MAC Protocol Data Unit of 802.11p

The structure of a MAC protocol Data Unit (MPDU) is quite a delicate and complex part of 802.11 standards, based on the fact that a MAC layer carries out most functions in WLAN communication, which generate a MPDU by wrapping MSDU received from LLC and then handing to the PLCP sublayer in PHY, as mentioned previously.

In fact, there are four types of frames in 802.11, namely management, control, data and reserved; however, an overall MPDU format is generally given regardless of frame type, since the structure remains the same across all frames, even though not all parts are used in every specific type. Meanwhile, the MPDU format can be further divided into two kinds - QoS supportive and non-supportive - which implies that the format of 802.11p will be the same as 802.11e, with an additional QoS field than normal 802.11 a/b/g standards. Figure 4- 8 illustrates the overall MPDU format.

Figure 4- 8 Overall MPDU Format of 802.11p/e

divided into several sub fields shown in Figure 4-9.

Figure 4- 9 Sub-fields of Frame Control

Currently, the protocol version is 00 and the remaining possibilities are reserved for future use. Frame types are used to indicate four types of frames, as mentioned above. Table 4-4 details the type field instance.

Table 4-4 Type Field Instance

Bits Frame Type Example

0,0 Management Association, Authentication, Probe

0,1 Control ACK, CTS, RTS

1,0 Data

1,1 Reserved

Furthermore, the sub type field indicates the specific type of frame, for instance, 0000, 1010 and 0001, mean association request, Power Save (PS)-Poll and Data +CF-ACK, respectively. As we know, fragmentation can be used in WLAN communication to support the division of MSDUs into smaller elements carried out by More Frag Field set to 1, if there is a fragment subsequently belonging to the same MSDU. However, it is necessary to point out that only unicast frames will be fragmented rather than broadcast or multicast frames. In this case, the MAC protocol will use the aFragmentationThreshold parameter in its management information base (MIB) to judge whether the size of generated MPDU will exceed the threshold by merely passing the whole MSDU to the MAC layer. If the answer is true, then

fragmentation will be conducted in order to break down MSDU and send each one separately. 1. Duration/ID and Address Fields: The former field lasting 16 bits long contains

information of NAV or short ID alternately. However, since only the power-save poll (PS-Poll) uses a short ID tightly related to the PCF mechanism, this field is generally representing duration for 802.11p application, under which Bit 15 is set to 0 and maximum value 32767 in microseconds can be obtained by manipulating Bit from 0-

14. The four MAC address fields in MPDU are selectively used according to different types of frames. In most cases, only addresses 1-3 contribute wireless communication and address-4 is only for special situations, such as DS, which is also the reason why it does not adjoin with the other three counterparts in MPDU. In short, Addresses-1/2 are always the recipient/ destination addresses or the under ToDS/ FromDS addresses of AP, respectively. Address-3 is generally the missing address, since under BSS, merely indicating an address of AP is not enough and an extra field needs to be used for the source or destination address. However, we can predict that in case there are two APs in DS, the communication has to account for the full usage of four address fields. Beyond that, a higher layer address mechanism, such as IP, will be needed. Under the 802.11p scenario, only addresses 1 and 2 join the procedure of communication in most cases.

2. Frame Body and QoS Control Fields: The frame body often referred to MSDU is passed from higher layer LLC. In fact, it is a bit difficult to find the maximum size of MSDU in 802.11 standards, due to the evolution and advancement. This can be explained twofold. Firstly, the 802.11-2007 standard defines that the maximum size for MSDU is 2304 bytes for non-security mechanisms applied or 2312 (WEP)/2324 (TKIP) [45] as overheads are increased. Thus, the derived max length has three possibilities, namely 2338 (non-security), 2346 (WEP), and 2358 (TKIP). Under the QoS supported Frame structure, another 2 bytes need to be added on the above results, thus making the theoretical maximum length 2360. On the other side, PSDU allows a 4095 maximum in length [46], although MPDU can only reach half of the size. It is not

latest 802.11n brought up two novel schemes - aggregate MSDU and MPDU (A-

MSDU, A-MPDU) – which extended the maximum sizes to 3839/7935 and 65,535 octets, respectively [47]. Based on what is stated above, we can conclude that the max MSDU for 802.11p should be less than 2304, due to no authentication mechanism required. Concerning the QoS control field, which is 16 bits long and which bit 7 is reserved, the first 3 bits are used to indicate TID and bit 4 is for End of service period (EOSP) purposes, through setting 1 for transmission/ retransmissions, or 0 otherwise. Bits 5-6 define the acknowledgement policy under different communication procedures. Finally, multi-functions are carried out by Bits 8-15, in general, TXOP limit, QAP PS Buffer State, TXOP duration requested and Queue size.

4.4 Summary

In this chapter, we introduced the mechanisms of MAC protocols for IEEE 802.11 standards. Meanwhile, 802.11p adopts EDCA as a more advanced one revolving from DCF, which is not only for collision avoidance but also for concentrating on providing QoS. Its traffic stream is granted different EDCAF according to its TID out of four kinds or channel types (CCH or SCH) in order to provide a priority-oriented service. Also, the structure of MPDU of 802.11p has been reviewed.