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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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    At the ISP I worked for, T1 was done over copper pairs. OC3 and OC12 utilize fiber as the medium for the the pipes, and are usually used for backbone pipes between backbone providers and ISPs, but it is possible to have the bandwidth for that pipe directly relayed to a single location if you're willing to pay enough. These terms really just indicate circuit types and ranges of bandwidth passed along the circuit. – MaQleod Dec 22 '17 at 0:23
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    T1 & T3 are not connections, but at their core are T-Carrier or Transmission System standards. A T-1 connections allows for 24 multiplexed voice grade circuits and a T-3 is a connections of 672 multiplexed voice grade circuits. DS1 is a 1.544Mbps signalling carrier which MAY carry a T1 or a ISDN PRI or similar circuit, and a DS3 is 44.736 Mbps signal carrier that carriers 28 multiplexed T1, ISDN PRI, or similar circuits. The actual physical transmission medium can vary but is often 2 copper pairs for DS1 and a coaxial or fiber for DS3. – acejavelin Dec 22 '17 at 0:38
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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.

A quick search yields this link on Wiki which shows the relations between the major standards. There is also this which details SONET/SDH.

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.

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