The IEEE 802 standards define two separate layers for the Data Link layer (Layer 2) of the OSI Reference Model. These two layers are the Logical Link Control (LLC) and the Media Access Control (MAC) sublayers. The IEEE 802.11 standards cover the operation of the MAC sublayer and the Physical layer of the OSI Model. This is illustrated below in Figure 9-4:
Fig. 9-4. IEEE 802.11 and the OSI Reference Model
The 802.11 frame consists of a 32-byte MAC header, a variable length body (0 to 2312 bytes), and 4-byte Frame Check Sequence (FCS). Although going into detail on the contents of the MAC frame format is beyond the scope of the SWITCH exam requirements, it is important to have a basic understanding of the fields contained in the MAC header, which include the following:
- The Frame Control field (2 bytes in length)
- The Duration / ID field (2 bytes in length)
- The Sequence Control field (2 bytes in length)
- The Four Address fields (6 bytes each)
- The Quality of Service field (2 bytes in length)
The Frame Control field contains control information that is used to define the type of 802.11 MAC frame. This field also provides information that is necessary for the following fields to understand how to process the frame. This information includes, but is not limited to, the frame type, whether the frame is fragmented, whether encryption and authentication are being used, or even whether the sending STA is in active mode or power-saving mode. The different 802.11 frames will be described in detail later in this chapter.
The Duration / ID field is used to indicate the remaining duration needed to receive the next frame transmission. Other STAs must look at the duration value contained in this field and wait that length of time before considering their own transmissions. The information that is contained is essentially used to avoid collisions in the wireless network. This will be described in detail later in this chapter.
The Sequence Control field contains the sequence number of each frame, as well as the number of each fragmented frame sent. This information is stored in the Sequence Number and Fragment Number subfields of the Sequence Control field.
802.11 uses 48-bit addresses, similar to the 802.3 standard. The MAC frame can include up to four addresses. The addresses used will depend on the frame type and include the following:
- The BSS Identifier (BSSID)
- The Destination Address (DA)
- The Source Address (SA)
- The Receiver Address (RA)
- The Transmitter Address (TA)
The BSS Identifier (BSSID) uniquely identifies each BSS. The BSSID is simply the MAC address of the AP. The Destination Address (DA) field contains the MAC address of the final destination for the frame. The Source Address (SA) field contains the source MAC address of the original source that initially originated and transmitted the frame. The Receiver Address (RA) field contains the MAC address of the next immediate STA on the wireless medium to receive the frame. Finally, the Transmitter Address (TA) field contains the MAC address of the STA that transmitted the frame onto the wireless medium.
The Quality of Service (QoS) field is based on the IEEE 802.11e amendment to the original 802.11 standard. The 802.11e amendment has since been incorporated into the published 802.11-2007 standard. While going into detail on WLAN QoS is beyond the scope of the SWITCH exam requirements, it is important to remember that Cisco Unified Wireless products support Wi-Fi MultiMedia (WMM), which is a QoS system based on IEEE 802.11e that has been published by the Wi-Fi Alliance.