Modern-Day Optical Network Physical Signal Encoding

(PAM-3 eye pattern; image from Wikipedia)

This blog posting is still in editing, and is posted just so I could talk to some students about elements of the contents.

Recently, I posted about how SDH optical networks encode bits at the physical level. My interest in the topic stems from a desire to know how to multiplex classical and quantum information on different channels/wavelengths, and part of that involves a basic understanding of the classical signals on the fiber. In both the demonstration network we are building and in the longer term as experimental specifications develop into standards, we may choose to put classical synchronization signals and the like for the quantum signals into the same fiber, or we may decide to carry full-on classical data traffic.

Of course, SDH is rather old now, going back to the 1990s, and optical networking has advanced considerably, especially for data center and local area networks. The most obvious place to look for newer developments is Ethernet, so here we are. First, let's look at almost-but-not-quite leading edge networks, where the technological decisions are more settled than in, say, Ultra Ethernet, which is still under development. (I do hope to come back to Ultra soon, but the draft specs are currently closed to the public.)

[tl;dr: PAM-4, with four distinct signal amplitudes, is common. Development of Ethernet using 16QAM was suspended after a draft specification was developed but not approved. As far as I can tell there is no standardized use of quadrature amplitude modulation in the optical regime, though it's common in RF.]

Many of Ethernet specifications are available for free from IEEE, including 802.3db-2022 - IEEE Standard for Ethernet - Amendment 3: Physical Layer Specifications and Management Parameters for 100 Gb/s, 200 Gb/s, and 400 Gb/s Operation over Optical Fiber using 100 Gb/s Signaling, and 802.3df-2024 - IEEE Standard for Ethernet Amendment 9: Media Access Control Parameters for 800 Gb/s and Physical Layers and Management Parameters for 400 Gb/s and 800 Gb/s Operation which mention PAM but not QAM.

400Gbps Ethernet has 11 separate physical layers that run over fiber (one still in development), two twisted-pair copper and one backplane form.  Let's focus on the fiber variants, since our interest here is photons in fibers (sometimes many of them, sometimes only one). Five of the variants listed at Wikipedia use PAM-4 (sometimes written PAM4 in the page), one is listed as 16QAM (but more below), and the rest don't say; perhaps they are simply on/off NRZ keying, like in earlier optical networks like SONET/SDH.  (It's nice that this information was much easier to find than the original SONET/SDH stuff!  Partly because I better understand what I'm looking for this time, I suppose.)

...So what are PAM-4 and 16QAM?

PAM, or Pulse Amplitude Modulation, is pretty straightforward: instead of using signal ON at full power and OFF to represent a single bit, if you use several voltages (or RF or optical signal strengths), then each symbol can represent more than one bit.   A related term is Amplitude Shift Keying; I'm not sure exactly why the Ethernet folks stick to PAM instead of ASK. The picture at the top of this posting is PAM-3: the level at the bottom, the level in the middle, and the level at the top. The sloping lines are transitions from one level to another; the cleaner those lines are, the bigger the "eye" is, indicating that your circuit is very stable. (If you don't know how to read an eye diagram, you should learn.)

PAM-4 uses four signal levels, carrying two bits per symbol. Prolabs and Samtec have nice explanations of PAM-4, including eye diagrams. I mentioned above that there are many different physical layers for Ethernet; PAM-4 is used in the 400GBASE-DR4, 400GBASE-FR8, 400GBASE-LR8, 400GBASE-FR4, and 400GBASE-LR4-6 variants, all running over four or eight single-mode optical fibers working together. (More on that multi-fiber concept some other time.)

That was easy. Whew! ...but what about 16QAM? Gotta mind your Ps and Qs...same thing? Nope! QAM is a lot more complicated, involving a lot of signal processing theory we're not going to get into in this blog, but let's take a quick look anyway.

QAM is Quadrature Amplitude Modulation. In QAM, we modulate the signal using two separate waves, one sine term and one cosine term. When using on-off keying or PAM on optical fiber, our carrier signal is just the laser light amplitude, and the system is relatively insensitive to the phase of the light. With QAM, however, the phase of the carrier is critical.

