You are here: CSP Developer’s Guide: Overview > 5 Layer 4 Call Control > Call Processing Guidelines

Call Processing Guidelines

During call processing, real-world scenarios sometimes cause variations to the normal message flows designed by the application developer. The following are problem scenarios, with guidelines for developing the host application to accommodate these variations.

Glare

For bidirectional interfaces, the host and the network attempt to initiate an outseizure on a channel at the same time. Each network signaling protocol has its own means of determining which call is dropped. If the host initiates the outseizure with a Route Control (or Outseize Control) message, a response status of Outseize Failure, Glare (0x1C) is returned, followed by a Channel Released message (if the outgoing call is dropped). The CSP then sends a Request for Service or Request for Service With Data message to the host for the incoming call.

Positive ACK Wait

When sending multiple call control messages, the host should wait for a positive acknowledgment (ACK) to each message before sending another. Call Control allows multiple outstanding messages to a channel, but they are queued and processed in order until previous messages have been acknowledged.

If a channel purges, the CSP does not acknowledge outstanding host messages. You can configure the CSP to acknowledge outstanding host messages upon a purge by changing PPL Config Byte 19 (0x13) of the CH component to 0x01.

More Status

The More Status field is valid for some connection management messages. (See Response Status Values in the API Reference). For example, when the value "Invalid Event" (0x1D) is returned, the More Status value indicates the state of the CH state machine.

Important! Some of the CH component call state values reported in the Status are not backward compatible with the values generated before System Software Release 5.3.

DS0 Status Change and Channel Released Messages

Design your host application so that all channel states can accept and process the DS0 Status Change message from the CSP. All non-idle channel states must accept and process the Channel Released message.

Release Channel Race

When the distant end and the host initiate a release concurrently, a status of Invalid Event (0x1D) is returned, indicating that the call is being released. The CSP sends a Channel Released message when the distant end releases.

Parking Channel Association

When the host initiates a Park Channel (A, B) message on a two-way connection, the acknowledgment message allows other messages to be directed to Channel A, and indicates only that Channel A has been parked. The host receives a DS0 Status Change message indicating that Channel B is parked.

Purge With No ACK

Sometimes the host initiates a message to the CSP, for example Outseize Control, to cause the purging sequence. Therefore, upon receiving the DS0 Status Change message of Out-of-Service, the CSP does not acknowledge the original message.

Outgoing call setup during real-time call processing on a specific channel.
Positive acknowledgment to a message
Setting up, monitoring, and tearing down calls.
See Programmable Protocol Language.