HDLC-Frame Protocol User's Guide (February 2004)

Using the SS7 Mode Feature
Receiving Frames
Appendix B 39
Receiving Frames
When enabled HDLC.FRAME begins receiving frames from the line. The
first frame received is passed to the application. After the first frame has
been received, the contents of received frames and the current state of
HDLC.FRAME are used to determine whether subsequent frames are
passed to the application.
When a LSSU frame is received HDLC.FRAME enters “LSSU receiving
mode”.
When a FISU frame is received HDLC.FRAME enters “FISU receiving
mode”.
When in “FISU receiving mode”:
• Any FISU with the same sequence numbers as the previous frame
are discarded and not passed to the application. However the
“discard-rate” parameter sets a limit for the maximum number of
FISUs to be discarded, with no intervening MSU or LSSU, and with
the same sequence numbers. When this limit is reached one FISU is
passed to the application, and the discard count is reset.
• If an LSSU is received, it is passed to the application and
HDLC.FRAME enters “LSSU receiving mode”.
When in “LSSU receiving mode” Any LSSU with the same sequence
numbers as the previous frame and the same status byte(s) as the
previous LSSU are discarded and not passed to the application. However,
the “discard-rate” parameter is processed in the same way as for FISUs
in FISU receiving mode above.
If a FISU is received, it is passed to the layer-2 protocol and
HDLC.FRAME enters “FISU receiving mode”.
All MSU frames are passed to the application regardless of
HDLC.FRAME’s receive mode. The receive mode is unchanged.
LSSU frames may have one or two bytes of status data.