ALL Internet connections share bandwidth at some point. DSL, dial-up, and most data services offered by the phone companies start to share bandwidth at the phone company central office. The Internet services offered by cable companies to most residential users start to share bandwidth earlier in the connection then that.
A node is a piece of equipment that connects the cable system fiber optic network of an area to the coax cable network distributed throughout the local neighborhood. Nodes are really just media converters, connected to similar equipment in the cable head-end, used to bring the signal from the cable head-end to a neighborhood without the long series of amplifiers previously needed to do so.
Nodes are important because due to the way cable systems are built, nodes are the common aggregation point for all the cable signal traffic in a particular neighborhood. This means all users connected to one node share the bandwidth available to that node.
The number of users per node depends on subscription rate, number of homes passed per node, and general system 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% cable modem subscription rates for an area. This means anywhere from 10-500 active modem customers are connected per node in most cases, depending on local cable system design.
Bandwidth limits in cable modem networks are not per node. Bandwidth limits in cable systems are defined by the upstream or downstream channels assigned. Nodes aren't the limit because they can be configured to use larger data channels or multiple data channels for more bandwidth.
Often due to low bandwidth utilization, several nodes are combined together in the cable "headend" before they are connected to the ports on the CMTS, so another way a provider can increase available bandwidth to an area is to "decombine" a number of nodes using the same data channel.
The bandwidth available per CMTS data channel in DOCSIS 1.0, 1.1, and 2.0 CMTSs is about 30 mbps per downstream channel using QAM 64 modulation and about 42 mbps using QAM 256 modulation. Total upstream bandwidth is limited to about 5 mbps per upstream channel using QPSK modulation and 10 mbps using QAM 16 modulation for DOCSIS 1.0, 1.1, and 2.0 CMTSs Up to 30 mbps total upstream channel bandwidth can be made available on DOCSIS 2.0 CMTSs using DOCSIS 2.0 modulation rates. Actual usable data bandwidth is 10-15% lower due to the bandwidth overhead of DOCSIS protocols.
Every cable provider using DOCSIS cable modem equipment has these same bandwidth limitations.
This works because everybody connected in a neighborhood isn't using their allowed allotment of bandwidth every second of every day and cable modem systems are designed to take advantage of that. This is also why every single cable ISP prohibits customers running servers on computers connected to the cable modem because constant use by a single modem reduces overall capacity available for everyone else on those CMTS ports. If more than a few servers are active very little bandwidth would be left for possibly hundreds of other users.
Obviously more customers per node and CMTS port is more cost effective for the cable company, but pushing it too far can cause bandwidth capacity overload.
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Try this article, its older but gives a good feel for the situation:
http://www.spectrum.ieee.org/WEBONLY/publicfeature/jun01/cmode.html
Here's a few good sites for an overall explanation:
http://computer.howstuffworks.com/cable-modem.htm
http://cable-modems.org/tutorial/
http://cabledatacomnews.com/cmic/
For a technical description of what a modem does to get online:
http://www.scte.org/chapters/cascade/DOCSIS_SCTE_detail_2.pdf
Here's an ACLU comissioned report on open access which details how cable systems work in the appendix:
http://www.aclu.org/Files/getFile.cfm?id=10520
This was taken directly from a post (and edited) by MacLeech  in the Adelphia forum here.
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by KeysCapt edited by MacLeech
last modified: 2005-09-01 12:57:02 |
