MAC Sublayer Functional Description

Distributed Coordination Function (DCF)   The fundamental access method of the IEEE 802.11 MAC is a DCF known as CSMA/CA. The DCF will be implemented in all STAs, for use within both IBSS and infrastructure network configurations.

For an STA to transmit, it will sense the medium to determine if another STA is transmitting. If the medium is not determined to be busy, the transmission may proceed. The CSMA/CA distributed algorithm mandates that a gap of a minimum specified duration exist between contiguous frame sequences. A transmitting STA will ensure that the medium is idle for this required duration before attempting to transmit. If the medium is determined to be busy, the STA will defer until the end of the current transmission. After deferral, or prior to attempting to transmit again immediately after a successful transmission, the STA will select a random backoff interval and will decrement the backoff interval counter while the medium is idle. A refinement of the method may be used under various circumstances to further minimize collisions — here the transmitting and receiving STA exchange short control frames (RTS and CTS frames ) after determining that the medium is idle and after any deferrals or backoffs prior to data transmission.

Point Coordination Function (PCF)   The IEEE 802.11 MAC may also incorporate an optional access method called a PCF, which is only usable on infrastructure network configurations. This access method uses a point coordinator (PC), which will operate at the AP of the BSS, to determine which STA currently has the right to transmit. The operation is essentially that of polling, with the PC performing the role of the polling master. The operation of the PCF may require additional coordination, not specified in the 802.11 standard, to permit efficient operation in cases where multiple point-coordinated BSSs are operating on the same channel, in overlapping physical space. The PCF uses a virtual carrier sense mechanism aided by an access priority mechanism. The PCF will distribute information within Beacon management frames to gain control of the medium by setting the NAV in STAs. In addition, all frame transmissions under the PCF may use an interframe space (IFS) that is smaller than the IFS for frames transmitted via the DCF. Using a smaller IFS implies that point-coordinated traffic will have priority access to the medium over STAs in overlapping BSSs operating under the DCF access method. The access priority provided by a PCF may be utilized to create a CF access method. The PC controls the frame transmissions of the STAs so as to eliminate contention for a limited period of time.

Coexistence of DCF and PCF  The DCF and the PCF will coexist in a manner that permits both to operate concurrently within the same BSS. When a PC is operating in a BSS, the two access methods alternate, with a CFCFP followed by a contention period (CP).

DSSS PHY Specification for the 2.4 GHz Band Designated for Industrial, Science, and Medical (ISM) Applications

Overview

The PHY for the DSSS system is described in this clause. The radio frequency (RF) LAN system is initially aimed for the 2.4 GHz band designated for ISM applications as provided in the United States according to FCC 15.247 and in Europe by ETS 300-328. The DSSS system provides a WLAN with both a 1 and 2 Mbps data payload communication capability. According to the FCC regulations, the DSSS system will provide a processing gain of at least 10 dB. This will be accomplished by chipping the baseband signal at 11 MHz with an 11-chip pseudonoise (PN) code. The DSSS system uses baseband modulations of differential binary phase-shift keying (DBPSK) and differential quadrature phase-shift keying (DQPSK) to provide the 1 and 2 Mbps data rates, respectively.

Scope   The PHY services provided to the IEEE 802.11 WLAN MAC by the 2.4 GHz DSSS system are described in this subsection. The DSSS PHY layer consists of two protocol functions:

• A physical layer convergence function, which adapts the capabilities of the physical medium dependent (PMD) system to the PHY service. This function will be supported by the PLCP, which defines a method of mapping the IEEE 802.11 MAC MPDUs into a framing format suitable for sending and receiving user data and management information between two or more STAs using the associated PMD system.

• A PMD system, whose function defines the characteristics of, and method of transmitting and receiving data through, a WM between two or more STAs each using the DSSS system.

DSSS PHY Functions   The 2.4 GHz DSSS PHY architecture is depicted in the reference model in Figure 5-9. The DSSS PHY contains three functional entities: the PMD function, the physical layer convergence function, and the layer management function. Each of these functions is described in the following section. The DSSS PHY service will be provided to the MAC through the PHY service primitives.

PLCP Sublayer   To allow the IEEE 802.11 MAC to operate with minimum dependence on the PMD sublayer, a physical layer convergence sublayer is defined. This function simplifies the PHY service interface to the IEEE 802.11 MAC services.

PMD Sublayer   The PMD sublayer provides a means to send and receive data between two or more STAs. This clause is concerned with the 2.4 GHz ISM bands using direct sequence modulation.

Physical Layer Management Entity (PLME)   The PLME performs management of the local PHY functions in conjunction with the MAC management entity. The models represented by figures and state diagrams are intended to be illustrations of functions provided. It is important to distinguish between a model and a real implementation. The models are optimized for simplicity and clarity of presentation; the actual method of implementation is left to the discretion of the IEEE 802.11 DSSS-PHY-compliant developer. The service of a layer or sublayer is a set of capabilities that it offers to a user in the next higher layer (or sublayer). Abstract services are specified here by describing the service primitives and parameters that characterize each service. This definition is independent of any particular implementation.