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Summary:

Conventional wisdom would say that more bandwidth is actually a good thing for VoIP. But that is not the case when it comes to ADSL2+, according to this article in the CommsBusiness. The article explains that in classic ADSL scenario when a file is being transfered […]

Conventional wisdom would say that more bandwidth is actually a good thing for VoIP. But that is not the case when it comes to ADSL2+, according to this article in the CommsBusiness. The article explains that in classic ADSL scenario when a file is being transfered or a VoIP connection is made, the bandwidth allocated to that connection remains constant, till say the transfer of files is complete or say the call is finished.

From what I understand it is like having a dedicated port and every subscriber gets a certain amount of bandwidth. In the case of ADSL2+, subscribers share the bandwidth at the local exchange level, and the bandwidth allocation is more dynamic. Carriers like ADSL2+, as it allows them to over provision, though it is not such a good deal for DSL subscribers. (I am sure some of you might have a more eloquent description of how this technology works.)

“This new technology uses a rate adaption policy which effectively means it can (and does) steal bandwidth dynamically from subscribers as and when required. We have seen upstream capacity decrease to lower than 57k in peak working hours which is simply not viable for a VoIP application. Period.” Scott Dobson, Managing Director of Newbury Based distributor Vcomm.

  1. Sorry, this is utterly and completely wrong. ADSL2+ is just ADSL with higher frequency bins in use (so top speed goes from 8mbit -> 24mbit and upstream from ~800k -> 3mbit).

    It has a feature called SRA (seamless rate adoption) but this is designed to prevent complete disconnects when there is noise, instead stepping down speeds until it becomes stable again. If SRA wasn’t there, you’d just get a disconnect and 30 seconds or so of no traffic whatsoever.

    Depressing this sort of misinformation can get published.

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  2. The ADSL or ADSL2+ it doesn’t matter because if the ISP does not provide enough bandwidth you will have call quality issues.

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  3. I think both Om and the commentor ‘Bill’ are right.

    Strictly speaking ADSL2+ is just a further iteration of the ADSL spec, but as Om mentions ISP’s are far more aggressive with the contention mechanisms they use…

    ADSL2+ may quadruple the ‘last mile’ bandwidth available, but that doesn’t mean that the ISP’s up-stream bandwidth is suddenly quadrupled too. ISP’s have to find the margin somewhere, especially as suddenly their customer’s bandwidth expectations have increased with their new account!

    The mechanism Om describes is a bit like riding a bus. Plain-jane ADSL is a bit like the bus services here you can only board if there is a free seat. Ie once you have started your journey (download) you know how much room (bandwidth) your seat will be for the duration.

    ADSL2+ is often implemented more like a bus during a busy standing-room only period. You get on the bus, but during the journey you might end up with less or more room as the bus fills up and empties up. Obviously with a VoIP connection you need a certain amount of room before things become ‘too uncomfortable’.

    ADSL2+ providers are like the bus companies who try to serve busier routes but can only increase the size of the bus (the overall bandwidth available) by a certain amount as other factors – such as the cost of building bigger buses and the maximum size a bus can be on the roads.

    Hope that explains things in a way that’s somewhat understandable. Or maybe not.

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  4. ‘Bill’ is right. This cuprit is the upstream aggrator. It has nothing to do with ADLS2+ itself. ADSL2+ has higher upstream bandwidths, so if the ISP doesn’t have enough upstream bandwidth at the DSLAM, then something downstream will have to be squeezed.
    What’s probably happening here is an ISP providing 2+ to customers without increasing the upstream pipe.

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  5. Something doesn’t compute. The following are extracted from the quoted news item:

    “Simply speaking, old ADSL used to use a contention policy which basically meant that if you got an amount of bandwidth of a file transfer or a VoIP call, you would keep that bandwidth for the duration of that transfer. Once the transfer was complete, the bandwidth would then be freed up for other users on the network.” Could this be true (even in an ATM based ADSL)?

