ETSI and IEEE 802.11 group cooperated to ensure that the PHY layers of 5 GHz WLAN standards are harmonized. IEEE 802.11a and HIPERLAN/2 differ mainly at the MAC layer.
Medium Access Control
IEEE 802.11a uses a distributed MAC protocol that doesn't require any centralized control. It uses the CSMA/CA protocol. The use of distributed MAC makes it more suitable for ad hoc networking and non-real time applications.HIPERLAN/2 medium access is based on TDMA/TDD using a MAC frame with a period of 2 ms. The frame consists of uplink, downlink and direct link (DiL, directly between two station) phases. The phases are scheduled centrally by the AP, which informs the mobile stations at which point in time in the MAC frame they are allowed to transmit their data. Time slots are allocated dynamically depending on the need for transmission resources. HIPERLAN/2 MAC is designed to provide QoS support, suitable for many multimedia and real-time application.
HIPERLAN/2 uses fixed length packet while IEEE 802.11a uses variable length packets.
IEEE 802.11a MAC
IEEE 802.11a provides two types of service:- Asynchronous — implements the CSMA/CA MAC protocol with binary exponential backoff, known as DCF. DCF defines a basic access method and an optional four-way handshaking technique, known as the RTS/CTS method.
- Contention-free — this is provided by the point coordination function (PCF) that supports time-bounded services. PCF is optional.
