The Network Modes have different values and interpretation in UTRAN and GERAN. In both the cases the Operation modes is decided based on the Gs interface between the CS CN (core network) a.k.a. MSC and the PS CN a.k.a. SGSN
In UTRAN:
Network Operation Mode I (NMO-I) is used when the Gs interface is present. In this case during the registration a Combined Attach (includes GPRS Attach & IMSI Attach procedures) procedure can be performed. A GMM Attach Request message with the attach type set to Combined Attach is used. Upon completion of this procedure, MM Status is IMSI Attached and GMM State is Attached.
In Network Operation Mode II (NMO-II) the GS Interface is not present. So the GMM attach procedure and the IMSI Attach (via Location Update) has to be performed separately. This causes additional signalling.
Basic air interface signalling in case of NMO2 is shown here.
In GERAN:
Network operation mode 1. A network which has the Gs interface implemented is referred to as being in network operation mode 1. CS and PS paging is coordinated in this mode of operation on either the GPRS or the GSM paging channel. If the mobile device has been assigned a data traffic channel then CS paging will take place over this data channel rather than the paging channel (CS or PS).
Network operation mode 2. The Gs interface is not present and there is no GPRS paging channel present. In this case, paging for CS and PS devices will be transferred over the standard GSM common control channel (CCCH) paging channel. Even if the mobile device has been assigned a packet data channel, CS paging will continue to take place over the CCCH paging channel and thus monitoring of this channel is still required.
Source: http://blog.3g4g.co.uk/2011/07/network-mode-of-operation-nmo.html
