Is ADSL2+ Bad for VoIP?

20 Comments

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.

20 Comments

Dallas

Just wondering. For those using personal experience as a base… are you referring to Australian connections, or to US connections? Australian internet plans (no matter what they be) are crap to begin with.

So… poor download rates may be because you’re using an inferior incarnation of the technology, rather than the technology itself. (I’m in Australia myself, so I can testify to the dodgy-ness of intarweb connections -_-; ).

Saurabh

Friends,

Oversubscription is everywhere: ADSL/ADSL2+/TDM/MOBILE CALLS.

You need to have the eyes to look for that. Without this a local call might have costed huge $$$$.

Do you think that your regional TDM exchange(called Remote station unit) has ports for all of you to make calls simultaneously? Nahhhhhhhh

John

“I believe many are missing the point of the contention at the DSLAM to the Metro network.”

No, they are not missing this point, most have pointed out that where congestion/contention exists VoIP quality may degrade.

Contention may exist in any shared backhaul, it is nonsense to suggest that ADSL2 suffers from this more (or less) than any other technology.

If you don’t have enough backhaul to a DSLAM, you may get congestion (dropped packets), VoIP quality will degrade as a result. This is not news.

John Egan, Infineon

I believe many are missing the point of the contention at the DSLAM to the Metro network.

This contention creates the same “battle for bandwidth” as exists with cable modems, just somewhat further into the network (and rarely discussed).

As the contention ratio can be quite high and peaks of upstream traffic can overwhelm such a contended link, there can be times where jitter and latency will effect a VoIP call. Experienced as far end complaints of voice quality, not the end user experiencing it.

This can be found on an ADSL, ADSL2+, or via a cable modem.

Steve Dean

The difference between ADSL and ADSL2+ is the modem chipset at the DSLAM (located at the central office) and the customer’s DSL modem (located at home). The ADSL2+ chipset allows for higher bandwidth speeds. There is no difference between the methods of traffic aggregation between the two chipsets. The ADSL line is not shared with any other customers although it is likely that two neighbors would both terminate on a DSLAM within the same central office. The DSLAM is typically backhauled via ATM to an internet protocol transit site router. That router is then connected to the internet to transit requests heading towards popular websites (like myearthlink.net, google.com, etc..). VOIP traffic is handled in the exact same manner from the home to the service provider (such as EarthLink’s True Voice service “ETV”). However, with EarthLink’s DSL and Home Phone Service, we have deployed new ADSL2+ technology with an even better twist to the story. We are utilizing the same technology used for line sharing by all the major telephone companies. There is a specific frequency used for voice and a different frequency used for DSL on the pair of wires traveling between a home and the central office. The DSLAM provides electricity to power the line and sends the POTS (telephone service) down the frequency designated for voice. This frequency is standard and is why ANY telephone will work with EarthLink’s DSL and Home Phone Service. The DSL frequency is the other band used to send down the ADSL2+/ADSL data service. The DSL modem can hear that frequency (your telephone set cannot because it’s not tuned to listen to it) and the signal is then synchronized with the modem within the DSLAM at the CO. There is good news with this new technology. Not only can you receive higher line speeds for your internet surfing (in most circumstances), but your phone service is completely separated from your surfing and you get to take advantage of the many advanced features of VOIP. The VOIP part of the service happens from the backside of the DSLAM towards EarthLink’s service. EarthLink and Covad have extensive private peering between our two networks. This ensures the voice traffic makes its way to us without contenting with other general internet data traffic. So there you go, DSL and Home Phone Service from EarthLink is different and provides customers with the best of both worlds. Fast internet connections and a standard line powered voice service with the advantages of VOIP.

Steve Dean, Vice President of Network and Systems, EarthLink.

Greg

Actually, ADSL2+ is beneficial to VoIP as it does provide more bandwidth for VoIP services. Let me clarify how rate adaptation works in DSL2+. Rate adaptation speaks to the DSLAMss ability to adjust the sync rate (and therefore line speed) based on interference on the line, not how many users are online in the DSLAM. The only bandwidth pooling that takes place is on the Internet side of the DSLAM, but this is true for all Internet services. Dial-up, DSL, and DSL2+ all share upstream connections to the Internet. ISPs then manage that bandwidth across all services to ensure that users can fully utilize the bandwidth made available to them on their last mile connection.

If you would like to further explore DSL2+ and ISP bandwidth management, I would be happy to talk you.

Greg Collins, Director of Network and Data Center Engineering, EarthLink.

Ram

The linked article is not factual. There is no dynamic allocation of bandwidth unless online reconfiguration (OLR) is in play.

Seamless Rate Adaptation (SRA) is meant to ensure that a stable link is retained. If conditions get worse and there is a need to switch to a lower data rate, the Modem and the Central Office switch agree to change the data rate. If conditions improve, a higher data rate ensues.

Dynamic Rate Repartioning (DRR) is used to partition data rates among multiple services. I think this is what the article is referring to. However, even this repartioning of rates goes through a negotiation protocol which cannot drastically and unreasonably drop the rates from 200-300k Upstream down to 57k. There is no basis to this claim. DRR in ADSL2+ is not practical for operator deployments. Thus, I am not sure where these analyses stem from.

Rest assured, there are challenges in guaranteeing 24+ Mbps in practical deployments in ADSL2+, however, it takes performance notches above ADSL1.

John

Absolute nonsense, disappointing to see such rubbish reported on GigaOm.

As others have already pointed out VoIP quality has nothing to do with ADSL or ADSL2+. Lack of bandwidth and congestion will affect VoIP regardless of the transport mechanism be it ethernet, fibre, wet string etc.

Jesse Kopelman

One thing I discovered when talking to an actual telco is that their ADSL DSLAMs did not support a lot of feature one would expect. Most amazing to me was lack of support for VLAN. While the ADSL2+ standard itself does not bring new QOS mechanisms (just changes to the PHY), apparently the newer DSLAMs that support ADSL2+ do bring those QOS mechanisms. So, I think the poorly expressed point of the CommsBusiness article is that the ability of an operator to oversubscribe the hell out of their customers just happens to come along for the ride with ADSL2+.

Allan Leinwand

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.

Don

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.

Richi Jennings

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.

Rui Carmo

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).

MIke

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.

Aswath

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?

Tom

‘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.

Ben Metcalfe

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.

bill

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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