T-Carrier Systems (T1, T2, T3, T4)
T-carrier systems are digital transmission systems used to send multiple voice or data channels over a single physical line. They are commonly used in North America for telephone and data networks.
The “T” stands for Trunk. Each level (T1, T2, T3, T4) represents increasing numbers of multiplexed channels and higher data rates.
T1 System
- Channels: 24 voice channels
- Data rate: 1.544 Mbps
- Multiplexing: Each channel = 64 Kbps → 24 × 64 Kbps + framing bits
- Purpose: Small to medium scale digital voice/data transmission
T2 System
- Channels: 96 voice channels
- Data rate: 6.312 Mbps
- Multiplexing: 4 T1 streams combined
- Purpose: Medium scale transmission
T3 System
- Channels: 672 voice channels
- Data rate: 44.736 Mbps
- Multiplexing: 7 T2 streams combined
- Purpose: Large scale trunking
T4 System
- Channels: 4,032 voice channels
- Data rate: 274.176 Mbps
- Multiplexing: 6 T3 streams combined
- Purpose: Very large backbone networks
Summary Table
| System | Channels | Data Rate | Multiplexing | Use Case |
|---|---|---|---|---|
| T1 | 24 | 1.544 Mbps | Base level | Small-medium scale voice/data |
| T2 | 96 | 6.312 Mbps | 4 × T1 | Medium scale transmission |
| T3 | 672 | 44.736 Mbps | 7 × T2 | Large scale trunking |
| T4 | 4,032 | 274.176 Mbps | 6 × T3 | Very large backbone |
Key Points
- Each level multiplies channels from the previous level.
- Used in telephone networks and digital backbone lines.
- Higher levels mean more complex multiplexing and higher data rates.
Calculating Capacity (Mbps) in T-Carrier Systems
T-carrier systems transmit digital voice channels over a single line. The bit rate (Mbps) is calculated using the number of channels, channel rate, and framing bits.
Basic Idea
- Each voice channel = 64 Kbps (PCM: 8 bits × 8,000 samples/sec)
- Framing bits are added for synchronization in the T-carrier line
T1 Calculation (1.544 Mbps)
- Channels = 24
- Channel rate = 64 Kbps
- Multiply channels × channel rate:
24 × 64 Kbps = 1536 Kbps - Add 1 framing bit per frame → total = 1.544 Mbps
T2 Calculation (6.312 Mbps)
- T2 is 4 T1s multiplexed:
1.544 × 4 = 6.176 Mbps - Add extra framing bits → total = 6.312 Mbps
T3 Calculation (44.736 Mbps)
- T3 is 7 T2s multiplexed:
6.312 × 7 = 44.184 Mbps - Add T3 framing → total = 44.736 Mbps
T4 Calculation (274.176 Mbps)
- T4 is 6 T3s multiplexed:
44.736 × 6 = 268.416 Mbps - Add T4 framing → total = 274.176 Mbps
General Formula
Bit rate = (number of channels × 64 Kbps) + framing bits
Summary
- T1 = 1.544 Mbps → 24 channels
- T2 ≈ 6.3 Mbps → 4 × T1
- T3 ≈ 44.7 Mbps → 7 × T2
- T4 ≈ 274 Mbps → 6 × T3