# Open RAN--Radio Access Network Evolution, Benefits and Market Trends
## 4 Open Xhaul Transport Network--Requirements and Solutions
Within these network segments, it is assumed that the packet switches are connected by means of high capacity and low delay point-to-point Ethernet links, which are realized using either fibre optic, wireless, or cable technologies. Below, we briefly discuss the main technologies considered for the underlying physical network. 1. Wavelength division multiplexing (WDM) allows to significantly increase the capacity of an optical fibre due to multiplexing of signals transmitted on different wavelengths using either passive or active WDM equipment installed at the link ends [50]. Passive WDM reduces about 4-6 times the cost of active WDM since it does not use signal amplification and dispersion compensation components as well as it involves a less expensive WDM equipment. In addition, its installation at a cell site is simplified due to reduced power supply requirements. However, passive WDM has some drawbacks related to limited management, fault detection, configuration, and maintenance capabilities. A trade-off solution for a large-scale deployment of 5G fronthaul networks connecting a huge number of sites is a semi-active WDM network, which uses a passive WDM equipment at the remote (cell) side and active WDM at the hub site. This solution has the advantages of both passive and active WDM, whereas it mitigates their drawbacks such as the cost of active WDM equipment and limited protection and management capabilities of passive WDM. Currently, depending on the spectral grids used, up to 6/12-channel (in MWDM) and 40-channel (in DWDM) systems are considered for O-RAN [40] with a transmission distance of 10-20 km (passive solutions) and transmission rates of 10-25 Gbps in access and 100 Gbps (per wavelength) in aggregation/core networks. The demand for fibre resources can be further reduced by means of bi-directional transceivers and the allocation of different wavelengths in opposite directions, which enables the transmission in a single optical fibre. 2. Passive optical networks (PONs) are point-to-multipoint access networks, widely used in \"fibre-to-the x\" (FTTX) applications, which make use of the optical fibre as a transmission media. PONs are based on a central optical line termination (OLT) equipment connected with remote optical network units (ONUs), which are installed at the client side. The access of PON clients to the transmission resources is achieved either using time-division multiplexing (TDM) techniques and/or by means of passive WDM technologies. The application of PONs for transport of 5G Xhaul traffic has been broadly discussed in the literature [50,51], and is considered in the O-RAN specification [40]. Since PONs are multi-service systems, they allow to carry mobile traffic (e.g., backhaul) along with non-mobile traffic (e.g., fixed access services). In fronthaul use cases, special attention should be placed on a latency vs. bandwidth efficiency issue arising in TDM-PON solutions that relay on a dynamic bandwidth assignment (DBA) mechanism. As WDM-PONs realize dedicated point-to-point links, they have advantageous features, such as high capacity, low latency, and operational simplicity [51], which make them a suitable solution for Xhaul networks. 3. Microwave (MW) and mmwave (mmW) radio transport technologies have been the primary solution for provisioning backhaul connectivity in previous generations of mobile networks, providing transmission capacities of a few Gbps in MW systems and up to 10 Gbps in E-band (70/80 GHz) systems, with transmission distances of up to 3-4 km in point-to-point and line-of-sight (LoS) configurations [52]. To cope with high bandwidth demands of centralized 5G RANs, radio transport technologies are evolving in several directions.