SNAX/XF and SNAX/APN Configuration and Management Manual

Configuring for Ethernet Support
SNAX/XF and SNAX/APN Configuration and Management Manual—425836-006
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Description of Frames
Description of Frames
At the E4SA, FESA, GESA, or G4SA adapter, data and SDLC control information are
encapsulated in a data frame suitable for transport on the Ethernet LAN. SNAX
supplies the information for the source and destination SAPs, and the destination MAC
address.
The first part of the encapsulation is to create the link protocol data unit (LPDU), shown
in Figure 10-8.
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 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.
Figure 10-8. LLC Protocol Data Unit
DSAP
SSAP Control Data
DDDDDDUI/G
SSSSSSUC/R
Information
Supervisory
Unnumbered
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