To multiplex upstream and downstream traffic typically frequency multiplexing or time multiplexing is used. E.g. an ADSL connection may use FDM and use 25kHz to 138kHz for upstream and 138kHz to 1104kHz for downstream connections. In this case the downstream has a substantially larger band of frequencies available and can thus reach larger transmission speeds.

However while most users use downstream more than upstream most of the time it is not like that all the time. E.g. online backup services need upstream most of the time. In that case it would be desirable that the frequency division is realigned to put an emphasis on upstream traffic. Not only is such a dynamic allocation of frequencies not possible with ADSL (or I have not found anyone offering that) but I cannot even call my ISP and tell them that I seldomly need downstream anyway and I would like to have a lot of upstream and I am very willing to sacrifice downstream for that.

For LTE I could not figure out how the duplexing is actually performed but the specification allows for larger download rates (300 Mbit/s) than upload rates (75 Mbit/s) rather than a combined connection speed which can be divided as needed.

Why does this happen? Is this for marketing business reasons or are there actually physical problem with dynamic allocation? In the later case: What are those reasons?

(I think server fault is the best place to ask because it is about networks but it the answer to my question has to do with physics or signal processing it may also be better on one of those sites. I am happy to migrate the question if in which direction I need to look for the answer)

  • I don't think this is too broad, nor do I think it's off topic, however this may benefit from being asked on NetworkEngineering.SE.
    – MDMoore313
    May 15, 2015 at 13:03

3 Answers 3


One reason is that although it is possible to do dynamic frequency allocation, it is more resource intensive, and as a result more costly because it has to be negotiated on both sides, which leads to a higher cost for the ISP and as a result, the end user.

For example, In order to multiplex traffic both sides have to agree on a frequency band before the data is transmitted, like any bidirectional protocol. The processors will have to translate that into new transfer functions and adjust the filters accordingly. This can be verified by the fact that ISPs typically have to "reboot" your modem after a package upgrade. The type of dynamic FDM you're referring to would likely require more hardware than just negotiating frequencies on rare occasion, like what is currently done. Since this would likely drive up modem costs, and since the average residential consumer is only interested in consuming (downloading) the internet, I have to guess that it benefits the business more to standardize the bandwidth in blocks (10 down/2up, 20 down/4 up, etc.) than to offer dynamic allocation.

As a result, TDM is better suited for what you are attempting to do. I read it stated that "TDM signals use all of the bandwidth some of the time, FDM signals use some of the bandwidth all of the time."

This is not just theory, ISPs such as Merit give you the amount of bandwidth you pay for, and you can use it however you want. I don't know the specifics but that sounds like a practical use of TDM vs. FDM.

  • You have some good points BigHomie although I partially disagree with the opening statement about frequency renegotiation being resource intensitve to the point it maybe too costly to deploy at a large scale. ADSL2+,VDSL,VDSL2,G.Fast are all technologies (that are productised) and allow frequency (re)negotions that have/are being rolled out to millions of subscribers across various countries. In my answer below I mentioned that I think that basically "yankee" ias asking this from the view of a home user or small business. Outside of the domestic market ADSL is much less common in my opinion.
    – Baldrick
    May 19, 2015 at 12:59
  • @jwbensley that's true. I think what I was aiming for was that the constant frequency (re)negotiation that would come from trying to constantly use bandwidth as much as possible either up or down would cause a lot of overhead, which might negate any perceivd benefit, and in turn trying to overcome that overhead may be costly.
    – MDMoore313
    May 19, 2015 at 13:04
  • 1
    Ah yes I see know what the OP was getting at. Constant re-negotiation would be a terrible idea. I think partially because if you know how much bandwidth you want you can just buy a connection with that much bandwidth available so its not really a useful requirement. If you need a car that is faster traveling in some directions than others you wouldn't want one that has a changing engine size, you just buy one what meets your largest requirements. Also how would the ISP plan the backhaul links if end users have varying link sizes and requirements. Nope that would be a terrible idea :)
    – Baldrick
    May 19, 2015 at 13:09

I think you have missed something more fundamental that has lead you to ask your question, but in fact the question isn't really valid.

ADSL isn't for hosting providers or people with large uploads requirements. It’s a cheap service aimed at home users and small businesses that can be quickly and cheaply deployed, it has low overheads, and works over a variety of physical line qualities and lengths. So that makes it great for domestic roll out, home users can't afford to have their street dug up to lay fibres.

