dslreports logo
site
spacer

spacer
 
    All FAQs Site FAQ DSL FAQ Cable Tech About DSL Distance DSL Hurdles »»
spc

spacer




how-to block ads



Information on signal levels: http://homepage.ntlworld.com/robin.d.h.walker/cmtips/signal.html#signal

For non-DOCSIS 3.0 modems:

Downstream power level (Receive): -15dbmV to +15dbmV
A value of -15 or worse indicates a poor downstream signal path. A tech would aim for a value close to the optimal 0 dBmV, but a good cable modem should be capable of working within the broader range of -15 to +15 dBmV, provided the downstream Signal to Noise Ratio remains good enough.

Upstream (Transmit): 35-52 dBmV. A value within the range +35 to +52 dBmV is within spec with the low to mid 40's the most common. If the cable modem is going offline, and the upstream signal strength is at or above +52dBmV, then a poor upstream path is probably the problem.

Carrier/Noise Ratio: 35 db and above The downstream Signal to Noise Ratio must be 35 dB or higher. The lower ratio the more noise and the poorer the performance. The Cable Modem will have to keep requesting retransmissions of packets with uncorrectable errors.

DOCSIS 3.0 modems:

Downstream power level (Receive): -15dbmV to +15dbmV
A value of -10 or worse indicates a poor downstream signal path. A tech would aim for a value close to the optimal 0 dBmV, but a good cable modem should be capable of working within the broader range of -10 to +10 dBmV, provided the downstream Signal to Noise Ratio remains good enough. Additionally all downstream channels should be within a +/-2db window with respect to each other.

Upstream (Transmit): 40-52 dBmV. A value within the range +40 to +52 dBmV is within spec with the low to mid 40's the most common. If the cable modem is going offline, and the upstream signal strength is at or above +52dBmV, then a poor upstream path is probably the problem.

Carrier/Noise Ratio: 35 db and above The downstream Signal to Noise Ratio must be 35 dB or higher on each channel. The lower ratio the more noise and the poorer the performance. The Cable Modem will have to keep requesting retransmissions of packets with uncorrectable errors.

Feedback received on this FAQ entry:
  • If you are using tests to look for those low downstream levels and high upstream levels, you are compensating for something that isn't standard which can cause future problems when things on the CMTS and around the infrastructure change (more CM's being applied to a CMTS). It is always essential to give best effort towards 0 on the downstream (stabilized in between the low and high threshold of -15 to 15+) and about 42.50 on the upstream (stabilized between the threshold of 33-52 with a 9.50 midpoint {33 + 9.50 = 42.50 and 42.50 + 9.50 = 52]). Never use a conventional CATV amplifier for your modem line. Only a powered return "splitter" should be used to provide a seperate return channel for both CATV and DOCSIS signal to balance out levels. Only apply attenuators or amplifiers on the CATV lines because the power range is more lenient and the way tuners work are different than a modem's handling and tight requirement for a clean data signal.

    2013-08-28 13:26:47

  • My job is testing and refurbishing cable modems with docsis 3.0 for a major cable company. Our tests look for a downstream between -15 dbmv to -9 dbmv and an upstream of 50 dbmv to 60 dbmv. Anything outside these ranges can cause connection problem and packet loss.

    2013-03-21 08:20:43

  • I would suggest that the requirement for allowable channel signal level unbalance for D3 connections be added to this FAQ answer.

    2011-06-23 19:35:47 (gwm See Profile)



Expand got feedback?

by redxii See Profile edited by CoxTech1 See Profile
last modified: 2011-06-24 08:23:51