    “This new technology uses a rate adaption policy which effectively means it can (and does) steal bandwidth dynamically from subscribers as and when required.” Since this is the normal mode of operation in an IP network, the real question is how much bandwidth is realized. The news item states that, “We have seen upstream capacity decrease to lower than 57k in peak working hours which is simply not viable for a VoIP application. Period.” Now, Global IP sound claims that their iSAC codec (yes that wideband codec made famous by Skype) works over dial up modem line ( http://www.globalipsound.com/solutions/FAQs.php ). So there shouldn’t be a problem, right?

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  6. ADSL2+ is 100% crap and I will explain why.

    I live in Europe and I used ADSL for 3 years. The phone wiring from the phone company to my home is so bad that my connection speed limit, in ADSL was around 5 MBPS.

    So, ADSL2+ arrived, offering speeds up to 24 Mbps! I am using one of these 24 MBPS ADSL2+ links exactly now and guess what is my speed… yeah, 5 MBPS!!!!!!!!!!!

    Your speed will not improve in ADSL2+. If you line is crap you will continue to have a crap speed. If you live far (more than 1 mile) from the telephone central your connection will be crap.

    I bought an ADSL2+ modem and guess what… I had to return the modem and get an ADSL modem, cause the ADSL2+ connection was so unstable that it was impossible to be continuously on the web.

    So, do not believe in speeds greater than 5 Mbps, unless you live on the building next to the telecom.

    We all must follow the example of South Korea. They put 100 Mbps bidirection fiber optical links in every home for 20 bucks a month.
    That works. ADSL is crap.

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  7. Bullshit. ISPs are able today to enforce QoS policies based on protocol at the DSLAM level. So each individual packet flow (for any subscriber) can be assigned a distinct priority. It works much better if you have a home gateway able to do QoS (and hence do the separation right at the customer premises), but that is not strictly necessary.

    This assumes, of course, that you’re using standard VoIP and not some Skype-like abomination, and that your tech staff knows what they are doing (i.e., beyond buying boxes and hooking up cables).

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  8. This article is so misconceived, it’s hard to know where to begin.

    Without wishing to repeat other comments, you should note that the article is talking about one particular implementation of DSL in the UK. It’s BT’s so-called “Max” service, which is resold to other ISPs who haven’t unbundled the “last mile” yet. It’s nothing to do with 2+, either — “Max” is a BT brand for regular RADSL, at adaptive downlink sync speeds of up to 8Mb/s (as opposed to the original BT ADSL service at fixed sync speeds of 256Kb/s, 512Kb/s, 1Mb/s, and 2Mb/s).

    The description of how bandwidth is allocated under the standard 20:1 or 50:1 contention is also laughably wrong.

    Nothing to see here. Move along.

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  9. Yep, tend to agree with the negative comments, most ADSL subs are getting mediocre service. My take is that the ISPs are very reluctant to tell us about their upstream bandwidth and contention. In the UK there’s three elements to ADSL service, the connex to the DSLAM – obviously, uncontended and it’s obvious to me why BT/Openreach wanted to do that – so they could setup a rival TV service to Sky ‘cos they own the last mile. From the DSLAM through BT’s Colossus network to the various ISPs backend infra I think BT are just putting their finger in the air and keeping contention in the 50:1 area. Then ‘dog’ knows what the multitude ISPs are delivering to their peering points. I’m lucky, small rural exchange about 150 metres away, solid 8Mb downstream/440Kb upstream, very little contention from my neighbours an my ISP seems well provisioned to LINX. I can VoIP, VPN and stream 192kps “radio” all day long and am still not getting anywhere near my 50GB/mth ISP limit. Different story in a middle sized town and an exchange serving 20K+ subscribers – contention, contention, contention. Same story as digital radio, digital TV: it’s often not better for the consumer than the preceding analog services but 90% are so ill informed that they think digital is nirvana.

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  10. I think what Om is describing here is that even though the last mile speed may increase when using ADSL2+, the issue is the connectivity from the DSLAM into the xSP backbone network. I doubt that service providers are increasing their backbones when they roll out ADSL2+, so that means that everyone can transmit faster over their last mile and then wait in a queue from there to get on the network. That’s why VOIP (and all other traffic) may suffer – more packets arriving faster at the DSLAM and getting queued up to get to the same backbone network as before.

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