so I decide to purchase this cable modem after be tell by Comcast that they charge $ 7 per month for the rental of they cable modem . I believe they rental price be reasonable , but give the cable modem would pay for itself within the year I decide to purchase my own . that be say Comcast 's website be very specific on which type of modem they support and I choose to purchase a DOCSIS 3.0 modem to be certain the modem I purchase would be support for at least the next several year . the Motorola sb 6121 support all my need for upload\/download speed and be easy to setup . I only have 25 mbp download and 4 mbp upload from Comcast but want to make sure my modem could support future speed increase -lrb- plus my wireless router support up to 150 mbp download speed so I figure the modem be able to supply that be a plus -rrb- . overall , the product function exceptionally and I be very satisfied with its performance and the internet speed I be get both wire and wireless throughout my home . as a side note , the sb6121 come with the power adapter , modem , and a ethernet chord but you will need to have a standard tv coaxial cable to connect it to you cable jack .