Optical transceiver question
Daniel.Jameson at tdstelecom.com
Wed Sep 7 22:03:07 UTC 2016
There are a bunch of variables that impact actual power needed vs. road-miles, number of cross-connects, type of fiber, amount of slack, type of connectors, frequency, dispersion, etc. The km notation simplifies the naming convention. As a general rule 40Km budget 20db, 60km budget 24db, and 80km budget 28db. Best practice is design and deploy to stay a minimum of 4db above the Minimum Input Sensitivity, and 4db below the Max Receive Level to account for the change as the optic and glass as it ages. If you are engineering a 60km fiber length, you'd step up to the next higher optic to account for other losses. 60KM is just a rough number.
There is another value to worry about as well. It's the De-Assert Level; an optic will run all the way down to its minimum input sensitivity, sometimes a couple db below. But once it hit's the De-assert level, the optic will stop forwarding what it receives. So there is a little more fudge in the numbers if you're not playing by the rules.
From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Robert Jacobs
Sent: Wednesday, September 07, 2016 3:33 PM
To: Eric Kuhnke; nanog at nanog.org list
Subject: RE: Optical transceiver question
Not buying fresh veggies here... All optics have about a 5 db range that the vendor will say it is good. The better venders stamp the output power on the optics but not all do this... What he said is to achieve the 60 Km selling point you would have to have all the optic be on the high side of the db TX power... I have never heard of a 60 Km rated optics and it would seem they should be saying 40 to 60 not just 60. It would be nice to say I only want the optics that have an output on the high side and will accept only a 1 db variance but have never seen that in reality. Most are in the middle..
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From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Eric Kuhnke
Sent: Wednesday, September 7, 2016 3:51 PM
To: nanog at nanog.org list <nanog at nanog.org>
Subject: Re: Optical transceiver question
What you're saying is if you purchase ten identical optics with the same SKU, and put them on a few hundred meters of coiled SC/UPC to SC/UPC simplex fiber and an optical power meter on the other end, they're showing varying real world Tx powers from between +0 to +5dBm?
That's not right at all, they're supposed to be sorted at the factory by their actual optical power output before they have labels put on them.
On Wed, Sep 7, 2016 at 1:23 PM, Frank Bulk <frnkblk at iname.com> wrote:
> We recently purchased some generic optics from a reputable reseller
> that were marketed to reach 60 km.
> But what we found, based on the spec sheets, is that it could only
> reach that distance if the optics were transmitting on the high side
> of the transmit power range.
> For example, if the TX range was 0 to +5 dBm and minimum RX power was
> -20 dB, the designed optical budget should be no more than 20 dB (0 - -20).
> Based on the wavelength the appropriate loss would be 0.4 dB/km and
> results in only 50 km, not 60 km. To get 60 km it would need 24 dB of
> link margin, and that would only be attainable if it was transmitting
> on the high side, at +4 dBm.
> Is it an industry practice to market distance based on the hot optics,
> not on the worst case, which is minimum TX power?
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