look this diagram:
Because i have lots of E1 from WAN, i want
to buy dsu/csu device to terminate the N*E1, the DSU/CSU has E3 interface to connect to the router 3640, but it seems that the 3640 has not E3 interface (it only has E3 FOR ATM-----NM-1A-E3), if connect to the NM-1A-E3,it could work? if not,
how can i do ? any idea is appreciated.