Logical Link Control (LLC) is an OSI protocol developed by the IEEE802working group and is the same for all LAN subsystems within the IEEE 802 standard. It is the control of data transmission on the upper sublayer of the link layer, which has been divided into the sublayers Logical Link Control and Medium Access Control( MAC) in the Ethernet layer model.
IEEE 802.2-LLC includes addressing of the end systems as well as error checking. According to its architectural embedding, the LLC specification consists of the subscriber interface, the LLC protocol specification and the MAC interface. The subscriber interface describes the services that the LLC layer can request from the MAC sublayer below it.
The four LLC service types
Logical Link Control recognizes four service types: Type 1 identifies a connectionless, unsecured datagram service. Type 2 denotes a connection-oriented service. Type 3 denotes a connectionless datagram service with confirmation, and Type 4 for point-to-point connections.
When LLC Type 1, Unacknowledged Connectionless Mode Service is used, a data unit passed from the network layer to LLC is simply placed on the transmission medium regardless of whether the receiving station is ready to receive or has accepted previous data packets. The LLC Type 1 service relies on the presence of appropriate higher-layersoftware that ensures completeness, error-freeness, sequence preservation of data packets, and the like.
In the LLC Type 2 service, the Connection Mode Service, part of these tasks can be performed in the LLC sublayer, since a logical connection is explicitly established and connection-related state can be maintained. The service separates the connection establishment, data transfer, connection termination phases.
With the Type 3 service, the Acknowledged Connectionless Mode Service, simple polling of other LAN stations can be achieved, as well as acknowledging transmissions across the LAN without the complexity of connection-oriented state.
LLC Type 4 is specifically designed for very high-speed point-to-point connections. The full duplex connection allows two completely independent connections.
The IEEE 802.2 standard includes for each service primitive the specifications concerning the general functions, the parameter list, the semantics, the allowed invocation time, the triggered effect at the receiver and other remarks if necessary. The LLC protocol is based on the bit-oriented HDLC protocol, which is used, for example, as a line procedure for X.25.
The frame structure of LLC
The structure of the control information of the Control field and the use of the LLC frame types, i.e. the layer 2b protocol elements, is essentially identical to that of HDLC, with the following exceptions: Logical Link Control uses only Asynchronous Balanced Mode( ABM), so does not assume unbalanced configurations, as assumed with Normal Response Mode( NRM); thus, any station can be a leader station (primary node). LLC supports a datagram service by using the Unnumbered Information Frame.
LLC allows multiplexing at Layer 2 by allowing multiple LLC service access points per station. This supports e.g. the connection ofterminal servers to the local area network (LAN). The address fields have the same length as in HDLC, but are coded differently. The first bit in the destination address field decides whether it is an individual or group address. The first bit in the source address field allows the distinction between commands and responses. LLC uses a 32 bit cyclic redundancy checksum. This also reduces the residual error probability for bits that are corrupted but not recognized as such in a transmission to the order of `2^-32`. LLC provides for flow control via a dynamic change of the window size in Addendum 1 to 802.2.