11e MAC [3] for QoS provisioning. The IEEE 802.11e
defines a single coordination function, called the hybrid coordination function (HCF). The
HCF combines functions from the DCF and PCF with some enhanced QoS-specific
mechanisms and QoS data frames. Note that the 802.11e MAC is backward compatible
with the legacy MAC, and hence it is a superset of the legacy MAC. The HCF is composed
of two channel access mechanisms: (1) a contention-based channel access referred to as the
enhanced distributed channel access (EDCA), and (2) a controlled channel access referred
to as the HCF controlled channel access (HCCA). The HCF sits on top of the DCF in the
sense that the HCF utilizes and honors the CSMA/CA operation of the DCF. In a QoSenabled
IBSS (QIBSS), only the EDCA can be used since the HCCA requires an AP for
channel control. Since we are considering the IBSS operation here, we will limit ourselves
to the EDCA operation in the following sections.
4.4.1 IEEE 802.11e EDCA
The EDCA is designed to provide differentiated and distributed channel access for frames
with 8 different user priorities (UPs) (from 0 to 7) by enhancing the DCF. Each MSDU
from the higher layer arrives at the MAC along with a specific user priority value.
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