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Model: Cable ModemChange
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2,626 of 2,688 people found the following review helpful
on September 29, 2011
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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2,063 of 2,147 people found the following review helpful
on February 4, 2012
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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804 of 835 people found the following review helpful
on March 17, 2012
... It seems that various ISPs are breaking FCC regulations and Federal Law by refusing to activate some newer modems, in an attempt to force users to use a "leased" modem.

There is an essentially IDENTICAL version of this device, the SB6141, which is the "ISP leased" version of this. You can't buy that, at least not "officially," mind you.

(EDIT - since I originally wrote this, it does seem that it's become possible to buy the "cable operator only" SB6141 directly...)

Time Warner Cable has just had me on the "loop" for the past two and a half hours, and I'm still on hold as I write this. They're telling me that the device isn't on the "approved list" and thus cannot be activated on their system. This, of course, is PATENT NONSENSE. All they require in order to access any device is (a) a device which speaks the same electronic "language" (in this case, DOCSIS 3.0, which TWC Austin supports fully, and brags about supporting fully), and (b) a valid, unique HFC MAC ID code.

The MAC ID is what allows the two devices (the ISP's "source" modem and your modem) to talk to each other, as the ISP needs to know that the ID it's talking to is an ID which it's supposed to talk to, and they need to speak the same language. THAT IS ALL THAT IS REQUIRED ON ANY TECHNICAL BASIS WHATSOEVER.

It is a violation of Federal Law and FCC regulations for an ISP to refuse to service any device which has a valid, unique, and "not stolen" MAC ID and which uses an approved communications protocol.

Those of you who have been told "Comcast won't support this" or, as I just was, "Time Warner won't support this," please realize, YOU ARE BEING ROBBED, every bit as much as if they put a gun to your head and told you to hand over your wallet. It is a CRIME for them to do this.

I am currently on hold, with a "customer advocate" at Time Warner, attempting to get my personally-owned, legally-required-to-be-supported device added to my account.


Okay, after a long dialog where I quoted letter and verse of Federal law to the "advocate," I got them to add my modem to my account. And it's working flawlessly.

I would not ordinarily post things not SPECIFICALLY about the device on a review of the device, but in this case, I felt like it was necessary, as some folks may believe they've got a defective piece of hardware (based upon what the ISP tells them) when this is NOT the case.

My ping times have improved... which is somewhat surprising to me, honestly... from about 58ms to around 20ms. I'm not sure why that's the case, but it's a good indication that I have a better connection (lower ping times means less delay between when you send something at one end and when it's received at the other end, basically). My throughput is much more consistent and level, and I haven't seen any of the disconnection errors I've seen repeatedly with my older (DOCSIS 1.0) modem. It seems to be performing flawlessly.

If you have an internet service provider (ISP) who has support for DOCSIS 3.0 (the latest iteration of the modem-interaction-language standard), this is a great choice. Just be prepared to have to argue a bit with your ISP, who seem, in many cases, to be doing everything possible to get you to lease one from them instead of buying your own. Once they give in and agree to activate the MAC code on their system, this will work just fine on ANY current ISP, and will provide full DOCSIS 3.0 support on any DOCSIS 3.0-supporting network.

Don't let the script-reading "support" types tell you otherwise.
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784 of 862 people found the following review helpful
on July 8, 2011
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.
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325 of 363 people found the following review helpful
on August 7, 2012
I had bought this modem about a month back for my connection with Comcast based on all the stellar reviews it has received. However, the modem just didn't stay connected on my line and kept rebooting every hour or two, and sometimes took a good 15 minutes to reestablish a connection. Comcast promptly blamed the modem for my connection problems, but I insisted they have a tech come and inspect the line and as it turned out, the upstream signal power was a tad more than what the recommended power range. It took 4 or 5 tech visits to bring down the power level within the acceptable limits, but the rebooting issue persisted. Comcast finally issued a firmware patch which kind of brought down the frequency of the reboots significantly, but it didn't entirely eliminate the issue.

So I was forced to return this modem and opt for the Zoom 5341 DOCSIS 3.0 Cable Modem 5341J which was recommended to me at the Comcast forums and that modem has stayed connected without a single disconnect ever since I bought it. I would've given the SB6121 a 4 star rating because when it stayed connected, the speeds were pretty good but the fact that the Zoom modem was able to handle signal strengths which this modem couldn't made me pause. For reference, my upstream power levels are around 47dbmv and my downstream is around 5 dbmv, which are within the acceptable limits for a modem to operate.

