IEEE 802.2

IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines Logical Link Control (LLC) as the upper portion of the data link layer of the OSI Model.[1] The original standard developed by the Institute of Electrical and Electronics Engineers (IEEE) in collaboration with the American National Standards Institute (ANSI) was adopted by the International Organization for Standardization (ISO) in 1998, but it still remains an integral part of the family of IEEE 802 standards for local and metropolitan networks.

LLC is a software component that provides a uniform interface to the user of the data link service, usually the network layer. LLC may offer three types of services:

Conversely, the LLC uses the services of the Media Access Control (MAC), which is dependent on the specific transmission medium (Ethernet, Token Ring, FDDI, 802.11, etc.). Using LLC is compulsory for all IEEE 802 networks with the exception of Ethernet. It is also used in Fiber Distributed Data Interface (FDDI) which is not part of the IEEE 802 family.

The IEEE 802.2 sublayer adds some control information to the message created by the upper layer and passed to the LLC for transmission to another node on the same data link. The resulting packet is generally referred to as LLC Protocol Data Unit (PDU) and the additional information added by the LLC sublayer is the LLC HEADER. The LLC Header consist of DSAP (Destination Service Access Point), SSAP (Source Service Access Point) and the Control field.

The two 8-bit fields DSAP and SSAP allow to multiplex various upper layer protocols above LLC. However, many protocols use the Subnetwork Access Protocol (SNAP) extension which allows using EtherType values to specify the protocol being transported atop IEEE 802.2. It also allows vendors to define their own protocol value spaces.

The 8 or 16 bit HDLC-style Control field serves to distinguish communication mode, to specify a specific operation and to facilitate connection control and flow control (in connection mode) or acknowledgements (in acknowledged connectionless mode).

Operational modes

IEEE 802.2 provides two connectionless and one connection-oriented operational modes:

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.

Each device conforming the IEEE 802.2 standard must support service type 1. Each network node is assigned an LLC Class according to which service types it supports:

Supported Service Types
1 2 3

LLC header

Any 802.2 LLC PDU has the following format:

802.2 LLC Header Information
DSAP address SSAP address Control
8 bits 8 bits 8 or 16 bits multiple of 8 bits

When Subnetwork Access Protocol (SNAP) extension is used, it is located at the start of the Information field:

802.2 LLC Header SNAP extension Upper layer data
DSAP SSAP Control OUI Protocol ID
8 bits 8 bits 8 or 16 bits 24 bits 16 bits multiple of 8 bits

The 802.2 header includes two eight-bit address fields, called service access points (SAP) or collectively LSAP in the OSI terminology:

LSAP Values

The DSAP and SSAP can have the following values:

Value Meaning
Dec Hex
0 00 Null LSAP[2][3]
2 02 Individual LLC Sublayer Mgt[2][3]
3 03 Group LLC Sublayer Mgt[2][3]
4 04 SNA Path Control (individual)[2][3]
5 05 SNA Path Control (group)[3]
6 06 Reserved for DoD IP[2]
14 0E ProWay-LAN[2][3]
24 18 Texas Instruments[3]
66 42 IEEE 802.1 Bridge Spanning Tree Protocol[3]
78 4E EIA-RS 511[2][3]
94 5E ISI IP[2]
126 7E ISO 8208 (X.25 over IEEE 802.2 Type LLC)[3]
128 80 Xerox Network Systems (XNS)[3]
130 82 BACnet/Ethernet[4]
134 86 Nestar[3]
142 8E ProWay-LAN (IEC 955)[2][3]
152 98 ARPANET Address Resolution Protocol (ARP)[3]
166 A6 RDE (route determination entity)
170 AA SNAP Extension Used[2][3]
188 BC Banyan Vines[3]
224 E0 Novell Netware[3]
240 F0 IBM NetBIOS[3]
244 F4 IBM LAN Management (individual)[3]
245 F5 IBM LAN Management (group)[3]
248 F8 IBM Remote Program Load (RPL)[3]
250 FA Ungermann-Bass[3]
254 FE OSI protocols ISO CLNS IS 8473[3][5]
255 FF Global DSAP (cannot be used for SSAP)[2][3]

Despite the SAP fields being 8-bit long, some bits have specific significance, so that there is room for only 64 distinguished SAP numbers, which are globally assigned by the IEEE to uniquely identify well established international standards.

The low-order bit of the DSAP indicates whether it contains an individual or a group address.

The low-order bit of the SSAP indicates whether the packet is a command or response packet;

The remaining 7 bits of the SSAP specify the LSAP from which the packet was transmitted.

The protocols or families of protocols which have assigned one or more SAPs may operate directly on top of 802.2 LLC. Other protocols may use the Subnetwork Access Protocol (SNAP) with IEEE 802.2 which is indicated by the hexadecimal value 0xAA (or 0xAB, if the low-order bit of the field is set) in SSAP and DSAP. The SNAP extension allows using EtherType values or private protocol ID spaces in all IEEE 802 networks. It can be used both in datagram and in connection-oriented network services.

Ethernet (IEEE 802.3) networks are an exception; the IEEE 802.3x-1997 standard explicitly allowed using of the Ethernet II framing, where the 16-bit field after the MAC addresses does not carry the length of the frame followed by the IEEE 802.2 LLC header, but the EtherType value followed by the upper layer data. With this framing only datagram services are supported on the data link layer.

Internet Protocol and 802.2 LLC

Although IPv4 has assigned SAP value of 6 and there is even a SAP value of 98 hex for ARP, IP traffic is almost never encapsulated in IEEE 802.2 LLC frames without SNAP. Instead, the Internet standard RFC 1042 is usually used for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers in FDDI and IEEE 802 networks. In Ethernet/IEEE 802.3 networks using Ethernet II framing with EtherType 800 hex for IP and 806 hex for ARP is more common.[6]

It is possible to use diverse framings in a single network. It is possible to do it even for the same upper layer protocol, but in such a case the nodes using unlike framings cannot directly communicate. IPX protocol used in the Novell NetWare networks allows an additional Ethernet frame format supporting 4 frame types in Ethernet and 2 frame types in IEEE 802 networks and FDDI.

Control Field

Following the destination and source SAP fields is a control field. IEEE 802.2 was conceptually derived from HDLC, and has the same three types of PDUs:

To carry data in the most-often used unacknowledged connectionless mode the U-format is used. It is identified by the value '11' in lower two bits of the single-byte control field.


  1. IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements. Part 2: Logical Link Control. New York: The Institute of Electrical and Electronics Engineers. May 7, 2008. ISBN 1-55937-959-6.
  2. 1 2 3 4 5 6 7 8 9 10 11 RFC-1700
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Miller, Philip; Cummins, Michael (2000). LAN Technologies Explained. Digital Press. p. 506. ISBN 1-55558234-6.
  4. The BACnet Standard—Standard 135-2012, Ashrae.
  5. Final Text of DIS 8473, Protocol for Providing the Connectionless-mode Network Service, RFC (994), IETF.
  6. LKML, 2011-07-27.

External links

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.

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