Not applicable

cable modem restart

I have read many, many questions and answers on T3/T4 time outs and they always lead to a hardware problem.

What about T3/T4 errors  and no hardware errors? That's a Spectrum tech coming to my house with his little black box. He found nothing wrong with  co-ax and looking over my shoulder at my own modem saw nothing wrong with the modem and verified that the signals on the black box were almost the same as my modem. You will want to ask about signal to noise ratio and power level downstream and power level upstream. Are all the channels bonded, etc.  SNR 37-38 dBmv. power level plus or minus 1 is 0 . upstream power 43-45 dBmv. at times the modem shows a restart but is not noticed.  event viewer on MS platform shows DHCPv6 lost address errors and DNS client time outs and at times DHCP sends a NACK message.

I've been told by tech support that that shouldn't be happening but it is. Oh, and the final kicker or the other shoe dropping. This shows up using two different modems.  Arris SB6121 which is mine and Technicolor TC8715D which is Spectrum's.


Does anyone know how to fix a problem that shouldn't exist?


Re: cable modem restart

T3/T4 errors are almost never hardware errors. They are the most obvious sign of a coax related issue, usually loose connections, damaged cable, water, corroded connections, unterminated runs that allow signal interference, etc.

Proven Sharer

Re: cable modem restart

The first step is to permanently get rid of your obsolete SB6121 modem.  That unIt is not technically compatible with Spectrum's channel assignment plan needed to allow Gigabit and 400 Mbps ethernet service over coaxial cable. 

Next, the first message pinned at the top of this forum is titled


It tells you what information the forum contributors need to see so they can offer educated guesses about the problem affecting your connection.   Copy each of the items and paste into a reply to this message.  

T3 and T4 errors are caused by the modem losing signal on on or more of the DS data channels  assigned to your specific modem connection.  Those signal drops are less than [T3] or longer than [T4] three seconds duration.  Anything in the T4 category will cause the modem to drop its old connection, request a new channel plan assignment from the CMTS, acquire all of those signals, and finally restart communications through the data modem.  Notr al home service techs understand how this works with the new high-speed digital cable infrastructure. 


Not applicable

Re: cable modem restart

It would be what I need to do, replacing the modem, if I had 300Mbps service. Not sure why I didn't say that. I'm using the plan that has download speeds of 60Mbps down and 6 up. I assumed, wrongly, that if I told the type of modem and it's inherent limits, it would be enough.  But even if I didn't, the fact that a Gateway modem and a Surfboard modem would be a clue,

Anyway, kids, I'm closing the call with Spectrum forums and in the future be  using a more friendly environment.

Spectrum, in and of itself, is bad enough. I don't need the same attitude on a "community forum." 


"We're fools whether we dance or not, So, we might as well dance."

Japanese Proverb


Re: cable modem restart

Unfortunately, those 4 downstream models are in fact going by the wayside. Spectrum is gradually bumping the standard plans from 60mbps to 100mbps, which cannot be properly supported with only 4 channels given the amount of congestion on their lines--it is even questionable whether 4 is enough for 60 in some markets (all depends how many users are active on a single node during peak times).

An 8 or 16 channel would actually serve you better not only in the short term, but going forward as well. 8 channels is their current minimum standard for that 100mbps tier they have been gradually moving standard plans towards.

Should note also that if you are on a Spectrum tier now (not grandfathered on a legacy TWC or other previous ISP's plan), they will provide you a modem with no rental fee--which also means you can get it swapped if the modem is suspect.

Aside from those technical details though, you may have some unwanted noise in the lines, considering your SNR levels. May need to take a closer look at what is going on with your in-house wiring. It can be caused by many things, some pretty obvious as has been mentioned (damaged cables, bad connections, etc.). Could be something commonly overlooked like rooms with active cable jacks that do not have any devices attached to them. Those need to be either terminated (resistor caps or other device installed), or removed from the feeds (maybe removing unnecessary splitters as well) in order to more properly balance the lines in use and (hopefully) remove some noise from the floor (basically, improve quality of signal, possibly strength as well).
Proven Sharer

Re: cable modem restart

I'm not sure why you have taken strong offense at my response where none was intended.   Contributors to these peer-to-peer forums are customers like you, with no financial or management connection to Spectrum, except for moderators who are identified as Spectrum employees. 

As a former owner and experienced user of a Motorola Surfboard SB6121 modem,  I can state that the situation is that older four channel modems which do not have full band receivers will not work in today's digital environment with Spectrum's new CMTS channel assignment algorithms. 

The SB6121 is designed to only tune over a 36 MHz operating window, which is 6 sequential channels from top to bottom.  For more than a year Spectrum has been upgrading to 16 or 24 DS channel blocks in much of their recently combined service territory, and 32 DS channel blocks in the mega-metro areas where 400 Mbps and Gigabit service is proving to be a popular offering.  Their CMTS plan tries to assign at least 8 to every modem, but yours and others of that vintage will only accept four, which must be nearly adjacent for the modem receiver to meet its performance specs. 

Spectrum no longer provides any 'rental' devices with narrow bandwidth receivers for any of their numerous speed tier plans.  Their CMTS does not check the maximum speed or channel spacing capabilities of customer provided devices, based on the model number you provided when registering your unit.   That was part of the risk when you opted to buy your own modem and avoid the former monthly modem rental charge.