Connecting to your ISP

Using the AT Command Set

Upgrading your Modem

Modes of Operation

Dialing, Answering, and Hanging Up

Working with Memory

Controlling Result Code Displays

Controlling EIA-232 Signaling

Accessing and Configuring the Courier 56K Corporate Modem Remotely

Controlling Data Rates

Dial Security

Flow Control

Handshaking, Error Control, Data Compression, and Throughput

Displaying Querying and Help Screens

Testing the Connection

Dedicated/Lease Line and Synchronous Applications




Alphabetic Command Summary

Flow Control Template

Result Code Meanings and Sets

Technical Information

V.25 bis Reference


Fax Information for Programmers

Viewing LEDs

Regulatory Information


Courier 56K Corporate Modem Command Reference

Handshaking, error control, data compression, and throughput

This chapter contains information about:


With each call, Courier 56K Corporate modems go through a link negotiation process with the remote device. Another name for the negotiation process is "handshaking."

The Courier 56K Corporate modem attempts to find both the highest compatible modulation and the highest rate, in the following order: V.90, x2, V.34, V.32 bis, V.22 bis, V.22 (Bell 212), and Bell 103.

Selective Reject

The Courier 56K Corporate modem supports Selective Reject for analogue calls. Selective Reject improves performance on noisy lines by reducing the amount of overhead incurred when the protocol must resend data due to errors.

When Selective Reject is active, only the frame that contained the error is re-sent, instead of the frame plus all of the following unacknowledged frames.

Selective Reject is an optional part of the ITU-T V.42 (LAPM) standard.

For your modem to
Enable Selective Reject
ATS51.6=0 (Default)
Disable Selective Reject

Attaining Speeds Above 28.8 Kbps

V.34 connections at 21.6, 24, and 26.4 Kbps are common. To get connections of 28.8, 31.2, and 33.6 Kbps, line quality must be pristine end-to-end.

Controlling the V.8 Call Indicate Tone

The V.8 protocol speeds call negotiation and specifies a call indicate tone. Providing the call indicate tone is optional. For compatibility, we ship the Courier 56K Corporate modem with the call indicate tone disabled.

For your modem to
Enable the call indicate tone

If you enable the V.8 call indicate tone, expect to hear a sound like a fast ringing signal while the call is being connected.


If the remote device has V.34 capability, the Courier 56K Corporate modem uses a line probing technique in order to determine if a V.90 connection can be established.

When the Courier answers a call, it sends out a series of answer tone signals until both devices negotiate the best connection rate.


V.90 Capabilities

The Courier 56K Corporate modem has V.90 capabilities. The Courier 56K Corporate modem can dial into V.90 servers to establish speeds up to 56K downstream.

For your modem to
Enable V.90
ATS58.5=0 (Default)
Disable V.90

x2 Technology

x2 client modems can receive data at speeds up to 56 kbps and send data at V.34 speeds. To use x2, the client x2 modem must connect to a server x2 modem. If clients attempt to connect to ISPs that do not use x2, the client modem will negotiate the next available modulation. For example, an x2 client modem calling into an ISP that only supports V.34, the modem will only negotiate the highest v.34 connection rate. The maximum V.34 connection speed is 33.6 kbps.

Other Protocols

Fast Class (V.FC) Handshaking

After trying V.34, the Courier 56K Corporate modem tries for the fastest possible V.Fast Class (28.8 Kbps) connection. In order to negotiate V.FC, V.8 has to be disabled. V.FC is not part of the ITU V.8 training sequence.

If the remote device is not V.FC capable, a connection is made using the highest compatible modulation scheme (V.32 terbo, V.32 bis, and so on, down to as low as Bell 103, or 300 bps).

If the remote device has V.FC capability, the Courier 56K Corporate modem uses a line probing technique to determine the highest speed possible under current line conditions, then completes the connection. If the remote analogue device does not have V.FC capability, the Courier 56K Corporate modem listens to the device's answer tones to identify what standard rate the remote analogue device is operating at, and adjusts to that rate.

When a Courier 56K Corporate modem answers a call, it sends out a series of answer tone signals until both devices negotiate the best connection rate.


We recommend that Courier 56K Corporate modems retain the default B0 and &N0 settings. This allows them to make analogue connections with "V." protocol and HST modems in both Originate and Answer modes at a variety of speeds.

When originating an analogue call, the Courier 56K Corporate modem set to B1 sends out a Bell answer tone, which is the prevalent standard in the U.S. and Canada for connections at 2400 bps and lower. At higher speeds, the Courier 56K Corporate modem also recognises the ITU answer tones necessary for connecting with V. protocol modems, and adjusts to the answering device.

