E1 Timeslots and Channels
Mapping of 30 voice channels plus signaling and alignment slots.
A Structured Approach to Time
As previously established, the E1 frame is a container for digital data that repeats 8,000 times per second, giving it a fixed duration of 125µs. To manage multiple channels within this short period, the frame is meticulously divided into 32 distinct time slots.
Think of the E1 frame as a train with 32 numbered compartments, departing precisely every 125µs. Each compartment is a , carrying a small piece of the total information. Understanding the specific role assigned to each of these slots is key to understanding how PDH works.
The Payload: User Channels (TS1-15 & TS17-31)
The primary purpose of the E1 frame is to transport user data. Thirty of the 32 time slots are allocated for this task, 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 a bit rate of:
This is why the system is commonly known as PCM 30/32: it provides 30 user channels within a 32-slot structure.
The Frame Anchor: Time Slot 0 (Synchronization)
Time Slot 0 (TS0) is the most critical slot for the physical operation of the link. It does not carry user data; its sole purpose is to provide synchronization and carry operational data.
The primary function of TS0 is to carry the . The receiver constantly scans the incoming bitstream looking for this specific pattern (e.g., `0011011`). When it finds the FAW, it knows it has located the start of a frame and can begin correctly identifying and routing the data from the other 31 time slots. TS0 is also used for carrying alarm indications and error checking codes (CRC), which will be discussed in the context of the multiframe.
The Control Channel: Time Slot 16 (Signaling)
Time Slot 16 (TS16) is also reserved for overhead and does not carry user data. It functions as a dedicated channel for . This control information is essential for managing the connections within the 30 user channels.
The 8 bits available in TS16 of a single frame are insufficient to manage all 30 user channels. To solve this, E1 systems group 16 frames together into a larger structure called a multiframe. The TS16 slot of each frame within this multiframe is then used to carry signaling information for two specific user channels at a time. This method is known as . The detailed operation of the multiframe and CAS will be explored in subsequent topics.