# Highest Cost First-Based QoS Mapping Scheme for Fiber Wireless Architecture
## 3 Distributed Channel Schemas of 802.11 ac and XGPON  
When the data packets arrive at the MAC layer of a wireless station (STA) from upper layers, the STA executes either the DCF or EDCA channel distributed method to transmit packets on the wireless medium. DCF is a basic distributed scheme of 802.11 ac that uses the Carrier Sense Multiple Access mechanisms with Collision Avoidance (CSMA/CA). A collision of transmission can occur when multiple nodes want to access channels simultaneously, therefore DCF uses the CA mechanism to control the collisions between transmissions. Figure 1 presents a basic flowchart of the DCF model. In DCF, the STA detects distributed inter-frame spacing (DIFS) before accessing the wireless channel [29]. When the wireless medium is not busy, the STA transmits frames directly to the wireless medium without waiting for the time-period termed as back-off time (T\({}_{\text{BO}}\)), resulting in a delay of transmission of a random time-period, T\({}_{\text{BO}}\). A value of the T\({}_{\text{BO}}\) is uniformly obtained using Equation (1), where the R\({}_{\text{Counter}}\), known as back-off (BO) random counter, is chosen between zero and CW-1 using Equation (2), and T\({}_{\text{Slot}}\) is the Slot-Time parameter for the BO process. In case of unsuccessful transmission, STA repeats the BO process and the size of CW by a factor of 2\({}^{\text{N}}\)-1 in each re-transmission up to the maximum contention window (CW\({}_{\text{Max}}\)) size, following Equation (3) [30]. CW is reset to the minimum contention window(CW\({}_{\text{Min}}\)) when the transmission is repeated up to a specific retry limit time or when the transmission is successful [31]. Figure 1 explains this process step by step using a flowchart. \[\text{T}_{\text{BO}}=\text{R}_{\text{Counter}}\times\text{T}_{\text{Slot}} \tag{1}\]  
\[\text{R}_{\text{Counter}}=\text{Pseudorandom}\ [0,\text{CW-1}] \tag{2}\]  
\[\text{CW}_{\text{Min}}\leq\text{CW}\leq\text{CW}_{\text{Max}} \tag{3}\]Another variant of DCF access mode is known as enhanced distribution channel access (EDCA). It offers distinguished broadcast facilities under four different access classes (ACs) and applies CSMA/CA on each queue in one STA [32]. At the MAC layer, each message is mapped into the ACs, as shown in Table 2. It shows the priority weightage of each access category for both DCF and EDCA channel access modes. EDCA has 4 AC categories named; Voice(VO), Video (VI), Background(BK) and Best efforts(BE). The higher the user priority number, the more chances to access the wireless channel [33]. Each AC will compete for access to the medium in an Arbitration inter-frame space (T\({}_{\text{AIFS}}\)) period in EDCA. The T\({}_{\text{AIFS}}\) is expressed by Equation (4) [34]:  
\[\text{T}_{\text{AIFS}}=\alpha\text{T}_{\text{Slot}}\times\text{AIFSN}_{\text{ AC}}+\text{SIFS} \tag{4}\]  
where T\({}_{\text{AIFS}}\geq 2\) in the case of QoS stations (QSTA) and T\({}_{\text{AIFS}}\geq 1\) in the case of QoS access points (QAP). In Equation (4), \(\alpha\text{T}_{\text{Slot}}\) represents the duration of a slot, and short interframe space (SIFS) is a short time duration (typically taken as 10 us) that helps to avoid the collisions, and it is maintained before  
Figure 1: The flowchart of distributed-coordination function (DCF) model in 802.11 ac. and after clear to send (CTS) frame in the IEEE 802.11 network. T\({}_{\text{BO}}\) value for access category (\(AC\)) in the case of EDCA is causally chosen from (0, CW) and it is decreased by one slot before the expiry of the T\({}_{\text{AIFS}}\). Figure 2 explains this process step by step using a flowchart. According to the priority mechanism in EDCA, the frames with the highest priority are sent first and the rest have to wait and complete the BO process with a higher CW value. When the number of activated ACs starting the BO process within the same node increases, it causes a virtual collision (VC). At this time, the BO counters are reset and VC management (VCM) handles collision.