IEEE 802.2 Logical Link Control

In the OSI Reference Model, the Datalink layer sits above the Physical layer and below the Network layer. When considering IEEE LAN technology the situation is a little more complex. There are a number of LAN systems such as Token Ring and Ethernet and the physical characteristics of these are defined in the Physical Layer of the OSI model. Characteristics such as the frame format for systems such as Token Ring and Ethernet are defined in the Datalink layer in standards such as 802.3, 802.5 etc. A common interface was required between these standards and the protocols in layer 3 and this is implemented in 802.2. A full description of IEEE 802.2 Logical Link Control is beyond the scope of this document; a brief overview is given below.

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 the appendix called Open Systems Interconnection (OSI) Reference Model Open Systems Interconnection (OSI) Reference Model

802.2 supports both connection-oriented and connection-less oriented communications. 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:

Some examples of DSAP and SSAP values are given below.

For IPX (the network protocol traditionally used with NetWare networks), DSAP = 0xE0 (224), SSAP = 0xE0 and Control is 1 byte 0x03 which denotes the 802.2 unnumbered format.

For SNAP (Sub-Network Access Protocol), DSAP = 0xAA (170), SSAP = 0xAA

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

Some IEEE 802.2 Numbers of interest can be found at "The Internet Assigned Numbers Authority" web site, "Protocol Numbers and Assignment Services" in "IEEE 802 Numbers":

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.