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.
DCF requires that a mobile station senses the medium for a specific time interval. If the medium is idle, it can start transmitting. Otherwise, the transmission is deferred and a backoff process begins.. After the backoff time expires, the mobile station can access the medium again. A positive acknowledgement is used to notify that a frame has been successfully received. If an acknowledgement is not received, the station retransmits the packet.
The transmission cycle consists of distributed interframe space (DIFS), backoff, data packet transmission, short interface space (SIFS) and ACK. A PHY layer convergence procedure (PLCP) maps a MAC PDU into a frame format. A complete packet contains the preamble, header, PHY layer service data unit (PSDU or payload):
The header is always transmitted using the lowest-rate transmission mode to ensure robust reception. It is mapped onto a BPSK modulated OFDM symbol. The header contains information about:
- The rate field specifies the type of modulation and the coding rate used in the rest of the packet.
- The length field takes a value between 1 and 4095, and specifies the number of bytes in the PSDU.
- The parity bit is a positive parity for the first 17 bits of the header.
- The 6 tail bits are used to reset the convolutional encoder and terminate the code trellis in the decoder.
- The first 7 bits of the service field are set to 0 and used to initialize the descrambler. The remaining 9 bits are reserved for future use.
- The pad bits are used to ensure that the number of bits in the PPDU maps to an integer number of OFDM symbols
HIPERLAN/2 MAC
The MAC frame structure consists of time slots for broadcast control (BCH), frame control (FCH), access feedback control (ACH), random access channel (RCH)and data transmission downlink (DL), uplink (UL) and direct link (DiL) phases. A mobile station has to request capacity from the AP before sending data, which is performed in RCH, where contention for the same time slot is allowed.
There are two types of PDU for the DL, UL and DiL phases: long and short. The long PDU is 54-byte long and contains control or user data. The payload is 48-byte long and the remaining bytes are used for the PDU type, a sequence number (SN) and CRC-24. Long PDUs are referred to as the long transport channel (LCH). Short PDUs contain only control data and is 9-byte long. It contains resource request, ARQ messages etc, and are referred to as the short transport channel (SCH).
Traffic from multiple connections to/from one mobile station can be multiplexed onto one PDU train that contains long and short PDUs. A physical burst is composed of the PDU train payload preceded by a preamble and is the unit to be transmitted by the PHY layer.
The Physical Layer
The PHY layer of both standards are similar and uses OFDM. The PHY layer modes with different coding and modulation schemes are selected by a link adaptation scheme.
Source:
A. Doufexi, S. Armour, M. Butler, A. Nix, D. Bull, J. McGeehan, "A Comparison of the HIPERLAN/2 and IEEE 802.11a Wireless LAN Standards", IEEE Communications Magazine, Volume 40, Issue 5, May 2002
No comments:
Post a Comment