application note 1998 mx com, inc. www. mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 doc. # 20830115.001 4800 bethania station road, winston-salem, nc 27105-1201 usa. all trademarks and service marks are held by their respective c ompanies. recommendations for v.23 1200/75bps full-duplex call set-up recommendations for v.23 1200/75bps full-duplex call set-up v.23 recommendations do not describe how a connection is established. the following information and figure describes how the connection should be established. note : one should not rely on other modems to follow this particular flow and the 1200/1200bps half-duplex connections are not covered by this application note. 1. the calling (originating) modem should connect to the line by operating the off-hook relay. 2. having seized the line the dial tone should be detected. the dial tone will typically appear approximately 1/3 seconds after going off-hook. if the dial tone does not appear after 5 seconds return to the on-hook state. if dial tone monitoring is not to be performed (blind dialing) a pause of several seconds is necessary after line seizure and before dialing begins, typically 2 seconds. 3. send dtmf digit sequence, tones should have on and off periods of typically 100ms. reference point a. 4. calling modem remains silent after dialing until it receives a response from the answering modem. the first response may be the v.25 answering procedure, this should be ignored until the signal returns after 75ms 20ms energy drop, reference point b. without v.25 answering procedure reference, point b will be the end of the 2 seconds answering modems billing delay. 5. when binary 1s (high band marks) have been detected from the answering modem for 155ms 50ms the calling modem can be configured ready to receive data, reference point c. start timers for 456ms 10ms and 765ms 10ms. 6. when the 456ms 10ms timer expires start transmitting binary 1s (low band marks) to the answering modem, reference point d. 7. when the 765ms 10ms timer expires the calling modem can change to data transmission mode and assert the cts handshaking line. the binary 1s (low band marks) at the transmitter should now be replaced by data, reference point e. send 2100hz answer tone silence 456ms 10ms wait detect binary ones (highbandmarks) from answering modem for 155ms 50ms 155ms 50ms wait 765ms 50ms 75ms 20ms silence delay 2.15s 0.35s 3.3s 0.7s connects to line remain silent answering modem calling modem send binary ones (highbandmarks) binary ones (low band marks) data cts data a b de c figure 1: timing diagram
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