Re: T1, T2 and T3

http://en.wikipedia.org/wiki/DS1
http://en.wikipedia.org/wiki/T-carrier
http://en.wikipedia.org/wiki/ISDN

... or to make it short: T1 is made up of twenty 64kbit/s channels and T2/T3 are made up of multiple T1 channels. ISDN with its two 64kbit/s channels is also related to T1!

Re: DSL

http://en.wikipedia.org/wiki/Digital_Subscriber_Line

The abbreviation DSL describes a large set of technologies and the numbers above are common for ADSL (Asymmetric DSL). Howsoever, the following DSL technologies are also quite often in use:

SDSL (Symmetric DSL)
VHDSL (Very high bit-rate DSL)

... and ADSL2/ADSL2+...

http://en.wikipedia.org/wiki/ADSL2

...in Japan with up to 50MBit/s