IEEE 802.11 Logical Architecture
A topology provides a means of explaining necessary physical components of a network, but the logical architecture defines the network's operation. As Figure 3.8 illustrates, the logical architecture of the 802.11 standard that applies to each station consists of a single MAC and one of multiple PHYs.
Figure 3.8 A single 802.11 MAC layer supports three separate PHYs: frequency hopping spread spectrum, direct sequence spread spectrum, and infrared light.
IEEE 802.11 MAC Layer
The goal of the MAC layer is to provide access control functions (such as addressing, access coordination, frame check sequence generation and checking, and LLC PDU delimiting) for shared-medium PHYs in support of the LLC layer. The MAC layer performs the addressing and recognition of frames in support of the LLC. The 802.11 standard uses CSMA/CA (carrier sense multiple access with collision avoidance), and standard ethernet uses CSMA/CD (carrier sense multiple access with collision detection). It is not possible to both transmit and receive on the same channel using radio transceivers; therefore, an 802.11 wireless LAN takes measures only to avoid collisions, not detect them.
IEEE 802.11 Physical Layers
The 802.11 standard specifies several Physical layers. The initial standard approved in 1997 included frequency hopping and direct sequence spread spectrum, delivering data rates of 1 and 2Mbps in the 2.4GHz band. This initial release also defined an infrared Physical layer operating at 1 and 2Mbps via passive ceiling reflection. The current 802.11 standard, released in December 1999, added an 11Mbps, high-rate version direct sequence standard commonly referred to as IEEE 802.11b. In addition, the current standard defines a Physical layer using OFDM (orthogonal frequency division multiplexing) to deliver data rates of up to 54Mbps in the 5GHz frequency band. Refer to Chapter 5, "IEEE 802.11 Physical (PHY) Layer," for more details on these standards.
As with the IEEE 802.3 standard, the 802.11 working group is considering additional PHYs as applicable technologies become available.