How does VoIP work on non-QoS DSL?

Posted on 2009-12-28
Last Modified: 2013-12-22
Hey Experts!

Please help out...
After completing a CCNA VoIP class I'm still confused about a "VoIP over DSL" topic:

1. Why? All throughout my Cisco VoIP class there was a HUGE emphasis on having a QoS - on your WAN side and to have ISP manage the QoS for VoIP and Data. It was to the point where it almost was  a LAW that (nobody should ever set-up a VoIP system without having VoIP QoS implemented by their ISP) And god forbid you set-up a VoIP phone system on anything but a dedicated QoS managed T1 circuit because you can't have VoIP calls go through the "Public Internet" which would ruin any VoIP quality...

After truly believing these concepts which I learned in the classroom, I go out to the real world and see a bunch of very CHEAP VoIP providers such as Access Line etc. who claim to provide VoIP service on an ADSL home-grade connection from ANY ISP, without ANY QoS. AND their pre-sales engineer telling me that QoS is not necessary on a WAN line and that they have 100k customers using it fine on DSL lines (Especially if a second DSL line only used for VoIP, and not both Voice and Data). And that QoS is NOT needed because of something related to 'Windowing', and that TCP/IP can figure it all by itself by adjusting the window size...

Was I mislead in Cisco VoIP curriculum or these "CHEAP PBX" guys are just getting lucky with good quality ADSL lines?

2. Why is that in Cisco VoIP class it's SO emphasized to have VLANs on your LAN unconditionally, but the "CHEAP PBX" guys laugh at it and say that VoIP traffic is so small that unless you're using a HUB or your LAN is overloaded, the VOIP traffic will be just fine and doesn't need to be prioritized, nor any QoS on LAN is needed!?

Again, I feel that I was mislead by Cisco's VoIP curriculum and makes me look stupid in front of $9.99 / month / line "Cheap PBX guys".

Please share your knowledge on this topic. It's been bothering me for a while now :)

Question by:Kaptain1
    LVL 7

    Expert Comment

    Dear Kaptain1,

    If you want to guarantee high quality voice for your voip calls its recommended that you do QoS. I don't think that you were mislead.

    When you talk about real-time systems like voip where any (slight) delay in the voice will lead to bad conversation and at the end, users will end up in hanging up the call, it's very important and a rule of thumb to do QoS. It's the default thing.

    You mentioned that the "cheap" voip providers use a second line for voip. Then that describes it. They separate voip and data traffic and so quality is maintained regarding voip calls.

    If you use one DSL line then you have to do QoS. I advise you also to check the internet speed and the voice codec (g.711, g.729, g.723, etc.) used.

    Voice codecs vary in their internet bandwidth they consume for each call. The codec you use plays a big part in the voice quality over a poor internet speed.

    I hope this shed some light.
    LVL 1

    Author Comment

    I see. Thank you for your reply! That helps a lot.

    I got a few more similar questions:

    1. So, it's OK to use a DSL line for VoIP as long as it's a dedicated line used for VoIP  only?

    2. If a budget ATT DSL line is 6000/768 kbit , and a single high-quality VoIP conversation takes about 80kbps, does that mean that I can use a single DSL line for about 9 simultaneous VoIP conversations w/ good call quality?

    3. How would a non-managed 1500/1500 T1 line compare to a 6000/768 kbit DSL line besides being able to support more calls due to a higher upload speed?

    4. There are SIP providers that offer unlimited VoIP and long distance for about $10/month/line . How would those be different from ISP's (e.g. Speakeasy who puts you on a Long Distance plan just like a regular PSTN line?). Is that only because Speakeasy manages QoS on your connection if you get Data+Voice from them, and does Speakeasy just tries to rip you off for Long Distance usage?

    Thank you once again!
    LVL 1

    Author Comment

    One more thing...

    "You mentioned that the "cheap" voip providers use a second line for voip. Then that describes it. They separate voip and data traffic and so quality is maintained regarding voip calls."

    Yes, that would take care of QoS for VoIP calls while they're leaving my office LAN. But wouldn't those VoIP calls use the "Public Internet" to get to the ISP's SIP server?

    This makes me think that in the "Public Internet" there's a very high chance of VoIP calls to get delayed because it's all together - especially on a cheap ADSL line which may be oversubscribed and the main bandwidth pipe would be shared with all of the other DSL subscribers in my area for VoIP calls to get to the ISP's SIP server?

    Am I getting this right? Am I too paranoid about this? :)

    LVL 7

    Accepted Solution

    Dear Kaptain1,

    I will try to answer all the questions:

    1. Using another line for voip (separated from data) would definitely "guarantee" a better voip quality. I put guarantee around  " " because there is no straight-forward answer for this.

    2. Probably the calculations you made are correct but what if the DSL line you have is shared and not dedicated? The calculations are right if your DSL line is dedicated and not shared. You have to keep that in mind.

    3. My experience regarding T1 is that, it has 23 channels. This would lead to 23 simultaneous calls. If enough dedicated bandwidth is allocated then calls will be guaranteed a good quality. So it's different from a DSL line where calls use the same route.

    4. I am not familiar with all the SIP providers and I don't know Speakeasy :S

    Regarding your second comment:

    I don't want to believe that the "cheap" voip providers have their servers on a DSL shared line. They have to have a dedicated line and they should have done provisioning regarding their capacity.

    You are not paranoid :) VoIP is all about quality after all so relax.

    To sum up, all I can say is, that everything depends on your scenario. Write down the scenario you want to implement: simultaneous calls, simultaneous users, international destinations, budget, etc...

    You have to do some estimated calculations also. By this you can narrow your options regarding the voip providers you want to work with.

    I hope I helped.
    LVL 1

    Author Closing Comment

    Thanks :)

    This cleared up a lot of my confusion.

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