My 1st thought after reading you exchanges was just to kill the thread and say don't use DSL for VoIP but i also know that there is a huge hole in the market for last mile for the micro SME , so let’s try and clear up a few points that are mentioned here.
1/ How good is a DSL line ? I have no idea! They are all different and this is controlled by a variety of factors , distance from the exchange, DSLAM contention, SNR ratio, line attenuation , copper quality, lack of available twisted pairs, the list goes on !
2/ Voice and data on the same line ? This is always a bad idea, all the QoS, ToS , DSCP markings are of limited use if the ISP is not looking for them , they have some but little value client side .Some guys rave about the use of an IPC account, a tunnel directly into the provider, set up on router side as PPPoE with multiple session routing. Sounds good hey ? Data from one ISP and a VPN into another provider, much easier to control the traffic, however, to achieve this you will have to use something like L2TP or IPsec tunnels, the problem is here that the SIP overhead is massive, can be 50% higher using a VPN , since our problem here is bandwidth starvation , we are not in the right direction,
OK, ViBE it was mentioned , ViBE uses byte level QoS and is the best ever QoS on Voice and data i have ever seen ! Alas a small hurdle exists , the voice and data have to be inside the ViBE tunnel for the QoS to work. Most VoIP providers are not ISP's so you have to run your data outside the tunnel and the QoS dont work. MWeb is one of the providers that can give you both voice and data but they do not support ViBe as an ISP.
3/ Dedicated the way to go ? Yes, yes, yes ! So lets assume we have a reasonably stable line ( because we read too much Alice in Wonderland) you should be ok for a few concurrent calls on a good day ! If you ViBe the connection it will be less acceptable to bandwidth starvation, IAX2 will achieve the same thing but is only valid for providers with Asterisk core networks .
4/ 3G failover , Brilliant ! Now we have redundancy ! used to think this was a great idea but lets look at the reality. 90% of tales of wow on DSL are about quality, not my DSL is down, somebody stole the cable ect, if they are complaining about quality it means the router is still synced to The DSLAM and it will not fall over to the 3G, also 3G has issues with restricted APN’s and can be a long task to get the associated provider to un-restrict them.
So in conclusion you see im not the biggest fan of DSL, try and avoid it at all costs , I hope my 2 cents worth has at least made you aware of the challenges .
Go the fibre route, its not as expensive as you think and is the perfect long term solution with increasing broadband demand and quality voice requirements .
PM me if you need more info.
Paul Colmer
