# A QoS-Enabled Medium-Transparent MAC Protocol for Fiber-Wireless 5G RAN Transport Networks
## 3 The QoS-Aware MT-MAC Protocol
### Express Traffic Priority Enhancements in qMT-MAC  
The PS creation sequence of the qMT-MAC protocol provides a clear priority to CEXP and EXP traffic, since packets of this type are always placed first in the PS, while BE packets are inserted in the PS only after the insertion of the CEXP and EXP traffic. However, since delay-bounded EXP traffic can arrive within the duration of an SF, i.e., while the network is executing a specific PS, the newly arrived packet is forced to wait until the next SF is transmitted, causing delays that could be detrimental to X-Haul or URLLC traffic. To this end, the qMT-MAC protocol proposes two additional priority policies for nodes that combine EXP and BE queues, as displayed on the lower right-hand side of Figure 1, to further enhance the EXP traffic priority over BE traffic. In total, a qMT-MAC client serving both EXP and BE traffic will execute the PS according to one of the following three PS execution policies:  
1. Default priority PS execution policy: This is the default policy, under which the qMT-MAC client follows the PS as it is constructed by the CO based on the PS creation algorithm shown in Figure 4. Under this regime, when a client receives a DATA_POLL for BE traffic, it checks the status of the BE queue, and in case there is a pending packet, the packet is transmitted. If the BE queue is empty, the client checks its EXP queue, and if an EXP packet is located there, it gets transmitted. Otherwise, the transmission slot remains empty.
2. EXP priority PS execution policy: Under this policy, when the qMT-MAC client receives a BE DATA_POLL, it checks the EXP queue first for outstanding packets, and  
Figure 5: Example operation of the CEXP-Fit algorithm for 3 CEXP flows with IFGs of 4, 8, and 16 slots, respectively.  
if one is found, it gets transmitted instead of the BE packet. If no EXP packets are waiting for transmission, only then does the client proceed in polling the BE queue for packets.
3. Pre-emption priority PS execution policy: This scheme is similar to the previous policy, since the client always polls the EXP queue first for a packet to transmit. If the EXP queue is empty, then it polls the BE queue. In addition, this scheme considers also packet pre-emption, meaning that when an EXP packet arrives and the medium is busy due to the on-going transmission of a BE packet, the queuing system allows the EXP packet to disrupt the transmission of the BE packet, pending the preemptable frame having at least 60 bytes already transmitted and at least 64 bytes still remaining until the end of the preemptable frame is reached [27]. Packet pre-emption has become increasingly important lately, since it was shown that a severe delay may be caused by starting a large, low-priority frame ahead of a time-critical frame [28]. Packet pre-emption has been adopted and standardized in the IEEE 802.3 Qbr and 802.3 Qbu standards, which deal with Time-Sensitive Networking [29; 30], and has been shown in various works in the literature to further reduce the average delay of the EXP traffic, while also reducing the observed delay jitter [27; 28].