As I know T1 and T3 are optical fiber connection in USA.
Is it the same in Europe and rest of the world?
And what is OC-3 and OC-12? Are they fiber for users or are these for governments, NSA, etc.?
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No. It depends where you live and what standard your telecoms providers follow. Generally it's the USA and EU/ETSI versions.
In the UK we have:
Nx64 - Anything from 9600kbit upwards
E1 - 2Mbit/Sec
E2 - 8Mbit
E3 - 34Mbit
Then jump into the ATM range of 'STM-x'.
These are all fractional services and don't have to be presented as an optical barer. We have something called CWSS which is delivered over 120 volt copper cables. Depending on the distance and number of pairs usually a max of an E2.
Most of these sorts of circuits are being phased out in the UK in favour of Ethernet. Ethernet is delivered over copper or optical paths up to 100Mbit/sec. Anything more is always an optical or radio presentation all the way up to 10Gbit/sec.
The bigger connections (OC3/STM1 and above) are used by lots of people (I have many); they're either used as part of ATM ring networks with drop and insert muxes or aggregation points where a number of 'tails' all terminate at a point of presence. They're also used for DWDM with wavelengths at 2.5Gbit/sec upwards; the other common one is 10x10Gbit.
Any of these circuits can be point to point (if you have the money). You're normally limited by what you can afford and what the telco can physically deliver to you.
There is much more reading you can do online if you have specific queries.