However, when answering a call, a Courier sending out the Bell answer tone (B1) won't be recognised by V. protocol modems. The calling modem, instead, will wait until it detects a tone it recognises (the V.22 bis tone used at 2400 bps).

If you want to have your Courier connect with V. protocol modems at high speeds, make sure it is set to B0 for the ITU answer tones. It will also connect with HST modems at speeds up to 16.8 Kbps.

USR V.32 terbo to USR V.32 terbo

On these analogue connections, Courier 56K Corporate modems have two features that result in outstanding performance: Quick Connect and Adaptive Speed Leveling (ASL).

  • Quick Connect allows two Courier 56K Corporate modems to connect in approximately 7 seconds, a far shorter time than with most devices.
  • ASL (described below in Other V.Protocol Operations) is used by Courier 56K Corporate modems operating in V.32 terbo and V.32 bis modes.

Lower-speed V. Protocols

Older, lower-speed "V." protocols do not employ line probing. Instead they use predefined answer tones to specify or identify speed capabilities. These protocols define the following maximum speeds:

  • V.32 terbo: 16.8 and 19.2 Kbps, with an additional USR device-to-USR device speed of 21.6 Kbps.
  • V.32 bis: 14.4 Kbps.
  • V.32, V.22 bis, etc.: 9600 bps and lower.

ASL (used in V.32 terbo and V.32 bis modes) is a technique that allows the Courier 56K Corporate modem’s receivers and transmitters to act independently of each other. One transceiver may slow down and then speed up without affecting the data flow on the other. The result is more efficient line operation.

Note: While most modems on the market now use higher speeds, there may be a problem in answering older V.32 modems at 9600 bps. Use Register S28 to modify the duration of the extra tones used in V.32 negotiations, in the rare instance that this may be necessary. Refer to the S-Registers appendix in this guide.

Error Control

Courier 56K Corporate modems can employ error-control techniques during data connections.

Note: High-speed data calls are vulnerable to errors unless the data is protected by error control. If your Courier 56K Corporate modem connects with a remote device at a high speed, but without error control, and if you are not using an error control protocol for your call, you may lose data.

Automatic Repeat Request (ARQ) is a method used in many error-control protocols to ensure that any data that has been corrupted in transit is retransmitted. We use the term to designate a connection under error control.

Error-Control Commands

You can use Error-Control commands to enable ARQ (error control) or enable synchronous protocols. The Courier 56K Corporate modem and the remote device must use the same protocol.

For your modem to
Have no error control (Normal Mode).

Due to the nature of phone line channels, this is never recommended for analogue calls above 2400 bps.
Enter Online Synchronous Mode without V.25 bis. External Courier 56K Corporate modems only.
Operate in Normal mode if an ARQ connection can’t be made (Normal/ARQ mode).
Hang up if it can’t make an ARQ connection (ARQ Asynchronous Mode).
Start V.25 bis synchronous mode, using the BISYNC protocol.
Start V.25 bis synchronous mode, using the HDLC link protocol.

Example: Sending AT&M4 allows your modem to operate in Normal Mode if an ARQ connection can’t be made.

Error control is available for calls at 1200 bps and above. It can be disabled, although high-speed calls (9600 bps or higher) should always be under error control. The operations defined in an error-control protocol include:

  • Establishing compatibility.
  • Formatting data frames.
  • Detecting errors using Cyclic Redundancy Checking (CRC).
  • Retransmitting corrupt data frames.

The Courier 56K Corporate modem is set at the factory to try for an error-control connection and, if that isn't possible, to proceed with the call in Normal mode. This means that &M4 is set.

V.42 Error Control

The Courier 56K Corporate modem first tries for a V.42 connection, then an MNP connection. The following information is based on the Courier 56K Corporate modem's setting of &M4.

This international standard includes a two-stage hand-shaking process:

  • A Detection phase that is based on an exchange of predefined characters.
  • A Link Access Procedures for Modems (LAPM) Negotiation phase, during which the devices identify their capabilities concerning ARQ .

MNP Error Control

The Microcom Networking Protocol (MNP) is supported by the ITU-T V.42 Recommendation. MNP is based on special protocol frames. If the remote device doesn't recognize an MNP Link Request, error control isn't possible. (In HST ARQ mode, USRobotics devices use a proprietary scheme similar to MNP.)

Error Control and Flow Control

Flow control of data from the computer is required under error control for two reasons:

  • The transmitting device saves a copy of each frame it transmits to the remote end until it is acknowledged by the receiving device.
  • If errors are encountered, retransmission activity can cause a steady stream of data from the computer to overflow the buffer.


