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1. 

   T. Hyatt on February 3, 2015 at 2:07 pm

   2,224 of 2,271 people found the following review helpful

   
   Does the job well/tip for Comcast users, September 29, 2011

   By 

   T. Hyatt –
   
     

   Verified Purchase(What’s this?)

   This review is from: Motorola Surfboard SB6121 DOCSIS 3.0 Cable Modem (Personal Computers)

   Out of nowhere, I was having excruciatingly slow internet speed on Comcast. The Comcast tech advised that my Motorola SB 4100 was at the “end of life” and that I should upgrade. Skeptical, I purchased this modem which is a DOCSIS 3.0 (and backwards compatible) modem. It worked. I went from a 2mbps download speed to over 17mbps. What a difference. There was one installation glitch that you need to be aware of. When you hook up the modem (connect the coax cable, connect the ethernet cable, and plug it in, that’s it), and then try to get online, it will direct you to a Comcast self-activation screen (no other site is available). After entering my account number and phone number and hitting the “next” button, I got a blank screen, and nothing happens. So I called Comcast and they said that usually happens if you are doing an upgrade; it mostly works only for a new service connection. However, it is a simple matter to give the tech your MAC address and the serial number. After about 5 minutes, while he stayed on the line, it came alive, no problem. BUT, here’s the thing: the serial number they need is the CUSTOMER serial number. This is only found on the bottom of the Motorola box the modem came in. This is different from the “S/N” serial number that is on the label that is on the instruction sheet and on the bottom of the modem itself. The customer S/N has letters in it; the regular S/N has only numbers in it and is too long. My first Comcast tech didn’t know that and it took a day to sort it out. Other than that, it works well with my iMac and Apple Airport Extreme Base wireless router. Enjoy

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2. 

   Kendoso on February 3, 2015 at 2:19 pm

   1,571 of 1,639 people found the following review helpful

   
   Modem NOT defective – Comcast conspiracy?, February 4, 2012

   By 

   Kendoso –

   Verified Purchase(What’s this?)

   This review is from: Motorola Surfboard SB6121 DOCSIS 3.0 Cable Modem (Personal Computers)

   I felt compelled to write this to help those who will encounter the problems I faced (and mentioned here). I’m one of the many people who wanted to avoid renting a modem from Comcast and bought this modem based on the stellar reviews, only to find that on the day of installation/activation that it “didn’t work”. Didn’t work meant experiencing the same symptoms that’s been mentioned here several times – “modem connects but then resets/power cycles/disconnects itself within a few minutes.” This happened to me on the day I activated my Comcast internet. The technician spent 1.5 hours trying to figure it out and ended up loaning me a Comcast modem (Motorola SB5120 DOCSIS2), which came up almost instantly and activated just as fast (go figure). He suggested I return this modem as it was defective, in his opinion. I didn’t like this since the probability of this was very small. But I saw what I saw and could not get past the first page of Comcast self activation screen with this modem. Before the web page for self activation could proceed, after typing my account info and phone number, the modem would conveniently lose connection. At one point I did get to the next screen where it was scanning for devices, but the modem cut out thereby hanging the process. The modem was packed up in the box for return to Amazon when I couldn’t shake how much this bothered me and by my bad luck. So I came back here and read pretty much every review. What I found was there’s several people who experienced the EXACT same symptoms while trying several of the same modem. There were two reviews that stood out. One was from a person who mentioned the Comcast technician moaned when he saw it was this modem, indicating that the technician was aware of difficulties using this modem. The person mentioned that after 70 minutes of trying to get the modem to connect the technician advised him to leave the modem powered on and hooked up for awhile and to try activating later. The person did this and was successful in activating the modem. The other review was from a person who was admirably persistent. His review talks of several attempts to connect and refusal to accept that the modem was defective. He ended up leaving the modem power on and connected overnight so the modem could “download firmware”. The next day he was able to activate after two attempts with Comcast self activation process. Lastly, I noticed for me that if I left the modem power on and connected but make no attempt to activate it via Comcast web site, the modem retains connection (via indicator lights) for prolonged periods of time. It only seems to lose connection when I try to activate. Based on this I felt something was broken with Comcast activation process specifically for this modem or perhaps Comcast was intentionally making it difficult to activate this modem to convince customers to rent a “reliable” modem from them. To be honest, it almost worked for me. After sitting there watching the technician agonize trying to get the modem to work, I was tempted to rent the modem and move on with life. So as one last hail Mary attempt, this is what I did:
   1. Took the modem back out of the box and left the Motorola SB6121 power on and connected to cable outlet overnight just to entertain the possibility that some firmware or info needed to be downloaded to the modem and that for some reason it took a long time to do so.
   2. The next morning I tried Comcast’s self activation process.
   3. Every time Win7’s network icon in the lower right corner of my screen indicated that a internet connection was established (vs. exclamation mark) I immediately opened a IE windows and started the activation process with my account and phone #. The modem still lost connection when I did this but I kept on trying.
   4. It took me about 4 tries to SUCCEED. Each time I tried, I started back on the account and phone # page. But each time it got further and further in the activation process.

   Attempt #1: Account/phone # screen -> Retrieving Device Information -> Modem lost connection.
   Attempt #2: Account/phone # screen -> Retrieving Device Information -> List of devices detected (screen showed my MOTR SB6121 modem and a DTV box both shown as “inactive” -> Modem lost connection. (A thing to note here is that Comcast’s activation system could specifically identify this modem. This meant it was in their system and their system was able to communicate with it)
   Attempt #3: Account/phone # screen -> Retrieving Device Information -> Listing devices detected -> Clicked “Rescan” device (modem)-> Hit “Next” to activate then got to screen where it said “Activating Device – This may take 10 minutes” -> Modem lost connection
   Attempt #4: This isn’t a real attempt because all I did is leave the web browser in the previous “Activating Device” screen and waited for the modem to re-establish connection again. I stepped away for a few minutes and…

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