You mentioned frequency division multiplexing, the division of frequencies can be moved with ADSL. With ADSL2+ for example Annex M reallocates some of the downstream frequency range for use as upstream frequencies sacrificing download speed to improve upload speed. Also with ADSL2+ the line speed can and will fluctuate through the day or weak as the physical line is susceptible to interface and the attenuation rate fluctuates. As a result the frequencies used by an ADSL2+ line for upload and download will be changing (more acccuractly, they will reduce to cope with periods of high interferance).

But like I said ADSL/SDSL/ISDN/3G/4G/LTE are cheap services (if you live in a developing country they maybe the best money can buy, I'm of course talking relatively within the scope of available technologies, relatively these are for home users or small offices at best).

There are products that bring symmetrical bandwidths to home users like EFM (bonded SDSL). VDSL brings asymmetric high bandwidths to home users here in the UK, up to 80Mbps download and 20Mbps upload delivered over the same telephone line that would have been used for ADSL/ADSL2+ previously but with a new modem.

If you are a hosting provider for example and need symmetrical bandwidth you can pay for, and this is the crux of my argument, FULL DUPLEX connectivity (probably Ethernet based). ADSL and as per you example use FDM so that it can be full duplex. These days 100BaseTX, 1000BaseTX, 1000BaseSX/LX etc 10GBaseSX and so on all have and transmit paths in both direction so they have full duplex communications and there is no need for FDM or TDM so “split” the physical bearer channel into an “upstream” and “downstream” path like ADSL.

The old saying, you get what you pay for. If you pay peanuts which home users tend to do, you get a single copper pair that has to use FDM to achieve bidirectional communications that is only asymmetric in available bandwidth. If you pay more money you get bidirectional high speed low latecny connectivity.

Why does this happen? Is this for marketing business reasons or are there actually physical problem with dynamic allocation? In the later case: What are those reasons?

So to be clear; both products are available, symmetric connections and asymmetric. Symmetric connections tend to be more expensive but it sounds like you are approaching this from a home user / small office point of view. Anyone with the money to spend who also has the need for lots of bandwidth isn't going to be using ADSL (or LTE). ADSL is everywhere because that’s what people can afford. There is no limitation here as such. Its more about what is commercially viable.

  • 1
    Just as a further point: I work as an engineer for an ISP. My job is to build WANs for our customers, they are all large business. We never use ADSL, only VDSL, EFM or Ethernet over fibre. Large WAN customers can afford that and ADSL is essentially a "poor mans" service. If they hahve more than 10 people in an office ADSL isn't going to be fast enough. We only use ADSL in rare cases such as one of their offices being way up a hill making all other connectivity types not commercially viable.
    – Baldrick
    May 19, 2015 at 12:54
  • I am not quite sure how to understand your answer. Are you saying that dynamic allocation needs more expensive DSLAM-Hardware? Let's say a potential customer of yours says he wants a connection with the usual end user downstreams but symetric and he'd be willing to pay 4-5 times as much as an end users connection is there anything you could offer?
    – yankee
    May 20, 2015 at 12:43
  • Are you saying that dynamic allocation needs more expensive DSLAM-Hardware? - No, I gave no hint of that to my knowleedge. Im curious as to why you think that?
    – Baldrick
    May 20, 2015 at 19:20
  • To anser the question in your comment: No I can't offer that using ADSL, but not becasue that might not be physically possible. All these technologies go othrough heavy standardisation and ratification processes and take years to develop. There is no standardised off-the-shelf DSLAM I can buy that will offer that but that doesn't mean it isn't possible. I can offer a user a symmetrical servicet though but instead of using ADSL I can use EFM or fibre. The idea of running symmetrical service at say 100Mbps for example over ADSL isn't a good one, otherwise we'd all be doing it right?!
    – Baldrick
    May 20, 2015 at 19:22

I'm no expert here, but I would assert that this is primarily because of business reasons. A DSL is basically symmetric in that you have a wire and two boxes on both ends of the connection that share the same medium. (Unlike, for example in case of WiFi, or WiMAX, etc.). Having said that the difference in price and hence quality of the equipment on both sides may be a factor here. (Sorry if I did not clarify this better, but I seem to be the only one with at least a vague answer so I decided to post an answer. Also, my 15-year-old experience with telcos would suggest that this is a human factor, as they tend to avoid per-user solutions, and concentrate on mass markets).

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