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During multiple meeting cycles, concern has been raised that
Transmitter Functional Test (TFT) uses a Functional Receiver (FRx) with an Optical Receiver (ORx) which can have variable characteristics, resulting in passing bad transmitters or rejecting good transmitters. Suggested remedies have included dropping TFT,
or adding separate FRx ORx optical characteristics, i.e. turning ORx into a reference receiver.
An optical receiver which meets 802.3dj optical receiver characteristics is interoperable with a transmitter which meets 802.3dj optical transmitter characteristics. TFT is one of many optical transmitter characteristics, and all have to be met to guarantee interoperability with a compliant Rx. Optics industry practice is to enhance the guarantee of interoperability by deploying Tx and Rx with an additional margin to 802.3dj optical characteristics, in particular Tx OMA and receiver sensitivity (RxS). TFT is also enhanced by additional test margin which ensures that only Tx performance is being measured and ORx noise is minimized. TFT is entirely based on existing IEEE 802.3 optics methodology and is derived from other 802.3dj specifications.
During extensive Tx functional testing, a problem was uncovered with 802.3dj optical receiver characteristics specifications, specifically RxS conformance test signal. This leads to
RxS variability [ieee802.org], which may lead to interoperability problems with an 802.3dj compliant transmitter. Since FRx ORx is a compliant 802.3dj optical receiver, this may also lead to TFT problems. In logic terms:
a = Optical receiver is 802.3dj compliant
b = Optical receiver sensitivity is correctly measured
x = Optical receiver is guaranteed to be interoperable with 802.3dj compliant optical transmitter
IF a AND b THEN x
IF NOT a OR NOT b THEN NOT x
c = Transmitter functional symbol error histogram criteria is met with the correct FRx VOA setting
y = TFT FRx optical receiver is guaranteed to be interoperable with 802.3dj compliant optical transmitter
IF a AND b AND c THEN y
If NOT a OR NOT b OR NOT c THEN NOT y
Stating that TFT can fail because FRx ORx has an interoperability problem is the same as stating that ORx is not 802.3dj compliant, or ORx RxS is not correctly measured. The solution is not to add separate FRx ORx optical characteristics, but rather to fix
the 802.3dj optical receiver characteristics, specifically RxS conformance test signal. This is particularly urgent because the available margin in 200G/lane optical links is shrinking.
During Electrical comments resolution last Wednesday, excellent guidance was provided by Kent Lusted and Adee Ran for best way to reach agreement and efficiently resolve comments during meetings. They recommended to hold discussions
off-line, develop consensus and proposals, and draft those into presentations with many supporters prior to meetings. The
Optical Track would also benefit from this guidance, including with respect to TFT.
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