E2 Frame Overview

Structure and overhead for aggregating four E1 streams.

Multiplexing E1 into E2

The E2 stream is the second level in the European PDH hierarchy. It is created by multiplexing four independent E1 tributary streams, each operating at 2.048 Mbit/s. While the combined data rate of the four tributaries is $4 \times 2,048 = 8,192 \text{ Mbit/s}$ , the total bit rate of the resulting E2 stream is 8.448 Mbit/s.

This additional capacity ( $8,448 - 8,192 = 0,256 \text{ Mbit/s}$ or 256 kbit/s) constitutes the necessary for frame alignment, alarms, and, most importantly, the bit stuffing (justification) required to synchronize the four plesiochronous E1 streams.

The E2 Frame Structure

The 8.448 Mbit/s E2 stream is organized into a repeating frame structure that is precisely defined to accommodate both user data from the tributaries and the required overhead.

Inside an E2 frame

Visualise how four E1 tributaries share the 848-bit structure and where justification overhead lives.

Block I

Bits 1–212

Overhead 12 bits (5.66%)

Opens each E2 frame with alignment and service signalling before the interleaved payload.

Frame Alignment Word (FAW)
10 bits
Fixed 1111010000 pattern ensuring the receiver locks to the 848-bit frame cadence.
Alarm bit (A)
1 bit
Signals remote alarms and maintenance states to downstream equipment.
National use bit (N)
1 bit
Reserved for operator-specific signalling defined by national authorities.
Interleaved payload
200 bits
Four groups of 50 bits carrying tributary user data in round-robin order.

Payload portion delivers four bursts of 50 bits, one from each E1 tributary in sequence.

Nominal bit rate

8.448 Mbit/s

Second-order tributary in the European PDH hierarchy.

Bits per frame

848 bits

Four blocks, each 212 bits long.

Frame duration

≈ 100.38 µs

848 bits / 8.448 Mbit/s.

Payload share

824 bits (97.17%)

Interleaved user data from the four E1 tributaries.

Overhead share

24 bits (2.83%)

Alignment, alarm and justification signalling.

Key Frame Parameters:

Total Frame Length: The E2 frame consists of 848 bits.
Frame Duration: The time to transmit one frame is approximately 100.38 µs ( $848 \text{ bits} / 8.448 \times 10^6 \text{ bits/s}$ ).
Structure: The frame is divided into four equal blocks or sets, each containing 212 bits ( $4 \times 212 = 848$ ).

Detailed Breakdown of the E2 Frame Blocks

Each of the four blocks within the E2 frame has a specific purpose, combining overhead information with interleaved data from the E1 tributaries.

Block	Overhead Bits (First part of block)	Data Payload Bits (Interleaved)
Block I (Bits 1-212)	FAW (10 bits): The Frame Alignment Word (`1111010000`). Allows the receiver to synchronize to the E2 frame. A (1 bit): Alarm bit. N (1 bit): Bit reserved for national use.	200 bits of payload, consisting of 50 interleaved bits from each of the four E1 tributaries.
Block II (Bits 213-424)	J1, J2, J3 (3 bits): Justification Control bits for Tributary 2. S1 (1 bit): Stuffable (Justification Opportunity) bit for Tributary 1.	208 bits of payload.
Block III (Bits 425-636)	J4, J5, J6 (3 bits): Justification Control bits for Tributary 3. S2 (1 bit): Stuffable (Justification Opportunity) bit for Tributary 2.	208 bits of payload.
Block IV (Bits 637-848)	J7, J8, J9 (3 bits): Justification Control bits for Tributary 4. S3 (1 bit): Stuffable (Justification Opportunity) bit for Tributary 3. S4 (1 bit): Stuffable (Justification Opportunity) bit for Tributary 4.	207 bits of payload.