802.11 Physical Layer Technologies

The ratification of the 1999 802.11a and 802.11b standards transformed wireless LAN (WLAN) technology from a niche solution for the likes of barcode scanners to a generalized solution for portable, low-priced, interoperable network access. Today, many vendors offer 802.11a and 802.11b clients and access points that provide performance comparable to wired Ethernet. The lack of a wired network connection gives users the freedom to be mobile as they use their devices. Although standardization has been key, the use of unlicensed frequencies, where a costly and time-consuming licensing process is not required, has also contributed to a rapid and pervasive spread of the technology.

802.11 as a standards body actually defined a number of different physical layer (PHY) technologies to be used with the 802.11 MAC. This chapter examines each of these 802.11 PHYs, including the following:

• The 802.11 2.4 GHz frequency hopping PHY

• The 802.11 2.4 GHz direct sequencing PHY

• The 802.11b 2.4 GHz direct sequencing PHY

• The 802.11a 5 GHz Orthogonal Frequency Division Multiplexing (OFDM) PHY

• The 802.11g 2.4 GHz extended rate physical (ERP) layer

802.3 Ethernet has evolved over the years to include 802.3u Fast Ethernet and 802.3z/802.3ab Gigabit Ethernet. In much the same way, 802.11 wireless Ethernet is evolving with 802.11b high-rate direct sequence spread spectrum (HR-DSSS) and 802.11a OFDM standards and the recent addition of the 802.11g ERP. In fact, the physical layer for each 802.11 type is the main differentiator between them.


Wireless Physical Layer Concepts

The 802.11 PHYs essentially provide wireless transmission mechanisms for the MAC, in addition to supporting secondary functions such as assessing the state of the wireless medium and reporting it to the MAC. By providing these transmission mechanisms independently of the MAC, 802.11 has developed advances in both the MAC and the PHY, as long as the interface is maintained. This independence between the MAC and PHY is what has enabled the addition of the higher data rate 802.11b, 802.11a, and 802.11g PHYs. In fact, the MAC layer for each of the 802.11 PHYs is the same.

Each of the 802.11 physical layers has two sublayers:

• Physical Layer Convergence Procedure (PLCP)

• Physical Medium Dependant (PMD)

Figure 3-1 shows how the sublayers are oriented with respect to each other and the upper layers.

The PLCP is essentially a handshaking layer that enables MAC protocol data units (MPDUs) to be transferred between MAC stations over the PMD, which is the method of transmitting and receiving data through the wireless medium. In a sense, you can think of the PMD as a wireless transmission service function that is interfaced via the PLCP. The PLCP and PMD sublayers vary based on 802.11 types.

All PLCPs, regardless of 802.11 PHY type, have data primitives that provide the interface for the transfer of data octets between the MAC and the PMD. In addition, they provide primitives that enable the MAC to tell the PHY when to commence transmission and the PHY to tell the MAC when it has completed its transmission. On the receive side, PLCP primitives from the PHY to the MAC indicate when it has started to receive a transmission from another station and when that transmission is complete. To support the clear channel assessment (CCA) function, all PLCPs provide a mechanism for the MAC to reset the PHY CCA engine and for the PHY to report the current status of the wireless medium.

In general, the 802.11 PLCPs operate according to the state diagram in Figure 3-2. Their basic operating state is the carrier sense/clear channel assessment (CS/CCA) procedure. This procedure detects the start of a signal from a different station and determines whether the channel is clear for transmitting. Upon receiving a Tx Start request, it transitions to the Transmit state by switching the PMD from receive to transmit and sends the PLCP protocol
data unit (PPDU). Then, it issues a Tx End and returns to the CA/CCA state. The PLCP invokes the Receive state when the CS/CCA procedure detects the PLCP preamble and valid PLCP header. If the PLCP detects an error, it indicates the error to the MAC and proceeds to the CS/CCA procedure.