How is registration timeout regulated?

Hi, everyone
I’m facing a problem with asterisk 1.4 in a cable network. Some EMTAs (cable modems with ATA built in) stop receiving calls. After some debugging I saw that they do not reregister. So, my question is: are there any config parameters in asterisk that would make a registration timeout longer. Or is there a way so that asterisk could “remind” EMTAs: your registration is about to expire.
Thanks in advance.