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Some functions and procedures performed over a CN logical interface are specific to a GSM, UMTS, LTE, or 5G system. Typical examples of such GSM, UMTS, and LTE system‐specific functions over their specific logical interface are mentioned below:

 BSSMAP [between GSM BSC – MSC]Speech transcoding from 64 kbps, at MSC end, to 16 kbps, at BSC end

 RANAP [between UMTS/UTRAN RNC – MSC]UMTS Radio Access Bearer management, i.e. establish, modify, release, create, and allocate radio access bearer to the UETransport and RNL link management functions

 S1‐AP [between LTE/E‐UTRAN eNodeB – MME]LTE/EPS Radio Access Bearer management, i.e. setup, modify, release, to create and allocate radio access bearer to the UETransport and RNL link management functions

 NG‐AP [between 5G NG‐RAN – AMF]5G PDU Session Management, i.e. setup, modify, and release, a PDU session to a UE, MM, UE context management, and so onTransport and RNL link management functions.

For more information on the specific functions and procedures, refer to TS 25.413 [54], TS 36.413 [97], TS 48.008 [134], and TS 38.413 [119].