Is DSL full duplex?

[Jerboy]

I ran a connection speed test program on bandwidthplace.com and my connection speed is a slower than rated speed. When I run the same test while I have full width upload(five people downloading stuff from my puter) my download speed goes down as well.. Is this normal? I thought DSL was full duplex.

 

[Nutz]

Some are and some aren't.

Also, the CIR (Committed Information Rate) is usually pretty low on DSL even though you are being sold on a given 768, 512, 256, etc. downlink/512, 256, 128, etc. uplink line.

Also, the only way you will ever get a reliable speed test is by hitting something within the providers network. And even that isn't guarenteed. As soon as you leave their network you are competing for bandwidth with everyone else that they provide service to. Its not uncommon to see a bottleneck at that point giving you a false read on those speed tests. Your line is only as fast as the fastest pipe you are riding to any given site.

 

[subflava]

AFAIK DSL is full duplex and I'm pretty sure of it.

Nutz- why do you say some DSL is not full-duplex? Can you explain?

I agree with the rest of what Nutz says about speed testing.

 

[Nutz]

Its just something I remember reading here or at dslreports.

Not all DSL is created equal, just look at bridged vs routed as an example.

 

[subflava]

I guess you are referring to the different flavors of DSL like ADSL and SDSL? Well, if that's what you are talking about then I can tell you they are indeed duplex.



<< Not all DSL is created equal, just look at bridged vs routed as an example >>



I'm not sure how this relates to something being full-duplex or half-duplex...I guess you are just making the point that there are different forms of DSL. However, even with routed/bridge DSL types it does not affect whether it's full-duplex or not.

 

[Saltin]

ADSL is generally half duplex, In my experience.

 

[Nutz]

<< guess you are just making the point that there are different forms of DSL >>



Yup.



<< I guess you are referring to the different flavors of DSL like ADSL and SDSL? Well, if that's what you are talking about then I can tell you they are indeed duplex. >>



Of course its duplex. Whether or not it's full or half is the question.

 

[Saltin]

<< guess you are referring to the different flavors of DSL like ADSL and SDSL? Well, if that's what you are talking about then I can tell you they are indeed duplex. >>



Do you mean full-duplex? If so you are incorrect. I know several ADSL setups that run at half-duplex only

 

[subflava]

<< Do you mean full-duplex? If so you are incorrect. I know several ADSL setups that run at half-duplex only >>



Ooops...yes, I meant to say "full-duplex".


About the ADSL being half-duplex...can you point me to some documentation of this? I didn't know that this existed.

 

[imported_TekWiz]

Well I have Verizon DSL and it's unable to maintain full upsteam and downstream simultaneously. If I upload, downloading slows to a crawl and vice-versa, which seems to indicate it's not full duplex.

 

[imagoon]

ADSL and SDSL use 2 separate frequency bands for upload and download. They are "full duplex" however in ADSL the channels are not the same size and under heavy upload conditions, the download channel will slow down because the upload channel is overloaded and prevents "ACK" packets from getting returned. Older DSL is even more problematic as the "DL" channel was consecutive and pushed the "UL" channel in to the higher frequencies making it more unstable. Newer DSL breaks these up in to sub groups and creates and destroys them based on the performance of each group. You end up with: DL:UL (channel1) DL:UL (Channel2) DL:UL (channel3) etc. DSL tech is inherently full duplex. You can run the channel in "half" but you are not retraining each time, simply using one channel at a time which is pretty inefficient.

 

[drebo]

Yeah...asynchronous != half-duplex.

As stated above, heavy upload traffic will affect your downstream TCP performance because congestion on the upstream will affect your ACK packets from getting back to the source of your download. If ACKs are missing or late or out of order, that will result in retransmissions, which will cause the sending server to slow itself down.

 

[VirtualLarry]

Well I have Verizon DSL and it's unable to maintain full upsteam and downstream simultaneously. If I upload, downloading slows to a crawl and vice-versa, which seems to indicate it's not full duplex.

No, that's just standard TCP. When you download, you also need to upload, you send a stream of ACK packets back upstream. When you constrict your upstream by uploading, then it constricts those ACK packets, and your downstream suffers as well. Most DSL lines don't have a lot of upstream bandwidth (eg not symmetrical).

 

[vshah]

this can be mitigated by prioritizing acks via your routers QOS, if supported.

 

[VirtualLarry]

this can be mitigated by prioritizing acks via your routers QOS, if supported.

True. My Westell 327W calls it "Turbo TCP".