Re: [802.3ae_Serial] Tx setup window with OMA
I have a different way of looking at this problem. For the LR/LW 1310nm serial
PMD the minimum extinction ratio allowed is 4dB. The actual extinction ratio
will vary somewhat with temperature, aging etc. Therefore setting the
extinction ratio to exactly 4dB is not possible. Let's assume instead that the
extinction ratio is set to 5.25dB (which is still significantly lower than any
of the SONET minimum specs). This allows the extinction ratio to vary a
reasonable amount and leaves a 5dB allowable power set window which is more
than adequate. I still think that the maximum output power should be reduced
to 0dBm as I think a 4.5dB Tx output power window is easier than asking for the
extra dynamic range in the receiver.
Note that the attachment showing the 1GBE LX clearly shows one of the problems
with the old specification method. If you hold the laser at constant set power
using Automatic Power Control and have changes in laser slope efficiency due to
termperature and aging it is very easy to fall out of the Extinction Ratio
specification (ie at constant power there is only 1dB variation allowed in
OMA.).
The same general arguement applies for the ER/EW and SR/SW. The intent of
setting the minimum extinction ratio was not to have implementers set their
parts to this it was at least for 1300nm and 1550nm to bound the interferometic
noise, and to not include in the standard areas of the plot that clearly no
implementer would want to use.
For the SR/SW case the maximum power is being limited by the laser safety
standards. If we want a wider Tx output power setting range we would have to
make the receiver more sensitive. I think this would be the wrong thing to
do. At reasonable extinction ratios (say 6dB) the setting window is 4.5dB for
the same point in the triple trade off curve you used. Again I think this is a
good compromise.
For the ER/EW case I think that increasing the maximum output power is probably
a good idea. (2-3dB would seem to me to be the right amount based on the
arguements above.) Here laser safety is not an issue, and as we are already
using attenuators we would not have to increase the receiver's dynamic range.
"DAWE,PIERS (A-England,ex1)" wrote:
> As I wrote two days ago, the setup window for the LR/LW, 1310 nm serial,
> PMD, at low extinction ratio is 4 dB when 5 dB was intended.
>
> For the ER/EW, 1550 nm serial, PMD, the window is around 3.5 dB if TPD
> (transmitter and dispersion penalty) is small and extinction ratio is its
> minimum 3 dB, or 0.4 dB (!) at maximum TPD, minimum extinction ratio.
>
> For the SR/SW, 850 nm serial, PMD, the window at 3 dB extinction ratio is
> very roughly 2 dB; it depends strongly on the triple trade off.
>
> The attachment Pave_OMA_LX.pdf is an example of an old style (mean power
> based) standard that works: the 1 Gigabit Ethernet LX standard. The setup
> window is 8 dB wide. Because both the upper and lower limits are measured
> on the same basis, the window is 8 dB wide for any extinction ratio. (P1
> means the power in the ones.)
>
> At 10 Gbit/s we don't want to set a very high receiver overload value, we
> want to enable lower extinction ratios, and to enable cost-effective
> transmitters we wish to specify them on an OMA basis. The attachment
> Pave_OMA_LRW.pdf shows where the LR/LW, 1310 nm serial, PMD is at in draft
> 3.1. The setup window is 4 dB wide at 4 dB extinction ratio.
>
> How can we fix this?
>
> Options are:
> 1. Reduce the Tx OMA spec. Would need better receivers and/or reduced
> link attenuation. Not attractive.
> 2. Raise the Tx mean power spec back to 1 dBm. This works.
> 3. Specify the Tx maximum power on a maximum-in-in-the-ones basis at
> e.g. +3 dBm. This seems to work too. It's a reasonable compromise between
> the transmitter's primary metric (OMA) and the receiver's overload concern
> (power in the ones? mean power? OMA? it may depend on receiver
> implementation).
> 4. Specify Tx maximum in OMA, at say +1 dBm. This makes the setup
> window independent of extinction ratio again but it gives the receiver a
> hard time.
> 5. A combination of 2 and 3 above, as illustrated in purple. Not sure
> that the benefit over the better of options 2, 3 outweighs the cost of the
> extra test involved.
>
> Option 2 isn't an original idea. It was proposed for 1550 nm in
> http://www.ieee802.org/3/ae/public/jan01/frojdh_2_0101.pdf and shelved
> because it wasn't essential to fix anything broken and we wanted to know
> more about how 1550 nm receivers overload before taking it up.
>
> Piers
>
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