# Open RAN--Radio Access Network Evolution, Benefits and Market Trends
## 4 Open Xhaul Transport Network--Requirements and Solutions  
The support for a new range of 5G mobile services, in addition to conventional 4G services, poses high and diversified requirements on the transport network infrastructure connecting the Open RAN components. Enhanced Mobile Broadband (eMBB) services have high capacity demands, Ultra Reliable Low Latency Communications (URLLC) applications are sensitive to latencies and require increased reliability, whereas massive Machine Type Communications (mMTC) services involve a huge number of devices which should be managed efficiently in the network. At the same time, the introduction of new radio access technologies in 5G, the use of higher frequency bands, and the densification of installed antennas and access points, requires scalable, flexible, and cost-effective transport solutions. The disaggregation and distributed placement of 5G RAN functions (discussed in Section 3), depending on operator's deployment needs and in accordance with particular service requirements, results in multiple user, control, synchronization, and management data flows that differ in terms of bandwidth and latency requirements. It is expected that these flows related to the fronthaul (FH, between O-RU and O-DU), midhaul (MH, between O-DU and O-CU), and backhaul (BH, between O-CU and CN) segments of the network will be accommodated using a convergent and flexible Open Xhaul transport network, namely, it could be carried in the same portion of the physical transport infrastructure [40]. To meet the above mentioned requirements, in particular, to assure convergence of services with low-cost connectivity, the adaptation of a well-known packet-based Ethernet technology has been proposed for the underlying Open Xhaul transport network [40]. Ethernet enables statistical multiplexing of data flows and, hence, increased utilization of link bandwidth. In addition, the use of Ethernet in Xhaul allows to support other services not related to 5G, such as legacy 2G/3G/4G wireless services, enterprise services, residential broadband services, and data centre interconnections. The multi-service support is one of the capabilities identified by ITU-T in [47] that is expected from the transport infrastructure. The convergence of services with provisioning of adequate quality of service (QoS) guarantees in the Ethernet-based Open Xhaul transport network is achieved among others with the aim of the following technologies:  
1. Enhanced CPRI (eCPRI) protocol [8] that supports the transport of radio data with different functional splits options, including Options 7.2x and 2 considered, respectively, for fronthaul and midhaul in O-RAN;
2. Radio over Ethernet (RoE) protocol specified in the IEEE P1914.3 standard [48], which defines the encapsulation and mapping of CPRI data transmitted between RRH and BBU in legacy 4G services (Option 8 split) in a form of Ethernet frames;
3. Time-sensitive network (TSN) features proposed for a fronthaul network in the IEEE 802.1 CM standard [49] that enable prioritized transmission of latency-sensitive Ethernet frames. The specification of Open Xhaul [40] is also consistent with the next generation fronthaul interface (NGFI) architecture, presented in the IEEE P1914.1 standard [48], which defines a packet-based fronthaul transport network connecting distributed RUs, DUs, and CUs. The frame loss ratio and latency requirements for particular classes of services (CoS) are assumed to be in accordance with the eCPRI Transport Requirements V1.2. The estimation of bandwidth demands for dimensioning of the transport network that takes into account a statistical multiplexing gain from packet-based transmission has been discussed in the technical specification concerning Xhaul Transport Requirements [40]. The end-to-end Open Xhaul transport covers the access network--at which the cell site is located-- the aggregation network, and core network (see Figure 6).