The data link layer (layer 2) of the OSI model, specified by IEEE standard 802.2 is divided into two sub-layers; the Media Access Control (MAC) and the Logical Link Control (LLC) sub-layers. LLC sub-layer is bounded above by the network layer (OSI layer 3) and below by the MAC sub-layer. It thus provides services to both of these layers.
LLC frames data into Protocol Data Units (PDUs) by adding a header that contains the control information used for error control and flow control. The header also contain the Source Service Access Point (SSAP) and Destination Service Access Point (DSAP) fields that define the network layer protocols at the source and destination that is making use of LLC. When a host receives a frame, it checks the LLC header to determine what protocol the packet is to be delivered to at the network layer.
The other functions of the LLC include error detection but not correction and flow control. LLC provides flow and error control for the upper layer protocols to facilitate the exchange of data between the sending host network layer and a remote receiving host. In performing this flow control service, LLC sub-layer employs three modes of connection establishment as expounded below. The first mode which is the unacknowledged connectionless-mode services facilitates the exchange of data (Link Service Data Units, LSDUs) between two hosts without the need to establish a data link connection. This mode of connection can be employed in point-to-point connections, broadcast or multicast connections. The second mode of connection is the connection-mode services where point-to-point link is established before data is exchanged between the two hosts. This mode thus provides services to control the connection including establishment, reset and termination of a link. The control of data flow for a specific connection highlights its significance in multiple connections between the network layer and data link layer. The third connection mode is the acknowledged connectionless-mode service. It differs from the unacknowledged mode of connection in its use of acknowledgements for the LSDUs that are exchanged and the connection is point-to-point.
In summary, the services provided by the LLC sub-layer of the data link layer of the OSI reference model are framing of PDUs, error detection and not correction, sub-layer addressing and flow control.
REFERENCES
Forouzan, B.A. (2007). Data Communications and Networking (4th ed.). New York, NY: McGraw Hill.
LAN/MAN Standards Committee of the IEEE Computer Society (2008). ANSI/IEEE Std 802.2 Part 2: Logical Link Control. New York, NY: The Institute of Electrical and Electronics Engineers, Inc.
Stallings, W. (2009). Business Data Communications (6th ed.). Upper Saddle River, NJ: Prentice Hall.Tanenbaum, A.S., & Wetherall, D.J. (2011). Computer Networks (5th ed.) Boston, MA: Pearson Education, Inc.
IEEE 802.2. Retrieved from https://www.princeton.edu/~achaney/tmve/wiki100k/docs/IEEE_802.2.html
