T1 vs. E1 Frames: Key Differences in Telecommunications

This article breaks down the differences between T1 and E1 frame structures, crucial components in PCM (Pulse Code Modulation) telecommunication systems. We’ll cover the number of bits, channels, and bit rates associated with each, providing a clear comparison of these two technologies.

Both T1 and E1 frames play a vital role in carrying multiple channels, 24 and 30 respectively. T1 handles a data rate of 1.544 Mbps, while E1 supports a higher rate of 2.048 Mbps.

T1 is commonly used for transmitting telephone calls between local central offices, typically spanning distances of 5 to 50 miles. For longer distances, T1 signals are often multiplexed into faster TDM (Time Division Multiplexing) systems.

T1 Frame Structure

Figure-1: T1 Frame Structure

Figure 1 illustrates the T1 frame structure used in PCM-based telephone systems.

The T1 system efficiently multiplexes 24 voice channels onto a single line using TDM techniques. Each analog voice channel is sampled at a rate of 8 kHz. This means the analog waveform is sampled every 125 µS. These samples are then converted into serial digital words using an ADC (Analog-to-Digital Converter).

The serial digital words from all 24 channels are transmitted sequentially. Each sample consists of an 8-bit word, with 7 bits representing the magnitude and 1 bit indicating polarity. The channel sampling rate is 125 µS / 24 = 5.2 µS per cycle, or 192 kHz.

This equates to a total of 24 * 8 = 192 bits.

A single additional bit is added as a synchronization pulse. This 193rd bit is crucial for synchronization between the transmitting and receiving ends.

E1 Frame Structure

Figure-2: E1 Frame Structure

Figure 2 depicts the E1 frame structure. The E1 frame captures 8 bits from each of the 64 kbps channels, one after the other. It then retransmits these channels at 2048 kbps.

The E1 frame can accommodate 30 voice channels, along with two additional 64 kbps channels. These two extra channels are dedicated to transmitting synchronization, service, and signaling data.

This results in an E1 transmission rate of 2048 kbps = 32 * 64 kbps. Each E1 frame comprises 256 bits, arranged into 32 timeslots of 8 bits each.

The frame repetition rate is 8,000 per second. Therefore, each timeslot supports a data rate of 64 kbps. The maximum number of timeslots available for user data is 31, as timeslot 0 is reserved for other purposes, as described below.

Each 8-bit sample or data word occupies a timeslot in the E1 stream. A cycle of 32 timeslots repeats every 125 µs. A complete cycle of the 32 timeslots is called an E1 frame. The timeslots in a frame are numbered TS0 to TS31.

The first timeslot sent (TS0) contains synchronization and service data. The timeslots that carry user data are called the payload.

A fixed 7-bit pattern “0011011,” known as the FAS (Frame Alignment Signal), is transmitted in timeslot 0 in each even frame. The second bit of timeslot 0 alternates between 0 and 1. When the receiver detects this pattern, it achieves frame alignment.

If three or more incorrect FAS patterns are received in a row, frame alignment is lost.

When frame alignment cannot be achieved, or no signal is received by the E1 equipment, it transmits RAI (Remote Alarm Indication) by setting the A bits of TS0 to 1 or by sending an all-ones pattern called AIS (Alarm Indication Signal). The AIS is usually sent by network equipment.

Also, refer to the difference between CAS (Channel Associated Signaling) and CCS (Common Channel Signaling), which describes Channel Associated Signaling and Common Channel Signaling.

T1 vs. E1: Key Differences Summarized

As explained above, T1 and E1 are PCM frame structures. Let’s compare them and summarize the key differences:

• Data Rate: T1 supports 1.544 Mbps, while E1 supports 2.048 Mbps.
• Distance: T1 is typically used over distances of 5 to 50 miles. E1 can cover longer distances.
• Voice Channels: T1 offers 24 voice channels, while E1 offers 30.
• Frame Structure: T1 frame structure contains 193 bits, while the E1 frame structure contains 256 bits.