Texas Instruments 990 User Manual page 236

Communications Structures
The format of the LCI assignment frame is as follows:
BYTE 0
2280620
where:
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
2 3 4 5
ADDR.
0001 GGGG
ecce ecce
is the start-of-frame flag, >7E.
is ,the swi tch address of the secondary station.
is the HDLC UI command with P-bit set to 1.
is the I (information) field descriptor. This is
normally set to zero but may be used for UI
control expansion, if required.
is set to 0001, the general format identifier.
GGGG
is the virtual channel group number of the
secondary station.
is the logical channel number of the secondary
station.
Bytes 6, 7 are the 16-bit CRC HDLC frame check.
Byte
8
is the end-of-frame flag, >7E.
When the second,ary sta tion recei ves this message its level 2
software activates the UI processor and responds with an
unnumbered acknowledge (UA) message to the primary station The
secondary station then assigns GGGG CCCC CCCC as its LCI. This
LCI will be present in every packet sent on the permanent virtual
circuit (PVC).
D.3 PACKET LEVEL (LEVEL 3) PROCEDURES
The following paragraphs describe the level 3 interface
procedures that are used for communications between primary and
secondary stations. Each packet transmission is contained in an
HDLC numbered information (I) field. Only one packet is
contained in each
I
field and only the data transfer packet is
accompanied by user data. The types of packet are as follows:
D-2 2270526-9701