Imagine you have a T1, PPP link from CompanyA (7200 route) to CompanyB (6509 L3 Switch). In this case companies are located in another city and I am just wondering whether T1's are always real dedicated circuit, or perhaps throughtout the path to the other end you could technically see other types of medium other than the T1 line itself.
A private line T1 between cities will surely be multiplexed onto higher speed links, but your data will always be virtually isolated from other traffic.
The 1.5 mb T1 (DS1) signal will be multiplexed into a DS3 45 mb signal (28 T1s), and the DS3 becomes an OC1 in a fiber system and gets muxed into higher OC levels, up to an OC-192 (9.9 gb/s).
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