Basic terms for Digital Telephony and associated links:

DSX -
    1) http://whatis.techtarget.com/definition/0,,sid9_gci212004,00.html.
    2) http://www.inetdaemon.com/tutorials/telecom/ds/index.shtml. (best)

SONET -
    1) http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci214223,00.html.
    2) http://en.wikipedia.org/wiki/Synchronous_optical_networking. (best)

CAS -
    1) http://en.wikipedia.org/wiki/Channel_Associated_Signaling.

ISDN -
    1) http://en.wikipedia.org/wiki/ISDN.
    2) http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm.

Signaling System 7 -
    1) http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci214365,00.html.