SNAX/XF and SNAX/APN Configuration and Management Manual

Configuring for Token-Ring Support
SNAX/XF and SNAX/APN Configuration and Management Manual—425836-006
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Description of Frames
The LPDU is created by the LLC layer.

The destination SAP (DSAP) consists of a six-bit address, a seventh bit (U) for a
user-defined address, and an eighth bit that specifies whether this is an individual
(0) or group (1) address.

The source SAP (SSAP) has a similar structure to that of a six-bit-plus-one
address, but the final bit signifies whether the LPDU is a command or a response.
The command/response refers to requests to establish or disconnect the data link.

The control field may be of a varying length—one or two bytes—and contains link
information. The structure and function of this field is similar to that of an SDLC
control field. There are three types of control field: information, supervisory, and
unnumbered.

The information field is used for transferring information. The control field is
used primarily to contain the send and receive counts. The send count is used
to ensure that frames are received in their proper order, the receive count is
used to confirm that received information frames have been accepted.

The supervisory field is used to assist in the transfer of information. This field is
used to acknowledge received frames, to convey ready or busy conditions, and
to report frame numbering errors.

The unnumbered field is used for establishing and disconnecting the data link.
Figure 9-10. LLC Protocol Data Unit
DSAP
SSAP Control Data
DDDDDDUI/G
SSSSSSUC/R
Information
Supervisory
Unnumbered
VST910.vsd