IEEE 802.11 Physical Layer
- The physical layer for IEEE 802.11 has been issued in four stages.
- The first part, simply called IEEE 802.11, includes the MAC layer and three physical layer specifications, two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1 and 2 Mbps. IEEE 802.11a operates in the 5-GHz band at data rates up to 54 Mbps.
- Moreover, IEEE 802.11b operates in the 2.4-GHz band at 5.5 and 11 Mbps.
- IEEE 802.11g also operates in the 2.4-GHz band, at data rates up to 54 Mbps.
The figure shows the relationship among the various standards developed for the physical layer.
- Direct sequence spread spectrum (DSSS) operating in the 2.4-GHz ISM band, at data rates of 1 Mbps and 2 Mbps.
- In the United States, the FCC (Federal Communications Commission) requires no licensing for the use of this band.
- The number of channels available depends on the bandwidth allocated by the various national regulatory agencies.
- This ranges from 13 in most European countries to just one available channel in Japan.
- Frequency-hopping spread spectrum (FHSS) operating in the 2.4-GHz ISM band, at data rates of 1 Mbps and 2 Mbps.
- The number of channels available ranges from 23 in Japan to 70 in the United States.
- Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950nm.
IEEE 802.11 Services
The service provider can be either the station or the distribution system (DS).
- Station services are implemented in every 802.11 stations, including access point (AP) stations.
- Distribution services are provided between basic services sets (BSSs); these services may be implemented in an AP or in another special purpose device attached to the distribution system.
Three of the services are used to control IEEE 802.11 LAN access and confidentiality.
- Six of the services are used to support delivery of MAC service data units (MSDUs) between stations.
- The MSDU is the block of data passed down from the MAC user to the MAC layer; typically this is an LLC PDU If the MSDU is too large to be transmitted in a single MAC frame, it may be fragmented and transmitted in a series of MAC frames.