# A QoS-Enabled Medium-Transparent MAC Protocol for Fiber-Wireless 5G RAN Transport Networks
###### Abstract
In all these cases, however, a fixed transmission window is granted to each RAU, while specifically in the case of [18], the various traffic classes are also granted a static portion of the static transmission window. However, the mobility of wireless users essentially causes wide fluctuations in the RAUs' load over time, leading to inefficient spectrum usage under a fixed transmission window due to the high number of idle slots. To this end, a more sophisticated MT-MAC version, called Client-Weighted MT-MAC (CW-MT-MAC), was proposed in [19]. In this protocol, the CO grants transmission windows to each RAU proportional to the number of active users. Although CW-MT-MAC was able to achieve fairness among the network users, it still did not consider the actual traffic load of each RAU, as an RAU with more users could in total have lower aggregate traffic load than an RAU with few users being heavily loaded. Motivated by the need for a per-packet-based optical/ wireless resource allocation, a traffic-aware MT-MAC scheme, termed gated-MT-MAC (gMT-MAC), was recently proposed [20], which employed the gated service paradigm for establishing the SF duration so as to maximize the network performance and spectrum efficiency. The gMT-MAC protocol, where each active user is authorized to transmit in an SF the amount of data it reports at the end of the previous SF, was shown to greatly outperform the original MT-MAC [12], as well as the CW-MT-MAC protocols [19] and achieve up to 20-times higher throughput, 2-times lower delay, and 5-times higher data wavelength utilization. However, in the context of disaggregated RANs, the future transport networks are expected to provide connectivity solutions for a combination of Fronthaul/Midhaul/Backhaul (X-Haul) traffic, as the MNOs will wish to exploit the flexibility to deploy all sorts of equipment combinations: from the predominant Centralized Unit/Distributed Unit/Remote Unit (CU/DU/RU) split, to the CU/DU + RU split, i.e., the CU is separated through the Midhaul connection to the joined DU and RU equipment, as well as the legacy all-in-one CU + DU + RU formations, i.e., a monolithic Base Station. The above combinations signify that Layer-2 protocols employed in the future FiWi RAN transport networks must support Backhaul/Midhaul and Fronthaul connections, while concurrently meeting their distinct bandwidth, latency, and jitter Key Performance Indicators (KPIs). The gated service regime implemented in the State-of-the-Art (SoA) gMT-MAC protocol [20] does not offer service classification or a way to prioritize packets within the SF, and hence, packet flows originating from X-Haul equipment are served identically; to this end, the gMT-MAC protocol cannot effectively accommodate multiple traffic classes featuring their aforementioned diverse bandwidth, latency, and jitter specifications. To this end, this paper proposes the QoS-aware MT-MAC (qMT-MAC) protocol, which is specifically designed to provide the necessary performance guarantees to the various flows that are expected to traverse the next-generation 5G and beyond transport networks, offering QoS support for three X-Haul traffic classes: (i) low-layer split Fronthaul (FH) traffic that produces Constant Bit Rate (CBR) loads and carries strict delay and jitter requirements, (ii) low-layer split FH that produces load-dependent traffic that carries strict delay requirements, and (iii) higher-layer split traffic that produces load-dependent traffic with more relaxed service constraints in terms of delay. The rest of the paper is organized as follows: In Sections 2 and 3, the system model and the operation of the proposed qMT-MAC protocol are described, respectively. In Section 4, the performance evaluation and simulation results of qMT-MAC compared to the SoA are presented. Finally, Section 5 concludes the paper.