This technical specification provides a detailed, practical guide to the synchronous frame structures defined in ITU-T Recommendation G.704. These structures are fundamental to the operation of digital transmission systems, establishing the standardized formats necessary for multiplexing digital signals and ensuring interoperability between equipment from different manufacturers. By defining the precise arrangement of bits within a digital stream, G.704 enables network nodes to correctly interpret payload data, signalling information, and essential operational overhead.

The scope of this specification is to define the functional characteristics of interfaces at the hierarchical bit rates of 1544, 6312, 2048, 8448, and 44 736 kbit/s. These definitions encompass frame length, frame alignment signals, Cyclic Redundancy Check (CRC) procedures, and the allocation of bits for various operational and maintenance functions.

The primary types of equipment to which this specification applies include:

• Synchronous digital multiplex equipment
• Digital exchanges for telephony and Integrated Services Digital Networks (ISDNs)
• Pulse Code Modulation (PCM) multiplexing equipment

This document will proceed with a detailed examination of each hierarchical level, beginning with the foundational 1544 kbit/s interface.