Two network components/entities are connected by a link, and signals are transmitted via this link between components. The physical properties of the link and function of signals over this link are collectively known as Interface or reference points. Usually, an interface is standardized. In UMTS various interfaces are defined and are illustrated in Figure 6. They are categorized as follows,

i) $C_{u}$: As UE consists of MT and USIM, the link between these two is described by $C_{u}$ interface.

ii) $U_{u}$: It is the interface between user equipment and UTRAN. The equivalent interface in GSM/GPRS network is the $U_{m}$ interface.

iii) $I_{ub}:$ It is the interface between Node B and RNC. It is used by RNC to control multiple Node Bs. The main signaling protocol used is RANAP. The equivalent interface in GSM/GPRS is the A interface.

iv) $I_{u}$: It is the interface between UTRAN and CN. The $I_{u}$ is split functionally into two logical interfaces, $I_{ups}$ and $I_{ucs}$.

$I_{ups}$ connects the packet switched domain (SGSN) to the radio access network and is used to carry data traffic and signaling. The main signaling protocol used is RANAP. The equivalent interface in GSM/GPRS is the Gb interface.

$I_{ucs}$ connects the circuit switched domain (MSC) to the radio access network. It is used to carry voice traffic and signaling. The main signaling protocol used is RANAP. The equivalent interface in GSM/GPRS is the A interface. The standards do not dictate that these are physically separate, but the user plane for each is same and the control plane may be different.

The $I_{ur}$ interface logically connects radio network controllers (RNCs) but could be physically realized by a direct connection between RNCs or via the CN. It supports inter MSC mobility. When a UE moves between areas served by different RNCs, The UE’s data is now transferred to the new RNC via Iur interface. The original RNC is known as the serving RNC and the new RNC is known as the drift RNC. The main signaling protocol used is RNSAP. There is no equivalent interface in GSM/GPRS networks.

v) $Z_{u}:$ The serving network domain and home network domain are connected by $Z_{u}$ reference point.

vi) $Y_{u}$: The connection between serving network domain and transit network domain is defined by $Y_{u}$ interface.