A critical function of the G.704 standard is to embed channels for network operation and signalling within the frame structures.

4.1. Performance Monitoring and Data Links

For several hierarchical levels (1544, 6312, and 44 736 kbit/s), the frame overhead is used to create a dedicated data link. This link is used to transmit crucial OAM information. A key application is the performance report, which allows a network terminal to send back a summary of detected errors to the originating end.

The 1544 kbit/s data link, for example, defines a comprehensive set of messages for various functions:

• Priority Messages: Loss of Frame Alignment (LFA), Loopback Retention.
• Loopbacks: Commands to operate and release various types of loopbacks for testing.
• Protection Switching: Commands to operate and switch to backup lines.
• Synchronization Status: Reports on the quality of the timing source.
• Performance Reports: Structured reports detailing error events like CRC errors, severely-errored framing events, and line code violations.

4.2. Signalling Accommodation

G.704 provides standardized methods for carrying signalling associated with voice or data channels.

• Common Channel Signalling: A full 64 kbit/s time slot is allocated exclusively for signalling. For the 2048 kbit/s interface, this is typically Time Slot 16.
• Channel-Associated Signalling (CAS): Signalling for each individual channel is carried in specific bits allocated within a multiframe structure. For example, in the 2048 kbit/s interface, a 16-frame multiframe is used where Time Slot 16 of each frame carries signalling bits for two user channels. This allows for lower-rate signalling to be sub-multiplexed efficiently.