E4 Frame Structure

The highest European PDH level: 139.264 Mbit/s

E4 is the fourth level of the European PDH hierarchy. ITU-T Recommendation G.751 specifies its nominal rate as 139.264 Mbit/s and places it above the 8.448 Mbit/s second-order level. It was used for long-haul, national, and international links before synchronous SDH and SONET systems became widespread.

The usual arrangement multiplexes four 34.368 Mbit/s E3 tributaries into 139.264 Mbit/s. G.751 also specifies direct multiplexing of sixteen 8.448 Mbit/s E2 tributaries. The two methods produce the same E4 signal, with frame fields carrying data from independently timed tributaries. The nominal rate tolerance is ±15 ppm.

The North American T4 (DS4) rate is 274.176 Mbit/s and combines six T3 streams. T4 and E4 use different regional numbering and bit-rate plans, so T4 is not an E4 interface equivalent.

E4 frame structure: six 488-bit blocks

G.751 divides each E4 frame into six blocks of 488 bits. The blocks carry cyclically interleaved bits from four E3 tributaries. Fixed positions provide frame alignment, service signaling, and positive-justification control.

Key E4 frame parameters

• Total Frame Length: 2928 bits.
• Frame duration: Approximately $21.0248 \mu s$, corresponding to a frame rate of about 47.563 kHz.
• Block structure: 6 blocks × 488 bits.
• Nominal bit rate: 139.264 Mbit/s with a tolerance of ±15 ppm.
• Tributary allocation: 723 frame positions per E3 tributary.
• Positive justification: Up to about 47.563 kbit/s per E3 tributary; the nominal justification ratio is 0.419.

E4 frame blocks

The standard numbers the six blocks as Sets I to VI. Cyclic bit interleaving follows the tributary numbering order. The distributed control fields let the receiver find the frame boundary and decide how to interpret each justification position.

• Block I, or Set I (bits 1 to 488):
  • FAS (Frame Alignment Signal): Bits 1 to 12 contain 111110100000 and identify the frame boundary.
  • Alarm bit A: Bit 13 carries an alarm indication to the remote multiplexer.
  • N bits: Bits 14 to 16 are reserved for national use. On a cross-border connection, they are fixed at 1.
  • Tributary data: Bits 17 to 488 provide 472 cyclically interleaved positions from the four E3 tributaries.
• Blocks II to V, or Sets II to V (4 × 488 bits):
  • Cj1 to Cj4: The first four bits of each block provide one justification-control bit for each E3 tributary. The five C bits for a tributary are evaluated by majority vote.
  • Tributary data: The remaining 484 positions in each block are cyclically interleaved positions from the four tributaries.
• Block VI, or Set VI (final 488 bits):
  • Cj5: Bits 1 to 4 provide the fifth justification-control bit for the four E3 tributaries.
  • Justification positions: Bits 5 to 8 provide one J position per tributary. Each position carries data when the majority decision is 00000 and a stuffing bit when it is 11111.
  • Tributary data: Bits 9 to 488 provide 480 cyclically interleaved positions. Together with the four J positions, the block contains 484 tributary positions.