Tech Briefs Magazine - May 2021 - PIT-15

IEEE 802.3ck Annex 120G*
Chip-to Module 100 Gbs/s one-lane Attachment unit Interface
(100GAUI-1 C2M), and 200 Gbs/s one-lane Attachment unit Interface
(200GAUI-2 C2M), and 400 Gb/s four-lane Attachment Unit
Interface (400GAU-4 C2M)

IEEE 802.3ck clause 140
Physical Medium Dependent (PMD) Sublayer and
medium, type 100GBASE-DR

53.125Gbaud PAM4
Optical

Electrical Tx test
SNDR, VEC, EH, EW
(Waveform analysis)

53.125Gbaud PAM4
Electrical

Optical Rx test
SRS, Jtol (BER)

Electrical Rx test
SRS, Jtol (BER)

Optical Tx test
TDECQ, OMA, OER
(Waveform analysis)

400GAUI-4

400GBASE-xR4

106G/λ

Figure 3. Typical 400G Electro/Optic test setup.

working backwards from the receiver,
accounting for the expected loss of the
fiber, then defines the minimum signal
power level a transmitter must produce. In reality, it is more complicated
as there are a variety of mechanisms
that can cause a system to generate bit
errors beyond simply having the
power drop below the receiver sensitivity limit.
From the perspective of a receiver,
two lasers operating at the same power
level may generate very different signals. State-of-the-art systems today operate in excess of 50GBaud (PAM4). That
is, the light must be turned on and off
at the transmitter at a rate of up to 50
billion times a second. The receiver
must detect that the light is either on
or off, and a lower quality transmitter
may be slow. The laser may generate a
signal that is not stable when the
receiver makes a decision. Thus, the
quality of the laser signal needs to meet
a minimum level. Similarly, we cannot
expect to have perfect transmitters, so
receivers need to have some tolerance
for non-ideal input signals. This leads
to some important requirements for
transmitters and receivers:
Optical Transmitter Evaluation
* Optical Modulation Amplitude
(OMA): The difference between the
transmitter logic levels.
* Relative intensity noise (RIN): A measure of the amount of noise a transmitter generates.
* Transmitter dispersion and eye closure: TDEC or TDECQ (for PAM4
modulation) is a statistical measure of

Figure 4. FEC Aware physical layer analysis.

the signal quality, indicating the likelihood that the signal will generate
errors in the receiver (Figure 1).
* Overshoot/Undershoot: New metrics
recently defined in IEEE 802.3cu to
protect receivers from severe transients on the input signal.
Optical Receiver Impaired Signals
Stressed receiver sensitivity (SRS):
The bit-error-ratio or expected frameloss-ratio below the expected level when
the signal going into the receiver is the
worst case expected signal from the
transmitter (and channel).
Test instruments have been developed
to offer instrument grade optical impairments (to specific TDECQ, ER, and
OMA targets) for stressed receiver testing. Figure 2 illustrates a typical optical
SRS signal which would be generated for
test purposes.

Photonics & Imaging Technology, May 2021

Transmitters are generally tested with
a specialized digital communications
analyzer oscilloscope. These instruments
have built-in optical reference receivers
and firmware to execute the measurements required by these standards.
Similarly, for receivers, SRS test systems
(Figure 3), including a calibrated
'impaired' signal and a bit-error-ratio
tester (BERT), are available to verify
standards conformance.
Link performance at 100Gbps-be it
electrical or optical-both operate at
higher bit error rates than their lower
speed 25 or 50Gbps counterparts.
Current 100Gbps interfaces operate at
native link error rates as high as 2E-4
BER and rely on modern Reed-Solomon
forward error correction (RS-FEC) techniques to correct for random and isolated bit errors that naturally occur in
transmission.

Cov

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Tech Briefs Magazine - May 2021

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