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Appendix A: LAPB Data Link Control protocol Page 891 of 894
Circuit Card Description and Installation
LAPB balanced class of procedure
Applications which use ports on the QPC513 are automatically designated as
BAC, 2, 8 (for example, balanced operation, asynchronous balanced mode
class of procedure with optional functions 2 and 8 implemented).
Balanced configuration
A balanced configuration is one in which two combined stations have
identical responsibilities for exchanging data and control information and for
initiating error recovery functions, as shown in Figure 188 on page 892.
Combined station
A combined station has balanced link control capability and transmits both
commands and responses to, and receives both commands and responses
from the other combined station.
Table 275
LAPB frame structure
Flag Address Control Information FCS Flag
01111110 8 bits 8 bits unspecified
(no. of bits)
16 bits 01111110
Legend:
Flag: Flag sequence – All frames start and end with the flag sequence. (A single flag is used as both the
closing flag for one frame and the opening flag for the next frame.)
Address: Station address field – In command frames, the address identifies the station for whom the
command is intended. In response frames, the address identifies the station from which the response
originated.
Control: Control field – This field contains commands or responses and sequence numbers.
Information: Information field – Information may be any sequence of bits, usually related to a convenient
character structure such as an octet, but may be an unspecified number (from 1 to 128) of bits unrelated to
a character structure.
FCS: Frame check sequence.