As is not considered in the DCF mechanism, the IEEE 802.11e EDCA scheme [4] sup-ports different traffic types and fulfills their corresponding QoS requirements. In order to achieve the advancement from the DCF method to the EDCA scheme, the E-ARCR pro-tocol is proposed as the enhanced version of the ARCR scheme in order to fulfill the QoS requirements as specified in the EDCA scheme. The E-ARCR protocol will support four ACs in a WS in order to serve various traffic types which possess different priorities for the competition of channel access. As specified in the standard, the access categories are denoted as AC[Z] with Z = 3, 2, 1, and 0, where AC[3] represents the highest priority and AC[0] has the lowest priority. The special control functions described in Subsection
3.2.1are designed to facilitate the implementation of E-ARCR protocol. The operations of proposed E-ARCR scheme is explained with an arbitrary network scenario in Subsection 3.2.2.
3.2.1 Functional Description
In order to provide prioritized ACs for different traffic, four queues in the same WS are utilized as four virtual stations to contend for channel access. Therefore, instead of adopting a single reservation table as in the ARCR scheme, the proposed E-ARCR protocol exploits four reservation tables in order to record different types of traffic from all the WSs in the network. Each of the four reservation tables will be labeled as TAC[Z] which matches with the AC[Z] traffic, where Z = 3, 2, 1, and 0. The control fields similar to Definitions2.1to2.4are utilized in the E-ARCR protocol associated with different AC[Z]s, including TAR(Z), NTO(Z, r), RTS-R(Z), and RFD(Z, r). For example, TAR(Z) is defined as a control field to inform the AP that AC[Z] of a WS is intending to join the reservation table TAC[Z].
Considering different priorities among the AC[Z]s, the initial window size WAC[Z] as well as the maximum backoff stage MAC[Z]will be different between the four AC[Z]s. Based on the information acquired from the control field NTO(Z, r), the random backoff number kearcr,AC[Z]for the specific AC[Z] within a WS will be selected as
kearcr,AC[Z] =
U [0, 20WAC[Z]− 1], r = 0,
U [2r−1WAC[Z], 2rWAC[Z]− 1], 1≤ r ≤ MAC[Z], U [ℓ· 2MAC[Z]−1WAC[Z], u· 2MAC[Z]−1WAC[Z]− 1], r > MAC[Z],
(3.1)
where ℓ = r − MAC[Z] + 1 and u = r − MAC[Z] + 2. Moreover, the AIFS value in the EDCA scheme for each AC[Z] is denoted as AIF SAC[Z] in order to govern different wait-ing time intervals to start the backoff process. Therefore, the parameters WAC[Z], MAC[Z], and AIF SAC[Z]for each of the four AC[Z] can be manipulated to affect different priorities among the ACs.
3.2.2 Network Scenarios
The operations of proposed E-ARCR protocol without packet collisions and channel noise are depicted in Fig. 3.1. To clearly visualize the network behaviors of proposed E-ARCR scheme, each of the two WSs is associated with two ACs including AC[1] for high
C
Figure 3.1: The timing diagram for the proposed E-ARCR protocol.
priority and AC[0] for low priority transmission. Therefore, there will be two reservation tables TAC[1] and TAC[0] exploited within the AP. At the beginning, all the four ACs con-tend for channel by adopting the EDCA scheme and there is no entry recorded in AP’s reservation tables, i.e., TAC[1] = TAC[0] ={}. As AC[1] of node A successfully acquires the channel at time t1, WS A will be added into the reservation table TAC[1]as the first entry, i.e., TAC[1] ={T0(A)}. After data packets have been successfully delivered from AC[1] of WS A to the AP, the AP will transmit the ACK+NTO(1, 0) packet to WS A which indicates that AC[1] of WS A has the transmission order of 0. At the time instant t2, all the four ACs will continue to contend for the channel access. As WS A has received the NTO(1, 0) packet from the AP, WS A will adopt the E-ARCR scheme with random backoff mechanism as defined in (3.1); while the other three ACs will employ conventional backoff scheme from the EDCA algorithm. Considering that AC[0] of WS B wins the channel contention and hence it will be recorded as a new entry in the reservation table as TAC[0] ={T0(B)}. Sim-ilarly, in the case that AC[1] of WS B acquires the channel access at time t3, it will join in the reservation table TAC[1] as the second entry, i.e., TAC[1] = {T0(A), T1(B)}, and finally receives the ACK+NTO(1, 1) packet from the AP after packet transmission.
Assuming that AC[1] of WS A obtains the channel access at time t4, the RTS-R(1) packet will be delivered by WS A to initiate the reservation period for AC[1] in both WSs A and B.
After receiving the DATA+TAR(1) packet from WS A, the AP will respond with the ACK +RFD(1, 1) packet where the ACK packet is targeting for AC[1] of WS A and the RFD(1, 1) packet is for AC[1] of WS B. According to the received RFD(1, 1) message from the AP, AC[1] of WS B can deliver DATA+TAR(1) packet without the requirement for channel contention. At the end of the reservation period, the AP will change the entry order within
the reservation table TAC[1] in a round-robin manner, i.e., TAC[1] = {T0(B), T1(A)}. It is noted that similar reservation period will be implemented for AC[0] of both WSs A and B. Moreover, the proposed E-ARCR scheme can also be implemented in a more realistic network scenarios with the existence of packet collisions and channel noises, which can be extended from the descriptions as addressed in Subsection2.3.3for the ARCR protocol.
It is noticed that packet collision will not happen in the original ARCR scheme if all the WSs reside within the reservation table under error-free network environments. In the E-ARCR scheme, however, collisions may still exist even though all ACs in WSs are recorded within their corresponding reservation tables in the AP. The reason is contributed to the usage of more than one reservation table in the network. The first entries in those reser-vation tables will still contend with each other which results in the occurrence of packet collisions. This is considered the tradeoffs by adopting the E-ARCR protocol as the QoS requirement is specified to be fulfilled. The performance of the proposed E-ARCR scheme will be evaluated and compared in Section3.4.