Thread: The invention of Imperial measurement

Originally Posted by Mr Brush

... Standard SM fibre (used in NBN for example) has 125 micron OD (the whole piece of glass), but the light only travels down the middle 10 microns. Not really for people with fat fingers.

This seems rather wasteful of the NBN, or am I missing something?

Are you really saying that the optical fibre cable is 92% redundant?

Originally Posted by GraemeCook

This seems rather wasteful of the NBN, or am I missing something?

Are you really saying that the optical fibre cable is 92% redundant?

Perhaps, the extra "fibre" diameter is to allow for a future upgrade in the fibre's signal capacity?

After all, the real cost for the NBN was the labour and equipment [and 20-30% profit] involved in getting the fibre installed. A few extra cents per metre for "future proof" fibre is chicken feed against the Billions the network cost to install. Besides, there is probably a minimum diameter below which the cost to manufacture the optical fibre skyrockets.

Originally Posted by ian

Besides, there is probably a minimum diameter below which the cost to manufacture the optical fibre skyrockets.

More 'n' likely, IMO. The raw material is surely only a fraction of the cost.

mike

One optical fibre looks like this:

Single-mode optical fiber - Wikipedia

In the picture at the top of this link, (1) & (2) are glass (just slightly different types) - light only travels in the bit marked (1). Region (3) is a protective plastic (acrylate) coating.

NBN use mostly ribbon cables, where groups of 12 fibres are stuck together into ribbons to speed up jointing processes. Fibres are joined by arc fusion - melting the glass bits together - so you can imagine the precision required to align the portions the light travels in (only 9-10 microns wide).

Ribbon Cables - Prysmian Australia Pvt Ltd

Fibre has almost infinite bandwidth, achieved by sending different wavelengths (or "colours" of light, although it is all invisible infra red 1550nm wavelengths) down one single strand. The current "state of the art is 200Gbit/s or 400Gbit/s per wavelength "channel", and there are >80 channels available in standard fibre . I routinely install equipment which transmits 8 x 200Gbit/s = 1.6 Tbps total on a single fibre.

That is the tragedy of NBN (appreciated by all who work in the fibre industry). Once you have an "all-glass" path to a premises, technological advances will allow bandwidth to keep scaling beyond what anyone could ever use. Put in a section of copper, anywhere in the link, and you've stuffed it .

Another example of politicians not heeding expert technical advice. Although, since Malcolm Turnbull "invented the internet".....grrrrrrr

Pricing? The cost of OPGW cable (a metal ground wire at the top of your power pylons) which has 72 optical fibres inside it is currently ~A$10/m. Most of that cost is the steel and aluminium too.....

This allows power companies like TransGrid to build extensive fibre networks all over NSW inside their ground wires. No trenches to dig, no rights of way to negotiate, quick and easy to install. I believe they lease a lot of fibres to NBN for backhaul from country centres around the state.

Originally Posted by Mr Brush

One optical fibre looks like this:

Single-mode optical fiber - Wikipedia

In the picture at the top of this link, (1) & (2) are glass (just slightly different types) - light only travels in the bit marked (1). Region (3) is a protective plastic (acrylate) coating.

so your "the light only travels down the centre 10 microns" wasn't quite correct. The surrounding donut shaped piece of glass (58 microns thick) has a different composition to that used for the central 10 microns, so is not really "spare" or "extra" or a "waste" -- it's real purpose is to support the 10 micron central fibre.



but good analogy Mr Bush

Originally Posted by ian

so your "the light only travels down the centre 10 microns" wasn't quite correct. The surrounding donut shaped piece of glass (58 microns thick) has a different composition to that used for the central 10 microns, so is not really "spare" or "extra" or a "waste" -- it's real purpose is to support the 10 micron central fibre.

I believe it's not just for support. The glass surrounding the middle has a variable/graded "refractive index" or bending power and is what keeps the light signals being transmitted in the centre of the fibre. As a fibre goes around a bend the light wants to travel in a straight line which means some light would strike the outside off the fibre and even the best reflectors would not cope with some signal losses. The variable/graded refractive index of the gas bends the light back towards the middle before it strikes the outside of the fibre. This approach has been a significant development in reducing signal loses.

The core (or light guiding portion) of a fibre has a higher refractive index, created by doping the silica in that reqion with small amounts of germanium, etc. This causes injected light to be guided only in the core region.

Two main classes of fibre are multimode (core typically 50 microns, outer cladding 125 microns) and single-mode (core 9-10 micron, cladding 125 microns). Multimode is used in datacoms, e.g. gigabit ethernet, short links between switches in datacentres, etc. Multimode allows many different rays, or paths, through the core region. As a data pulse travels along the fibre these many rays (or modes) get slightly "out of sync." - the ones travelling along the axis have less distance to travel than the ones which bounce around in the outer core. Result is the pulse spreading out as it travels, or smearing, until at some point you can't differentiate '1's from '0's.

Single-mode fibre, in contrast, has a core so small that only one ray, or mode, can propagate. These have much lower loss/km, and much higher bandwidth, hence use in telecom/NBN applications. There are other causes of pulse spreading in single-mode fibres (chromatic dispersion, polarisation mode dispersion), but they are orders of magnitude smaller than the modal dispersion in multimode fibres.

For some pretty pictures explaining this, see

Optical fiber - Wikipedia

Funny story - I used to run training courses for Optus, and in one class a student wasn't really paying attention at a critical point. He got it into his head that the core was hollow, not doped glass. He managed to fit everything we covered over the next day or so into his view of the world, e.g. loss in the fibre (actually caused by impurities in the glass) he just put down to bits of dust getting into the hole/core. On Day 3 we got on to practical fusion splicing, i.e. the method of aligning two fibres, melting the ends in an arc between electrodes, and pushing them inwards to fuse the two glass fibres together. At this point his whole understanding unravelled, and he asked "But won't that block the hole in the middle?". D'oh.