User manual
Computer Gateway User Manual 5-5 5/96
5.4
5.4 HDLC LINK PROTOCOL
Operation of the HDLC link follows the Link Access Procedure, Balanced (LAPB),
according to C.C.I.T.T. document AP VIII-58-E June 1984 (Recommendation X.25) and
complies with FIPS 100 certification.
To provide a compatible HDLC interface with the CG, host processor software must meet
the following requirements:
• Link protocol is HDLC LAPB.
• Station identifier is configurable as either 1 or 3; default is 1.
• Maximum message size is 1024 bytes. Maximum frame size is 1030 bytes, consisting
of: Start Flag (1 byte), Frame Header (2 bytes), Message (1024 bytes), CRC (2 bytes),
and End Flag (1 byte).
• Window size (maximum number of outstanding frames) is set to 1.
• Time to wait (T1) for acknowledgment to a command frame varies from 0.1 second to
25.5 seconds with a default of 1.6 seconds. This time must be long enough to
exchange a complete frame.
• Maximum number of transmissions of a command frame following the run-out of timer
T1 ranges from 2 to 50 with a default of 10. After the specified number of retries with
no response, a disconnect occurs.
The communication interface software must support interface routines to
• communicate with driver routines
• control DTR and detect DSR state
5.5 MESSAGE STRUCTURE
The message structure is similar for both Bisynch and HDLC protocols. Each message is
packeted into 512-word (1024-byte) blocks.
Each block begins with an 8-word header. The remainder of the block contains message
information words up to the maximum block size. The maximum number of blocks in a
message is 13 blocks. This provides for a maximum of 6552 information (nonheader)
words for each message.
The 8-word header with each block contains the following information:
Word 1—Transaction Code (integer)
Word 2—Number of Words (integer, 1..504)
Word 3—Number of Blocks (integer, 1..13)
Word 4—Block Number (integer, 1..13
Word 5—CG Identifier (integer)
Word 6—Host Task Identifier (2 ASCII characters)
Word 7—Message Tag (integer)
Word 8—Message Counter (integer, 1..30000)
Header words 1, 5, and 6 are assigned at the application level and are not modified by the
communications package. See paragraph 4.1.2.1 for a description of header contents.