Cable Modem

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Motorola SB5101

Explicacion de los valores de la señal

  • Downstream: Signal to Noise Ratio
  • Downstream: Power Level
  • Upstream: Power

A: Downstream (Rx) Receive Power Level:

For all modems:

  • -15 dBmV to +15 dBmV maximum.
  • -12 dBmV to +12 dBmV recommended.

0 dBmV is the "optimal" level.

Upstream (Tx) Transmit Power (a.k.a. Return Signal) level:

  • +8 dBmV to +58 dBmV for QPSK. (DOCSIS 1.x)
  • +8 dBmV to +55 dBmV for 8 qam and 16 qam. (DOCSIS 1.x)
  • +8 dBmV to +54 dBmV for 32 qam and 64 qam. (A-TDMA DOCSIS 2.0)
  • +8 dBmV to +53 dBmV for S-CDMA DOCSIS 2.0 modulation rates.

Recommended upstream signal levels are between +35 dB and +50 dB.

A cable modem running a higher upstream modulation rate may downgrade itself to a lower modulation rate (i.e. 64 qam to 16 qam or 16 qam to QPSK) if the upstream transmit level is higher than the maximum signal level allowed for the higher modulation rate and the CMTS is configured to allow such a change. This downgrade can cause slow speed, packet loss, and connection loss issues depending on the condition of the upstream channel.

A house or drop amplifier will NOT fix upstream signal problem because most house amplifiers don't amplify the upstream signals, they only pass the upstream signal through with some loss.

SNR (signal to noise ratio) levels: DOCSIS specifications list minimum CNR (carrier to noise ratio) levels not SNR levels. The SNR levels listed here are based on commonly recommended MER levels for digital cable signals. Not all qam demodulator chipsets accurately calculate SNR levels that approximate actual MER levels, so these levels may vary depending on which chipset and/or firmware is used in the equipment.

QPSK: 12 dB minimum. 15 dB or higher recommended. (often used in upstream channels) 16 qam: 18 dB minimum. 21 dB or higher recommended. (often used in upstream channels) 64 qam: 24 dB minimum. 27 dB or higher recommended. (often used in downstream channels) 256 qam: 30 dB minimum. 33 dB or higher recommended. (often used in downstream channels)

There is no upper SNR limit, although in practice 40 dB is about the highest seen.

Downstream SNR levels are read at the modem on the downstream data channel.

Upstream SNR levels are read at the CMTS on the upstream data channel, not the modem or the modem diagnostic screens. The end-user cannot get the upstream SNR directly. Only the provider can read the upstream SNR level, directly from the CMTS. Also, the upstream SNR level provided by the CMTS is not specific to any single modem, but is an averaged, aggregate level from all modems on that upstream channel on the upstream port.

Explicacion del Channel ID

For every downstream channel, there might be up to six upstream channels on which cable modems send data to the UBR. These channels are normally allocated to frequencies below 42 MHz (DOCSIS) or below 65 MHz (EuroDOCSIS). Your cable modem discovers the upstream frequencies by listening on the downstream channel for Upstream Channel Descriptor (UCD) packets. The UBR's service area might be physically segmented so that not all upstream channels are available on all segments. The cable modem will choose (or be allocated to) one of the available upstream channels at your particular location.

Resumen de valores

  • Frequency = 603000000 Hz
  • Channel ID = 2
  • Frequency = 35000000 Hz
Downstream Upstream
Descripción Power Level Signal To Noise Ratio Power [4]
Como lo dejo Fibertel con un atenuador FAM-6 - 6 DB -4.2 dBmV 37.3 dB 46.3 dBmV
Sin el atenuador (directo y sin nada mas) 11.0 dBmV 37.6 dB 44.0 dBmV
Con Atenuador, divisor y 15 m de cable nuevo [1] -9.0 dBmV 36.4 dB 49.8 dBmV
Con Atenuador, divisor, filtro para TV y 15 m de cable nuevo y TV [2] -6.9 dBmV 37.1 dB 49.3 dBmV
SIN Atenuador, divisor, filtro para TV y y 15 m de cable nuevo y TV [3] -0.9 dBmV 37.7 dB 43.3 dBmV
  • [1] Cable de buena calidad y divisor 30 DCM6 5-1000 Mhz
  • [2] Filtro para TV HPF 54HRL
  • [3] Siguiendo las recomendaciones esta es la mejor configuracion, y queda aun mejor que como me lo dejaron
  • [4] Según el técnico de fibertel Upstream Power debería estar entre 40 y 45 dBmV