IEEE 802.2 Logical Link Control

IEEE 802.2 Logical Link Control frames often provide the data link layer support for implementations of NetBIOS. This is the case when NetBIOS frames are being carried "on the wire" rather than encapsulated in another protocol. The relationship is illustrated in the Open Systems Interconnection (OSI) Reference Model . A full description of IEEE 802.2 Logical Link Control is beyond the scope of this document.

The Logical Link Control offers services to the Network layer through Service Access Points (SAPs). The SAP is used to identify the process at the Network layer.

IEEE 802.2 frames have the following form:

DSAP 1 byte: Destination Service Access Point
SSAP 1 byte: source Service Access Point
Control 1 or 2 bytes: field length depends on the service
Information: This variable length field carries any data

For IPX, DSAP = 0xE0 (224), SSAP = 0xE0 and Control is 1 byte 0x03 which denotes the 802.2 unnumbered format.

For SNAP, DSAP = 0xAA (170), SSAP = 0xAA

For NetBIOS, DSAP = 0xF0 (240) , SSAP = 0xF0

Some IEEE 802.2 Numbers of interest can be found at:

http://www.iana.org/assignments/ieee-802-numbers

In 1985 IBM implemented NetBIOS over Token Ring and established the way in which NetBIOS frames would map to 802.2 frames.

When NetBIOS is implemented over Token Ring, the NetBIOS frames are mapped directly on to the 802.2 frames; the NetBIOS frame is contained in the information field of the 802.2 frame:

DSAP 1 byte Destination Service Access Point 0xF0
SSAP 1 byte source Service Access Point 0xF0
Control 1 or 2 bytes field length depends on the service
Information:
- NetBIOS header
- Optional data

The above scheme is general to implementations of NetBIOS over 802.2 where other underlying technologies are used such as Ethernet.