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Re: open fiber control in PAM-5


No doubt there are schemes to overcome any
difficulty such as the Class 1 power limitations
imposed by the IEC and CDRH.

The option of using Open Fiber Control does not
seem to be particularly attractive.  During the
FC-0 days of Fibre Channel it was used to overcome
the same Class 1 power limitations.

A great deal of time and effort was spent trying to
create an OFC protocol which would guarantee multi-
vendor interoperability.  We even had an OFC working
group to address these issues.  The FC-0 equipment
which is still in service today is in many cases, not
interoperable because of OFC timing incompatibilities.

This is especially true at longer distances, as the 
latency requires longer and longer OFC related outages
as the interested parties wait for the other partner to
respond with the correct sequence of pulses before they 
initiate full duty cycle transmission.  I believe Fibre
Channel correctly eliminated an obstacle to interoperable
systems in FC-1.  Sometimes simple is better.

Best Regards,

Pat Gilliland


At 02:55 PM 2/22/00 -0800, you wrote:
>Hello 10G'ers,
>I would like to eliminate another misconception
>regarding the serial at 5 Gbaud vs the 4-WDM
>at 1.25 Gbaud approaches in PAM-5: the supposed
>signal power advantage of the serial approach
>due to eye-safety limits.
>I will show below that the launched power
>PER CHANNEL in 4-WDM can be safely set at the
>same level as the total launched power in the
>serial approach, due to the redundancy in
>its receiver (no single point of failure).
>   ---> Since the launched power level per
>        transmitter of the serial and 4-WDM
>        approaches will be the same, 4-WDM will
>        enjoy a 12 dB advantage in SNR, due to
>        its receiver bandwidth being 4 times
>        smaller (same signal power, much smaller
>        noise power).
>The key is in the "open fiber control" method.
>How do I envision this method in the PAM-5
>specific case ?
>In a serial approach using, for example, 850 nm
>lasers, the maximum launched power is -4 dBm.
>In a 4-WDM approach we could use the following
>procedure to keep the launched power PER CHANNEL
>at -4 dBm (for a total launched power of +2 dBm)
>and still ensure a safe-eye environment:
>1) When a near-end node is connected to a link
>and powered-up it will transmit a signal at -4 dBm
>using only ONE transmitter and keep the other
>three transmitters off. In this way the eye-safety
>limit is satisfied.
>2) The near-end node will remain in this state
>for as long as it does not sense a signal in
>any of its four receivers.
>3) Also the far-end node, when it is powered-on,
>will do the same: transmit on only one channel
>using the maximum -4 dBm allowed for eye-safety
>   It makes sense to use only one transmitter
>   after power-up to send a life signal to a
>   potential partner on the other side of the
>   link, just to conserve power consumption
>   as long as there is no answer from the other
>   side.
>   Which transmitter should be used for sending
>   this life signal ? For reasons that will
>   become obvious later, the best choice is
>   the transmitter that sends the PAM-5 encoded
>   TA symbols, following the nomenclature of
>   the 1000BASE-T standard (*)
>   For the following, remember that each
>   receiver has four channels, named RA, RB,
>   RC, RD.
>4) If any of the two partners senses and recognizes
>   a signal in its RA-receiver, it will go
>   to the next state of its state machine: it
>   will switch-on the other three transmitters and
>   begin transmitting IDLES, with each transmitter
>   using a full -4 dBm launched power.
>   The advantage of using only the TA-transmitter
>   during the first step of establishing a link
>   becomes now obvious: in the 1000BASE-T,
>   the RA-receiver has the capability to
>   synchronize the receiver descrambler to
>   the transmitter scrambler, by just using
>   only the information embedded in the
>   transmitted TA-symbols.
>   Hence, there is no danger that the receiver
>   will confuse a spurious signal with the real
>   signal: it has to be able to synchronize
>   its descrambler and verify that the
>   synchronization is indeed correct using just
>   the embedded information in the transmitted
>   TA-symbols, in order to make a positive
>   identification. Without this identification
>   it will not switch 'on' the other three
>   transmitters.
>5) During normal operation, if any partner suddenly
>   ceases to receive signals on ANY of its four receivers
>   (that are tuned to four different wavelengths)
>   during more than, say, 1 millisecond, it will switch
>   back to its previous state, that is, shut off
>   three transmitters and send a life signal
>   using only one transmitter at - 4 dBm. This
>   might be the case, for instance, when a
>   technician opens the link at any point
>   between the two partners.
>   (the loss of signal is easier to detect than
>   the existence and validation of a real signal,
>   hence, a very simple and robust no-signal-detect
>   circuit may be used).
>   Notice that the four receivers, that are tuned
>   to four different wavelengths, must malfunction
>   in order to miss the "open link" event. It is
>   this redundancy in a 4-WDM system that allows
>   the use of a total + 2 dBm launched power during
>   normal operation.
>The above procedure could also be a replacement of the
>PHY Control State Diagram, Figure 40-15, of the
>1000BASE-T standard (with further details to be added
>later), since in 10 GbE we do not need the concept of
>"master" and "slave" and loop timing used in 1 GbE
>(that was used there to eliminate the Echo and NEXT
>(*) during normal operation, after the link has been
>    established, a transceiver sends through its four
>transmitters quartets of PAM-5 symbols, {TA,TB,TC,TD},
>with TA = {+2,+1,0,-1,-2} and similar for TB,TC,TD.
>Jaime E. Kardontchik
>Micro Linear
>San Jose, CA 95131
>email: kardontchik.jaime@xxxxxxxxxxx