# Planning of Optical Connections in 5G Packet-Optical xHaul Access Network
## 1 Introduction  
Telecommunication networking is undergoing a profound transformation related to the deployment of 5G networks [1]. This transformation is accompanied by evolution of the mobile network towards centralized and virtualized radio access network (C-RAN/vRAN) architectures [2]. Already in centralized 4G/long term evolution (LTE) network implementations, the base station has been disaggregated into a remote radio head (RRH), located close to the antenna at a remote site (cell), and a baseband unit (BBU) placed at a central site (hub). Distributed RRHs and centralized BBUs are connected using the common public radio interface (CPRI) protocol in such networks. In 5G, the radio frequency processing functions performed by a BBU are realized by a distributed unit (DU) and a central unit (CU), whereas RRH is replaced by a radio unit (RU), which performs low-level physical functions [3]. The DUs and CUs may be placed at different network locations in accordance with particular requirements of diverse 5G services, which can be classifiedas enhanced mobile broadband (eMBB), ultra-reliable and low-latency communications (URLLC), and massive machine-type communications (mMTC). For instance, the whole radio processing stack (i.e., DU/CU) might be placed at a remote site in mMTC applications, whereas the DU processing for eMBB services may be performed at a hub site, as mentioned in Reference [4]. The disaggregation and distributed placement of 5G RAN functions, results in multiple data flows that differ in terms of bandwidth and latency requirements. These flows, related to fronthaul (FH--between RU and DU), midhaul (MH--between DU and CU), and backhaul (BH--between CU and a 5G core network) connections, should be carried using a convergent xHaul transport network, as mentioned in Reference [5]. In particular, the transport of traffic from remote sites and its aggregation into a hub site is realized through an xHaul access network. 5G networks are expected to make use of a much larger amount of installed antennas and access points than previous generations of mobile networks, which is frequently referred to as network densification. To decrease the capacity requirements of transport links in RANs, different techniques for compression of radio data have been proposed [6; 7; 8]. Still, the use of conventional transport solutions, based on dedicated point-to-point CPRI links, is neither scalable nor cost-effective in dense 5G access networks as it results in huge demand for high-bandwidth links between the antennas and the hub site. Therefore, to assure a convergent, scalable, and low-cost transport of radio traffic, the adaptation of well-known packet-based Ethernet technology has been proposed for xHaul transport networks in the IEEE standards 802.1 CM [9] and 1914.1 [4]. Ethernet enables statistical multiplexing of data flows and, hence, increased utilization of link bandwidth. The encapsulation of 5G radio data into Ethernet frames is achieved by means of the enhanced CPRI (eCPRI) protocol [10]. The use of Ethernet in xHaul allows for support, as well other services not related to 5G, such as legacy 4G, enterprise, and residential services. In particular, the CPRI data related to 4G services is encapsulated and mapped into Ethernet frames using Radio over Ethernet (RoE) protocol specified in the IEEE 1914.3 standard [11]. Eventually, time-sensitive networking (TSN) features specified in Reference [9] enable prioritized, low-latency transmission of Ethernet frames carrying latency-sensitive fronthaul traffic. The Ethernet switches located at remote sites, where traffic from local antennas/access points/other sources is aggregated, and a hub site, where certain baseband processing functions are performed and traffic is forwarded towards the network core, will be principally connected by means of high capacity and low delay links.