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IEEE 802 LOGICAL LINK CONTROL SUBLAYER

The logical link control is a sublayer of the OSI data link layer and is designed to support a common set of services to the network layer. It resides at the upper part of layer two and is concerned with establishing, maintaining, and terminating logical links between communicating stations. The logical link control sublayer provides:

  The interface with the stations, specifying the services the logical link control (and hence the LAN) offers to the network subscriber.
  The logical link control protocol, specifying the logical link control functions.
  The media access control interface, specifying the media access control services the logical link control requires to perform its function.

The logical link control supports three types of operation (see Exhibit 1-1-3). These are discussed in the following paragraphs.


Exhibit 1-1-3.  Logical Link Control Procedures and Standards

Type 1: Unacknowledged Connectionless (Datagram)

In type 1 operation, peer entities transmit data to each other without the establishment of a connection. The data transfer can be point-to-point, multicast, or broadcast. There are no flow control or error-recovery mechanisms, and frames are unnumbered. Connection-oriented service, if needed, is provided by the upper layers.

Type 2: Connection Oriented

Type 2 operation permits peer entities to establish, use, reset, and terminate connection services at the data link layer to exchange information. It provides data flow control at the network layer—data link layer interface. The connection establishment mechanism allows a network entity to request or be notified of the establishment of a data link layer connection.

Data transfer is the means by which a network entity can send or receive link service data units over a data link layer connection. This service also provides sequencing, flow control, and error recovery at the data link layer.

Connection reset is the means by which established connections can be returned to the initial state. Connection termination is the means by which a network entity can request or be notified of the termination of data link layer connections. Flow control is the mechanism that controls data movement associated with a specified connection, across the network layer—data link layer interface.

Type 3: Acknowledged Connectionless (Single Frame)

This per-mits a peer entity to send data and requests to other peer entities and to receive acknowledgment and data.

High-Level Data Link Control

The logical link control protocol mimics the high-level data link control developed by ISO. The frame is composed of three parts—header, body, and trailer (see Exhibit 1-1-4). An 8-bit flag sequence, 01111110, at the beginning of the header and at the end of the trailer indicates the frame boundaries; it establishes and maintains synchronization. The header identifies the source and destination, as well as the frame type. (The source and destination addresses are required because LAN links support multiple sources and destinations.) The trailer is used in transmission-error detection.


Exhibit 1-1-4.  Logical Structure of High-Level Data Link

For LAN configurations, IEEE 802.2 establishes three types of frames for data communication between service access points. The control field identifies the frame type and the role it plays during information transfer and the management of that transfer. IEEE 802 recommended link access protocol-balanced (LAP-B) for these tasks. LAP-B allows only one logical link with bidirectional flow to exist across one interface.

The LAP-B frame format is shown in Exhibit 1-1-5. The IEEE 802 recommendation calls for two 8-bit address fields—the destination service access point and the source service access point (see Exhibit 1-1-6). These have the following features:


Exhibit 1-1-5.  Link Access Protocol-Balanced Frame Format


Exhibit 1-1-6.  Protocol Data Unit and Address Field Format

  The address fields are 8 bits long.
  The least-significant bit of the destination service access point is used in two ways. If it is 0, the address field represents an individual address; if it is 1, it indicates a group address. The least-significant bit of the source service access point identifies the logical link control protocol data unit as a command (0) or a response (1).
  When the destination service access point address field is all 1s, it is predefined as the global address.
  Addresses 01000000 and 11000000 are designated as the individual and the group address, respectively, for a logical link control management function at the station. All other addresses with the second lowest-order bit set to 1 are reserved for future definition by IEEE 802.

The control field can be either 8 or 16 bits long. It is used to facilitate the numbered information and supervisory transfer, including the unnumbered control and information transfer functions. The two least-significant bits of the control byte identify the frame format (see Exhibit 1-1-7).


Exhibit 1-1-7.  Structure and Associated Meanings of Control Field

I Format

The information transfer command and response I-format protocol data unit is used to carry numbered information transfer in type 2 operation. The functions N(S), N(R), and P/F (poll/final) are independent entities and are required for each protocol data unit, where N(S) is the transmitter send sequence number and N(R) is the transmitter receive sequence number used to confirm or authorize transmission of additional I-format protocol data units. The poll/final bit is set to 1 or 0, on the basis of operation-type parameters.

S Format

The S format or supervisory frame command/response is used to manage data link supervisory and control functions in type 2 operation. N(R) and P/F are independent; N(R) identifies the sequence number to confirm or authorize transmission of additional I-format frames at the receiving logical link control and a P/F bit to be set to 0 or 1.


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