# C-RAN Traffic Aggregation on Latency-Controlled Ethernet Links
## 1 Introduction  
Centralized/Cloud Radio Access Network (C-RAN) is a promising technique enhancing mobile network performance and flexibility [1]. Improving survivability and power saving are the two main key designs in this architecture [2]. C-RAN is based on the splitting of functionalities which are possibly remotized for the purpose of centralization of the higher layer baseband processing related to multiple Remote Radio Heads (RRHs), by the introduction of the fronthaul network segment [3]. In addition, this approach takes significant advantage of Network Function Virtualization (NFV) and Software Defined Networking (SDN) control and management to add flexibility and adaptability to 5G transport networks [4] and to allow migration of functionalities over the nodes of C-RAN for optimization purposes. In recent proposals [1], the highly demanding traffic, generated by the above functional splits, is carried together with packet-based backhaul (BH) traffic over a common optical infrastructure on different wavelength channels of the same fiber links [5]. This solution can be designed to meet the latency requirements of fronthaul (FH) traffic but may limit the system scalability and lead to low resource utilization with consequently high deployment costs. Hence, techniques for improvingthe optical channel utilization while meeting the strict performance requirements of FH traffic are needed and investigated in this paper. Ethernet-based links can be adopted to implement FH and standardization bodies are recently very active on the definition of the requirements to support FH traffic on this widely deployed interfaces. Delay requirements are identified as quite challenging, the Ethernet not being originally designed for delay-sensitive applications, such as fronthauling [6]. In this work, an integrated hybrid architecture, previously presented in [7], is reviewed and extended with pre-emption to be applied to the C-RAN optical transport network. An integrated Ethernet-based interface, where FH and BH traffic are multiplexed to achieve high utilization of the wavelength resources, is proposed and evaluated to meet the delay requirement of FH traffic, while offering throughput capability to BH traffic. A converged fronthaul/backhaul scenario, as depicted in Figure 1, is considered for evaluating the additional BH throughput that can be obtained as a consequence of the application of the mechanism to wavelength channels. In Figure 1, a sample C-RAN topology is shown where Integrated Hybrid Nodes (IHNs) are interconnected by optical links, possibly forming a mesh or other topologies. IHNs are assumed to be equipped with Ethernet interfaces, which ensure high backward compatibility and low cost. RRH are connected to IHNs which host baseband unit (BBU) functionalities. The set of BBUs available in an IHN form the BBU Hotel. These functionalities can be virtualized in a C-RAN and moved throughout the network to optimize the access service. The management of migration of virtual BBU functionalities is performed by the SDN control/management plane, not shown in the figure. As a consequence of different possible locations for BBU Hotels in IHNs, FH and BH traffic can be present on each link of the C-RAN. FH traffic is assumed according to the Common Public Radio Interface (CPRI) standard as encapsulated in Ethernet frames. The traffic characteristics of the CPRI traffic as generated by the different standardization options are taken into account and managed by a suitably extended integrated hybrid mechanism. As a consequence of the deterministic behavior of the encapsulated CPRI traffic, the benefits of introducing a segmentation policy on the BH traffic is also considered. Segmentation is the act of carving up BH traffic into smaller pieces so they can be inserted into the possible FH traffic gaps. The paper is organized as follows: in Section 2, research in this context is described. In Section 3.1, the application of the integrated hybrid concept in C-RAN is described.