Hi Peter,

NM-2CE1U is Channelized E1/ISDN PRI Unbalanced (75-Ohm) Network Module.

NM-2CE1B is Channelized E1/ISDN PRI Balanced (120-Ohm) Network Module.

Depending on the termination provided by the SP you'll have to decide whether you need to use the balanced or unbalanced module.

This represents the impedance of the termination either 75 ohms or 120 ohms.

http://www.cisco.com/en/US/docs/routers/access/interfaces/nm/hardware/installation/guide/ConntPRI_ps2797_TSD_Products_Module_Installation_Guide.html#wp1022505

-> Sushil