Hi Joseph

If any one can answer that why two people can't have 100% similar opinion ?

North American Digital Signal Hierarchy

The North American signal hierarchy was created by the old US 'Bell system' (AT&T) in the early 1960's and was the world's first digital voice system. It is based on multiples of the DS0 signal with a little bit of overhead to show its age. The fiendishly cunning Europeans who waited longer to define a digital hierarchy were able to live without the small overhead largely due to improved electronics.

The signal hierarchy defines the levels of multiplexing, that is, the first level of the hierarchy multiplexes (combines) a number of DS0s into a single digital signal (with a DSx designator) which is then placed on a carrier (with a T-x designator). The DSx defines an abstract signal or speed and the T-x defines a physical 'pipe' or format. The DSx and T-x series specs and most other telecom related specifications are standardized by the ANSI accredited Committee T1 (T1E1), now part of Alliance for Telecommunications Industry solutions - ATIS, which in turn represents, via the US State Department, the US at ITU standard sessions

European Digital Signal Hierarchy

The fiendish Europeans left the US to blaze the digital voice trail, so when they came standardize things they could forget all this 'frame separator' stuff. Euro Telecom standards are defined by CEPT (a Euro Telecom 'club'). Here in all its glory is the super simple European hierarchy. Again all based on our good friend, the ever popular, 64,000 bit DS0

For More Detail : Please refer the below link.

http://www.zytrax.com/tech/data_rates.htm

Regards

Chetan Kumar