Figure 3 in this open-access paper [0] provides historical scaling trends in optical fiber communication (transport) compared to generation and processing. Depending on the time period under study, bandwidth increases at between 20% and 100% per year.

Though the improvement in transistor economics has definitely benefited transport, the large bulk of improvement over time is due to breakthroughs in manufacturing, materials science, semiconductor optics, and signal processing.

[0] https://ieeexplore.ieee.org/document/7839935

Yes. At least a general trend of increasing the demonstrated transmission rates year after year.

In the past five to ten years, there has however been a discussion of wether this can continue and if we hit upon the speculate “capacity crunch“ – aka reaching the operational fiber capacity. This is all considering a single fiber strand.

The limit to the rate through the fiber is not well understood and a quite active research area. Previously, the “limit” has been broken by technology shifts, such as coherent transmission, different amplifier technology, more advanced signal processing. The big question is what the next big shift will be. Combs, as presented in the article are a promising direction.

More so when you work out how much data is inflight, maybe we see fiber reels used as storage/memory. Kinda like what we had in the early days of wire memory. May well see that come back into play with speeds like this for some uses cases.

It's the normal unit/measurement of spectral efficiency. You have X Hz bandwidth, and you multiply it by the spectral efficiency to get the data rate.

The fact that the time-unit cancels out just shows that spectral efficiency is independent of any sense of time one might have.

Measuring "spacing" in GHz makes sense if you consider the way heterodyne mixing shifts the signal around, without affecting the spacing of sub-carriers.

Spectral efficiency is also a nifty metric because, given some bandwidth and signal-to-noise ratio, there is an upper bound to which you can compare a result [0].

Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.

[0] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

The metric is a measure of how much of the frequency spectrum a particular modulation format takes up. For example traditional OOK (on-off-keying) has a spectral efficiency of 1bit/s/Hz. In coherent optical communications we take ideas from radio to send more bits in the same amount of BW by encoding information in the phase and amplitude. For QPSK, we send 2 bits/s/Hz. In optics we use 2 orthogonal polarizations at the same time so we call this DP-QPSK. This doubles the spectral efficiency to 4bits/s/Hz.

One can keep going to higher order modulations to improve the spectral efficiency but the SNR required increases exponentially with constellation order.

Don't the electronic:optical and optical:electronic transitions introduce quite a bit of latency? I've seen estimates of around 500 microseconds per conversion. That's not enough to matter for networking, but RAM chips have internal latencies measured in nanoseconds.

Of course, "one day perhaps" this could change.

> and it’s not uncommon for systems to have pre-FEC BER 1e-3.

Old guy here: I'm wondering when that changed. Early OC-768 requirements were 10^-12 BER preFEC (e: at least in short-haul), which was down from 10^-9 for OC-192.

For them like me who don't know https://en.wikipedia.org/wiki/Forward_error_correction as I didn't know what the 'forward' part meant.

It seems be error correction add to the sending stream to 'mend' broken data, vs error correction by the receiver detecting an error and requesting a re-send.

FEC does not perform miracles. It only trades off some throughput for a few orders of magnitude in BER. FEC can also be defeated by common types of distortions. If you get a spike of alien crosstalk then you aren’t going to just get one bit error, so you really do need that high fidelity connection if you don’t want your link to shit the bed in the real world.

That quoted number might be 'before non-forward error correction', ie. Before resending the data, which involves an extra roundtrip and degrades service.