No ranging response received - t3 time-out error and other Docsis timeout errors and fixes

May 11, 2021

Once in a while, cable modem signals can fall out of their specific operational range. This usually means that the hardware is degrading and slowly losing performance. It’s one of the first things technicians will look for when attempting to correct an Internet connection problem.

There are numerous things that can go wrong to cause an error of this kind. They include the T1 through T4 and T6 timeout errors. It could also be caused by DHCP Warning, a DCHP failure, a Sync Timing failure, or a number of others.

Here, we are concerned with the 5 possible timeout errors, but before you move on, below are some words and their abbreviations you may want to familiarize yourself with so you can get a better understanding of your modem's problem and how to troubleshoot it.

CM - Cable Modem
CMTS - Cable Modem Termination System
RNG-REQ - Ranging Request
RNG-RSP - Ranging response
DOCSIS - Data Over Cable Service Interface Specifications
VSIF - Vendor-specific information fields
MAC Layer - Media Access Control Layer
FEC - Forward Error Correction

T1 Timeout: “No UCD’s Received”

The cable modem has received no periodic Upstream Channel Descriptor (UCD) communications from the CMTS in the timeout period.

This is not an error that happens often. You will have to contact your ISP to get a solution to this problem.

T2 Timeout: “No Maintenance Broadcasts for Ranging Opportunities Received”

The cable modem received no broadcast maintenance opportunities to transmit a Ranging Request (RNG-REQ) in the T2 timeout period (+/- 10 seconds). The cable modem resets its cable interface and restarts the registration procedure.

T3 Timeout: “Ranging Request Retries Exhausted”

This error code indicates that the cable modem has sent 16 Ranging Request (RNG-REQ) messages and received no ranging response (RNG-RSP) message in return from the CMTS. The cable modem will reset the cable interface and restart the registration procedure. This is usually caused by noise on the upstream triggering the loss of layer messages. Noise might also raise the signal-to-noise ratio on the upstream until the cable modem’s power level is inadequate to transmit messages. If the upstream transmit power level cannot be raised to a level that permits communication in the max timeout period, it will reset the cable interface and restart the registration procedure.

T4 Timeout: “Received Response to Broadcast Maintenance Request, But no Unicast Maintenance Opportunities Received”

The cable modem received no station maintenance opportunities to transmit a Ranging Request (RNG-REQ) message in the timeout period, about 30 to 35 seconds. The cable modem will then reset the cable interface and restart the registration procedure. This means a temporary loss of service, but if the problem continues, check for service outages or maintenance activity on the system.

T6 Timeout: “REG RSP Not Received”

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 its cable interface 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. 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.

Why Codeword Errors Matter

If you are hoping to get a quality user experience without throwing your old hardware out and replacing it, it’s important to know what these code errors mean. DOCSIS codes are the best way to tell whether there is a data issue such as slow-loading web pages, rendering problems, poor sound quality, a data network problem, or an RF plant problem.

A codeword error alerts the user to corrupted data in the flow or transmission and there are two types, those that can be corrected and those that cannot.

Correctable VS Uncorrectable Codeword Errors

A correctable codeword is one that can be repaired using the applicable FEC data. If your decoder detects any bits in the codeword that have been corrupted, it will use the correction data to try to repair the corrupted bits. If the bits are able to be repaired, the decoder will report back with a codeword that is correctable, since the codeword was saved due to the error correction. That means the user never even knew that RF impairment took place. However, even after repairs, if too many correctable errors have been generated, it will also reduce performance as the modem and CMTS are laboring extra hard to correct the data, increasing the total workload of the machine.

Uncorrectable codeword errors indicate damage beyond the FEC’s ability to fix it. Uncorrectable errors necessitate that the data be repeated. They are usually the greatest concern. If the bits cannot be repaired, the decoder will report an uncorrectable codeword. This means that your device will need to re-transmit the data, and the quality of experience will begin to diminish as the number of uncorrectable codewords begins to pile up. This is especially the case for real-time services like gaming.

The way correctable errors are fixed is through an automated process rendered by an error protection algorithm called the Reed Solomon Forward Error Correction, usually abbreviated as “FEC.”

The FEC has two components; the encoder in the cable modem and the decoder in the CMTS. When the system is bombarded with correctable errors, it corrects them. When too many correctable errors appear, quality is degraded. When uncorrectable errors appear, the system will try to ride out the turbulence, but the quality will be degraded even more.

Regardless of the type of error, the system can only handle so many before the connection becomes unusable. But when it comes to answering the question of how much is too much, the only answer is that it depends on the device.

If you are getting uncorrectable codewords, that means someone is losing data. Generally, there are two ways to deal with this; to replace the modem or call your ISP to send out a tech.