EDCA for 802.11p

As IEEE 802.11p is designed exclusively for a Vehicle Environment, it adopts EDCA, while maintaining some modifications. Multi-channel operation is the main difference of 802.11p at a MAC layer from its counterparts, where all channels (CCH and SCH) are provided a set of traffic categories for QoS guarantee purposes. Figure 4-6 provides an illustration of an EDCA mechanism at 802.11p MAC layer.

Figure 4-6 EDCA at 802.11p MAC [43]

There are still four ACs labeled from AC[0] to AC[3]. Under the CCH interval, each one can be possibly explained, as follows:

AC[3]: with the highest priority, it is used for emergency information, especially for collision avoidance application. Other instances can be obstacles, e.g. wet road surface, traffic light break down.

AC[2]: concerns presence and speed information broadcasted by vehicles.

AC[1]: concerns the information from other vehicles for help, but is not strictly related to safety issues.

AC[0]: concerns information aimed at establishing new non-safety-related connections via SCHs.

However, we need to notice that the 802.11p standard was just finalised and that its application is still being processed. The definition of each AC, either on CCH or SCH, will largely rely on upper layers, especially the application layer designed by different manufacturers and thus may be various.

Meanwhile, as 802.11p is used for time-sensitive application for safety purposes mainly and because CCH and SCH are all allowed to perform real-time traffic, it is obviously beneficial

if we assign more time to CCH. However, for the sake of overall satisfaction, the tradeoff of time allocation of the QoS guarantee mechanism has to be made in order to cope with large and regular data traffic on SCHs, whilst meanwhile keeping CCH delay at an acceptable range. According to what is mentioned above, each AC represents a specific EDCAF at the MAC layer, which is further defined by corresponding parameters. Apparently, these EDCA parameters, even with the same AC, will vary in different channel types. There are two adaptive forms at IEEE 1609.4 for CCH and SCH, respectively, as shown in Table 4-2 and Table 4-3.

Table 4-2 EDCA parameter for CCH [44]

AC Traffic Type CWmin CWmax AIFSN

AC1 background 15 511 9

AC0 best effort 7 15 6

AC2 video 3 7 3

AC3 voice 3 7 2

We can see from the above table that the AC1 is actually with the lowest priority not only with the biggest CWmin and CWmax within the whole table but also the highest AIFSN, which indicates its AIFS and backoff window will be the longest one. Meanwhile, AC0 is at the middle priority among its counterparts, whose CWmin is just the CWmax of AC2 and AC3 but AIFSN is doubly comparing with AC2. Concerning AC2 and AC3, with the same contention windows, their AIFSN has only 1 slight difference ensuring AC3 is the top priority.

Table 4-3 EDCA parameter for SCH [44]

AC Traffic Type CWmin CWmax AIFSN

AC1 background 15 511 7

AC0 best effort 15 511 3

In contrast to Table 4-2, we can see that the EDCA parameter for SCH is more loosened than what is applied on CCH. However, the AC3 for SCH at the highest priority is exactly the same as its counterpart of CCH, maybe because SCH sometimes involves safety-oriented application. There is a tradeoff between AIFSN and the contention window made in SCH. As the AIFSN in SCH are all lower than their corresponding ones in CCH, it indicates that all AIFS in SCH will be shorter than CCH, except AC0. Such a mechanism makes nodes get ready to send frames by quickly entering into the backoff stage. By shortening the AIFS, the collision probability might increase but it will be accepted as the deployment happens in SCH, rather than strictly related to time-critical transmission. At the same time, the contention windows, both min and max for SCH ranging from AC1 to AC2, are all greater than CCH, which compensates for the shortcoming, due to the related lower AIFSN. For instance, The CWmax value of AC0 in SCH proving the best effort traffic type is set at 511, which means that EDCAF here is willing to ensure transmission reliability by doing retransmission and waiting longer, in order to have ACK. To be more specific, Figure 4-7 illustrates the AIFS plus minimum backoff time for each AC in both channels.

Since the channel environment for transmission is unpredictable, only minimum backoff time can be given, since it is irreverent to the transmitting situation. The time parameter on the chart matches what was mentioned previously. The time intervals of AIFS and backoff time for the highest priority in both channels are the same. Followed by the AC2 and AC1, their AIFS values in CCH are longer than SCH but backoff times are in a contrary situation, which makes the total time interval shorter than SCH. The last AC0 turned out to be a bit different, due to the same minimum backoff time in these two channels.