Input Channels (PCM Inputs)
TDM Multiplexing Process
E1 Line Output & Analysis
E1 System Theory
Overview
E1 is a digital transmission link with a capacity of 2.048 Mbps, standardized by ITU-T G.703/G.704. It uses Time Division Multiplexing (TDM) to combine 30 voice channels (plus 2 control channels) into a single high-speed data stream. The E1 system is widely used in Europe, Asia, and most of the world (except North America and Japan which use T1).
Frame Structure
- • Frame Duration: 125 microseconds (8000 frames/second)
- • Timeslots: 32 timeslots per frame (TS0 to TS31)
- • TS0: Frame alignment word (synchronization)
- • TS16: Signaling channel (CAS - Channel Associated Signaling)
- • TS1-15, TS17-31: 30 voice/data channels (64 kbps each)
PCM Encoding
- • Sampling: 8 kHz (Nyquist rate for 4 kHz voice)
- • Quantization: 8 bits per sample (256 levels)
- • Compression: A-law (Europe) or μ-law (USA)
- • Data Rate: 64 kbps per channel (8k × 8)
Multiframe Structure
16 consecutive frames form a multiframe (2 ms duration). This structure is essential for signaling:
Line Coding: HDB3
High Density Bipolar 3 (HDB3) is used to ensure clock recovery and DC balance:
- Replaces sequences of four consecutive zeros with special patterns
- Maintains bipolar violation rules for error detection
- Ensures sufficient transitions for clock recovery
- Prevents baseline wander in AC-coupled systems
Laboratory Procedure
Setup and Initialization
Launch the E1 Multiplex System simulation. Observe the default configuration with 30 active channels. Note the frame structure display showing 32 timeslots.
Observe Input Channels
Examine the analog input signals (voice channels). Notice the 4 kHz bandwidth limitation. Observe how each channel is sampled at 8 kHz rate (every 125 µs).
PCM Encoding
Watch the PCM encoding process. Each analog sample is converted to 8-bit digital words. Observe the quantization levels and note the 64 kbps rate per channel.
Time Division Multiplexing
Observe how the multiplexer rotates through channels, taking one 8-bit sample from each channel every frame. Note that TS0 carries synchronization and TS16 carries signaling information.
Frame Assembly
Watch the frame assembly process. Each frame contains 32 timeslots × 8 bits = 256 bits. At 8000 frames/second, this results in 2.048 Mbps.
Line Coding (HDB3)
Observe the HDB3 encoding of the output bit stream. Note how long sequences of zeros are replaced to maintain clock synchronization. Examine the bipolar waveform.
Vary Parameters
Experiment with different numbers of active channels (2, 4, 8, 30). Change simulation speed to observe details. Switch between voice and data input types.
Analyze Statistics
Record the bit rate, frame duration, and efficiency metrics. Calculate the overhead percentage (2/32 = 6.25% for TS0 and TS16).
Laboratory Report Guidelines
Required Components
- 1 Title Page: Course name, experiment title, student name, date, roll number
- 2 Objectives: Clear statement of what the experiment aims to demonstrate
- 3 Theory: Brief explanation of TDM, PCM, E1 frame structure, and HDB3
- 4 Procedure: Step-by-step description of simulation activities performed
- 5 Observations: Screenshots of frame structure, waveforms, and bit streams at different stages
- 6 Calculations: Bit rate verification, efficiency calculations, bandwidth calculations
- 7 Results: Tabulated data showing different configurations tested
- 8 Conclusion: Summary of findings and understanding of E1 multiplexing
Key Questions to Address
Q1:
Why is the sampling rate 8 kHz for voice channels? Prove using Nyquist theorem.
Q2:
Calculate the total bit rate of E1 system showing all steps (32 channels × 8 bits × 8 kHz).
Q3:
What is the purpose of TS0 and TS16? Why can't they carry user data?
Q4:
Explain why HDB3 encoding is necessary instead of simple NRZ.
Q5:
Compare E1 (2.048 Mbps) and T1 (1.544 Mbps) systems. Why the difference?