E3 Frame Structure

Level 3: The 34 Mbit/s Stream

The E3 stream is the third level in the European PDH and is formed by multiplexing four E2 tributaries. The North American T3 (DS3) level uses a different rate and multiplexes seven T2 streams.

Four plesiochronous tributaries operating at 8.448 Mbit/s are combined into a stream with a bit rate of 34.368 Mbit/s. The difference from the input sum ($4 \times 8.448 = 33.792 \text{ Mbit/s}$) is $34.368 - 33.792 = 0.576 \text{ Mbit/s}$ and provides rate budget for framing, control, and positive justification.

E3 Frame Structure

The E3 stream is organized into a 1536-bit frame divided into four 384-bit blocks. Its layout follows the E2 frame in using FAS, service bits, and positive justification.

Key frame parameters

• Total frame length: 1536 bits.
• Frame duration: Approximately $44.69 \mu s$.
• Block structure: The frame is divided into 4 blocks, following the E2 frame structure.
• Nominal bit rate: The E3 stream operates at 34.368 Mbit/s with a tolerance of ±20 ppm.
• Justification: In the nominal structure, each E2 tributary contributes 378 bits per frame. The maximum justification rate is 22.375 kbit/s and the nominal justification ratio is 0.436.
• Justification control: Cj1-Cj3 repeat the decision for each E2 tributary and are evaluated by majority. The code 111 indicates positive justification and 000 indicates no justification.

E3 frame block by block

Each block contains control fields and interleaved tributary positions. The fixed layout carries the frame alignment signal, service bits, and justification-control bits.

• Block I (First 384 bits):
  • FAS (bits 1-10, 10 bits): The Frame Alignment Signal (1111010000) also used in E2 for frame synchronization.
  • A/N bits (bits 11-12, 2 bits): Service bits for alarms and national use.
  • Data bits (bits 13-384, 372 bits): Interleaved data from the four E2 tributaries.
• Block II and III (next 2 x 384 bits):
  • Cj1 (bits 1-4, 4 bits): The first justification-control bit for each of the four E2 tributaries.
  • Cj2 (bits 1-4, 4 bits): The second justification-control bit for each of the four E2 tributaries.
  • Data bits (bits 5-384, 380 bits): Interleaved data positions from the four E2 tributaries in each block.
• Block IV (Final 384 bits):
  • Cj3 (bits 1-4, 4 bits): The third justification-control bit for each of the four E2 tributaries.
  • J positions (bits 5-8, 4 bits): E2 tributary-data positions available for positive justification. Without justification, they carry data.
  • Data positions (bits 5-384, 380 positions): Interleaved positions from the four E2 tributaries, including four J opportunities and 376 fixed data positions.