Multi-Tech Systems MT2834MR6 manual Dial-Up Automatic Answering, Dial-Up Handshaking Details

Chapter 2 - Dialing and Answering

2.1Introduction

This chapter describes the dialing and answering capabilities of the MT2834MR6 modems, which are standard ITU-T V.34 full duplex dial-up modems.

2.2Dial-Up Automatic Answering

The MT2834MR6 can answer calls automatically. The MT2834MR6 is in originate mode in its idle state. When it detects an incoming call, it automatically switches to answer mode. You can program the modem to answer a call after a certain number of rings by using register S0 (see Chapter 4).

2.3Dial-Up Handshaking Details

This section briefly explains what happens between two modems in a normal call. We are assuming that there are MT2834MR6s at both the originating and at the answering end of the telephone line. (If other brands of modems are used with our modems, they will still communicate, but each manufacturer tends to use slightly different delay timings and sequences, and many of the brands vary in their indicator designations.)

Before V.34 negotiation takes place, remote and local modems transfer their functional capabilities using modulated calling and answering tones. First, the Calling Menu (CM), a data sequence using V.21 low-band modulation (a handshaking technique originally developed for 300 bps modems), is sent from the originating V.34 modem to the answering V.34 modem. This describes the range of functional capabilities the originating modem supports. The answering V.34 modem responds to the CM with a Joint Menu, or JM (using V.21 high- band modulation), indicating the common capabilities of the modem at each end.

Once this is done, a probing signal is passed between the modems to identify impairments in the telephone channel. After receiving the results of the probing signal, the modem receivers and transmitters will start with the lowest speed and move up until reaching the highest speed at which they can exchange data. All this occurs in about five seconds.

Note: In order for the called modem to be able to answer the call, it must have a high DTR (Data Terminal Ready) signal on pin 20 of the RS-232C interface. This signal comes from the computer or terminal to which the modem is connected.

Note: If you are using reliable or auto-reliable mode, some additional handshaking takes place. This is explained in Chapter 5, in the “Auto-Reliable Mode” section.

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