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3GPP defines a larger number of protocol logical interfaces, starting with the alphabet “A”, stacks, and layers which are central to the interworking of a network element with another network element of mobile communications systems and networks. In this chapter, we have covered only the following logical interfaces of mobile communications networks:

 Air interfaces, i.e. Uu, Um, between UE/MS and RAN of GSM, UMTS, LTE, and 5G systems.

 Network interfaces (A, Gb, S1, Iu, X2, Gn, NG, and so on) between the GSM, UMTS, LTE, and 5G RANs and their respective CNs.

In fact, the majority of the logical interfaces are found only in the CNs domain along with the other CN elements such as the Home Location Register/Home Subscriber Server (HLR/HSS), Visitor Location Register (VLR), and Policy Charging and Restriction Function (PCRF). Other logical interfaces are also available that can be configured to support interworking and interoperations, e.g. CS fallback, Single Radio Voice Call Continuity, and so on, between two mobile communications networks. Some of these interoperation facilities may be configured as an optional and separately licensed feature.

A developer must put the focus on a particular network element and its logical interfaces at a time. The air interface and its protocol stack is the most interesting one that consists of advanced wireless communications theories. The air interface differentiates one system from its predecessor. As a starting point, the reader is advised to go through and familiarize themselves with the list of 3GPP TSs mentioned in the Reference section of this book. There are 3GPP TSs describing the protocol layers of the respective air interface of the GSM, GPRS, UMTS, LTE, and 5G systems. The reader may, then, proceed gradually toward the other logical interfaces and their protocol stack.