I suspect you have a setup similar to this one:
A strong signal enters the house at the ISRA point. If you split the line you get two weaker signals.
Traditionally the line is split and two filters are applied. One to block anything but the sounds/frequencies needed for an analog telephone. And one with a different frequency range for DSL.
That that I used thinner lines after splitting to indicate a weaker signal.
If the line which leads to the ADSL filter and the ADSL modem gets split a several times then its signal will also get weaker. It will either need more resends (in case a packet gets lots) or it will need to work at a slower speed.
Note that most of this is guess work. I will need more then 3 lines in a post to actually answer that. (Also your DSL modem, switch and Wireless access point may be a single device).
Have you got DSL filters installed?
– Ƭᴇcʜιᴇ007 – 2013-08-26T17:59:44.033@techie007 yes, i have filters, for both phone and router (as they are in different sockets) – user902383 – 2013-08-26T18:01:01.713
@techie007 btw, if i didnt use filter, will be any difference between main socket and other sockets? – user902383 – 2013-08-26T18:02:27.800
2What is a "main socket"? – David Schwartz – 2013-08-26T18:04:38.870
@DavidSchwartz my bad, i ment master socket – user902383 – 2013-08-26T18:10:04.097
@Hennes, i know i don't NEED to, but is it normal that affects signal so badly? – user902383 – 2013-08-26T18:11:50.947
Have you replaced the filter on the faulty socket? Have had an electrician replace/check the socket? – user 99572 is fine – 2013-08-26T18:35:01.743
@user99572isfine no i didn't replace anything, i just want to know is it fault, or expected behaviour – user902383 – 2013-08-26T18:40:50.630
@user902383 What is a "master socket"? – David Schwartz – 2013-08-26T18:41:24.180
@DavidSchwartz master socket is where line enters property – user902383 – 2013-08-26T18:45:26.370