IEEE 802.2
Encyclopedia
IEEE 802.2 is the IEEE 802
standard defining Logical Link Control
(LLC), which is the upper portion of the data link layer of the OSI Model
. 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, which is dependent on the particular medium being used (Ethernet
, token ring, FDDI, 802.11, etc.).
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 packet regardless of network type. There is also an 8 or 16 bit control field for use in auxiliary functions such as flow control
. There is room for 64 globally assigned SAP numbers, and the IEEE does not assign them lightly. IP does not have an assigned SAP number, because only “international standards” could be given globally assigned SAP numbers. Protocols which are not international standards can use a SAP number from the locally administered SAP number space. The Subnetwork Access Protocol
(SNAP) allows EtherType
values to be used to specify the protocol being transported atop IEEE 802.2, and also allows vendors to define their own protocol value spaces.
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.
s or SAPs in OSI terminology; they are the destination SAP (DSAP), and the source SAP (SSAP). The low-order bit of the DSAP indicates whether it contains an individual or a group address. If the low-order bit is 0, the remaining 7 bits of the DSAP specify an individual address, which refers to a single local service access point (LSAP) to which the packet should be delivered. If the low-order bit is 1, the remaining 7 bits of the DSAP specify a group address, which refers to a group of LSAPs to which the packet should be delivered. The low-order bit of the SSAP indicates whether the packet is a command or response packet; if it's 0, the packet is a command packet, and if it's 1, the packet is a response packet. The remaining 7 bits of the SSAP specify the LSAP from which the packet was transmitted.
Some protocols, or families of protocols, have one or more SAPs assigned to them; for example, IPv4
has a SAP value of hex 06. Those protocols operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented network services.
The Subnetwork Access Protocol
(SNAP) can be used with IEEE 802.2; it allows EtherType
values to be used with all IEEE 802
protocols, as well as supporting private protocol ID spaces. When both the DSAP and the SSAP are set to the hexadecimal value 0xAA (or 0xAB, if the low-order bit of the field is set), the SNAP service is requested.
Most IEEE 802 protocols, as well as FDDI, always use 802.2. IEEE 802.3
is an exception; Ethernet
originally used Ethernet II framing, with a 2-octet type field containing an EtherType
value, and no 802.2 header, but the original IEEE 802.3 specification used a framing wherein the 2-octet field following the 6-octet destination and source MAC addresses contained a frame length, with an 802.2 LLC header following the length field. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.
There exists an Internet standard
, RFC 1042, for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers. It is almost never implemented on Ethernet, although it is used on FDDI and on token ring, IEEE 802.11
, and other IEEE 802
networks.
IP traffic can not be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is an LLC protocol type for IP, there is no LLC protocol type for ARP
. IP Version 6 can also be transmitted over Ethernet using IEEE 802.2 with LLC/SNAP, but, again, that's almost never used (although LLC/SNAP encapsulation of IPv6 is used on IEEE 802 networks).
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 PDU
s:
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.
IEEE 802
IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...
standard defining Logical Link Control
Logical Link Control
The logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
(LLC), which is the upper portion of the data link layer of the OSI Model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
. Beneath the LLC sublayer is the Media Access Control
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) sublayer, which is dependent on the particular medium being used (Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, token ring, FDDI, 802.11, etc.).
The IEEE standard adds this sublayer which adds the standard 8-bit DSAP (Destination Service Access Point
Service Access Point
A Service Access Point is an identifying label for network endpoints used in Open Systems Interconnection networking.When using the OSI Network Layer , the base for constructing an address for a network element is an NSAP address, similar in concept to an IP address...
) and SSAP (Source Service Access Point
Service Access Point
A Service Access Point is an identifying label for network endpoints used in Open Systems Interconnection networking.When using the OSI Network Layer , the base for constructing an address for a network element is an NSAP address, similar in concept to an IP address...
) labels to a given packet regardless of network type. There is also an 8 or 16 bit control field for use in auxiliary functions such as flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
. There is room for 64 globally assigned SAP numbers, and the IEEE does not assign them lightly. IP does not have an assigned SAP number, because only “international standards” could be given globally assigned SAP numbers. Protocols which are not international standards can use a SAP number from the locally administered SAP number space. The Subnetwork Access Protocol
Subnetwork Access Protocol
The Subnetwork Access Protocol is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point fields. SNAP supports identifying protocols by Ethernet type field values; it also supports vendor-private protocol...
