E1 and T1 lines are used to carry data and voice between carrier links. Historically, the T1 circuit(24 channels) has been used in North America and the E1(32 channels) circuit in Europe. T1 and E1 lines are at the lowest level.so it can carry either ISDN or CAS on top of it.It also explains the difference in channel numbers on ETSI ISDN(32 channels) and ANSI ISDN(24 channels).for more queries ottawaindia2000@yahoo.ca