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open fiber control in PAM-5

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

   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

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