Controlling Data Rates

This section contains information about:

Overview

You can set the Courier 56K Business Modem to use fixed or variable serial port rates and fixed or variable connection rates. Serial port rates pertain to data transferred between your computer and the Courier 56K Business Modem. Connection rates pertain to data transferred between the Courier 56K Business Modem and the device at the other end of a connection.

Serial Port Rates

You set a fixed serial port rate to get the highest possible throughput and the best performance. Set a variable rate to allow the Courier 56K Business Modem to match the connection rate.

Your software must support fixed or variable serial port rates.

Note: Your software may use terms such as locked serial port (fixed rate) or autobaud (variable rate).

To allow your modem Set the serial port rate as Command
To change its serial port rate to match the connection rate. Variable AT&B0
To always communicate with an attached device at the rate at which you have set the terminal or software, regardless of the connection rate. Fixed AT&B1 (Default)
For the greatest throughput, set the serial port to 230400, 115200, 57600, or 38400 bps for high-speed calls, and to at least 9600 bps for 2400-bps calls.

To shift its serial port rate to a rate that you specify using your communications software (for example, 38.4 Kbps) when the Courier 56K Business Modem connects in ARQ mode.

If the connection is not under error control, the Courier 56K Business Modem behaves as if it was set to &B0 and switches its serial port rates to match the connection rate of each call.

Fixed for ARQ calls AT&B2

To implement this feature, first set your software to the desired rate. Then send AT&B2&W to the modem.

The Courier 56K Business Modem stores the rate of the command in NVRAM along with the current settings. The Courier 56K Business Modem checks NVRAM for the specified serial port rate each time it makes an ARQ connection.

When saving subsequent configurations to NVRAM, be sure your software is set to your selected serial port rate so the correct rate is maintained.

Variable for non-ARQ calls when answering only.

Refer to the Handshaking, Error Control, Data Compression, and Throughput section of the Contents section for more information about ARQ

Note: The serial port rate MUST be equal to or higher than the Connection rate (&Nn).

Connection Rates

You can set the Courier 56K Business Modem to a fixed or variable connection rate for data calls. Set a variable rate to have the Courier 56K Business Modem negotiate with the remote device for the highest possible connection rate. Set a fixed rate to connect only at a specified rate. You might use a fixed rate to filter calls for security or other reasons.

To allow your modem to Set connection rate as Command
Negotiate for the highest possible rate. Variable AT&N0
Connect only if the remote device is operating at the rate you specify. See below. Fixed AT&N1-AT&N38

 

To connect at this rate Command To connect at this rate Command
300 bps AT&N1 32.0 Kbps AT&N20
1200 bps AT&N2 33.333 Kbps AT&N21
2400 bps AT&N3 34.666 Kbps AT&N22
4800 bps AT&N4 36.0 Kbps AT&N23
7200 bps AT&N5 37.333 Kbps AT&N24
9600 bps AT&N6 38.666 Kbps AT&N25
12.0 Kbps AT&N7 40.0 Kbps AT&N26
14.4 Kbps AT&N8 41.333 Kbps AT&N27
16.8 Kbps AT&N9 42.666 Kbps AT&N28
19.2 Kbps AT&N10 44.000 Kbps AT&N29
21.6 Kbps AT&N11 45.333 Kbps AT&N30
24.0 Kbps AT&N12 46.666 Kbps AT&N31
26.4 Kbps AT&N13 48.0 Kbps AT&N32
28.8 Kbps AT&N14 49.333 Kbps AT&N33
31.2 Kbps AT&N15 50.666 Kbps AT&N34
33.6 Kbps AT&N16 52.0 Kbps AT&N35
28.0 Kbps AT&N17 53.333 Kbps AT&N36
29.333 Kbps AT&N18 54.666 Kbps AT&N37
30.666 Kbps AT&N19 56.0 Kbps AT&N38

Example: Sending AT&N8 will only allow connections with remote devices that are operating at 14.4 Kbps.

 

Controlling Link Speeds With &N and &U

You can use the &N and &U commands to control link speeds.

Controlling Link Speeds

Use the following table to determine how to use &N and &U commands:

To limit the Use
Specific connect speed AT&N
Range of possible connect speeds AT&N and AT&U

Note: The default values for &N and &U are 0. If you change these values, you will limit the speeds at which you can connect. U.S. Robotics recommends that you do not alter these values.

Specifying the Connect Speed

The &N command allows you to specify the connect speed. If a remote modem Attempts to connect to your Courier 56K Business Modem at a speed different from &N, your Courier 56K Business Modem will not allow it to connect.

To limit the Use this command Where x is
Highest possible connect speed AT&Nx A value from 0 to 38

Note: For a complete list of connect speeds, see the Command Values section.

Limiting a Range of Possible Connect Speeds

By setting &N and &U values, you can limit the range of speeds at which your Courier 56K Business Modem connects. If a remote modem does not connect to your Courier 56K Business Modem at a range between the speeds designated by the &N and &U commands, your Courier 56K Business Modem will not allow it to connect.

The link speed associated with the &U argument CANNOT be greater than the link speed associated with &N argument.

Use the following table to understand the relationship between &N and &U commands:

If &U And &N Then your modem
Equals zero Equals zero Connects at the highest possible speed.
Equals zero Is greater than zero Connects at the &N speed only.
Is greater than zero Is greater than zero and greater than &U Connects at the highest possible speed in the range from &U to &N .
Is greater than zero Is greater than zero and less than &U Not allowed.

&N and &U Command Values

Use the following table for a complete list of &N and &U link speeds and their associated indexes:

Link Speed Index Link Speed Index
Highest 0 32000 20
300 1 33333 21
1200 2 34666 22
2400 3 36000 23
4800 4 37333 24
7200 5 38666 25
9600 6 40000 26
12000 7 41333 27
14400 8 42666 28
16800 9 44000 29
19200 10 45333 30
21600 11 46666 31
24000 12 48000 32
26400 13 49333 33
28800 14 50666 34
31200 15 52000 35
33600 16 53333 36
28000 17 54666 37
29333 18 56000 38
30666 19    

Configuring The High Speed LED

You can configure your Courier 56K Business Modem to alert you when it reaches 56K speeds. Use the following S69 setting to configure the HS (High Speed) LED:

To do this Command
Configure the HS LED to turn red when your modem reaches speeds over 33.3 kbps. ATS69=12

Setting DTE Rate To 230 Kbps

The DTE rate of your Courier 56K Business Modem has been increased to 230 kbps to enhance throughput. Your modem must be attached to a serial port capable of this speed to achieve this rate.

Note: 230 Kbps DTE rate is only supported with modems attached to high speed serial cards.