IEEE 802.2

IEEE 802.2 is the IEEE standard defining Logical Link Control (LLC), which is the upper portion of the data link layer for local area networks. 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.

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 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.
• Type 3 is an acknowledged connectionless service. It supports point-to-point communication only.

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.

External link

• IEEE 802.2 online

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.