Comcast Cable modem upgrade how to. Motorola SB6120 & Zoom 5341 modems and the slow upload speed problem

Summary: this article is a tutorial on how to upgrade your cable modem as well as my experience upgrading my Comcast (Colorado) cable modem to a Motorola SB6120 and then a Zoom 5341. My experience (low upload speed) may be related to my location on Comcast’s network so other Comcast users outside the Denver / Boulder area may not experience the same problems with upload speed with the Motorola SB6120.

Comcast recently jacked up it’s modem rental fee for my Motorola SB5120 to $7 per month so it now makes sense to buy your own modem as they can be purchased online for around $80 – $90. Another benefit would be upgrading to the new cable modem standard DOCSIS 3.0 that promises 100Mps speed from good old copper wire (your coaxial cable). Of course if you have a 2Mbps up and 10Mbps down plan from your cable company you’re not magically going to jump up to 100Mbps but you may get better download speed as happen with me even though my plan didn’t change. So I knew I wanted to get a new cable modem but what modem should I buy? After some 5 minutes of googling it was obvious that a lot of folks love the Motorola SB6120 – Amazon has a ton of positive reviews on this modem. I ended up ordering it from Newegg. Note: it’s important to make sure your Cable provider supports the modem you buy but most of the latest popular cable Modems work on major US cable networks.

Preparation – run a speed test!

Before I installed the modem I did a speed test with the old modem. Remember if you do a speed test over WIFI you will only get a max speed of 10Mbps down if you have an older wireless G router (wireless N routers should be able to get full speed) so make sure to do the test with an ethernet cable plugged in to you computer and your wifi off to get the true speed of your connection. In my case, I’m a Colorado Comcast user with the base high speed internet package that I believe is 2Mbps up and 10Mbps down. My speed test with the old modem Motorola SB5120 showed me I was actually getting 4Mbps up and 15Mbps down – not too bad but I do pay $67 per month (soon to be $60pm)!

Unboxing and what next?

I un-boxed the new modem and plugged it in. Make sure to disconnect your wireless router and simply plug the new modem into the ethernet port of your computer when first setting it up. I was unsure what to do next: a buddy had mentioned that you just fire up your browser and a screen magically appears asking your to enter some details on your new modem. This was not the cast with me and the docummentation was usless. An install CD proved to be useless too even though it contained a Mac compatiable installer program.

Installation is actually quite simple: all you need to do is plugin the cable modem to the cable TV wire, plug in the ethernet cable into the modem and the other end into your computer and call Comcast and a nice man in a far off land, India in my case, will ask you to give him a little number off the back / bottom of the modem called the MAC address ( nothing to do with Steve Jobs or Apple Computers). That’s all there is too it. The lights on the modem will flash a few time and he’ll tell you that it’s connected when it’s not, but after about 15 mins the modem will actually be online and you’ll be able to navigate over to you oh so important page and run a speed test.

The results of the Motorola 6120 – Upload speed fail!

With much excitement I ran the obligatory speed test on and sure enough I was now getting 20 Mpbs down BUT, and this was a big BUT, only 1Mbps up! I emailed my buddy who recommend the Motorola SB6120 modem and he said that perhaps the Comcast network was busy as it was in the evening and that my upload speed would probably improve. The next day I nervously tried the test again and got the same lousy results! Going back online it seemed that some folks were having the same problem too. Some suggested another modem, a Zoom 5341 and they reported that this modem had solved the lousy upload problem. Being an Amazon prime user I found a Zoom modem for $80 and clicked the free two day shipping button. Sure I could of called Comcast and sat on the phone for several hours with a nice man in a far off land whike he asked me question such as “is you PC turned on” but they’d probably told me that I only have a 2 Mbps up service so the Motorola was indeed working correctly. So I opted to try the Zoom instead.

Zoom zoom zoom – Zoom 5341 to the rescue

The Zoom arrived on Saturday morning and after spending slightly less time on the phone with Comcast we got it up and running and I was happy to see my upload speed was now back to normal and I was getting 20 Mpbs down now too. Some readers may well be asking why bother with all the fuss and that I should of stuck with the Motorola. Good point but I happen to do quite a bit of uploading via ssh to a server that I run so the faster the upload the better for me and I was dammed if I was going to spend $90 and have a worse upload speed. I’m a video producer too so it’s not uncommon for me to upload a 200meg file to YouTube or Vimeo.

In Summary

Upgrading your cable modem makes financial sense due to the rate increase of the cable modem rental fee but you’re gonna have to spend some time and effort making sure that you get the same speeds that you got before. In debugging the problem i do what i always do; use a search engine and then post to related forums to get folks more knowledgeable than me to help me out. In my case DSLreports forum had info about the upload speed issue. Make sure you run the speed tests listed below both before and after you do the upgrade to see if you are not running into the upload speed issue I encountered. The Motorola seems like a better built unit compared to the tiny Zoom. There are some reports that the Zoom gets very hot (with an older model i think) but I’ve been using it for a few days now and have not noticed this. Upgrading also makes sense as it will allow you to take advantage of the higher speed offerings eg 50mbps (and more costly) that your cable company provides.


Comcast speedtest page:
Speakeasy Speed test: speed test page:


MOTOROLA SB6120 160Mbps in DOCSIS mode and 195Mbps in EuroDOCSIS mode DOCSIS 3.0 SURFboard eXtreme Cable Modem – link to pdf datasheet:
Zoom 5341 DOCSIS 3.0 Cable Modeml – link to PDF datasheet –