T1 frame vs E1 frame | Difference between T1 and E1 frame structure

This page compares T1 frame vs E1 frame structure and mentions difference between T1 and E1 frame structure used in PCM telecommunication systems. It mentions number of bits, channels and bit rates used in E1 and T1 frame structure.

Both T1 and E1 frames are related to PCM 24 channels and 30 channels respectively. T1 supports 1.544 Mbps data rate and E1 supports 2.048 Mbps data rate. T1 is widely used to transmit telephone calls between local central offices over distances of about 5 to 50 miles.

For long distance transmission, T1 signals are multiplexed into other faster TDM systems.

T1 frame structure used in PCM telephone system

PCM T1 frame structure
Figure-1 : T1 Frame Structure

The figure-1 depicts T1-frame structure used in PCM based telephone systems. As shown, T1 system multiplexes 24 voice channels into a single line using TDM techniques. Each analog voice channel is sampled at 8KHz rate. In other words, analog waveform is sampled at every 125 µS.

The samples are converted to serial digital words by ADC. These serial digital words produced from total 24 channels are transmitted one after the other. Each sample is an 8 bit word, 7 bits of magnitude and 1 bit representing polarity.

Channel Sampling rate is 125µS/24 = 5.2 µS per cycle or 192 KHz. This represents a total of 24 ( 8 ) = 192 bits. 1 more bit is added as sync pulse. This 193rd bit will help in synchronization between transmission and reception.

E1 frame structure used in PCM telephone system

PCM E1 frame structure
Figure-2 : E1 Frame Structure

The figure-2 depicts E1-frame structure. E1 frame takes 8 bits from each of the 64 kbps channels one by one. It retransmits these channels at 2048 kbps. E1 frame accomodates 30 voice channels including two additional 64 kbps channels. These two additional channels are used to transmit synchronization, service and signaling data. This results into E1 transmission rate of 2048 kbps = 32 * 64 kbps.

Each E1 frame consists of 256 bits. These bits are arranged in 32 timeslots of 8 bits each. The frame repetition rate is 8,000 per second. Hence data rate supported by each timeslot is 64 kbps. The number of timeslots available for user data is maximum 31 as timeslot 0 is always reserved as explained below.

Each 8-bit sample or data word occupies a Timeslot in the E1 stream. A cycle of 32 timeslots repeats every 125 us. A complete cycle of the 32 timeslots is called a E1-Frame. The timeslots in a frame are numbered TS0 to TS31. The first timeslot sent (i.e. TS0) contains synchronization and service data. The timeslots that carry user data are called payload.

A fixed 7-bit pattern "0011011" known as FAS (i.e. 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, the frame alignment is lost.

When frame alignment can't be achieved or no signal is received by the E1 equipment it transmits RAI (Remote Alarm Indication) by setting the A bits of the TS0 to 1 (see table 6) or by sending all ones pattern called AIS (Alarm Indication Signal). The AIS is usually sent by network equipment.
Also refer difference between CAS and CCS➤➤ which describes Channel Associated Signaling and Common Channel Signaling.

As explained above, T1 and E1 are PCM frame structures. Let us compare T1 vs E1 and summarize difference between T1 and E1 frame structures.
➤T1 supports 1.544 Mbps data rate where as E1 supports 2.048 Mbps.
➤T1 is used over distances of about 5 to 50 miles. E1 cover longer than T1.
➤T1 offers 24 voice channels where as E1 offers 30 voice channels.
➤T1 frame structure contains 193 bits, E1 frame structure contains 256 bits.

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