I posted on another thread. but here it is again in different words..
1. Amongst others there following 2 parts that enable you to browse internet at home
a. The connectivity. Lets assume it is DSL.
b. The service - Internet service
In slightly technical terms on can say for above case that Internet service is being provided over DSL. It can also be provided over other connections like Cable, wireless etc.
DSL is a medium/ Internet is service over that medium.
2. Similarly IPTV is a service. Its a service that runs in parallel to the internet service. It is provided over DSL connection and not provided over internet service. It does not use ANY bandwidth of internet service and as such has no relation to internet service. Technically you can have only IPTV over DSL WITHOUT internet.
Another point to note is that IPTV is provided over LOCAL LOOP. The content (channels) is fed to a local DSLAM. Hence there are less number of variables as compared to world wide web (internet service).
In layman terms its like content being fed from your local telephone exchange. It needs to travel only the distance from your home and the exchange which is very well possible over a telephone line.
3. Now the capacity issues :
ADSL2 downstream speed can be upto 24 Mbps.
The speed you are talking about 2Mbps is of internet service. So remaining 22mbps is still available for other services.
This is used by IPTV. IPTV requires 3.5 Mbps for SDTV and 18-20 Mbps for HDTV using mpeg2 compression. source
An introduction to IPTV: Page 1
Although there could be issues for HDTV, SDTV is very well possible.
Also note that in internet service the data is traveling from anywhere and everywhere. In case of IPTV its very much local.