# Highest Cost First-Based QoS Mapping Scheme for Fiber Wireless Architecture
## 3 Distributed Channel Schemas of 802.11 ac and XGPON
The VCM guarantees throughput for the higher priority AC but puts the lower priority AC on a bandwidth  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mode** & **AC** & \(\alpha\text{CW}_{\text{Min}}\) & \(\alpha\text{CW}_{\text{Max}}\) & \begin{tabular}{c} **DIFS or** \\ **AIFS** \\ \end{tabular}  & \begin{tabular}{c} **TXOP** \\ **Priority** \\ \end{tabular}  & \begin{tabular}{c} **User** \\ **Priority** \\ \end{tabular}  \\ \hline EDCA & \begin{tabular}{c} BK \\ BE \\ VI \\ \end{tabular}  & \begin{tabular}{c} \(\alpha\text{CW}_{\text{Min}}\) \\ \(\pm\) 2 - 1 \\ VO \\ \end{tabular}  & \begin{tabular}{c} \(\alpha\text{CW}_{\text{Min}}\) + 1 \\ \(\pm\) 4 - 1 \\ \end{tabular}  & \begin{tabular}{c} \(\alpha\text{CW}_{\text{Max}}\) \\ \(\pm\) 2 - 1 \\ \end{tabular}  & \ \begin{tabular}{c} 7 \\ 3 \\ \(\pm\) 504 ms \\ \end{tabular}  &
\begin{tabular}{c} 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 0 \\ 0 \\ 0 0 \\ 0 \\ 0 \\ 0 0 \\ 0 \\  
shortage, which is an unfair allocation of resources. Also, the latest EDCA of 802.11 ac supports Transmit Opportunity (TXOP), which is a contention-free access mechanism for one duration of any AC [35]. The typical FiWi access network architecture is shown in Figure 3, where ONUs connect the wireless access points (WAPs) and WAPs connect to wireless users via a wireless channel. The optical line termination (OLT) offers the backbone access to the ONUs [36]. There is a P2 MP passive splitter between OLT and ONUs that shares an individual optical fiber channel with many optical fiber channels for ONUs and vice versa (v.v). The communication going from OLT to ONU is called downstream (DS), and when it is going from ONU to OLT it is called upstream (US) in the passive optical network (PON). A dynamic bandwidth allocation (DBA) mechanism acts as an arbitrator for US bandwidth management. The DS frames are used to query ONUs for their bandwidth demand. The DBA process helps to avoid the collisions on the common shared US medium of PON, and additionally, it distributes bandwidth among all the traffic classes of each ONU, identifying each with the help of allocation identity (AllocId). Ten-gigabit PON (XGPON) is an upgraded version of GPON that supports two bitrates (e.g., 2.5 and 10 Gbps) for the US channel and it supports 10 Gbps for the DS channel. The US and DS communication between OLT and ONU take place in a synchronized manner in 125 us duration of frames in XGPON. The DS frame contains a payload and physical control block (PCBD) header. The PCBD contains a bandwidth map (BWMap) for bandwidth allocation of each allocation id that is utilized in the next cycle of US traffic by ONU. There are five different QoS classes for US traffic of XGPON, also termed as transmission containers (TCONTs), which were originally introduced in the broadband passive optical network (BPON) by ITU-T in G984.3 standards [37]. TCONT represents a traffic flow uniquely identified by an allocation id. OLT assigns bandwidth with the help of DBA to all the TCONTs as follows:  
1. Class 1 deals with constant bit rate (CBR) applications that require stringent performance, latency, and delay variation. 2. Class 2 has efforts to deal with variable bit-rate (VBR) traffic and is suitable for video application that has predefined throughput requirements. 3. Class 3 offers a guaranteed minimum transmission rate. 4. Class 4 deals with the best effort (BE) connections. 5. Class 5 is the combination of all classes.