This topic provides an overview of Barr line trace data. A typical frame from a Barr line trace follows. Sections of the data are separated by vertical bars. Before the first vertical bar is the link header. Between the first and second vertical bars is the transmission header (TH). Between the second and third vertical bars is the request header (RH). After the final vertical bar is the data portion of the frame.
For specific bit values, check the SNA Formats manual.
C1 is the SDLC address of the connection.
CC is an SDLC command frame. This indicates whether the frame is Supervisory (RR, Rnr, etc.), Informational (Data), or a non-sequenced Command (SNRM, Xid, etc.). CC means this is an information frame, in this case a BEGIN.
2C is TH0, byte 0 of the transmission header. This delineates the frame type (normally FID2), what part of a segment is being sent, and whether a frame is expedited. The 2 is for FID2; the C means the whole segment is being sent.
00 is TH1, which is reserved for the system. This is always 00.
03.01 is the destination address followed by the source address (typically this is an LU address). When the host sends data, it will always be in the format 03.01; when the Barr software sends data, it will be 01.03.
00_01 is the sequence number field.
The first byte of the request header is RH0. It indicates:
whether this is a request or response
the level of the request or response – whether the frame is FM data (data), Network Control, Data Flow Control, or Session Control
the format of the frame – character coded or formatted
whether sense data is included
the state of the chain – beginning, middle, or end
0B means that the frame is a request containing FM data, that the RU is formatted, and that it is a single frame in its own chain.
The second byte of the request header is RH1. It indicates:
whether a definite response is required
whether an exception response is included
whether a pacing response is required
81 indicates that a definite response is required and that a pacing reponse is also required.
The third byte of the RH is RH2. It indicates:
the state of the bracket
whether a change direction flag is set to allow the other end of the connection to communicate
80 indicates a BEGIN bracket.
The data portion of the frame can be anything such as the BEGIN command listed here (06), sense data, or print data. If the frame is a command, the first byte of data will be the command. Some common commands are 11 for ACTPU, 0D for ACTLU, 06 for BEGIN/END, and 31 for BIND. These are always listed by name in the Barr software, but commands will be the same in other packages' traces.
