When Imaging a DCT with a New Nk.bin, CM Reaching Online Status Can Take Up to 20 Minutes (320303)

MORE INFORMATION

The maximum time that the CM takes to range is largely hardware dependent.

Assume that the cable modem termination system (CMTS), cable plant, and CM hardware are functional and meet the specifications. You can compute the maximum time as follows:

• The frequency range in a modern cable plant has downstream channels spaced at 6 megahertz (MHz) intervals from 91 MHz to 770 MHz. So the total number of channels is:
  
  ((770 MHz - 91 MHz)/(6 MHz/channel) = 113 channels
• There are three possible channel plans, so the maximum number of channels (or center frequencies) that must be scanned is:
  
  (113 channels) x (3 channel plans) = 339 total channels to scan
  
  The limiting factor in scanning these channels is the settling time of the digital tuner. Our experience in the lab with the DCT5000 set top box indicates that the settling time is between 70 and 200 milliseconds (ms). Additionally, as soon as the CM detects the presence of the downstream channel, the modem needs to lock onto the carrier and go through the connection handshake. This, again, is really a function of the hardware (in this case the CM chipset), but our experience is that it takes from 25 to 45 seconds.
• Assuming, in the real world, a worst case settling time of 500 ms, the calculation returns a maximum scan time of:
  
  (339 channels to scan) x (500 ms/channel) + (45 seconds connect time) = 723 seconds ~= 12 minutes

If the CMTS is busy or goes down during this time, or if there are other network interruptions, this time might be longer. Additionally, builds that pull data over PPFS can block the CM from functioning while modules or other data are being pulled over the Ethernet (or parallel) port. Therefore, maximum time to connect cannot be guaranteed.