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To identify and understand a particular protocol stack architecture, its layers, and their corresponding 3GPP technical specifications, the following steps may be taken:

 Choose a particular mobile communications system such as the GSM, GPRS, and UMTS, LTE, or 5G as your area of interest. Use the information available in the 3GPP site [2] (second, third, fourth column) as mentioned in Section 2.5.6.

 Next, decide the particular network element of interest such as GSM MS, BTS, BSC, and MSC; UMTS NodeB, and RNC; or LTE eNodeB, MME, and S‐GW or 5G gNB and 5G core.

 Now, look at the logical interfaces supported by the chosen network element. Pick a particular logical interface and look at its protocol stack and layers. A logical interface and its protocol stack cover different subjects and specifications area. Look at the subject and specifications areas mentioned in the 3GPP site [2], first column, and pick a particular subject area. Against this chosen subject area, e.g. signaling, requirements, and so on, or a particular protocol layer, attempt to identify the corresponding technical specifications series and its specifications from the column 2, 3, or 4, 3GPP site [2].

Study the protocol layer architecture, its functions and procedures, and other details from the identified technical specification.

Example 3.14 describes the typical steps to be used to derive the 3GPP technical specification of a protocol layer and its sub‐layer.

Figure 3.21 shows the protocol stack of the A‐interface on the CN side.