Compared to GSM, the major difference of UMTS is the air interface. The air interface uses GRAN, or Generic Radio Access Network. This means it can be connected to a wide range of networks, such as the internet, ISDN, GSM, or to another UMTS network its self. GRAN uses the lower 3 layers of the OSI model, layer 3, the network layer is the RRM, or Radio Resource Management protocol. The RRMís manage the channels between the base stations and the fixed network.

UMTS is not compatible with GSM, but UMTS phones which are currently sold are dual mode, which means they canít work on GSM as well as UMTS. If a user wants to make a call, but is out of a UMTS area, the device can use GSM. Even if this happens while on a call, the phone can be switched over to the GSM network, completely uninterrupted, transparently to the end user. Although the newer wave of 3G phones can be used on both networks, GSM phones canít be used on UMTS networks.