From the first releases of the UMTS standard, the UTRAN architecture was initially very much aligned with 2G/GSM Access Network concepts. The general architecture follows the good old 2G/GSM ‘star’ model, meaning that a single controller (the RNC, Radio Network Controller) may possibly control a large number – the typical number in commercial networks is about several hundreds – of radio Base Stations (the NodeB) over the Iub interface. In addition, an inter-RNC Iur interface was defined to allow UTRAN call anchoring at the RNC level and macro-diversity between different NodeB controlled by different RNCs.
The initial UTRAN architecture resulted in a simplified Node B implementation, and a relatively complex, sensitive, high capacity and feature-rich RNC design. In this model, the RNC had to support resource and traffic management features as well as a significant part of the radio protocols.
Compared with UTRAN, the E-UTRAN structure is quite simple. It is only composed of one network element: the eNodeB (evolved Node B). The 3G RNC inherited from the 2G BSC (Base Station Controller) has disappeared from E-UTRAN and the eNodeB is directly connected to the Core Network using the S1 interface. As a consequence, the features supported by the RNC have been distributed between the eNodeB or the Core Network MME or Serving Gateway entities.
From a high-level perspective, the new E-UTRAN architecture is actually moving towards WLAN network structures and Wifi or WiMAX Base Stations functional definition.
So functional definition eNodeB (as WLAN access points) support all L1 and L2 features associated to the physical interface, and they are directly connected to network routers. There is no more intermediate controlling node (as the 2G/BSC or 3G/ RNC was). This has the advantage of a simpler network architecture (fewer nodes of different types, which means simplified network operation) and allows better performance over the radio interface.
From a functional perspective, the eNodeB supports a set of legacy features, all related to physical layer procedures for transmission and reception over the radio interface:
Besides, the eNodeB includes additional features, coming from the fact that there are no more Base Station controllers in the E-UTRAN architecture. Those features, which are further described in Chapter 4, include the following: