E4 Frame Structure

2928-bit frame with six blocks and multiple justification opportunities.

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.

Inside an E4 frame

Explore the six-block, 2928-bit frame with its K/D control bits and Y service channels.

Frame Alignment Word (FAW) • 12 bitsAlarm bit • 1 bitNational use bit • 1 bitY1 orderwire channel • 1 bitY2 orderwire channel • 1 bitJTributary payload • 472 bitsBlock IBits 1–488Justification control bits K1–K4 • 4 bitsJTributary payload • 484 bitsBlock IIBits 489–976Justification control bits K5–K8 • 4 bitsJTributary payload • 484 bitsBlock IIIBits 977–1464Justification control bits K9–K12 • 4 bitsJTributary payload • 484 bitsBlock IVBits 1465–1952Justification control bits • 4 bitsJTributary payload • 484 bitsBlock VBits 1953–2440Justification control bits • 4 bitsJustification opportunities D1–D4 • 4 bitsJTributary payload • 480 bitsBlock VIBits 2441–2928Scaled in bits

Block I

Bits 1–488

Overhead 16 bits (3.28%)

Opens the 2928-bit frame with the 12-bit FAW, service signalling and two 10 kbit/s supervisory channels before the first tributary burst.

  • Frame Alignment Word (FAW)

    12 bits

    Fixed 111110100000 sequence keeping multiplexers synchronised at 139.264 Mbit/s.

  • Alarm bit

    1 bit

    Conveys remote alarm indication downstream for maintenance.

  • National use bit

    1 bit

    Reserved for operator-specific signalling mandated by national regulators.

  • Y1 orderwire channel

    1 bit

    One bit per frame provides a 10 kbit/s ancillary channel for voice/data supervision.

  • Y2 orderwire channel

    1 bit

    Second 10 kbit/s service/data channel sharing the block lead-in.

  • Tributary payload

    472 bits

    First E3 tributary burst. When justification is idle, all 472 bits carry user traffic.

Feeds tributary 1. The Y channels transport orderwire/data traffic while maintaining alignment for the 140 Mbit/s stream.

Nominal bit rate

139.264 Mbit/s

Fourth-order signal in the European PDH hierarchy.

Bits per frame

2,928 bits

Six blocks, each 488 bits long.

Frame duration

≈ 21.03 µs

2928 bits / 139.264 Mbit/s.

Payload carried

2,888–2,892 bits

When no stuffing occurs the D slots revert to payload (98.63–98.77%).

Frame overhead

36–40 bits

FAW, service bits, Y channels and K control. D slots (4 bits) can trade places with payload.

Key E4 Frame Parameters:

  • Total Frame Length: 2928 bits.
  • Frame Duration: Approximately 21.03μs21.03 \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.

E4 Overhead Explained

Within the 2928 bits of an E4 frame, a total of 36 to 40 bits are reserved for overhead. This small fraction of the total bandwidth is critical, as it carries the framing pattern (FAW), service bits, and the justification control and stuffing bits necessary to align and merge four independent 34 Mbit/s data streams into one coherent 140 Mbit/s signal.

    E4 Frame Structure | Teleinf Edu