Data Link Layer Technologies

 
Data Link Layer TechnologiesThe Data Link layer is divided into two sub-layers — Logical Link Control(LLC) and Media Access Control (MAC). From the Data Link layer down,data packets are addressed using MAC addresses to identify the specificphysical devices that are the source and destination of packets, rather thanthe IP addresses, URLs or domain names used by the higher OSI layers.Logical Link ControlLogical Link Control (LLC) is the upper sub-layer of the Data Link layer(Figure 2-3), and is most commonly defined by the IEEE 802.2 standard.It provides an interface that enables the Network layer to work with any A frame produced by the LLC and passed down to the MAC layer iscalled an LLC Protocol Data Unit (LPDU), and the LLC layer managesthe transmission of LPDUs between the Link Layer Service Access Pointsof 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 ensurethat each LPDU is passed on receipt to the correct Network layerprotocol.The LLC layer defines connectionless (Type 1) and connection oriented(Type 2) communication services and, in the latter case, the receivingLLC layer keeps track of the sequence of received LPDUs. If an LPDU islost in transit or incorrectly received, the destination LLC requests thesource to restart the transmission at the last received LPDU.The LLC passes LPDUs down to the MAC layer at a logical connectionpoint known as the MAC Service Access Point (MAC SAP). The LPDUis then called a MAC Service Data Unit (MSDU) and becomes the datapayload for the MAC layer.Media Access ControlThe second sub-layer of the Data Link layer controls how and when adevice is allowed to access the PHY layer to transmit data, this is theMedia Access Control or MAC layer.In the following sections, the addressing of data packets at the MAC levelis first described. This is followed by a brief look at MAC methods applied in wired networks, which provides an introduction to the morecomplex solutions required for media access control in wireless networks.MAC AddressingA receiving device needs to be able to identify those data packetstransmitted on the network medium that are intended for it — this isachieved using MAC addresses. Every network adapter, whether it is anadapter for Ethernet, wireless or some other network technology, is assigneda unique serial number called its MAC address when it is manufactured.The Ethernet address is the most common form of MAC address andconsists of six bytes, usually displayed in hexadecimal, such as 00-D0-59-FE-CD-38. The first three bytes are the manufacturer’s code (00-D0-59 inthis case is Intel) and the remaining three are the unique serial number ofthe adapter. The MAC address of a network adapter on a Windows PC canbe found in Windows 95, 98 or Me by clicking Start, Run, and thentyping “winipcfg”, and selecting the adapter, or in Windows NT, 2000,and XP by opening a DOS Window (click Start, Programs, Accessories,Command Prompt) and typing “ipconfig/all”.When an application such as a web browser sends a request for data ontothe network, the Application layer request comes down to the MAC SAPas an MSDU. The MSDU is extended with a MAC header that includesthe MAC address of the source device’s network adapter. When therequested data is transmitted back onto the network, the original sourceaddress becomes the new destination address and the network adapter ofthe original requesting device will detect packets with its MAC address inthe header, completing the round trip.As an example, the overall structure of the IEEE 802.11 MAC frame, orMAC Protocol Data Unit (MPDU) is shown in Figure 2-5.The elements of the MPDU are as shown in Table 2-6.