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Is DSL dedicated, while Cable modems shared bandwidth ?

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A common misconception is that residential DSL is dedicated bandwidth, while Cable modems provide shared medium.

This is only partly true - for the segment between you and the ISP's central office, and that is rarely the bottleneck of the connection. From the Central Office out to the Internet, both Cable and DSL share your ISP's backbones, whatever they are. Residential broadband is oversubscribed, whether cable or DSL - usually with 20+ times as many subscribers as the maximum backbone capacity. Since your ISP's backbones and peering arrangements are often the bottleneck of the connection, and it is shared medium, both residential DSL and Cable may experience slowdowns at peak times.

One can argue that DSL is dedicated between you and the Central Office (and shared from there on), while Cable is shared for that "last mile" segment of the connection as well. However, cable technology is able to push much higher bandwidth over that last mile to support multiple clients, and the signal does not deteriorate nearly as fast because of distance. Because of this, cable modem technology may be somewhat more prone to variations in speed than DSL, however, it usually offers higher average throughput.

Note: One can compare a speed test from their ISP's servers (local/nearby test) vs. a speed test from a distant location to determine whether the speed limitation is in the last mile.


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by bsdGuru - 2012-03-13 09:18
This is fundamentally wrong. By the logic used here, all bandwidth is "shared", unless you have a direct line to the server you're accessing. A cable modem is the same as a dedicated line. Of course it's shared at the router or at the ISP backbone, but the cable companies have such large backbones that it's effectively dedicated. DSL is shared by definition; a DSLAM is a physical multiplexor that shares by design; DSL is oversold by a factor of 30 or so. The actual bandwidth that is dedicated is undefined; providers will oversell at different levels.

Cable is vastly superior to DSL for the same advertised bandwidth.
by anonymous - 2013-06-13 12:35
Thats funny because Cable companies can segment areas in which all customers on the segment use the same bandwidth to the local box up to the cable company. How do I know this , because the base I live on has overloaded the cable companies lines and at 7pm you can only pull 2.5 mps of the 60 paid for. When the tech came out he explained there was nothing that could be done because of the infrastructure. I switch to DSL and now get 16 mps of the 20 I pay for. Comcast loses, Qwest Wins!
by anonymous - 2014-02-06 22:09
When people say they are sharing a line, they are referencing the last mile. Bullshitting them by saying that everyone shares the same line anyhow at the ISP is just a way to avoid saying the truth. Cable modems last mile solutions are a SHARED line(more traffic usage, less speed). Where as DSL's last mile is a dedicated line directly from you to them. Referencing the connection from the ISP to the internet and then pointing and saying see everyone's sharing the same line is complete bs and these internet companies know it.

The backbone is not the bottleneck of a connection, completely false. It has always been the last mile. In which case DSL vastly superior to cable as the last mile for DSL is a dedicated line to your home, the last mile for cable modem is a shared line. Neighbors get online, your shared line is being used and maximum data transfer rate is now split in half. On DSL if my neighbors get on, my data doesnt slow down. The reason for this is my dedicated line can send the data to the ISP unfettered by other traffic. Only when the ISP recieves and forwards the data on is there a shared line being used. The shared line the data is sent on is at that point, no different than a cable isp. But if my data is slowed down before it gets to that point which is the case, DSL would be better as it is a dedicated last mile solution.
by anonymous - 2014-02-06 22:19
The term Last mile was created as a direct result of the shared connection from isp to home. The problem is the last mile with bandwidth. Completely false to say it bottlenecks at the isp hardware.
by anonymous - 2014-11-08 08:47
Cable can be superior because often the highest available bandwidth is much higher, but typically has a wide range of experienced speeds, due to the shared bandwidth. However, dsl speeds are very consistent because of the dedicated line.

Sharing bandwidth at the CO isn't any different from sharing bandwidth at the cable company and both face similar challenges when there's an increase in subscribers that out paces the existing equipment. What is very different is that shared line to your house with cable versus the dedicated line with dsl. Peak hours can suffer a 10-20x reduction in speed in my experience. There are no "peak hours" with dsl.
by anonymous - 2016-06-03 22:59
Then I wish someone would factually explain why my 20Mbps DSL, once again on a Friday night, is currently only hitting 1.2Mbps. Come morning, it'll be back to 19-20.
by Philip - 2016-06-04 07:23
Congestion... Everyone is streaming Netflix 5pm through midnight. It usually means the capacity in your area is not adequate, it can happen to both Cable and DSL. As described in the FAQ, DSL is not immune to congestion as both technologies are oversubscribed.

I was suffering such severe congestion issues at peak times with cable only once in the past 20 years, and my experience is detailed here: http://www.speedguide.net/articles/network-adapter-brand-affects-latency-4925
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