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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 .


OPERATIONAL MODES

IEEE 802.2 provides two Connectionless and one connection-oriented operational modes:
  • Type 1 is an unacknowledged connectionless mode. It allows for sending frames

  • --- to a single destination (point-to-point or unicast transfer),

  • --- to multiple destinations on the same network (multicast),

  • --- or to all stations of the network (broadcast).

    • 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-oriented 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. A Sliding Window flow control procedure prevents fast senders from flooding slow destinations.

  • Type 3 is an acknowledged Connectionless service. It supports point-to-point communication only.



IEEE 802.2 HEADER CONTROL WORDS AND FRAME FORMATS

To confuse matters further, there can be three kinds of IEEE 802.2 PDU, in so-called U, I or S frames.

  • U frames, with an 8-bit control field, are intended for connectionless applications

  • I frames, with a 16-bit control and sequence numbering field, are intended to be used in connection-oriented applications

  • S frames, with a 16-bit control field, are intended to be used for supervisory functions at the LLC 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. IEEE 802.2 was conceptually derived from HDLC , which explains these aspects of its design.


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