ITU-T G.704 Synchronous Digital Transmission Frame Structures

3. Detailed Frame Structure Specifications by Hierarchical Level

G.704 provides precise specifications for the frame structure at each of the five bit rates.

3.1. 1544 kbit/s Interface

This interface, commonly associated with T1/DS1 lines, is characterized by a compact frame structure.

Parameter	Specification
Frame Length	193 bits (numbered 1 to 193).
Overhead	A single bit, the F-bit (bit 1 of each frame).
Channel Capacity	24 octet-interleaved 64 kbit/s channel time slots.
Multiframe Structures	Two methods are defined for allocating the F-bit:
	24-Frame Multiframe: The F-bit pattern across 24 frames is used to provide FAS, a CRC-6 check for error monitoring, and a 4 kbit/s data link. This data link carries OAM messages for functions like loopback, protection switching, synchronization status (SSM), and detailed performance reports.
	12-Frame Multiframe: An alternative structure used primarily for carrying channel-associated signalling.

3.2. 2048 kbit/s Interface

This interface, commonly associated with E1 lines, uses a dedicated time slot for overhead functions.

Parameter	Specification
Frame Length	256 bits (numbered 1 to 256), organized into 32 time slots of 8 bits each.
Overhead	Time Slot 0 (the first 8 bits of the frame) is reserved for overhead.
Channel Capacity	31 octets for user data channels (Time Slots 1-15 and 17-31). Time Slot 16 is typically used for signalling.
Overhead Functions	Frame Alignment: A FAS word is transmitted in Time Slot 0 of alternate frames.
	CRC-4 Procedure: A 16-frame multiframe structure (divided into two 8-frame Sub-Multiframes, or SMFs) is defined to enable a CRC-4 check for robust error monitoring. The CRC-4 bits are transmitted in bit 1 of Time Slot 0.
	Synchronization Status: Spare bits (Sa4 to Sa8) in Time Slot 0 are used on international links to transmit Synchronization Status Messages (SSM), indicating the quality of the network’s timing source.

3.3. 6312 kbit/s Interface

Parameter	Specification
Frame Length	789 bits.
Overhead	The last five bits of each frame are designated as F-bits.
Channel Capacity	98 octet-interleaved 64 kbit/s channel time slots.
Overhead Functions	The F-bits are allocated to provide frame alignment, remote end alarm indication, a CRC-5 check, and a 4 kbit/s data link.

3.4. 8448 kbit/s Interface

Parameter	Specification
Frame Length	1056 bits.
Channel Capacity	132 octet-interleaved 64 kbit/s channel time slots.
Overhead Functions	The frame includes a frame alignment signal and service digits for alarm indication.
	CRC-6 Procedure: An end-to-end quality monitoring procedure is specified, using a CRC-6 check. The bits for this procedure are transmitted within one of the 64 kbit/s channels (Time Slot 99).

3.5. 44 736 kbit/s Interface

This interface, associated with DS3 lines, uses a more complex multiframe structure for its overhead.

Parameter	Specification
Frame Structure	A 4760-bit multiframe.
Sub-Structures	The multiframe is divided into 7 M-subframes of 680 bits each. Each M-subframe is further divided into 8 blocks of 85 bits (1 overhead bit and 84 payload bits).
Overhead Bits	The multiframe overhead is distributed across several types of bits:
	F-bits: Provide M-subframe alignment.
	M-bits: Provide multiframe alignment.
	P-bits: Used for performance monitoring (parity check).
	X-bits: Used to signal an error condition from the remote end.
	C-bits: Path application bits that can be used for various purposes, including unchannelized bulk data, channelized multiplexing, and Far-End Alarm and Control (FEAC) signals.
Special Signals	The recommendation defines specific bit patterns for Alarm Indication Signal (AIS) and Idle Signal to be used on the 44 736 kbit/s signal.