One of the most basic types of QAM is 16QAM.  When first getting oriented, I found this web page to be helpful, but keep in mind that it's talking about the use of 16QAM for radio signals, not optical. In 16QAM, we use two signals, I and Q, that are both sinusoids, 90 degrees out of phase. Each of those two signals is modulated to carry two bits, then the two are combined, so that I+Q is the signal transmitted. The modulation involves two choices of amplitude, and two choices of phase -- either 0 or 180 degrees added to the already 0 or 90. Since there are four possible choices for each of I and Q, we have a total of 16 possible waveforms, hence 16QAM. The 16 waveforms are shown below.

The wild thing is that you can recover the four bits cleanly from that.  I won't go into the math here, but you can check Wikipedia.

Those sixteen waveforms are arranged in a particular way, corresponding in order to what is called a constellation diagram. The constellation diagram helps you understand the noise in the system and figure out whether or not your hardware can reliably reconstruct the original set of bits.

A helpful, if not super-high quality, video:
https://www.youtube.com/watch?v=6BIqEWEe5-I

The 400Gbps Ethernet variant 400GBASE-ZR was supposed to use 16QAM. BUT:

The 802.3cw website says, "The work of the IEEE P802.3cw 400 Gb/s over DWDM Systems Task Force concluded with the withdrawal of IEEE P802.3cw PAR on 22 May 2024." Apparently, they published P802.3cw/D3.0, Dec 2023 - IEEE Draft Standard for Ethernet Amendment: Physical Layers and Management Parameters for 400 Gb/s Operation over DWDM (dense wavelength division multiplexing) systems as "Active - Draft" in Dec. 2023.  It's not available in the freely accessible documents yet (which only include approved standards at least six months old, as I understand it), and even my university account seems to be unable to reach it.  Too bad, that's definitely where I wanted to be looking!  https://www.ieee802.org/3/dj/public/24_03/motions_3cwdj_2403.pdf says the PAR (Project Authorization Request) was withdrawn by unanimous consent on 13 March 2024. This was foreshadowed a month earlier in an email from John D' Ambrosia, chair of the TF.

According to the Wikipedia page on terabit Ethernet, it was proposed in 802.3cw to use dual-polarization 16QAM, which might add an extra bit but sounds even more complicated to me.

I don't know yet where the carrier for reconstructing the signal comes from...if it's just the laser itself, that's about 200THz for 1.5um light, and we need some reference to find the right phase for the carrier. One research paper on carrier recovery:
https://opg.optica.org/jlt/abstract.cfm?uri=jlt-27-15-3042
No idea if that's what's used...

And another survey paper, heavily cited in papers and in (at least) 20 patents:
https://ieeexplore.ieee.org/abstract/document/5464309

As a companion to this posting, I am developing a Jupyter notebook on 16QAM that made the plot above.

Julia Parsons, "Code Girl", 1921-2025

(Image taken from Seattle Times, where it is credited the World War II Foundation.)

 The New York Times reported last week that Julia Parsons passed away. (The Seattle Times has a copy not behind a paywall.) She was probably the last living member of the WWII Naval Communications Annex team responsible for deciphering Enigma messages sent to and from German U-boats. She joined the WAVES in 1942, right after graduating from Carnegie Tech (now Carnegie Mellon University), and was assigned to work in the unit from 1943 through the end of the war.

I don't recall if she was mentioned by name in Liza Mundy's Code Girls, but she was definitely part of that crew.  If you haven't read that book, you really should.

As one of the youngest members of the group, her initial task was to work directly on the deciphering of the messages from the U-boats. She worked with the US Navy Bombe, feeding it possible plaintext and ciphertext. The Bombe would then produce a "menu" of possible Enigma wheel settings that had to be checked to determine which (if any) of them would correctly decrypt the message. The Wikipedia article has an excellent description of the workflow.

Because the work she did was classified, she didn't talk to anyone about it until 1997, when she discovered it had been declassified in the 1970s. We probably lost a lot of history that way, as even by the 1970s many of the senior people involved had doubtless passed away.

Thank you, Ms. Parsons, for what you did for democracy and freedom. I know it came with a cost.