Comcast XFINITY → Re: [Connectivity] Modem is reporting continuous T4 errors.

FWIW, they happen for downstream issues as well. Any RF impairment which causes a loss of "handshake" / communication between a modem and the CMTS will yield them.

Maybe in your system they're a downstream problem - but where I'm at its 99.9% an upstream issue.

I'm inclined to believe they are upstream for a few reasons:

1. Historically the majority of issues with Internet are upstream signal problems.
2. I'm not seeing problems with my cable channels which are all digital.
3. The signal level and SNR for my downstream channels is very good.

From what I've read the T4 time out is 30 seconds and that's what I'm seeing every 30 seconds since yesterday I get a "Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out" error in my log. So this is a continuous problem.

Quote;

"3-Critical H501.8 HFC: T4 Timer Expired:

Explanation. The cable modem did not receive a station maintenance opportunity in which to transmit a Ranging Request (RNG-REQ) message within the T4 timeout period (30 to 35 seconds). The cable modem is resetting its cable interface and restarting the registration process. Typically, this indicates an occasional, temporary loss of service, but if the problem persists, check for possible service outages or maintenance activity on this particular headend system.
Recommended Action for your Provider: Check the configuration on the CMTS. Check the cable plant for RF connector or cabling issues that could be generating noise on the downstream and upstream."

"T4 ( Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received )

Explanation: The cable modem did not receive a station maintenance opportunity in which to transmit a Ranging Request (RNG-REQ) message within the T4 timeout period (30 to 35 seconds). The cable modem is resetting its cable interface and restarting the registration process. Typically, this indicates an occasional, temporary loss of service, but if the problem persists, check for possible service outages or maintenance activity on this particular headend system. This error message is DOCSIS event message is R04.0, Ranging Request.

Quote;

"Why Do Cable Modems Drop Offline?
A Cable Modem requires three main things to remain online once it is connected and operational:

Clean RF plant with a carrier-to-noise consistently above 30dB in the Upstream, and above 33dB in the downstream.

Unicast polls from the CMTS every 30 seconds (keepalives). These are unicast transmit opportunities for this modem's assigned SID, in which it can send a RNG-REQ to the CMTS. If the cable modem does not receive a unicast transmit opportunity within T4 seconds (30 seconds) it has to time out and re-initialise it's MAC layer. So if there is a problem (RF) in the downstream, the cable modem might not "see" this unicast transmit opportunity, and drop offline.

If the CMTS does not get a reply from the CM to the unicast transmit opportunity, the CMTS will poll the modem 16 times in short succession in order to try and get an answer. The modem is considered offline by the CMTS if there is no reply after these retries."

T3/T4 explained in a lot more techy detail than most arm chair techs will ever remember.

FWIW - »volpefirm.com/docsis-101 ··· tenance/

I've seen that before. It's a good article, thanks.