E4 Frame Structure

The Pinnacle of the PDH: The 140 Mbit/s Stream

The E4 stream represents the fourth and highest standardized level in the European PDH. It was the primary rate for long-haul national and international trunk connections, designed to carry massive amounts of aggregated traffic before the widespread adoption of synchronous (SDH/SONET) systems.

The E4 stream multiplexes four E3 (34.368 Mbit/s) tributaries, resulting in a total bit rate of 139.264 Mbit/s. The North American equivalent, T4 (or DS4), achieved 274.176 Mbit/s by multiplexing six T3 streams. The E4 stream required a significantly more complex frame structure to manage the multiplexing of these high-speed plesiochronous tributaries.

E4 Frame Structure: A Six-Block Design

A key difference in the E4 frame compared to the E2 and E3 frames is its organization into six distinct blocks instead of four. This structure accommodates the data and increased overhead required at this higher rate.

Key E4 Frame Parameters:

• Total Frame Length: 2928 bits.
• Frame Duration: Approximately $21.03 \mu s$.
• Block Structure: Divided into 6 blocks of 488 bits each.

Inside the E4 Frame Blocks

The 2928-bit frame is meticulously structured to interleave data from four E3 streams while providing robust synchronization and control.

• Block I (First 488 bits):
  • FAW (Frame Alignment Signal): A longer, 12-bit pattern (111110100000) is used to ensure reliable frame synchronization at this high bit rate.
  • A/N Bits: 2 service bits for alarms and national use.
  • Y1/Y2 Channels: Two bits per frame provide auxiliary 10 kbit/s service/data channels.
  • Information Bits (J): 472 data bits belonging to the first E3 tributary stream.
• Blocks II, III, IV, & V (4 x 488 bits):
  • K Bits: Each block starts with four justification control bits that announce any stuffed payload bits.
  • Information Bits (J): The remaining 484 bits in each block carry payload data from the tributaries.
• Block VI (Final 488 bits):
  • K Bits: The final quartet steers justification for the fourth tributary.
  • D Bits (4): This is a unique feature of the E4 frame—four justification slots for the fourth tributary that usually carry payload but absorb phase corrections when required.
  • Information Bits (J): Up to 484 data bits from the fourth tributary, four of which may be replaced by the D bits.