# A QoS-Enabled Medium-Transparent MAC Protocol for Fiber-Wireless 5G RAN Transport Networks
###### Abstract
FiWi networks are in general classified into two groups: (i) Radio-and-Fiber (R&F) and (ii) Radio-over-Fiber (RoF). In R&F networks, the two distinct optical and wireless media are being addressed separately, forming two discreet networks, each one implementing its own MAC layer scheduler with protocol translation transpiring at their interface. As a direct consequence of the above, the wireless MAC frames are transmitted solely in the wireless network segment and do not have to traverse the optical fiber to /from the centralized location, therefore avoiding the fiber propagation delay impact. However, R&F networks are applicable only in Decentralized Radio Access Networks (D-RANs), which are inefficient for use in dense urban areas with highly concentrated traffic requirements and more frequent site expansion and densification needs. The above is due to the necessity to deploy multiple antenna sites in close proximity, which dictates the need for having simpler APs with centralized control to ensure proper coordination [9]. Regarding RoF networks, initial works focused mainly on the wireless Layer-2 protocol functionality while considering predominately only the fiber length in their performance analysis. In [10], the authors studied the application of the 802.11 Point Coordination Function (PCF) and proposed a modification specifically targeting broadcast distributed antenna systems where multiple Remote Antenna Units (RAUs) are employed to serve as the antennas of a single AP, taking into account in their system model the added propagation delay due to the fiber connection between the RAUs and AP. The use of Analog-RoF (A-RoF) techniques for converged FiWi networks has received high research interest due to its high spectral efficiency benefits and the low complexity needed at the RAUs [11]. Nevertheless, A-RoF FiWi networks necessitate the joint study of the optical and wireless domains in order to offer optimal end-to-end performance. Consequently, new efficient medium-transparent protocols capable of optimally allocating both wireless and optical resources should be designed. To that end, in [12], a Medium-Transparent-Medium Access Control (MT-MAC) protocol was proposed for the first time for converged FiWi networks exploiting A-RoF, which efficiently manages both optical and wireless resources dynamically. In MT-MAC, the Central Office (CO) communicates directly with the MTMAC nodes (clients), without the intervention of the RAUs. As a result, the spectrum allocation decision takes place at the CO, i.e., the CO allocates the optical resources to the RAUs and the wireless resources to the users. In MT-MAC, the data traffic exchange is divided into Superframes (SFs), which contain two types of frames: (i) contention frames used for initial connection setup and status reporting and (ii) the frames containing the actual data exchange. In the initial form of MT-MAC [12; 13; 14; 15], the SF size constituted a fixed number of transmission slots, which were the same for all RAUs, regardless of the number of active users served by each RAU at each time instance. With this scheme, resource allocation fairness among RAUs was the predominant target, and hence, all RAUs were allocated transmission windows of the same size by the CO to grant to their users independently of pending traffic. Focusing on the same MT-MAC version, in [16], a wireless multi-channel resource allocation technique was proposed for 60 GHz RoF local access networks, which takes into account predictions on the residual capacity of each wireless channel to reduce the handover latency. In [17], a method targeting at the reduction of the number of polling packets required to identify all active users was presented. A first attempt to provide Quality-of-Service (QoS) characteristics to the MT-MAC protocols was performed in [18], with the introduction of the QoS-aware Medium-Transparent (QMT) resource management scheme, which considered the existence of various traffic classes per node, with each traffic having a distinct priority attribute.