Worksheet 1

Modem Connection Types

The following commands tell your modem what type of connection you want it to try. Do not use them during an active connection.

AT\N0 Normal Connection

If you use AT\N0, the modem still acts just like a regular modem, except that you won't have to change your computer rate to match the connection rate. You'll be using "flow control". (Flow control is explained later in this chapter.)

AT\N1 Direct Connection

If you use AT\N1, the modem will act just like a regular modem that doesn't support error correction or data compression. Your computer rate and connection rate must match.

AT\N2 Reliable Mode

If you use AT\N2, the modem will try to make a V.42 or MNP connection. If it can't, the modem will hang up. Reliable Mode features flow control.

AT\N3 AutoReliable Mode

If you use  AT\N3, the modem will try to make the best connection possible. If it can make a V.42bis connection, it will. If not, it "falls back" through V.42 and MNP, trying each one until it is able to connect. (It will not attempt an MNP 5 connection if you've issued the AT%C0 command.) The connection the modem actually makes can be Reliable or Normal. In most situations, you will want to use AT\N3.

AT\N4 LAPM Mode

If you use AT\N4, the modem tries to make a V.42 connection, and if it can't, it hangs up.

AT\N5 MNP Mode

If you use AT\N5, the modem tries to make an MNP connection, and if it can't, it hangs up.

Flow Control

Normal and Reliable connections use a technique called "flow control" that lets your computer rate (computer - to - modem rate) be different from your connection rate (modem - to - modem rate). Flow control is useful with data compression because your throughput is higher than the connection rate and because the throughput changes depending on the compressibility of the data being sent. Flow control lets your modem and computer tell each other when they can accept more data for transmission and when to wait. This "handshaking" prevents data from being lost because the computer or modem was busy.

Hardware flow control (also called hardware handshaking) uses two of the modem's RS - 232 lines for signaling. Software flow control uses special XON and XOFF characters to communicate between your computer and the modem. Hardware flow control is the preferable method because of its speed and reliability.

To use hardware handshaking flow control, you must have:

• Hardware flow control enabled in your telecom software.
• Flow control enabled by the AT&K3 command.

This extract has been taken from my manual for my modem - SupraExpress 336i PnP - produced by Diamond Multimedia.

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After reading through the text, try to test yourself.