Data Compression

Courier 56K Corporate modems employ different data compression techniques during data connections.

For the modem to
Disable data compression.
Auto-enable/disable data compression. The Courier 56K Corporate modem enables compression if the serial port rate is fixed (&B1) and disables compression if the serial port rate follows the connection rate (&B0).
AT&K1 (Default)
Always enable data compression.
Selective data compression. Use this setting to transfer compressed files. The Courier 56K Corporate modem negotiates only for V.42 bis compression, and disables MNP Level 5 (MNP5) compression.

Example: Sending AT&K3 is used to transfer compressed files. The modem will only negotiate for V.42 bis compression. This command also will disable MNP Level 5 (MNP5) compression.

If a Courier 56K Corporate modem successfully establishes a V.42 error control connection with a remote device, it also negotiates for V.42bis data compression.

If a Courier 56K Corporate modem successfully establishes an MNP connection with a remote device, it also negotiates for MNP5 data compression.

The type of compression for a call, if any, is reported in the ATI6 display and in the CONNECT message if the modem is set to &A3.

V.42 bis versus MNP5 Data Compression

Courier 56K Corporate modems using V.42 bis compression negotiate the following options and report them in the ATI6 display:

  • Dictionary size, that is, the amount of memory available for compression table entries. (Entries are codes devised for redundant data. User data is encoded into codewords and decoded by the receiving device).

    Possible dictionary sizes:


    Your modem uses a 12-bit, or 4096-entry dictionary, but can reduce its size to accommodate a remote modem that uses a 9-, 10-, or 11-bit dictionary.

  • Maximum string length of each entry. As the dictionary fills, your modem deletes the oldest unused strings.

V.42 bis compression is more efficient than MNP5 compression, in part because it dynamically deletes entries that are no longer used. In addition, it works better with files that are already compressed. These include .ZIP files and 8-bit binary files.

MNP5 compression should NOT be used with binary files because it expands the data size which lessens throughput. When transferring such files, it's best to set the modem to &K3 (See the information in data compression). This allows V.42 bis compression to work dynamically with the compressed data, but disables MNP5.


Getting Maximum Throughput

The following guidelines should help you to make the most of the Courier 56K Corporate modem's advanced performance features. In many instances, experimentation and experience will indicate what works best for your applications.

Maximum throughput results when:

The communications software allows fixing the serial port rate higher than the connection rate, by setting the software to 115200, 57600, or 38400 bps and setting the Courier to &B1.

  • If the software automatically switches serial port rates to follow the connection rate, the Courier 56K Corporate modem's serial port rate must be also set to follow the connection rate for each call, &B0, and throughput will be limited.

    Installations with specialised software may want to enable a fixed serial port rate for ARQ calls and a variable serial port rate for non-ARQ calls. See the &B2 command in the Alphabetic Command Summary appendix in this guide.

  • The call is under data compression.
  • The data is made up of text files rather than binary files such as .EXE or .ZIP files.
  • MNP5 compression is disabled for files that are already compressed, and 8-bit binary files that appear to modems to be already compressed. Disable MNP5 compressing by sending the Courier 56K Corporate modem AT&K3.
  • The file transfer is not slowed down by a file-transfer protocol. Many non-text files require a file transfer protocol, but the results vary.

Note: For the best throughput on error-controlled connections only and with hardware flow control we recommend the most current version of ZMODEM file transfer protocol.

For example, certain public domain file transfer protocols have the following effects:

Public Domain
Newer versions support packets up to 9K and a sliding window design to eliminate turnaround delay. With earlier versions, however, throughput may be severely reduced due to short block lengths (possibly under 128 bytes) and acknowledgment turnaround time.
Throughput may be reduced if your version uses short block lengths (128 bytes). Some versions use larger blocks (1K blocks). Throughput is also reduced by overhead (error control protocol information).
There is an improvement over XMODEM, due to larger block lengths (1K bytes), but throughput is still reduced by the protocol's error control overhead.

The protocols listed above further reduce throughput when an error control connection is established. The accuracy of the data is checked both by the file transfer protocol and the Courier 56K Corporate modem. To avoid redundancy, use the above protocols only for non-ARQ connections, and only at speeds of 2400 bps and below.

Overhead is minimal with ZMODEM, resulting in throughput that is almost equal to that obtained with no file-transfer protocol. ZMODEM should also be used for non-ARQ connections. Leave the Courier 56K Corporate modem at its &M4 and &K1 settings for both error control and data compression.

YMODEM-G is another good choice, but never use it unless both the local and remote devices are using error control: if YMODEM-G detects an error, it aborts the transfer. Do not use either protocol with software flow control (XON/XOFF signaling).