6.2.1.4CCHC Frame Structure

After the beacon, four TXOPs are related to four resource allocations in Figure 6.4. The first TXOP is a resource allocation by the CCHC during the optional CFP. The CCHC uses a Controlled Access Phase (CAP) to allocate the channel, as part of the controlled channel access of the HCF. Because the CAP is allocated by the CCHC and not through contention-based channel access by any other 802.11 station, the maximum duration of this resource allocation is not restricted by the TXOPlimit. The only limit is that the resource allocation has to be finished before the end of the CFP. However, the length of CFP is under control of the CCHC. This first resource allocation (in the figure indicated as CAP) is used to operate HiperLAN/2 by means of two MAC frames. As explained before, in 802.11e the CFP is not mandatory and not needed for QoS support by the HCF.

Another three resource allocations, which fall in the CP after the CF-end frame, are shown in Figure 6.4. The second and the third resource allocations (in the figure indicated as TXOPs) are allocated by stations through contention-based channel access, and therefore their durations must not exceed the TXOPlimit. Based on what is defined for the HCF, the CCHC may use various techniques to allocate resources during the CP. One way is illustrated for the fourth resource allocation (indicated as CAP in Figure 6.4). There, the CCHC uses its high priority in channel access as part of the controlled channel access of the HCF and transmits a QoS CF-Poll frame addressed to itself in advance right after the end of the preceding TXOP.

time

Figure 6.4: The structure of the CCHC superframe. One superframe between two TBTTs is illustrated. HiperLAN/2 MAC frames are scheduled within periodically repeated TXOPs.