One ethernet cable in my home network gives me very low signal strength. The lenght of the cable is approximately 60 ft. (18 meters), does this influence the signal strength in any way?
I use a Cisco E3000 and the WiFi is tremendous.
You don't explain exactly which cable is the long one, are you referring to a long Cat5/Cat5E/Cat6 Ethernet cable? The max length for Cat5 is 100 meters.
The length of the DSL cable shouldn't matter - it is basically a phone cable and you can have ADSL signals over kilometers of phone cable. However it is important to use a suitable ADSL microfilter to separate any analogue phone handsets from the ADSL broadband signal.