This section contains information about:
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.
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. | ||
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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 |
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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 |
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Note: The serial port rate MUST be equal to or higher than the Connection rate (&Nn).
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.
You can use the &N and &U commands to control 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.
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.
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. |
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 |
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 |
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.