# A QoS-Enabled Medium-Transparent MAC Protocol for Fiber-Wireless 5G RAN Transport Networks
## 2 System Model  
We considered a FiWi 5G RAN transport network in a Point-to-Multipoint formation, meaning the topology comprises a single central location connected to multiple remote locations, as depicted in Figure 1. At the central location, the MNO places the centralized equipment, which, depending on the level of disaggregation, can be either a 5G-Core unit, a Base Band Unit, a CU, or a CU + DU module. The centralized MNO equipment is connected through an Ethernet connection to the qMT-MAC CO with protocol translation taking place at their interface. The CO in turn extracts the Ethernet payload, inserts it into qMT-MAC packets, and modulates the latter on top of A-RoF carriers, which in the downstream traverse the optical fiber towards the RAUs. The RAUs transmit the wireless signal towards the qMT-MAC nodes, which in turn are connected through Ethernet to the MNO's remote equipment. Depending again on the disaggregation level, the remote equipment can be either a 5G Base Station (gNodeB/Small Cell), a DU+RU module, or an RU module. The qMT-MAC equipment is assumed to be mounted on top of street furniture, such as street light lampposts, since this is the predominant model considered for facilitating the RAN densification and installing the necessary Small Cells and other MNO remote equipment. However, the lamppost antennas are used here solely as an illustrative example, as in general the qMT-MAC nodes are considered to be collocated with the MNO remote equipment. A collection of data wavelength pairs is considered, with one wavelength from the pair used for Uplink (UL) and the other used for Downlink (DL) communication. In this way, the qMT-MAC fiber portion of the network operates under a Wavelength Division Multiplexing (WDM) scheme, allowing many wavelengths to be sent over the same fiber at the same time, resulting in higher field-fiber resource utilization efficiency. A dedicated wavelength pair is also employed as a common control channel, via which all control information is sent, while data wavelengths are primarily used for data packet exchange. Wireless communications on the UL and DL use distinct frequencies. In this work, without loss of generality, we focus on the UL traffic generation, i.e., from the nodes towards the mobile gateway. As shown in Figure 1, the wavelength allocation is considered to employ a wavelength selectivity device at each RAU, such as a Reconfigurable Optical Add-Drop Multiplexer (ROADM) or a Wavelength Selective Switch (WSS). These types of devices have been shown to offer very fast switching speeds that lie in the order of a few us down even to a few tens of ns [21; 22; 23]. This enables dynamic network capacity reconfiguration based on actual load, resulting in an increased optical wavelength utilization factor. Because optical wavelengths are only allocated when pending traffic exists, higher energy- and cost-efficiency are also attained. It is worth mentioning, however, that the proposed protocol works independently of the wavelength switching speed. However, the faster the wavelength switching speed, the lower the additional switching overhead on the packet delay and, as a result, the greater the protocol performance is. The Ethernet-based communication between the MNO equipment and FiWi qMT-MAC network facilitates the use of enhanced Common Public Radio Interface (eCPRI) messages for data exchange as the predominant protocol for X-Haul communication. In order to encompass and simulate the main eCPRI splits in our study, we considered that the MNO equipment produces the following three types of payloads:  
* Constant Express traffic (CEXP): This type of traffic simulates a low-split FH traffic that has strict delay and delay jitter constraints. Examples of this kind of traffic are the 3 GPP splits 7 and 8. The traffic pattern exhibited by these splits is the Constant Bit Rate (CBR), which does not fluctuate with the number of users present in the cell [24].