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PN 88300150
2.1
Introduction
This chapter describes the dialing and answering capabilities of the MT2834MR. Since the modem can
be either a leased line with dialback modem or a standard ITU-T V.34 full duplex dial-up modem, both
modes are described (in terms of dialing features) in this chapter.
2.2
Automatic Leased Line Restoral Operation
When the MT2834MR is in the dial back-up mode of operation, it will periodically check the leased line to
see if it's operational and try to restore the leased line if possible. This is the automatic leased line
restoral feature.* The frequency of the restoral feature attempts are determined by the contents of S-
Register S18. The default for S18 is 30 minutes and can be set in one minute increments from 10 to 255
minutes. Setting the restoral frequency under 10 minutes causes excessive breaks in the dial-up
operation.
2.3
Manual Dial Backup Call Termination
If your MT2834MR is in the leased line mode (DIP Switch #10 in the Down ("Closed") position) and with a
dial back-up operation in process, there are two ways to manually terminate (other than automatic leased
line restoral) the dialback call. In each of these cases, you will be attempting to establish the leased line
connection because it is back in operating condition.
*Both local and remote modems should have S-Register S18 set identically.
The methods of dial-back call termination are:
1. DTR Control. If DTR (Data Terminal Ready) is turned off for 50 milliseconds or more, a disconnect
will occur. This is probably the most common method used by computer systems at the automatic answer
end of the line to cause the answering modem to disconnect after logging off procedures.
2. Command Mode Control. It is possible to enter Command Mode while remaining on-line by
entering the Escape Sequence (+++AT<CR> in most cases). After you have entered Command mode,
you may either return to the On-Line by typing ATO, or terminate the call with the H command. Typing
ATH will hang up the line (bring it on-hook), bringing the modem out of the On-Line mode. So, the
command to hang up a call is three plus signs, and then ATH.
3. Loss of Carrier. After a data connection has been established, the modem will disconnect if a loss of
carrier occurs for 700 milliseconds (.7 seconds) or more. The 700 mSec time can be configured for any
time from 0 to 25.5 seconds, in 100 mSec increments, with S-Register S10. Note that one cause of carrier
loss would be if one of the modems were to disconnect normally. S10 works for speeds of 2400 bps or
below, otherwise carrier loss will take about 20 seconds in a V.34 connection.
2.4
Dial Back-up Leased Line Restoral
The dialing associated with the MT2834MR when it's operating in leased line with dialback mode,
involves the placing of a call from the originating modem due to a leased line failure. The call can be
placed automatically by the modem when the below described conditions are met. The dialback condition
is indicated by the OOS LED blinking. After a preset period of time (determined by S-Register S18), the
modem will try and restore the leased line automatically.
The parameters used to determine whether a leased line is down (so that automatic dialback can occur),
is based on the MT2834MR doing a retrain on the leased line due to an error condition in the
transmission. An error condition is defined as a "hit" on the line (the Carrier gets interrupted).
The retrain is a "handshake" procedure between the modems to establish the Carrier again. If the retrain
fails, both modems (originate and answer modems) start their Dialback timers. The time is determined by
the setting of S-Register S19. The default for the time is one minute. During that minute, the originate
modem will try to establish the leased line link. If during that time, the leased line is established, the timer
is cleared and everything is back to normal. If the timer expires the modems will go to dialback mode.
4/3/01
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