(SNAP) allows EtherType
EtherType
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the PayLoad of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.EtherType...
values to be used to specify the protocol being transported atop IEEE 802.2, and also allows vendors to define their own protocol value spaces.
Operational modes
IEEE 802.2 provides two connectionlessConnectionless mode transmission
In packet switching networks, CL-mode or connectionless communication is a data transmission method in which each data packet carries information in a header record that contains a destination address sufficient to permit the independent delivery of the packet to its destination via the network.A...
and one connection-oriented operational modes:
- Type 1 is an unacknowledged connectionless mode for a datagramDatagramA datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
service. It allows for sending frames- to a single destination (point-to-pointPoint-to-pointPoint-to-point or point to point may refer to:Computing* Point-to-point construction, an electronics assembly technique* Point-to-point * Point-to-Point Protocol , part of the Internet protocol suite...
or unicastUnicastright|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
transfer), - to multiple destinations on the same network (multicastMulticastIn computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
), - or to all stations of the network (broadcastBroadcastBroadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
).
- to a single destination (point-to-point
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-orientedConnection-orientedConnection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
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. - Type 3 is an acknowledged connectionlessConnectionless mode transmissionIn packet switching networks, CL-mode or connectionless communication is a data transmission method in which each data packet carries information in a header record that contains a destination address sufficient to permit the independent delivery of the packet to its destination via the network.A...
service. It supports point-to-point communication only.
LLC header
The 802.2 header includes two eight-bit address fields, called service access pointService Access Point
A Service Access Point is an identifying label for network endpoints used in Open Systems Interconnection networking.When using the OSI Network Layer , the base for constructing an address for a network element is an NSAP address, similar in concept to an IP address...
s or SAPs in OSI terminology; they are the destination SAP (DSAP), and the source SAP (SSAP). The low-order bit of the DSAP indicates whether it contains an individual or a group address. If the low-order bit is 0, the remaining 7 bits of the DSAP specify an individual address, which refers to a single local service access point (LSAP) to which the packet should be delivered. If the low-order bit is 1, the remaining 7 bits of the DSAP specify a group address, which refers to a group of LSAPs to which the packet should be delivered. The low-order bit of the SSAP indicates whether the packet is a command or response packet; if it's 0, the packet is a command packet, and if it's 1, the packet is a response packet. The remaining 7 bits of the SSAP specify the LSAP from which the packet was transmitted.
Some protocols, or families of protocols, have one or more SAPs assigned to them; for example, IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
has a SAP value of hex 06. Those protocols operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented network services.
The Subnetwork Access Protocol
Subnetwork Access Protocol
The Subnetwork Access Protocol is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point fields. SNAP supports identifying protocols by Ethernet type field values; it also supports vendor-private protocol...
(SNAP) can be used with IEEE 802.2; it allows EtherType
EtherType
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the PayLoad of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.EtherType...
values to be used with all IEEE 802
IEEE 802
IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...
protocols, as well as supporting private protocol ID spaces. When both the DSAP and the SSAP are set to the hexadecimal value 0xAA (or 0xAB, if the low-order bit of the field is set), the SNAP service is requested.
Most IEEE 802 protocols, as well as FDDI, always use 802.2. IEEE 802.3
IEEE 802.3
IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
is an exception; Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
originally used Ethernet II framing, with a 2-octet type field containing an EtherType
EtherType
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the PayLoad of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.EtherType...
value, and no 802.2 header, but the original IEEE 802.3 specification used a framing wherein the 2-octet field following the 6-octet destination and source MAC addresses contained a frame length, with an 802.2 LLC header following the length field. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.
There exists an Internet standard
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
, RFC 1042, for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers. It is almost never implemented on Ethernet, although it is used on FDDI and on token ring, IEEE 802.11
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
, and other IEEE 802
IEEE 802
IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...
networks.
IP traffic can not be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is an LLC protocol type for IP, there is no LLC protocol type for ARP
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
. IP Version 6 can also be transmitted over Ethernet using IEEE 802.2 with LLC/SNAP, but, again, that's almost never used (although LLC/SNAP encapsulation of IPv6 is used on IEEE 802 networks).
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 PDU
Protocol data unit
In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
s:
- Unnumbered format PDUs, or U-format PDUs, with an 8-bit control field, which are intended for connectionless applications;
- Information transfer format PDUs, or I-format PDUs, with a 16-bit control and sequence numbering field, which are intended to be used in connection-oriented applications;
- Supervisory format PDUs, or S-format PDUs, with a 16-bit control field, which are intended to be used for supervisory functions at the LLC (Logical Link Control) 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.