E1 Frame Structure

E1 Stream Composition

The European PDH hierarchy uses the E1 stream to combine individual voice or data channels into one digital stream. Transmission is organized as a repeating data unit called a frame.

The slot numbers and their assigned roles let a receiver recover frame boundaries, user data, and signaling information from the TDM (Time-Division Multiplexing) 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) carries eight bits during each frame period.
• Bits per time slot: Each time slot contains 8 bits. In a voice slot, these bits form one PCM sample (one byte). TS0 and TS16 carry control and signaling bits.
• Voice coding and rate tolerance: G.711 A-law represents each sampled voice channel with an 8-bit PCM word. The nominal 2.048 Mbit/s interface has a tolerance of ±50 ppm.

E1 Frame Layout: The PCM 30/32 System

In the PCM 30/32 configuration, TS0 carries framing and service information, TS16 carries signaling, and TS1-15 plus TS17-31 carry 30 voice or user-data channels.

Time Slot 0 Framing Fields

In the basic E1 frame, TS0 carries the Frame Alignment Signal (FAS) 0011011 in bits 2-8 of alternate frames. The receiver tracks this pattern in the incoming bitstream to locate the boundaries of each 256-bit frame. The pattern is called Frame Alignment Signal (FAS). In the other frames, bit 2 is fixed at 1, bit 3 carries the alarm bit A, and bits 4-8 carry Sa4-Sa8 service bits. Bit 1 (Si) is reserved for international use when CRC-4 is not configured. With CRC-4 enabled, successive bit 1 positions of TS0 carry CRC-4 check bits in FAS frames and the six-bit multiframe alignment pattern 001011 in selected non-FAS frames. TS0 also carries other service information.

Time Slot 16 Signaling Fields

In the PCM 30/32 configuration, TS16 carries signaling, the control information used to establish, maintain, and terminate calls. It includes dialing digits, line state, ringing, and billing information. The standard also allows common-channel signaling, and TS16 may carry a 64 kbit/s channel when channel-associated signaling is not required.

For channel-associated signaling, E1 groups 16 consecutive frames into a multiframe. In frame 0, bits 1-4 of TS16 form the multiframe alignment signal 0000. The complete field is 0000xyxx, with y used for remote-alarm information and x bits reserved. In frame n from 1 through 15, the first four-bit group a, b, c, d serves channel n and the second serves channel n+15, forming pairs 1/16 through 15/30. This method is known as Channel Associated Signaling (CAS). Each four-bit group serves one traffic channel, and each a, b, c, or d signaling channel has a bit rate of 500 bit/s.

Comparison with the North American T1 Frame

The North American T1 format and the Japanese J1 designation use 1.544 Mbit/s and differ from the European E1 frame.

• Channel capacity: A T1 frame carries 24 user channels called DS0, while E1 carries 30.
• Frame structure: A T1 frame has 24 time slots of 8 bits each plus one framing bit, 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 corresponding to E1's TS16. It uses a technique called Robbed-Bit Signaling where signaling bits are periodically inserted into the user data channels themselves.