6.0 Frame Structure Specification at 44 736 kbit/s
The 44 736 kbit/s (commonly referred to as DS3) interface serves as a high-capacity trunk in the digital hierarchy, used for multiplexing multiple lower-rate signals, such as twenty-eight 1544 kbit/s streams.
The overall multiframe structure contains 4760 bits and is divided into seven M-subframes, with each M-subframe containing 680 bits. Each M-subframe is further divided into eight blocks of 85 bits. The first bit of each 85-bit block is an overhead bit, and the remaining 84 bits are for payload. This results in 56 overhead bits per multiframe.
- Multiframe: 4760 bits total.
- M-subframes: 7 M-subframes per multiframe (680 bits each).
- Payload Blocks: Each M-subframe contains 8 payload blocks of 84 bits each, preceded by a single overhead bit.
The 56 overhead bits within the multiframe are allocated to five distinct categories, each serving a specific operational purpose.
- X-bits (X1, X2): There are two X-bits, which are used to indicate received errored multiframes to the remote end. They are set to binary 1 (X1=X2=1) for a dark (no signal) or error-free condition and to binary 0 (X1=X2=0) in case of a Loss of Signal (LOS), Out of Frame (OOF), Alarm Indication Signal (AIS), or Slips.
- P-bits (P1, P2): Two P-bits are used for performance monitoring. They carry parity information calculated over the 4704 payload bits of the preceding multiframe. P1 and P2 are set to 1 if the digital sum of all payload bits is 1, and to 0 if the digital sum is 0.
- M-bits (M1, M2, M3): Three M-bits form the multiframe alignment signal (010). They are used to locate the seven M-subframes within the larger multiframe.
- F-bits (F1, F2, F3, F4): Four F-bits in each M-subframe form the M-subframe alignment signal (1001). This allows the receiving equipment to identify the boundaries of the 680-bit M-subframes.
- C-bits (C11 to C73): There are 21 C-bits in total, with three in each M-subframe. These bits can be used for various embedded operations channels, including C-bit Parity applications for justifying lower-rate tributaries.
The Far-End Alarm and Control (FEAC) mechanism uses a specific C-bit (FEAC bit) for operational communication between the near-end and far-end terminals. It is used to send alarm/status information and control signals, such as loopback commands.
Table 12: FEAC Alarm/Status Codewords
| Alarm/status condition | Codeword |
| Out of frame at 44 736 kbit/s | 0000 0000 1111 1111 |
| RDI for HEC-based ATM mapping in 44 736 kbit/s | 0000 0010 1111 1111 |
| LCD for HEC-based ATM mapping in 44 736 kbit/s | 0000 0100 1111 1111 |
| Equipment failure at 1544 or 2048 kbit/s (NSA) | 0000 0110 1111 1111 |
| Equipment failure at 1544 or 2048 kbit/s (SA) | 0000 1010 1111 1111 |
| LOS/HBER at 44 736 kbit/s | 0001 1100 1111 1111 |
| Equipment failure at 44 736 kbit/s (NSA) | 0001 1110 1111 1111 |
| Multiple LOS/HBER at 1544 or 2048 kbit/s | 0010 1010 1111 1111 |
| AIS received at 44 736 kbit/s | 0010 1100 1111 1111 |
| Equipment failure for 44 736 kbit/s (SA) | 0011 0010 1111 1111 |
| Idle received at 44 736 kbit/s | 0011 0100 1111 1111 |
| Common equipment (NSA) | 0011 1010 1111 1111 |
| Single LOS/HBER at 1544 or 2048 kbit/s | 0011 1100 1111 1111 |
Table 13: FEAC Control Codewords
| Command | Codeword |
| Activate loopback | 0000 1110 1111 1111 |
| De-activate loopback | 0011 1000 1111 1111 |
| All 1544 or 2048 kbit/s Lines | 0010 0110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #1 | 0100 0010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #1 | 0100 0100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #1 | 0100 0110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #1 | 0100 1000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #2 | 0100 1010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #2 | 0100 1100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #2 | 0100 1110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #2 | 0101 0000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #3 | 0101 0010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #3 | 0101 0100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #3 | 0101 0110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #3 | 0101 1000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #4 | 0101 1010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #4 | 0101 1100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #4 | 0101 1110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #4 | 0110 0000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #5 | 0110 0010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #5 | 0110 0100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #5 | 0110 0110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #5 | 0110 1000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #6 | 0110 1010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #6 | 0110 1100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #6 | 0110 1110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #6 | 0111 0000 1111 1111 |
| 1544 or 2048 kbit/s Line No. 1, Group #7 | 0111 0010 1111 1111 |
| 1544 or 2048 kbit/s Line No. 2, Group #7 | 0111 0100 1111 1111 |
| 1544 or 2048 kbit/s Line No. 3, Group #7 | 0111 0110 1111 1111 |
| 1544 or 2048 kbit/s Line No. 4, Group #7 | 0111 1000 1111 1111 |
Data-link messages utilize a structure based on LAPD (Link Access Procedure on the D-channel), which is defined by Q.921.
Table 14: LAPD Message Structure
| Octet no. | Octet label | Octet content |
| 1 | Flag | 01111110₂ |
| 2 | SAPI CR EA | 00111100₂ or 00111110₂ |
| 3 | TEI EA | 00000001₂ |
| 4 | Control | 00000011₂ |
| … | Information field | Path Identifier, Idle Signal Id, or Test Signal Id |
| N – 1 | FCS | Frame Check Sequence |
| N | FCS |
Table 15: Information Field Contents for Data-Link Messages
| CL path identification |
| Data elements |
| Type |
| LIC |
| FIC |
| EIC |
| Unit |
| CL-facility ID |
| Idle signal identification |
| Data elements |
| Type |
| LIC |
| FIC |
| EIC |
| Unit |
| Port no. |
| Test signal identification |
| Data elements |
| Type |
| LIC |
| FIC |
| EIC |
| Unit |
| Gen. no. |
Special patterns are defined for the Alarm Indication Signal (AIS) and the Idle Signal to maintain signal integrity during fault conditions.
- Alarm Indication Signal (AIS): This is a signal with valid multiframe and M-subframe alignment signals. The information bits are set to a 1010… pattern, P-bits are valid, C-bits are set to binary zero, and X-bits are set to binary one.
- Idle Signal: This signal also has valid multiframe and M-subframe alignment signals. The information bits are set to a 1100… pattern, P-bits are valid, and C-bits and X-bits are managed according to specific rules.
The final section of this document provides supplementary information contained in the annexes.