twork layer to work with any type of Media Access Control layer. Logical Link Control layer (LLC) Layer 2 Data Link layer Medium Access Control layer (MAC) Layer 1 Physical layer (PHY) Physical layer OSI model layers IEEE 802 specifications Figure 2-3: OSI Layers and IEEE 802 Specifications Wireless Network Logical Architecture OSI Network layer LLC SAP Logical Link Control layer (LLC) MAC SAP Medium Access Control layer (MAC) Figure 2-4: Logical Location of LLC and MAC Service Access Points A frame produced by the LLC and passed down to the MAC layer is called an LLC Protocol Data Unit (LPDU), and the LLC layer manages the transmission of LPDUs between the Link Layer Service Access Points of the source and destination devices. A Link Layer Service Access Point (SAP) is a port or logical connection point to a Network layer protocol (Figure 2-4). In a network supporting multiple Network layer protocols, each will have specific Source SAP (SSAP) and Destination SAP (DSAP) ports. The LPDU includes the 8-bit DSAP and SSAP addresses to ensure that each LPDU is passed on receipt to the correct Network layer protocol. The LLC layer defines connectionless (Type 1) and connection oriented (Type 2) communication services and, in the latter case, the receiving LLC layer keeps track of the sequence of received LPDUs. If an LPDU is lost in transit or incorrectly received, the destination LLC requests the source to restart the transmission at the last received LPDU. The LLC passes LPDUs down to the MAC layer at a logical connection point known as the MAC Service Access Point (MAC SAP). The LPDU is then called a MAC Service Data Unit (MSDU) and becomes the data payload for the MAC layer. Media Access Control The second sub-layer of the Data Link layer controls how and when a device is allowed to access the PHY layer to transmit data, this is the Media Access Control or MAC layer. In the following sections, the addressing of data packets at the MAC level is first described. This is followed by a brief look at MAC methods Chapter