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FIBER -- internal run, dB loss ?

Posted on 2014-12-23
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Last Modified: 2014-12-28
I have about 1,000 feet of internal 62.5 micron multi-mode fiber with 6+ patches and a total combined loss of 3 db, making this about a 50% retransmit per http://www.thefoa.org/tech/lossbudg.htm

 1. Will this be OK for voice (VOIP) traffic ?
 2. If not, what is the max db loss for voice (VOIP) traffic that I should be willing to have ?
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Question by:finance_teacher
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by:Phonebuff
ID: 40516558
Not an expert in this area, but I would think it's going to have a lot to do with the hardware on either end of the Fiber run and what it's tolerances / capabilities are.
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by:agonza07
agonza07 earned 333 total points
ID: 40516674
I agree, I think there's some confusion between db loss and actual packet loss. They are not the same thing.

As long as the fiber run and equipment operate within their required margins, it should not affect transmission signals and proper operation.

To simplify things (a whole lot,)  if you get a link light you can run anything over that link whether it be video, voice, or anything else.

Think about it like a water hose. When you experience db loss the hose shrinks a little in diameter, but in the end the water (IP Packets) gets through all the way. Even though the diameter may be a little smaller than how it started, you're not losing any water.

In regards to db loss, all you are doing is making sure that that hose still has an opening at the end. Too much db loss and the hose closes up and no water gets through (no link light...)
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by:finance_teacher
ID: 40517084
So a 20% dB loss does not mean that 20% of the packets needs to be resent ?

It just means the pipe can only use 80% of its possible size ?
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by:agonza07
agonza07 earned 333 total points
ID: 40517168
they have nothing to do with each other, I'm sorry if I confused the matter more.

In order for the link to work, DB loss must fall within a certain margin which is dependent on the equipment used.

Once the link is up you should have full capability of that circuit.

Db loss has nothing to do with packet loss or bandwidth.
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Accepted Solution

by:
JRSCGI earned 167 total points
ID: 40520118
the dB loss on a fiber link is the measurement of the strength of the light source at the far end compared to the originating light source.  If the originating light source in the electronics is the basic LED used for an FDDI-grade 62.5-micron fiber (160 MHz-km at 850 nm) the light output will be a specific level of "brightness" - the loss will "dim" the light at the far end.  If the total loss is less than the allowable loss of the electronics (the published loss budget by the electronics manufacturer), the signal will work fine without performance degradation of the signal.  Most electronics can tolerate a 3 dB loss and work just fine.  However, remember that additional loss can be introduced by the connectors and patch cables, especially if they get dirty.

As stated above, fiber dB loss has nothing to do with packet loss.
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