Logical link control

Logical Link Control (LLC) is the upper portion of the data link layer of a local area network, as defined in IEEE 802.2. 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.

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 IP packet regardless of network type. There is also an 8 or 16 bit control field for use in auxiliary functions such as flow control.

Function of LLC

The SAP labels identify the protocol stack that is sending the encapsulated data, and the protocol stack which the encapsulated data is for.

The LLC also encapsulates the packet in its own PDU (protocol data unit). The MAC layer adds further data and encapsulates it in its own way. The frame is the end PDU of the data link layer, which is the second layer of the OSI model.

This may help illuminate its purpose. As part of the encapsulation chain the the LLC is used to enable layer 2 to talk to the higher layers in a standardized way, regardless of which lower-level technology is used. (Examples of lower-level technologies are Token Ring, Ethernet, FDDI or ATM).

The LLC is nowadays usually used together with the more versatile Subnetwork Attachment Point scheme to create so-called LLC/SNAP encapsulation. In this case, both the SSAP and DSAP values are set to AA indicating the use of SNAP, and the control field is set to the 8-bit value 3, specifying a U-format PDU.

LLC header control words and frame formats

To confuse matters further, there can be three kinds of LLC PDU, in so-called U, I or S frames.

U frames, with an 8-bit control field, are intended for connectionless applications
I frames, with a 16-bit control and sequence numbering field, are intended to be used in connection-oriented applications
S frames, with a 16-bit control field, are intended to be used for supervisory functions at the LLC layer.

The format of a PDU frame is identified by the lower two bits of the first byte of the control field. Of these three formats only the U-format is commonly used. LLC was conceptually derived from HDLC, which explains these aspects of its design.

In general use only the U-format frame is used.

Note: This article (or an earlier version of it) is derived from material from FOLDOC, used with permission.