I purchase the sb6121 base on review here at Amazon.com . and of course when it come to installation day with a Comcast installer ... the tech could not get the modem to work . the modem would reset itself every couple of minute and he could not activate it . the installer tell I the modem be defective and to return it . he even call a person at Comcast who be look at the line who say the same thing ... not work right ... return it . of course the tech go out to he Comcast van and return with a older 2.0 sb5150 surfboard which install in seconds and I agree to pay the $ 7 rental monthly fee to get it up and run . I pack the modem up to reship it but come back on Amazon to read the review once again and low and behold ... read numerous post by people with the same issue with Comcast who have to rent a modem on the spot to get up and run . so I unpack the modem and hook it up myself follow the direction in the box . the Comcast activation screen come right up and get I to the last window `` activate device '' . I leave it on there for a half hour and it would not finish so I call a live person at Comcast . the nice girl have I up in run in seconds and I have have no problem since . why be it that thier tech and online rep could not activate it two day prior after mess around for a hour and a half ?? who know ... but do not give up on the modem ... its a Comcast problem .