No kind of expert at all, but I understood that better control over / perception of narrower bandwidths of light have allowed fiber-optic cables to improve their data throughput immensely. Is that incorrect? Or are you using a narrower, technical definition of "bandwidth" that I've not understood?
A large factor for the efficiency, is how “thin” you can make channels in the cable via frequency of the light. Multiple signals can be sent over a single fiber line if they use different frequencies. One system I was working on at the time was constructing 88 channels at 100gbs for a total throughput of 8.8tbs.
Another very interesting aspect, is that service providers might share a lease of the same underwater cable, not actually owning the cable themselves. The companies enter an agreement on the frequency ranges assigned to them. Any mess ups in settings can cause disruption in your neighbor on the cable, resulting in millions of dollars of fines.
The levels of buying internet service simplified:
1. Pay for best effort traffic level. E.g. A residential user paying for 1gbps service
2. Pay for guaranteed traffic level. E.g. a business paying for guaranteed 10gbps. Often this maps to paying for a dedicated frequency on a cable, but that's hidden from you, you are just provided an Ethernet port or two.
3. Pay for a dedicated frequency on a cable. Also called "lease a wave". As you noted this is the most common way to go undersea. But it's common over terrestrial cables too. In this case the purchaser is often responsible for their own optical networking equipment, and the service provider muxes together all their customers. Sounds like you know more about this than me. The service provider's equipment doesn't have protection if your neighbor is accidentally stepping on your frequency range?
4. Pay for an entire cable. Also called leasing dark fiber. It's 100% your responsibility to "light" it. To make it worth it you're usually using optical networking equipment to mux together several different waves, just like the above scenario undersea, just all different waves belong to you. Commonly used by big tech companies and ISPs to connect their datacenters and points if presences in big cities. Lots of times the company you lease the cable from is also your contractor to repair cuts and maintain the amplifiers every ~50km.
You've got 3 types of use cases merged in one there.
First, The generic business internet which gives you uncontended bandwidth usage as far as an internet exchange point and an ISP which ensures its peering and transit is not oversubscribed.
Second, point to point ethernet connectivity. This morning I had an issue with a point-to-point connection I have from Beijing to the UK for example, where latency had jumped overnight from 215ms to 430ms. This is provided as point to point ethernet by I would assume something like MPLS or VXLAN over the providers network. I've had SDH and ATM backed ethernet services in the past too.
Third, I have link with an optical service from, which does map to a dedicated frequency as in your 3rd option, although it's provided to me as a standard ethernet handoff.
This is different to "pay for a dedicated frequency on a cable", as the service I have has an ethernet handoff. I couldn't put non-ethernet traffic on the frequency.
I've also got dedicated cables with a service provision at an ethernet layer (the provider gives me an ADVA which they control). The actual light goes from the ADVA in my equipment box to another ADVA in another equipment box
In none of these situations do I need to worry about frequencies, high power SFPs, etc, the handoff from the provider is always ethernet, they handle that.
My company does have some leased fibre between campuses too, I tend not to get involved in that, but I know one department runs some 800G sfps over a leased fibre between cities about 200 miles long. I don't think we have any owned fibre other than between buildings on the same campus
For short-ish cables (a couple hundred kilometers), driving the amplifiers via light (interestingly this can be done from both the data-sending and receiving end!) is possible, but not for trans-oceanic distances.
If I'm reading this right, BPSK - used on the longest transpacific cables - can't really be upgraded to have higher bandwidth but QPSK and other modulation schemes used for shorter distances can be (?).
Huh, I had no idea it needs to be done so frequently.
> These days optical cable repeaters are photon amplifiers that operate at full gain at the bottom of the ocean for an anticipated service life of 25 years.
> Repeaters are a significant cost component of the total cable cost, and there is a compromise between a ‘close’ spacing of repeaters, every 60km or so, or stretching the inter-repeater distance to 100km and making significant savings in the number of repeaters in the system. On balance it is the case that the more you are prepared to spend on the cable system the higher the cable carrying capacity.
https://blog.apnic.net/2020/02/12/at-the-bottom-of-the-sea-a...
Optical amplifiers simply boost the signal. A photo is absorbed and then re-emitted with more power, but the same frequency and phase.
There is no optical/electrical/optical conversion.