IEEE 802.2

IEEE 802.2 is the name given to a subsection of the IEEE 802 standard that describes a software component of a computer network. It defines Logical Link Control (LLC), which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer. Beneath the LLC sublayer is the Media Access Control (MAC) sublayer, which is dependent on the particular medium being used (Ethernet, token ring, FDDI, 802.11, etc.).

The IEEE 802.2 sublayer adds some control information to the message created by the upper layer (typically, the network layer) and passed to the LLC for transmission to the next hop node. The resulting packet is generally referred to as LLC Protocol Data Unit (PDU) and the additional field added by the LLC sublayer for the so-called LLC HEADER. The LLC Header consist of three fields, namely DSAP (Destination Service Access Point) and SSAP (Source Service Access Point), Control. S-SAP is an 8-bit long field that represents the logical address of the network layer entity that has created the message. D-SAP is an 8-bit long field that represents the logical addresses of the network layer entity intended to receive the message. Control field may be either 8 or 16 bit long and, besides defining the packet format, its used to carry some further control information for auxiliary services, such as flow control. Despite the SAP fields are 8-bit long, some bits have specific significance, so that there is room for only 64 distinguished SAP numbers, which are globally assigned by the IEEE to uniquely identify well established international standards. IP does not have an assigned SAP number, because only “international standards” could be given globally assigned SAP numbers. Protocols which are not international standards can use a SAP number from the locally administered SAP number space. The Subnetwork Access Protocol (SNAP) allows EtherType values to be used to specify the protocol being transported atop IEEE 802.2, and also allows vendors to define their own protocol value spaces.