Asynchronous Control Character Map (ACCM)

Definition

ACCM, fully Asynchronous Control Character Map, is a parameter in the Point-to-Point Protocol (PPP) used to identify control characters specified in a data frame of the serial data transmission mode. This function is especially crucial in communication over asynchronous serial interfaces, as it allows special characters to be identified and correctly interpreted to ensure interference-free data transmission.

More information (Wikipedia)

Operating principle

The ACCM is a 32-bit long field used to detect control characters in a data set. Each bit in the field represents a control character. If the bit is set to 1, it means that the corresponding control character is rejected (escaped) by the PPP software. The process starts with a standard ACCM that rejects all control characters except the ASCII null character (which is disabled) and the control-Q character (which is enabled). However, after connection establishment, the ACCM can be configured through Link Control Protocol (LCP) options.

Practical examples

• Data transmission between two computers via a modem.
• Terminal server applications where multiple terminals are connected to a host computer.
• VPN connections in which the ACCM value helps to optimize data transmission.

Advantages

• Facilitates the trouble-free transmission of data over serial interfaces.
• Provides the ability to customize the control characters used in a specific data stream.
• Enables efficient use of bandwidth by optimizing the data stream.
• Supports interoperability between devices from different manufacturers.
• Increases data security through precise identification and handling of control characters.
• Helps with troubleshooting and fixing network configurations.
• Offers flexibility through the ability for dynamic configuration during connection setup.
• Offers stability and reliability for long-term connection requirements.

challenges

• The incorrect configuration of the ACCM can lead to communication difficulties.
• The use of standard ACCM settings can be problematic in some cases.
• The need to detect asynchronous control characters can increase the complexity of network design.
• The ACCM configuration can vary between different operating systems.
• The dynamic configuration of the ACCM during connection setup requires proper synchronization.
• The incorrect interpretation of control characters can lead to data loss.
• A security risk may exist if critical control characters are not interpreted properly.
• The need to optimize the ACCM for specific requirements can lead to additional costs.

Best Practices

• Conduct a thorough analysis of communication requirements before implementation.
• Regularly review and update the ACCM configuration.
• Use individually customized LCP options to configure the ACCM if needed.
• Ensure the interoperability of the devices before ACCM is implemented.
• In case of communication difficulties, consider the ACCM settings as a potential factor.
• Systematically document and manage the ACCM settings.
• Use secure test environments to test the configuration before implementation.
• Perform continuous monitoring and logging of the ACCM settings.

Conclusion

ACCM is a key parameter in PPP connections and plays a crucial role in reliable and efficient data transmission over serial interfaces. Although the configuration and management of ACCM can present some challenges, they still offer significant advantages in terms of flexibility, interoperability, and network performance. With careful planning, monitoring, and adjustment, ACCM can help ensure optimal communication and data transfer between devices in a network.