DOCSIS T1, T2, T3, T4 Timeout Errors?

Below is an overview of the various DOCSIS cable modem timeouts, while the cable modem

T1 Timeout - No UCDs received
DOCSIS event error message U01.0, Upstream Channel Descriptor
Cable modem has not received any periodic Upstream Channel Descriptor (UCD) messages from the CMTS within the timeout period.


T2 Timeout - No Maintenance Broadcasts for Ranging opportunities received
DOCSIS event error message R01.0, Ranging Request
Cable modem did not receive a broadcast maintenance opportunity in which to transmit a Ranging Request (RNG-REQ) within the T2 timeout period (approximately 10 seconds). The cable modem is resetting its cable interface and restarting the registration process.


T3 Timeout - Ranging Request Retries Exhausted
DOCSIS event error message is R03.0, Ranging Request
Cable modem has sent 16 Ranging Request messages (RNG-REQ) without receiving a Ranging Response message (RNG-RSP) in reply from the CMTS. The cable modem is therefore resetting its cable interface and restarting the registration process.

This typically is caused by too much noise on the upstream that causes the loss of MAC-layer messages. Noise could also raise the signal-to-noise ratio (SNR) on the upstream to a point where the cable modem’s power level is insufficient to transmit any messages. If the cable modem cannot raise its upstream transmit power level to a level that allows successful communication within the maximum timeout period, it resets its cable interface and restarts the registration process. T3 timeouts can also be caused by a bad amplifier on the node, bad tap, coax issues, etc.

Some cable modem models/brands may have cleaner upstream signal reducing the number of T3 timeouts (reportedly Zoom 5341-00-00J, for example)


T4 Timeout - Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received
DOCSIS event error message R04.0, Ranging Request
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 and restarting the registration process.

Typically, this indicates an occasional, temporary loss of service, but if problem persists, check for possible service outages or maintenance activity on this particular headend system.


T6 Timeout - Cable Interface Reset
The cable modem has sent 3 Registration Requests (REG-REQ) to the CMTS without receiving a Registration Response (REG-RSP) within the T6 timeout period (3 seconds). The cable modem is therefore resetting and restarting the registration process. This problem can also occur if the DOCSIS configuration file is corrupt, or if it contains a large number of vendor-specific information fields (VSIF). If the configuration file contains a large amount of VSIF information, the cable modem might generate a Registration Request (REG-REQ) that exceeds the maximum size of DOCSIS MAC-layer management messages (1514 bytes plus the header). The CMTS considers this an invalid MAC-layer management message and drops it, without replying.


See also:
What cable modem signal levels are considered good?
Unicast DSID PSN Startup Error?