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The modem scans for a digital QAM channel (which can be digital TV or modem downstream channels) and "listens" for upstream channel descriptor (UCD) information on it (which is only found on modem downstream channels). If it doesn't get the UCD after a few seconds of receiving data on that channel, a T1 timeout occurs and the modem looks for another digital QAM channel.
Once the modem finds the correct downstream channel and receives the upstream channel descriptor, it then waits for its initial opportunity to "talk" to the CMTS. If it doesn't get that chance within 12 seconds, a T2 timeout occurs.
So during initial modem connections, T1 and T2 timeouts are perfectly normal to see in the modem logs.
After locating a downstream data carrier channel, the modem gets its chance to "talk" to the CMTS and sends data to negotiate the proper upstream signal levels and timing. This negotiation process takes several data exchanges between the cable modem and the CMTS. If at any point the modem doesn't receive a response from the CMTS within about 200 milliseconds, a T3 timeout occurs. Several T3s have to occur before a modem resets.
The modem then goes through the DHCP, TFTP, TOD processes and once all are completed the modem is online.
Once the modem is online, the CMTS sends "Periodic Ranging" or keep-alive requests to it every 30 seconds. If the modem doesn't receive one, a T4 timeout occurs. Once 16 of the T4 timeouts have to occurred, a modem resets.
If there is a downstream communication problem, the cable modem might not receive these "keep-alive requests". If the upstream utilization is too high, or too many modems are connected to the same upstream port, it is possible that some modems will not get the required bandwidth or transmit opportunities to respond to the keep-alive requests. Either of these issues can result in T3 or T4 timeouts occurring.
See these web pages for more info:
Cisco Document ID: 16510 - Troubleshooting uBR Cable Modems Not Coming Online
Cisco Document ID: 22543 - Cable Modems Dropping Offline in a 2-way Cable Network