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Optical to electrical signal conversion time

Michal Matej
Level 1
Level 1

Hi guys,

 

I have one specific question. Could you tell me about time between conversion from optical signal to electrical with SFP module on Cisco switch (C6500, 3750,..) please?

 

Thank you very much.

 

Regards,

Michal

5 Replies 5

Peter Paluch
Cisco Employee
Cisco Employee

Michal,

This is a rather unusual question. What do you need the information for?

In any case, I do not have any datasheets to corroborate this but we can always make an upper bound estimate on the conversion time. Assuming we are using 1 Gb/s Ethernet, this Ethernet uses the 8b10b coding to encode words of 8 bits into symbols of 10 bits. As a result, the true data rate on fiber is 1250 Mb/s. Now, because you cannot store an optical impulse for later processing, the time to convert a single optical impulse into a corresponding electrical signal must be shorter or equal to the duration of a single optical impulse. Because the optical impulses come at a rate of 1250*10**6 impulses per second, a single optical impulse must be processed in a time of 1 / (1250 * 10**6) = 0.8 nanoseconds. This time of 0.8 nanosec can be therefore considered to be the upper bound of converting a single optical impulse into an electrical value, otherwise the SFP would be unable to keep pace with incoming optical impulses.

Assuming that a backward conversion from the 10b symbol into 8b word can take place only after all 10 bits have been received, a decoded word needs to be produced by the SFP each 8 nanoseconds. This would be a very rough estimate on the data latency of a SFP in Gigabit Ethernet.

Roughly the same logic could be applied to faster Ethernet versions as well but it always needs to take into account various details about the physical layer and the coding that is used by the respective Ethernet variant.

Best regards,
Peter

Hi Peter,

 

thank you very much for your explanation to this unusual question. Your information is good for me.

 

Best regards,

Michal

Joseph W. Doherty
Hall of Fame
Hall of Fame

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Posting

Like Peter, an unusual question.

I would suspect latency would be very, very minimal.  I would think it unlikely to need to delay the conversion by more than 1 bit time, and as Peter has also described, for gig, that's about nanosecond.

Thank you, Joseph for your answer.

 

Best regards,

Michal

zozz6092
Level 1
Level 1

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