[j-nsp] acceptable/good laser receive power in case of different interfaces

Chris Tracy ctracy at es.net
Mon Aug 8 13:58:31 EDT 2011 

On Aug 7, 2011, at 6:11 PM, Joel Jaeggli wrote:

> 
> On Aug 7, 2011, at 2:03 PM, Martin T wrote:
> 
>> Lane,
>> while browsing the specifications of the optical modules listed in
>> this "Optical Interface Support—EX 3200 and EX 4200 Switches.pdf"
>> file:
>> 
>> http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/reference/specifications/optical-interface-support-ex-series.pdf
>> 
>> ..all the modules have minimum and maximum launch power which differ
>> from each other quite a lot. What does this mean? Shouldn't the launch
>> power be consistent? In addition, what is a "Maximum Receiver
>> Sensitivity"?
> 
> you're looking at emitters with rather different properties... a 1000sx or 100fx multimode gbic is an 850nm emitter (that's visible wavelength red) and probably uses an led and is meant for 50 or 62.5micron multimode, a 850nm or 1310/1550 nm qsfp+ gbic uses a vertical cavity emitting laser.
> 
> Maximum reciever sensitivity I think would normally be characterized as dynamic range. acceptable input power on the rx side would be between -1 and -25dBm on 1000baselr if it's higher than that, you may need to attenuate it.. for example if you put the 10Gb's ER gbics back-to-back you'll need an attenuator, to keep them from blinding each other.

Most transceiver data sheets that I have seen explicitly call out two properties for the receiver input:

* [minimum] receiver sensitivity @ X bps (what is the lowest optical signal, at a given rate for multi-rate optics, that can be converted to electrical without errors)

* maximum input power (what is the hottest optical signal that can be received before taking errors and/or causing physical damage)

Some long-range optics which use avalanche photodiodes (APD) are particularly sensitive, they have extremely low receiver sensitivity and can be permanently destroyed if you exceed the maximum input power, e.g. > -7dBm for the FTLX3611M3xx or FTLX1811M3.  The much more common PIN-based receivers can tolerate signals more than twice as bright without being damaged, but do not have such low receive sensitivities.

The maximum receiver sensitivities given in the Juniper EX doc look much, much lower than the maximum input power rating in data sheets for the same types of optics.  I'm guessing this is officially the range that Juniper wants the input power to fall between to guarantee correct performance.

Regarding the minimum/maximum launch power mentioned earlier in this thread:

Even for one model of transceiver for a given PHY standard (e.g., a 10GBASE-LR), you will see significant variation between the output power of one module to the next.  If you order ten 10GBASE-LR modules and measure the output power of each one, the output power should fall somewhere in-between the minimum/maximum launch powers, and (ignoring any long-term degradation) this output power is essentially fixed and will not change.

-Chris



>> David, Keegan,
>> thank you for explanation!
>> 
>> 
>> In addition, there isn't some sort of connection between Rx power and
>> Tx power, is there? I mean for example in case the received signal is
>> low, the transmit signal of the SFP/XFP is increased automatically?
> 
> remember that the TX and RX are on different fibers which (you hope) have similar properties.
> 
>> As
>> far as I know and as Lane confirmed, the Tx signal should be always
>> consistent..
>> 
>> 
>> regards,
>> martin
>> 
>> 
>> 2011/8/3 Keegan Holley <keegan.holley at sungard.com>:
>>> 2011/8/2 Joel Jaeggli <joelja at bogus.com>
>>> 
>>>> 
>>>> if these are sr multimode optics, the -15 number is low the -7 number is
>>>> marginal and everything else is decent.
>>>> 
>>>> either the -15 one is quite long ( for sr) or needs to be
>>>> replugged/cleaned/reterminated
>>>> 
>>>> 
>>> Yea I agree.  The -15 is a bit low unless it's is at the end of a really
>>> long, low-quality fiber run I'd clean it and or replace the XFP.  It's
>>> blasting out at +1 and receiving much less, there could also be a mismatch
>>> of some sort.  There are lots of ways to mismatch optics and cabling and
>>> still get link.
>>> 
>>> 
>>> 
>>>> On Aug 2, 2011, at 2:53 PM, chip wrote:
>>>> 
>>>>> Depending on whose optics you're using there should be a data sheet
>>>>> that shows the acceptable Tx/Rx levels for each type available from
>>>>> your vendor.  I can't seem to locate a document for Juniper at the
>>>>> moment.  But I assume they shouldn't be that far off from Cisco stuff.
>>>>> For example, here's a data sheet for the XENPAK module:
>>>>> 
>>>>> 
>>>> http://www.cisco.com/en/US/prod/collateral/modules/ps2797/ps5138/product_data_sheet09186a008007cd00_ps5455_Products_Data_Sheet.html
>>>>> 
>>>>> Check Table-2.
>>>>> 
>>>>> As far as I know, an optic will output power within a specified range
>>>>> as according to what type it is, SR, LR, ER, ZR, etc...
>>>>> 
>>>>> Hope that helps a bit.
>>>>> 
>>>>> 
>>>>> On Tue, Aug 2, 2011 at 5:26 PM, Martin T <m4rtntns at gmail.com> wrote:
>>>>>> What is the acceptable Rx power in case of SFP/XFP? For example, here
>>>>>> are XFP Tx and Rx signals from six FXP's:
>>>>>> 
>>>>>> 1:
>>>>>> Laser output power                        :  1.2920 mW / 1.11 dBm
>>>>>> Laser rx power                            :  0.0285 mW / -15.45 dBm
>>>>>> 
>>>>>> 2:
>>>>>> Laser output power                        :  0.6420 mW / -1.92 dBm
>>>>>> Laser rx power                            :  0.3054 mW / -5.15 dBm
>>>>>> 
>>>>>> 3:
>>>>>> Laser output power                        :  0.4230 mW / -3.74 dBm
>>>>>> Laser rx power                            :  0.5092 mW / -2.93 dBm
>>>>>> 
>>>>>> 4:
>>>>>> Laser output power                        :  0.4180 mW / -3.79 dBm
>>>>>> Laser rx power                            :  0.4208 mW / -3.76 dBm
>>>>>> 
>>>>>> 5:
>>>>>> Laser output power                        :  1.0920 mW / 0.38 dBm
>>>>>> Laser rx power                            :  0.1801 mW / -7.44 dBm
>>>>>> 
>>>>>> 6:
>>>>>> Laser output power                        :  0.7680 mW / -1.15 dBm
>>>>>> Laser rx power                            :  0.3337 mW / -4.77 dBm
>>>>>> 
>>>>>> 
>>>>>> Is there some sort of pattern? It looks like if the Rx signal is
>>>>>> lower, the Tx is higher? And what can one consider a decent Rx laser
>>>>>> power level?
>>>>>> 
>>>>>> 
>>>>>> regards,
>>>>>> martin
>>>>>> _______________________________________________
>>>>>> juniper-nsp mailing list juniper-nsp at puck.nether.net
>>>>>> https://puck.nether.net/mailman/listinfo/juniper-nsp
>>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> --
>>>>> Just my $.02, your mileage may vary,  batteries not included, etc....
>>>>> 
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>>>>> 
>>>> 
>>>> 
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--
Chris Tracy <ctracy at es.net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory

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