Here is my setup...
Outside Line -> Comcast-provided splitter/booster
From the splitter, internet Coax goes to Motorola SB6183. (Have Blast)
Other connections go to: Xfinity modem + 2 TVs
Xfinity modem is only used for phone service.
I added the Xfinity Beta channel to my Roku 2. I was able to successfully activate it online. However, after entering the code successfully, I get the error message: createdrmsession.403-102
Is there a way to get it to work when using your own modem?
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I am doing exactly what you mentioned. Yes, you can use both. What you will want to do is first put the current Comcast modem into 'bridge' mode. You can find that info by searching the Internet. It will require you to access the settings of your current router. Next, you can remove the xfinity hotspot, if you would like. You will be completely disabling the wifi and routing capabilities on your Comcast modem. Remove the Comcast modem from your cable. Next, install the new modem on the cable. Hard-wire a PC directly to the modem and access http://www.xfinity.com/activate. Here, you can provision your new modem by logging in with your Comcast id. Once you are connected and its working, you can reconnect your Comcast modem to a cable connection. You can restart the modem and your phone line should be working. Finally, you will want to connect your new router to the modem. Remember, whichever device is the 'first' connection will need to be your router that can assign IP addresses and that type of thing. Test the router by using a network cable and then you can check the Wifi.
As far as purchasing your own telephony modem. It's not easy. The Arris modem that Comcast rents is available at Best Buy for around $200. It's excessive and will take some time to recoup the cost vs renting (but will eventually pay off). You will also want to hang onto the modem for a little while until you are sure the new modem/router is working. If you plan on purchasing one online, be careful. Some are stolen or no longer servicable and may not activate.
One other thing to remember... the further away your modem is from the 'street' connection, the more signal loss you will experience. So, you may find that performance is slower depending on how far away you are from the street. My advice, use one split from the street... one to the cable modem and one to the rest of your house. If you have a booster, Comcast should be able to provide you with one that has a filter for the VOIP.
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