Logical link control
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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.
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Function of LLC
The SAP labels identify the protocol stack that is sending the encapsulated data, and the protocol stack which the encapsulated data are for.
The LLC also encapsulates the packet in its own protocol data unit (PDU). 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 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 such lower-level technologies are Token Ring, Ethernet, FDDI or ATM).
The LLC is nowadays usually used together with the more versatile Subnetwork Attachment Point (SNAP) scheme to create so-called LLC/SNAP encapsulation. In this case, both the SSAP and DSAP values are set to hexadecimal AA indicating the use of SNAP, and the control field is set to the 8-bit value 3, specifying a U-format PDU.
Operational modes
LLC provides two connectionless and one connection-oriented operational modes:
- Type 1 is an unacknowledged connectionless mode. It allows for sending frames
- to a single destination (point-to-point or unicast transfer),
- to multiple destinations on the same network (multicast),
- or to all stations of the network (broadcast).
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 3 is an acknowledged connectionless service. It supports point-to-point communication only.
- 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. A sliding window flow control procedure prevents fast senders from flooding slow destinations.
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.
Of these three formats only the U-format is commonly used. The format of a PDU frame is identified by the lower two bits of the first byte of the control field. LLC was conceptually derived from HDLC, which explains these aspects of its design.
See also
References
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.