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Is DSL dedicated, while Cable modems shared bandwidth ?

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.

  User Reviews/Comments:
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:
by anonymous - 2016-10-25 13:41
Why is then that when I checked the wireless speed yesterday I was pulling about 3.4 Mbps and today, at about the same time I'm pulling 10.2 Mbps? No more or no less devices currently running on the network. I pay for 12 Mbps and have difficulty streaming stuff, and my speeds and frequency of "buffering" does change. I have DSL and, if my neighbors usage doesn't impact my broadband availability, why the dramatic change from yesterday to today?
by Philip - 2016-10-25 14:03
Not only your neighbors, but everyone on your node, in your area, and using your ISP can have some effect on the connection. With wireless, your neighbors can affect your connection just by being in the same coverage area and overlapping channels / causing interference even without using the same ISP.
by Trex - 2018-07-30 03:08
I've heard the expression "there are no stupid questions" more than a few times over the past 3 decades, but ... aside from the 30 years experience with computer systems, (yeah... since 1985...) I also hold an Associates degree in Computer Networking. Needless to say, I feel pretty ignorant about the inquiry I'm about to post here... oh well... thank God for 'partial' anonymity, eh?

Anyway, with my limited budget, I've remained on a 10 Mbps DSL connection for a solid 2 years now, at least. One 'phenomenon' in particular confuses me as it seems to contradict any and all information I've heard, learned, or read about DSL internet service. At one point in time, there were 3 wired and 3 to 4 wireless devices on my network, and, being a 'geek' at times now and then, I would tinker around, monitor different systems, connect one computer to the other through the command line, etc. etc. I couldn't help noticing each device seemed to have its OWN 10 Mbps bandwidth.

Am I losing it, or does this sound totally wrong? I opt for the cheapest internet package with bandwidth most would consider laughable... and its not 10 Mbps for the entire household? I know DSL is a DEDICATED subscriber line, but uhh... I don't think this is what my ISP intended.

I've confirmed such occurrences, but have never truly invested time in researching or investigating it. Though, I can assure you... whether the throughput is temporary or remains constant unbeknownst to me... I HAVE clearly seen it happen. Any explanations on this? Maybe the ISP slipped up on the throttle and just never realized their mistake?

I'm open to any reasonable hypothesis'
by Philip - 2018-08-02 12:29
In this case, dedicated vs. shared refers to the line between you and the first hop, (or couple of hops) to your ISP. There is not dedicated bandwidth per device, your DSL modem is usually the only device your ISP sees.

Sometimes ISPs are generous with their overhead calculation, so if your DSL line is capable of higher bandwidth, you may be getting more than 10 Mbps. Your ISP may also be providing higher bandwidth to their servers, and capping it onto the internet. I attribute your "per device" speed measurement to either temporary buffering on your client devices, or at your local node.

To properly measure, you'd have to choose distant servers to download from (not in your city, and not using your /ISP). Then you'd have to initiate transfers on multiple clients on your end simultaneously, and wait for the initial speed fluctuation to settle (usually up to 20-30 seconds). After that, your total available bandwidth should be apparent.
by Trex - 2018-09-15 05:11
Ahh... sounds like a reasonable explanation. Thank you for educating me in an area of technology in which I should be particularly knowledgeable about considering my degree. I appreciate your time and willingness to help with such an inquiry, not to mention how considerate your response was... being absent of any snide comments or insults :) I don't frequent here, so perhaps this site is an exception, but with the abundance of rudeness that seems to be rampant in countless forums, I feel showing my gratitude is the least I can do for those who have any kind of consideration! :)

Now... with that in mind, I would like to respectfully disagree with one thing you mentioned in your post. Perhaps not 'disagree' ... rather, maybe say... I harbor some skepticism regarding the number of devices logged by internet providers. I can't imagine they only see one device, that being ... what? the modem/router they supplied at the time of installation?

I could be wrong, but consider this hypothetical... let's say my son (or myself, or anyone in the household) consistently engaged in piracy via torrent client software. Let's also say this activity eventually caught the ISP's attention, and action had to be taken. The ISP sends a letter to inform the customer of copyright infringement which... I would have to think included logged IP addresses, MAC addresses,dates, timestamps, etc. It would be hard to believe that without this kind of evidence, they could prove it wasn't just someone cruising around, laptop in tow, looking for local WiFi connections, intentionally attempting to avoid this same type of repercussion.

Now, I might be inclined to believe a default gateway is all that is *initially* seen ... like... on a "home screen" of their logs, but like directories and subdirectories on our hard drives, I would imagine they have the capability to delve into each gateway further, allowing them to view different devices connected TO that gateway.

Admittedly, I am unable to provide any real proof of this, but I have to say I'm almost 100% certain this or something very similar has to be possible. If it wasn't, certain instances of a more pressing nature, such as serious criminal investigations involving child predators, identity theft, security breaches into large corporations, etc... would find it a crippling hinderance.
by Philip - 2018-09-15 14:31
This is a bit off topic.. ISPs have the capability to detect when you are downloading torrents, and there have been cases where the RIAA has forced ISPs to send out warning notices to customers detected to download copyright content. This discussion would be better suited for your forums though.
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