Maxim Integrated Teridian V.22 bis Linux Softmodem for 73M1822/73M1922 MicroDAA - Manual (Page 65)

UG_1x22_043 V.22 bis Linux Softmodem for 73M1x22 MicroDAA User Guide
Rev. 1.5 65
SMS Applications continued
15. Wait per SMS timing and DCD turn off (SM-SC switched to receive mode)
16. Turn on RTS (to switch modem to transmit mode)
17. Wait for CTS on
18. Transmit DLL_SMS_ACK1 message to SM-SC
19. Turn off RTS (to switch modem to receive mode)
20. Wait for DCD on
21. Receive DLL_SMS_REL message from SM-SC
22. Wait per SMS timing and DCD turn off (SM-SC switched to receive mode)
23. Turn on RTS (to switch modem to transmit mode)
24. Wait for CTS on
25. Transmit DLL_SMS_ACK0 message to SM-SC acknowledging the end of the
session
END
A couple of things should be noted in the procedures above. Although detailed only in the
SM_SC to SM-TE connection, the TSC Softmodem is put into a mode that sends patterns
during the handshake to generate the alternating 0/1 and Marks (steps 9 and 10 above).
This is also necessary every time the line is turned around and the carrier is sent. The
reason for this using the test pattern generator for the alternating pattern is that the modem
only sends asynchronous data and to send this by using the equivalent character, “U”,
there can be some distortion of the waveform. By using the transmit alternate pattern a
symmetrical continuous waveform is guaranteed. The pattern transmission stops
automatically when the modem is put back into the data mode. To send the Marking
pattern simply do not send data for 70 ms (constant marks are sent when there is no data).
If a normal frame were being sent instead of the DLL_SMS_EST signal, the Message
Type, Message Length, Payload and Checksum would follow the Marks. This same
procedure must be done each time a pattern is sent.
It may take some time to fully comprehend the SMS protocols, but after studying the ETSI
documentation it will all start to fall into place. Any robust half duplex protocol should
contain a similar structure to SMS, but there is a lot of room for variation. It is important
that enough time is allowed for the carriers to turn around and there should be a way to
easily recognize the beginning of the data packets. If these functions are missing, the
chances for errors increase.