E2 Frame Overview

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. Their combined rate is $4 \times 2.048 = 8.192 \text{ Mbit/s}$, while the resulting E2 stream has a bit rate of 8.448 Mbit/s.

The rate difference ($8.448 - 8.192 = 0.256 \text{ Mbit/s}$, or 256 kbit/s) forms the overhead for frame alignment, alarms, and positive justification that adapts the four independent plesiochronous E1 tributary clocks.

Each 848-bit frame contains 24 fixed control and service bits. These comprise 10 FAS bits, the A and N bits, and three four-bit Cj fields. Block IV also contains four J positions, one for each tributary. Without positive justification, they carry tributary data. When justification is signaled, a selected position carries stuffing instead.

The E2 frame structure

The 8.448 Mbit/s E2 stream is organized into an 848-bit frame divided into four 212-bit blocks. The layout carries E1 tributary data and the overhead required for synchronization and justification.

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, each containing 212 bits ($4 \times 212 = 848$).
• Nominal bit rate: The E2 stream operates at 8.448 Mbit/s with a tolerance of ±30 ppm.
• Nominal payload per tributary: Each E1 tributary contributes 206 bits per E2 frame in the nominal structure, for 824 bit positions for tributary data in total.
• Justification: The maximum justification rate is 10 kbit/s per tributary, and the nominal justification ratio is 0.424.
• Justification control: For each tributary, Cj1-Cj3 form a control signal evaluated by majority. The code 111 indicates positive justification and 000 indicates no justification.

Detailed breakdown of the E2 frame blocks

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