IEEE 802.2 is the IEEE 802 standard defining 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 standard adds this sublayer which adds the standard 8-bit DSAP (Destination Service Access Point) and SSAP (Source Service Access Point) labels to a given packet regardless of network type. There is also an 8 or 16 bit control field for use in auxiliary functions such as flow control. There is room for 64 globally assigned SAP numbers, and the IEEE does not assign them lightly. 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.
IEEE 802.2 provides two connectionless and one connection-oriented operational modes:
- Type 1 is an unacknowledged connectionless mode. It allows for sending frames
The use of multicasts and broadcasts reduce network traffic when the same information needs to be propagated to all stations of the network. However the Type 1 service provides no guarantees regarding the order of the received frames compared to the order in which they have been sent; the sender does not even get an acknowledgment that the frames have been received.
- Type 2 is a connection-oriented operational mode. Sequence numbering ensures that the frames received are guaranteed to be in the order they have been sent, and no frames are lost.
- Type 3 is an acknowledged connectionless service. It supports point-to-point communication only.
The 802.2 header includes two eight-bit address fields, called service access points or SAPs in OSI terminology; they are the destination SAP (DSAP), and the source SAP (SSAP). The low-order bit of the DSAP indicates whether it contains an individual or a group address. If the low-order bit is 0, the remaining 7 bits of the DSAP specify an individual address, which refers to a single local service access point (LSAP) to which the packet should be delivered. If the low-order bit is 1, the remaining 7 bits of the DSAP specify a group address, which refers to a group of LSAPs to which the packet should be delivered. The low-order bit of the SSAP indicates whether the packet is a command or response packet; if it's 0, the packet is a command packet, and if it's 1, the packet is a response packet. The remaining 7 bits of the SSAP specify the LSAP from which the packet was transmitted.
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