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Network elements of mobile communications networks are designed and developed based on a set of standard protocols and specifications as defined by the 3GPP. A protocol interface is a communication path that is established between two network elements. Each network element and its protocol layers communicate with the peer network element and its protocol layers through a particular interface. The related protocol layers or the protocol stack, supported by a network element, are organized into a protocol interface. Over a particular protocol interface between two network elements, they exchange various signaling messages corresponding to a particular function and procedure such as the following ones:

 Call Control Management, i.e. a call establishment, maintenance, and its releasing;

 Mobility Management and Session Management (SM);

 Radio Resource Management, Handover, and Power Control Management; and

 Interworking, Interoperations, Roaming Management, and so on.

A protocol interface also carries user data or traffic between two network elements. A protocol interface between two network elements could be physical as well as logical as described below. A logical interface may be a point to point, i.e. direct, or may span through several network elements. A logical interface works on top of a physical interface.