The basics of fibre internet

[Guineafowl]

TL;DR

What is the basic nature of the signal that provides internet access to my home? Online accounts of the GPON seem to skip over this.

Previously, my internet access was via ADSL (copper twisted pair) broadband. I gained a fairly good understanding of it, sadly because it had so many faults, and my ISP allowed quite a lot of diagnostics and settings, including changing SNR margin and adding interleaving. We are now on fibre to the property.

Now, the ‘broad band’ mentioned above referred to the 26 kHz - 2.2 MHz (or thereabouts) frequency range used over the copper phone line.

It’s my understanding that ‘fibre broadband’, a term used even in many ISP adverts, is a misnomer, since the optical network uses just one or two frequencies of light. However, accounts such as this: https://www.cisco.com/c/en/us/suppo...tand-gpon-technology.html#toc-hId--1514518579 seem to skip over the basics. I don’t need a detailed understanding, just an appreciation of what the flashing thing on the wall is doing.

Here is my understanding so far:
Right from the bottom, light on/off is 1/0. Because the electrical bits are transduced directly to optical bits, without modulation, it’s called baseband. There’s one light frequency for upload, one for down. There must be some kind of expected frequency of bits, or clocking, in order to tell the difference between 11 and 1, for example. Is this the sync speed of 2.5 Gbps up/1.25 down?

If so, is my connection of 115/20 Mbps up/down simply clocked slower?

 

[Baluncore]

If so, is my connection of 115/20 Mbps up/down simply clocked slower?

No. The clocking will be significantly greater than the data rate you purchase. The 115/20 is a maximum limit for the total number of bits that will be communicated per second. The packets of data may be very fast, but there will be long gaps between the packets, to limit the data rate, to what you have agreed to pay for.

 

[Guineafowl]

No. The clocking will be significantly greater than the data rate you purchase. The 115/20 is a maximum limit for the total number of bits that will be communicated per second. The packets of data may be very fast, but there will be long gaps between the packets, to limit the data rate, to what you have agreed to pay for.

Ok, so during a packet, if the sync speed is 2.5 Gbps, the receiver is expecting a bit every 2.5 billionths of a second. The way it can tell between a 11 or just a 1, is that the former pulse lasts twice as long? Very basic stuff, I know.

 

[Baluncore]

The way it can tell between a 11 or just a 1, is that the former pulse lasts twice as long?

The packet is transported at a fixed bit rate with frames and synchronisation bits. '11' would fill two bit positions, '1' would need only one bit position. '11' is the same as two '1's.

 

[DaveE]

Data modulation can be a pretty complicated subject. There are a lot of different schemes. Which is my excuse for not knowing exactly what a specific ISP will use, especially for the "last mile" where bandwidth is relatively small. There is really a lot to read on the web about this subject (maybe too much), so you would do well to read about the parts you care about. You can start with Wikipedia. But here are some very general comments:

- Optical fibers can transmit many different frequencies of light, each a data channel. They are combined at the transmitter and separated again at the receiver. This is called Wavelength Division Multiplexing (WDM) and isn't too different than your old FM radio with different channels. High performance fibers can operate with over 100 channels on each fiber.

- Light is better than cable because it can be switched (modulated) very quickly. Radio (WiFi) is better because you don't have to have cables.

- Any high bandwidth data link can be shared use Time Division Multiplexing (TDM). Your data may only have access for a portion of the time, then it switches to your neighbor's data. This can be done fast enough that you don't notice any delay.

- Modulation over fiber isn't very different than modulation schemes for radio. There are a lot of choices, but the simple schemes like AM, FM, PM, etc. aren't used much in modern broadband systems. Look into coherent modulation (detection) schemes, like QAM, or OFDM. These are the type of schemes also used by your WiFi and cell phone.

- Modulation schemes will settle on standard definitions, of which there are many. Your ISP didn't invent theirs they bought it (switches, modems, etc.). There is lots of jargon and abbreviations because standards are similar and rely on the same basic concepts.

- Ultimately it's a cost/benefit trade off. Tons of data can be sent on a fiber if you pay enough money for the hardware.

 

[DaveE]

I thought this article was a good summary of the business side of the last mile problem, although it is 15 years old now.
https://arstechnica.com/tech-policy/2010/03/fiber-its-not-all-created-equal/

 

[harborsparrow]

The digital channels within fiber telecom systems in the USA are a hierarchy beginning with the smallest channel (DS0), of which 28 channels are muxed into DS1, of which a fixed number of channels are muxed into a DS3 digital channel. You can start here: https://en.m.wikipedia.org/wiki/Digital_Signal_0

Fiber systems reaching all the way into the home have to juxtapose internet packets onto these basic digital channels or pipes. Combined with the Ars Technica link provided by DaveE in a prior post, you can begin to understand what must be happening.

Originally, fiber to the home intended to give each household one or two voice lines (DS 0 channels), but when internet became the only required service, more bandwidth needed to be allocated to each residence. Still, ultimately, internet packets are always riding somehow on these standard digital pipes or streams provided by the equipment deployed by telecom companies.

Europe, Japan and other parts of the world each have differing standards for digital pipelines, whether the pipeline flows over satellite, fiber, cellular networks or even copper. You don't need to know how data is modulated within each system, which is essentially proprietary. You need to know the digital standard that each system implements. So building blocks of fiber networks can be regarded as black boxes that deliver data streams using a standard protocol. Internet packets are the highest application layer in the stack. The signal a fiber system delivers to the home is likely one of the lower level pipelines, and the in-home equipment is what loads or unloads internet packets into these standard digital pipes.