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Recall the protocol layering principles of the OSI 7 protocol reference model, having a single protocol at each of the individual Layers #1–7. In the case of a mobile communications system protocol architecture, a particular protocol layer may contain sublayers also, performing its functions and procedures. For example, consider the air Interface Layer 3 protocol stack. The GSM Layer #3, i.e. the network layer in terms of the OSI reference model, has three sublayers, as mentioned below:

 CM

 MM

 Radio Resources Control and Management (RR)

Similarly, the UMTS and LTE system air interface protocol stack, Layer #2, i.e. the data link layer in terms of the OSI reference model, has three sublayers, as mentioned below:

 Packet Data Convergence Protocol (PDCP)

 RLC

 MAC

In the case of the GPRS/Enhanced Data for Global Evolution (EDGE) system protocol stack also, Layer #2, i.e. the data link layer in terms of OSI reference model, has three sublayers, as mentioned below:

 Logical Link Control (LLC)

 RLC

 MAC

The different protocol sublayers mentioned above are illustrated in Figure 3.15.

Note that in the case of UMTS and LTE systems, sublayers of a protocol layer may spread across the AS as well as NAS groups of protocols. For example, consider the UMTS and LTE air interface Layer 3 protocol and its sublayers. Here, the RRC is the Layer 3 protocol that terminates at the UTRAN or E‐UTRAN, but it is placed as part of the AS group of protocols. On the other hand, the sublayers of LTE/EPS or GPRS SM, MM, and CM are part of the NAS group of protocols that terminates at the CN, i.e. GPRS SGSN or LTE/EPC MME. Further, as illustrated, the 5G New Radio (NR) Layer 2 contains a new sublayer called the Service Data Adaptation Protocol (SDAP).