All these stellar reviews for the SB6121 cannot be wrong, but just be aware that there are issues with using this modem on Comcast (if you are thinking of getting this modem, search on Google for "Comcast SB6121 rebooting issues" and run your own diagnostics to see if your setup is similar to the ones that experience this issue). Caveat Emptor.
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470 of 529 people found the following review helpful
on June 29, 2011
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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140 of 155 people found the following review helpful
on October 23, 2012
I bought this modem to avoid the new Time Warner cable modem rental fee in Charlotte, NC. I'm extremely happy with it so far.

The item arrived from Amazon. I unplugged my existing TWC modem and plugged this one in. I then called Time Warner and told them I bought a new modem. They asked for the MAC address and switched my account to the new modem. The call took about 15 minutes and was very easy.

One of the nice things about this modem is it has very detailed status and diagnostic information. While connected to the network this modem is connected to, point your browser at [...] and you can view items like signal strength and the number of channels in use.
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194 of 217 people found the following review helpful
on December 14, 2011
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.
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36 of 39 people found the following review helpful
on November 9, 2013
You are a Comcast customer.
You have read all of the reviews here that have Comcast in them.
You are ready to buy this modem and save money over the rental.

Carefully look at all of the Comcast success stories here and notice the dates. It appears either, Comcast installs vary by location or the process has changed.

Here are the steps that worked for me.

Get these ready before you start as you won't be able to look them up online once you start.

1.) Your Comcast account number.
2.) The phone number Comcast has in it's records for you (look at your account online to see this if you don't remember).
......Even if you do not have phone service through Comcast, they still ask for this in the sign up screen.
3.) The HFC MAC ID written on the bottom of the modem and bottom of the box the modem came in.
4.) The S/N written on the bottom of the modem and bottom of the box the modem came in.
......Note, not the Customer S/ need the longer one labeled S/N.
5.) Print out this review

Got all of that ready? No?....okay, I'll wait....


- Remove the old modem.
- Put in the new modem.
- Unplug at the router your computer's ethernet cable.
- Plug in ethernet cable directly into the modem for the setup. You will move this back to the router later.
- Plug in the power.
- Go get something to drink and enjoy it for a minute (sans adult beverages, not time to celebrate yet)
- Now that you have waited a few minutes look at the modem and the top four lights should be solid. They may be green-blue-blue-green or all green, but you are hoping for all solid.
- On your computer open your browser and try to go to say
- You get a Comcast activation screen.
- Fill in the requested MAC ID: (HFC MAC ID: on the box/bottom)
- Fill in the rest of the form hit Next, a couple of screens later you could be up and running.

I was not up and running at this point. My activation would not complete.

I called Comcast and told them I bought a new modem and needed them to register the MAC ID for me.
I was informed that they would also have to add this modem to my account.

REALLY? Okay, nobody mentioned that in the reviews. No problem. I have all of that.

I read them the MAC ID: (don't mention it says HFC MAC ID, it will confuse them)
I read them the long S/N:

Clickety, clickety, clickety on their side. "Let me put you on hold while I activate that modem"..."Okay, back now. It will take twenty minutes then it should work fine."

I hang up, 20 minutes later...nothing.

I unplug the power from the modem, plug it back in, get another drink and the activation screen is on my screen again.

I fill it out again, this time it goes through and the modem works.

I unplug the ethernet cable (goes to my computer) from the back of the modem.
I plug the ethernet cable back into the router.
I plug the ethernet cable that used to go between the old modem and the router into the new modem.

In a couple of minutes everything in the house is online again and enjoying faster downloads.

In looking at all of the most recent reviews with Comcast in the review, it appears we all had to call Comcast, so I'm guessing the "No call install" is a thing of the past.

Now...about those adult beverages...

P.S. results before modem - 35 Mb/s downloads. After installing the modem 58 Mb/s downloads.
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21 of 21 people found the following review helpful
on August 18, 2013
I've been paying Comcast $7 a month for 11 years on a cable modem rental (Yes that is $924.00). I assumed I would be getting the most up to date modems and have a replacement warranty. I've only had one Modem updated and it was the bottom of the barrel. If you are still renting your modem, please STOP! I figure if Comcast does not increase there rental fee I'll save $350.00 over the next 5 years ($7mo X 12 = $84 year X 5years = $420 - the cost of this Modem = $350 saving). I also posted screen shots of the before (30.5 / 10) and after speed (63.5 / 11) tests in Houston TX. Easy set up and works great!
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