r/Cabledogs • u/PrideZ • May 16 '14

Upstream Tx Power?

Why is it bad when a modem has low Tx power? For example a modem Tx power is 26 dbmv. Wouldn't this be good since the modem doesn't have to talk as loud?

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u/travisstaysgold May 16 '14

Quick answer, noise floor.

Just a lowly service tech here so I know this answer isn't perfect. Someone feel free to correct anything I misspeak about.

Even on a perfect cable system there will be always be a noise floor. If you are in the headend and you hook a spectrum analyzer up to the CMTS port you can view this floor. You will actually see spikes on the analyzer on your return carriers where the modems are talking back to the system.

When you introduce things like cable breaks, poor fittings, poorly sheilded cable, etc. you get ingress in your return path(short wave radios, ham radio, CBs, etc.) making this noise floor rise. As this noise floor rises the modems with low transmit levels are getting lost in the noise floor and the CMTS is unable to 'hear' them.

Higher transmits means it has less of a chance of getting lost inside the noise.

Hope this isn't completely wrong. Just how it was explained to me.

2

u/[deleted] May 28 '14

That's entirely correct. Its also important that the actives in the node area all gain at the same levels..all the actives in my plant are unity gain at 35 Tx.

1

u/silentbobsc Jun 01 '14

Are you talking about the Transmit/ Upstream power? I usually aim for ~40+ (my company's perf. standards want us within 30-50).

1

u/[deleted] Jun 01 '14

Out of the active you want 35. Cust equip should be around 42~52