Nortel Networks Circuit Card 311 - Manual (Page 1130)

1130 Appendix A LAPB Data Link Control protocol
Frame structure
All transmissions are in frames and each frame conforms to the format
shown in
Table 459 "LAPB frame structure" (page 1130) . In particular,
frame elements for applications using a port on the QPC513 follow these
LAPB conventions:
• Zero information field is permitted.
• Inter-frame time fill is accomplished by transmitting contiguous flags.
This is compatible with AT&T Technical Requirement BX.25 and ADCCP
standards.
• Extensions for the address field or the control field are not permitted.
This requirement imposes constraints to satellite operations.
• Individual station addresses are assigned in service change for
balanced configuration. The default ESDI address is 10000000. The
far-end default address is 11000000.
• The LAPB basic control field (modules 8) format is implemented.
• Frame check sequence is implemented in accordance with LAPB
procedures.
Table 459
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.
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 01.04 Standard
Release 5.0 23 May 2008
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