I have opportunity to test two different cable modem on Comcast business service . one be Motorola SB6121 and the other Zoom 5341 . both be DOCSIS 3.0 , which be require for high speed cable service . Comcast tech support be not thrill about zoom while they praise Motorola SurfBoard high . as it turn out , average Comcast technician , who suppose to activate the modem , have a trouble with the zoom serial number . my configuration of Comcast service in this case be 2 time 50 mb\/s download and 10 mb\/s upload speed . this service suppose to supply over 1000 user with internet access , load balance mode . first line be activate with zoom modem and everything go well for several day . to the second line I attach Motorola modem ; Comcast manage to deactivate Zoom modem on the first line in process , though . initial test of Motorola show expect value in download and upload speed . however , with network connect to it , Motorola download speed do not reach more than 10 mb\/s download with upload of 2 mb\/s . after 24 hour of 1000 people complain , Comcast Technician come to check the installation and re-activate Zoom modem . test of Motorola modem with a single laptop connect do not show any abnormality : download and upload speed be at nominal value . since I could not put in place my load balancer yet , I switch the network to the Zoom modem and make 1000 surfer happy again . in a couple of day my load balance configuration should go live , use one Motorola and one zoom modem . I intend to update this review with the result of Motorola performance in this configuration , for potential buyer interested in the Motorola sb6121 . well , I decide to buy Comcast Internet service for home and I do not have dilemma which modem to choose ; zoom it be . I order on-site installation because my cable coax have to be replace and expect confusion with zoom serial number -lrb- that happen as expect , too -rrb- . anyway , Comcast get it online and my modem be happily blinking , ever after -lrb- two week in service -rrb- . the one order here have slightly different housing than the first one I deal with -lrb- buy at Microcenter -rrb- ; I assume that zoom make a change in design and this one do not get as warm -lrb- as the other one -rrb- . price difference between Zoom and Motorola on Amazon be not worth mention ; while Motorola be a big name in communication equipment I feel more comfortable for go with zoom this time .