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API Message Guidelines

Applications must conform to the following guidelines when using API messages:

Downloading System Software

A matrix fault will occur if you allow more than 25 outstanding messages remain unacknowledged simultaneously while you are downloading system software.

Matching Messages and Responses

Every message must receive a response, regardless of whether the CSP or the host sent the message. The response contains a status byte, and you should design your application to check the status bytes of host-initiated messages to determine if the message succeeded or failed.

The message flows between the host and the CSP are not synchronized. The host can send multiple messages, while the CSP simultaneously sends multiple messages to the host. Sequence numbers are used to match messages with responses. The originator of the message selects the sequence number. For host-initiated messages, the host communications driver typically selects the sequence number, and provides a counter for each message type. The host allows up to 255 outstanding messages for each message type.

Message Trace Format

When you see message traces in hexadecimal format, the first byte of each message (the framing byte, 0xFE) is not displayed.

Call Processing Messages

When a host application sends call processing messages to the CSP, the host must be aware of associations between channels and between spans and channels. The host can send only one call processing message at a time to a channel. Design your application to track channel information such as state and configuration.

Configuration Messages

Many of the configuration messages use a range to configure multiple channels. When you specify channel ranges, do not exceed eight spans. It is important to read the description for each message.

A positive acknowledgment (0x10) to a configuration message can indicate that the configuration is complete. But sometimes the CSP sends a separate message to indicate that the configuration is complete. For example, if the host sends a Release Channel message, the CSP sends back a Channel Released message.