The Anatomy of the E1 Stream

The fundamental building block of the European PDH standard is the E1 stream. Its primary purpose is to combine multiple individual voice or data channels into a single digital stream for transmission. To do this in an organized way, the E1 stream is structured into a repeating unit of data called a frame.

This frame structure is the key to how TDM (Time-Division Multiplexing) works in practice, allowing a receiver to correctly identify and separate the original data channels from the high-speed aggregated stream.

Core Parameters of an E1 Frame

The entire structure of the E1 frame is derived from the requirements of digitizing a single voice channel using PCM.

• Frame Duration: The sampling rate for a voice channel is 8000 Hz. This means a new sample is taken every $1 / 8000$ seconds. Therefore, the duration of one frame, which must carry one sample from each channel, is precisely 125 µs (microseconds).
• Time Slots: Each frame is divided into 32 time slots, numbered from 0 to 31. A Time Slot (TS) is a small time window dedicated to carrying information from a specific channel.
• Bits per Time Slot: Each time slot contains 8 bits, which corresponds to the size of a single PCM-encoded voice sample (one byte).

E1 Frame Layout: The PCM 30/32 System

Although an E1 frame has 32 time slots, not all of them are used for user data. Two are reserved for essential control and synchronization tasks, leaving 30 channels for voice or data. This gives rise to the name PCM 30/32.

Special Role of Time Slot 0 (TS0)

Time Slot 0 is the frame's anchor. Its primary job is synchronization. The receiver continuously scans the incoming bitstream, looking for a special bit pattern located within TS0. This pattern, known as the Frame Alignment Word (FAW), allows the receiver to identify where each 256-bit frame begins and ends. Without this, the receiver would just see an undifferentiated stream of bits and wouldn't be able to separate the channels. In addition to the FAW, TS0 is also used to carry alarm signals and other maintenance information.

Special Role of Time Slot 16 (TS16)

Time Slot 16 is typically reserved for signaling. Signaling refers to the control information needed to set up, manage, and tear down calls. This includes information like phone number dialing, line status (busy, ringing), and billing. Since the 8 bits in a single TS16 are not sufficient to carry signaling for all 30 channels at once, E1 uses a multiframe structure. This method, known as Channel Associated Signaling (CAS), will be discussed in detail later.

Comparison with the North American T1 Frame

It is important to note that the E1 frame is the European standard. In North America and Japan, the equivalent is the T1 frame, which has a different structure:

• Channel Capacity: A T1 frame carries 24 user channels (called DS0), not 30.
• Frame Structure: The T1 frame consists of 24 time slots of 8 bits each, plus one additional framing bit for synchronization, for a total of 193 bits per frame.
• Bit Rate: $193 \frac{\text{bits}}{\text{frame}} \times 8000 \frac{\text{frames}}{\text{second}} = 1.544 \text{ Mbit/s}$
• Signaling: T1 does not use a dedicated signaling timeslot like E1's TS16. Instead, it uses a technique called Robbed-Bit Signaling where signaling bits are periodically inserted into the user data channels themselves.