een a REC and RE, multiple REs can be connected to one REC, and multiple RECs can be connected to one RE. Three types of topologies are supported: Chain topology (Figure 4.1.4) Tree topology (Figure 4.1.5) Ring topologies (Figure 4.1.6). In the physical (L1) layer, CPRI can use electric and optical transports and supports bitrates that range from 614.4 Mbit/s to 24 330.24 Mbit/s. Figure 4.1.4 CPRI chain topology. Figure 4.1.5 CPRI tree topology. Figure 4.1.6 CPRI ring topology. 4 NG-RAN Architecture While CPRI defines In-phase & Quadrature (I/Q) data transfer, synchronization, and a few other mechanisms in rigorous detail, many important pieces (such as management and control, for example) are intentionally left unspecified. These vary greatly between different vendors and therefore CPRI is not a fully multi-vendor interoperable standard. This gap is addressed by O-RAN specifications, described in Section 4.5. The latest CPRI version at the time of writing this book is CPRI v7.0, which sup- ports GSM, the Universal Mobile Telecommunications System (UMTS), LTE (including LTE-Advanced), and Worldwide Interoperability for Microwave Access (WiMAX). In order to support NR, CPRI released a new specification referred to as eCPRI. While eCPRI (eCPRI v2.0) has many similarities to CPRI, it is a complete redesign com- pared with the previous generation, with the following main objectives: Reduction of the required bandwidth of the transport network; Usage of Ethernet-based transport. It supports NR and is capable of other gNB functional split options other than PHY/RF split. The key word is "capable," as eCPRI specification does not actually define messages and procedures needed to support these functional splits. In terms of architecture, eCPRI defines eCPRI Radio Equipment Control (eREC) and eCPRI Radio Equipment (eRE). Further, there is the notion of eCPRI/CPRI Interworking Function (IWF), providing a bridge between eCPRI and CPRI nodes. This is illustrated in Figure 4.1.7. As one can see from the architecture Figure 4.