Using the SS7 Mode Feature

Receiving Frames

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.

Appendix B

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