E1 is the 2Mb link, generally assumed to be 30 B channels, a D channel and a timing/framing channel, 32 in total.
CAS = channel associated signalling, the B channel is subdivided into time-slots, 1 for each B channel. These time slots are used to pass the signalls associated with each B-channel.
CCS= Common Channel Signalling. Signals associated with each B channel are sent as ‘packets’ as and when a signal is required
ISDN is an advanced form of CCS and operates over an E1 bearer. It is not an alternative to E1 it is just a different sinalling protocol over E1