E1 Frame Timing Structure
An E1 frame contains 256 bits and repeats 8,000 times per second, so its duration is 125 µs. These bits form 32 time slots of 8 bits each. The gross E1 bit rate is 2.048 Mbit/s.
Each slot is numbered from 0 to 31 and carries 8 bits of user data or overhead information. Its role depends on the slot number. A carries eight bits during each frame period.
The payload: user channels (TS1-15 and TS17-31)
The E1 frame allocates 30 of its 32 time slots to user data, divided into two blocks.
- Time Slots 1 through 15
- Time Slots 17 through 31
These 30 slots are often called traffic channels or, in the context of ISDN, B-channels (Bearer channels). Each of these slots carries one 8-bit sample from a digital voice call or a byte of data every 125 µs. This creates 30 independent digital channels, each with the following bit rate.
The PCM 30/32 system therefore provides 30 user channels within a 32-slot structure. Together they provide 1.920 Mbit/s of channel capacity at 64 kbit/s per channel. The two overhead slots handle framing and signaling within the 2.048 Mbit/s stream.
Each traffic slot contributes one byte per frame, so 30 traffic slots carry 240 payload bits in every 125 µs frame. The resulting 30 × 64 kbit/s equals 1.920 Mbit/s, while TS0 and TS16 account for the remaining 128 kbit/s of the 2.048 Mbit/s interface rate.
Time Slot 0: Framing and Synchronization
In the PCM 30/32 configuration, Time Slot 0 (TS0) carries frame overhead, including the alignment signal, alarms, and service information.
The primary function of TS0 is to carry the . The pattern 0011011 occupies bits 2-8 of alternate frames. The receiver checks the repeated FAS position, locates the frame start, and interprets the other 31 time slots. 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.
Time Slot 16: Signaling
In the PCM 30/32 configuration, Time Slot 16 (TS16) carries overhead and . This control information manages connections within the 30 user channels. TS16 can also carry common-channel signaling or a 64 kbit/s channel when channel signaling is not required.
CAS groups 16 consecutive frames into a structure called 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. Each four-bit group serves one channel, and each a, b, c, or d signaling channel has a bit rate of 500 bit/s. This method is known as , assigning fixed signaling bits to each traffic channel.