Do I need DOSCIS3 modem for Cable 18 or some old one would do ?
You don't have to get a Docsis 3 modem for 18, no. 18/512 is going to become 25/2 sometime this year though, and you will need Docsis 3 for that. It is a very good idea to get Docsis 3. -- electronicsguru.ca
