E3 Frame Structure

1536-bit frame blocks, FAS, justification control bits.

Level 3: The 34 Mbit/s Stream

The E3 stream is the third level in the European PDH, designed to carry even greater data volumes by multiplexing four E2 tributary streams. In the North American hierarchy, the analogous (though different in rate) level is the T3 (or DS3) stream, which multiplexes seven T2 streams.

The process follows the same principle as E2 multiplexing: four plesiochronous 8.448 Mbit/s tributaries are combined into a single higher-speed stream. The final bit rate of the E3 stream is 34.368 Mbit/s. The difference between this and the simple sum of the inputs (4×8,448=33,792 Mbit/s4 \times 8,448 = 33,792 \text{ Mbit/s}) constitutes the necessary overhead for framing and justification.

E3 Frame Structure

To manage this higher level of aggregation, the E3 stream is organized into a 1536-bit frame. This frame structure is conceptually similar to the E2 frame but scaled to handle larger tributaries.

Inside an E3 frame

Explore how the 1536-bit frame is split into blocks, K/D bits and payload bursts.

Frame Alignment Word (FAW) • 10 bitsAlarm bit • 1 bitNational use bit • 1 bitJTributary payload • 372 bitsBlock IBits 1–384Justification control bits K1–K3 • 3 bitsJustification bit D2 • 1 bitJTributary payload • 380 bitsBlock IIBits 385–768Justification control bits K4–K6 • 3 bitsJustification bit D3 • 1 bitJTributary payload • 380 bitsBlock IIIBits 769–1152Justification control bits K7–K9 • 3 bitsJustification bit D4 • 1 bitJTributary payload • 380 bitsBlock IVBits 1153–1536Scaled in bits

Block I

Bits 1–384

Overhead 12 bits (3.13%)

Starts every 1536-bit frame with the FAW pattern and two service bits before payload bursts.

  • Frame Alignment Word (FAW)

    10 bits

    Fixed 1111010000 sequence locking the receiver to the 34 Mbit/s cadence.

  • Alarm bit

    1 bit

    Signals remote alarm indication (RAI) downstream.

  • National use bit

    1 bit

    Operator-specific signalling defined by national regulators.

  • Tributary payload

    372 bits

    First E2 tributary burst. When justification is idle, all 372 bits carry user data.

Carries the first E2 tributary. The FAW keeps regenerators phase-aligned at 34 Mbit/s.

Nominal bit rate

34.368 Mbit/s

Third-order tributary in the European PDH hierarchy.

Bits per frame

1536 bits

Four blocks, each 384 bits long.

Frame duration

≈ 44.69 µs

1536 bits / 34.368 Mbit/s.

Baseline payload

1512 bits (98.44%)

Interleaved bursts from the four E2 tributaries.

Baseline overhead

24 bits (1.56%)

FAW, service bits, K control and D opportunities. D may convert to payload (24–28 bits).

Key Frame Parameters:

  • Total Frame Length: 1536 bits.
  • Frame Duration: Approximately 44.69μs44.69 \mu s.
  • Block Structure: The frame is also divided into 4 blocks, mirroring the structure of the E2 frame.

E3 Frame Block-by-Block

The content of each block is dedicated to either overhead or tributary data, following a pattern that allows for robust synchronization and control.

  • Block I (First 384 bits):
    • FAW (Frame Alignment Signal): The same 10-bit pattern (`1111010000`) used in E2 for frame synchronization.
    • A/N Bits: 2 service bits for alarms and national use.
    • Information Bits (J): 372 data bits belonging to the first E2 tributary stream.
  • Block II & III (Next 2 x 384 bits):
    • K Bits: Justification control bits for the second and third E2 tributaries, respectively.
    • Information Bits (J): 380 data bits from the second and third tributaries. Each of these blocks contains justification opportunity slots.
  • Block IV (Final 384 bits):
    • K Bit: Justification control bit for the fourth E2 tributary.
    • D Bit: The justification (stuffing) bit slot for the fourth E2 tributary.
    • Information Bits (J): 380 data bits from the fourth tributary.

The E3 Overhead

Within the 1536 bits of an E3 frame, between 1508 and 1512 bits are payload data from the four E2 tributaries. The remaining 24 to 28 bits constitute the frame's overhead. This overhead is dedicated to framing, alarms, and the essential justification bits that allow for the seamless merging of plesiochronous data streams.

    E3 Frame Structure | Teleinf Edu