture in most cases.
said by justice1776:
...and each node providing 38 megbits?
Is this up and down? or 38 for download?
The bandwidth limits are not technically per node, they are per port on the CMTS. Ports is this case refer to the physical connections on the CMTS (modem controller in the cable office headend) in which 1 or several nodes can connect to. Number of ports on a CMTS can vary widely depending on manufacturer and design, but normally for every downstream port there are 4-8 upstream ports.

As far as bandwidth, DOCSIS 1.0, 1.1, & 2.0 can do a little less than 30 mbps per downstream port using QAM 64 modulation and about 38 mbps using QAM 256 modulation. Upstream is limited to about 5 mbps per upstream port using DOCSIS 1.0, 10 mbps using DOCSIS 1.1, and 30 mbps using DOCSIS 2.0.

Altogether, this means anywhere from 10-1000 customers might be sharing the bandwidth of 1 pair of ports (keep in mind every node connects to at least 1 downstream port and 1 upstream port) and a single CMTS might serve 10 to 20,000 customers.

Obviously more customers per port per CMTS is more cost effective, but pushing it too far can cause bandwidth overload.

Try this article, its older but gives a good feel for the situation:
»www.spectrum.ieee.org/WEBONLY/pu···ode.html

Here's a few good sites for an overall explanation:
»computer.howstuffworks.com/cable-modem.htm
»cable-modems.org/tutorial/
»cabledatacomnews.com/cmic/

For a technical description of what a modem does to get online:
»www.scte.org/chapters/cascade/DO···il_2.pdf

Here's an ACLU comissioned report on open access which details how cable systems work in the appendix:
»www.aclu.org/Files/getFile.cfm?id=10520
--
Adelphia SoCal, Cable modem manual (PDF), CATV Broadband Training Online


[text was edited by author 2003-04-19 16:35:33]


SOLdesign
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niiiiiiiice! Mac rocks
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RR Conductor
NWP RR Inc.,serving NW CA
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Redwood Valley, CA
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reply to MacLeech
Thanks for all the info Mac! That diagram would need to be adjusted for our area though, we only have one headend, and that's the one in Ukiah. The Ukiah headend serves the entire county of Mendocino(Ukiah, Willits, Redwood Valley, Calpella, Fort Bragg, Little River, Mendocino, Albion, Caspar), and is both a local and regional one, did I get all that right Mac?
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