Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem

• Up to 160 Mbps downstream, up to 120 Mbps upstream
• Front panel, easy to read operational LEDs to indicate status and simplify troubleshooting
• IPV6 support - latest Internet communication protocol
• Rear panel color coded for ease of installation

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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I have tried both the SB6120 and the SB6121 and the signal levels and speeds were the same on both.

The SB6121 is a little smaller than the SB6120 but runs much warmer than the SB6120. Looks like they tried to cram too much hardware into too small of a space.

My SB6121 did not come with the power save button (it's listed as an optional feature), so about the only difference between the two models is the design. Both the SB6120 and the SB6121 use the same firmware: labeled SB612X.

So unless you want the different looking LED lights and a smaller and warmer box for a higher price, stay with the SB6120.

As far as speeds are concerned: the multiple channel bonding on these DOCSIS 3.0 modems is nice for sustained constant speeds. It also makes it easier for your ISP to enforce the speed cap if they choose to do so (Comcast in my case does). I was getting 20+ Mbps down and 3+ Mbps up on my old Linksys DOCSIS 2.0 modem, however that was the peak when the transfer first started, then it would taper off.

The DOCSIS 3.0 modem stays right at the 12/2 ceiling of my speed tier, but at least the speeds stay right up there and do not drop off after a few seconds.

Read Best Reviews of Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem Here

Bottom line: this is a great modem and it works with Comcast. Comcast just doesn't have its act together. This modem is not only on their approved list, it has a three star rating, which, according to Comcast, is "the highest and most thorough level of testing." When the cable installer got here (I got a new line installed), he said that this modem was NOT on the Comcast list. Since I still had my DSL line working, I showed him that not only did Comcast approve the modem but that Comcast thoroughly tested it. He had to call the "office" to activate the internet service. The "office" didn't want to set up internet service for this modem; the "office" stated that this wasn't an approved modem. Well, he recalled the "office" and got someone else and they set up the service. I've been happy ever since. I'm supposed to get up to 20 Mbps download and 4 Mbps upload. My computers are connected wirelessly. Speednet states that I am getting 20 to 25 Mbps downloads and the upload speed is between 3 and 3.5 Mbps. I've gotten similar values with speakeasy and other internet speed testing sites. I have a lot of devices connected: three computers (wireless), two TVs (one wired, one wireless), three different game consoles (connected to the wired TV), two Nintendo 3DS (wireless), one printer (wireless), and an iPod Touch (wireless). At times all of these devices are running at the same time the computers are streaming You Tube videos, one of the TVs is streaming Netflix, etc. The modem handles all of it effortlessly. I'm really pleased with this modem.

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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 when I came back I got a screen that stated "Device Activate" (and some other instructions that didn't matter) Hallelujah! Also note, NO phone calls had to be made to Comcast to activate this modem.

At this point, this modem which was losing connection all the time before, now had a seemingly rock solid connection!!! I'm using the connection now with this modem as I type this. I've did some basic testing, streamed video, surf the net,etc. NO LOSS OF CONNECTION! I did a speed test and I'm getting 25Mbps down and 3.8Mbps up.

Based on my experience so far, it is in my opinion that this modem works and is as good as what the majority of the reviewers here have stated. The problem lies in Comcast's self activation process which is suspicious. What doesn't make sense to me is the fact that modem can hold a solid connection if you didn't attempt to activate and after it's successfully activated. It was only when I tried to go through the self activate process that I saw problems. I can't say much about whether step #1 helped but it can't hurt to try. I mentioned it only to inform people of what exactly I did.

I truly hope this helps people. Amazon is top notch. This modem is now doing what it's suppose to do with no issues. I'll come back and update this review in about a month to report if it still holds true. I think it will but we'll see. And lastly, thanks for the people who shared their experiences here. If it weren't for them I would have returned the modem back to Amazon and go through the hassle of finding another modem or giving in and renting one from Comcast. I'm trying to pay it forward with this review.

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12/14/2011 Just received the Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem today. Followed the self-install instructions included with the modem, using Comcast internet, and a Medialink Wireless N router, and was up and running in 20 minutes. Note; also used a Windows OS laptop for the setup, with Internet Explorer.

Here's how my setup went. Disconnect your old modem. Connect the SB6121 modem to your internet coaxle cable. Now connect the ethernet cable to both the SB6121 modem and your computer. Power up the SB6121 modem and wait for all lights to stop flashing (must have solid lights), then power up your computer. Now, open Internet Explorer browser and wait for the Xfinity screen to automatically show up on your Internet Explorer's screen. Enter your Comcast account number and the phone number associated with your Comcast account on the screen, where asked, and press next. The SB6121 modem will then be automatically identified and recognized. Press next to accept the Motorola SB6121 cable modem, then wait a few minutes for the next screen to show up while the information is being processed, then press finished. 3 screens total. Disconnect the ethernet cable from your computer and connect it to your router. Now, power both the SB6121 and your router off and wait 10 seconds. Now, power the SB6121 modem on and wait for all the lights to show themselves. Now, power on your router. You should be able to go online wirelessly. Literally that simple. The Comcast Xfinity self-install screens did not ask me for a MAC address or a serial number. Total time about 20 minutes to remove the old modem and install the SB6121. Best part, no having to call tech support and no frustration!

I was quite surprised at how easy the SB6121 was to install, as it seems like I'm one of those guys who takes 2-3 hours to hook anything up. Seems like there's always that one thing I didn't know, or think, about, and I'm usually on the phone with tech support for a while having a lackluster time. I replaced an old Comcast RCA modem which I had no problems with, but was tired of renting from Comcast at $84 per year. The SB6121 will pay for itself in a year's time, is a 3 docsis modem versus 2 on my old modem, and works well thus far.