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From the illustration shown in Figures 3.5 and 3.19, it has been observed that each of the concerned protocol stacks between the GSM BSS and its CN or the UMTS RNC and its CN has the following characteristics, i.e. multiple transport and data link networks, as described in Examples 3.12 to 3.13.

 Provisions for multiple Data Link layers

Example 3.12/Figure 3.20 illustrates the data link layers used for the UMTS Iu‐interface. The Iu‐interface transport network options are used to exchange the control plane and user place messages of both the Iu‐CS and Iu‐PS interfaces.

 Provisions for Separate Transport Protocol in an IP Transport Network

In the case of UMTS, LTE, or 5G system, if the IP transport network option is used, the IP network transport protocol layers used are listed in Table 3.8.

SCTP [17] transport network is also used for different logical interfaces in the 5G system.