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The UMTS Iu‐interface is divided into two parts: Iu‐CS, to support the CS domain, and Iu‐PS, to support the PS domain. The Iu‐interface may use the ATM or IP‐based transport network as the data link layer protocol, as illustrated in Figure 3.20. For more information, refer to TS 25.412 [53].

Figure 3.20 Iu interface transport network: data link layers.

Table 3.8 Protocol layer in UMTS (Iu), LTE system (S1), and 5G NG over IP.

IP Transport Layer	Purpose	System/Interface
UDP	To transport Iu or S1 or NG user plane information over IP	UMTS (Iu), LTE (S1), 5G (NG)
SCTP [17]	To transport Iu or S1 or NG control plane information over IP

 Provisions for Multiple Physical Layer Interfaces

Depending on the data link layer being used, the GSM BSS, UMTS RNC, and the core network element transport network may provide provisions for configuring multiple physical layer interfaces. This ensures a backup, in case one physical interface fails the other physical interface can be brought into service to continue communications services. The upper layers of a protocol stack are independent of the type of underlying data link layer and its physical interface being used to transport higher‐layer data.