A serial transmission T1 uses 8 information bits, 2 start bits, 1 stop bit and 1 parity bit for each character. A synchronous transmission T2 uses 3 e

Question

**A serial transmission T1 uses 8 information bits, 2 start bits, 1 stop bit and 1 parity bit for each character. A synchronous transmission T2 uses 3 eight bit sync characters followed by 30 eight bit information characters. If the bit rate is 1200 bits/second in both cases, what are the transfer rates of T1 and T2?**

Accepted Answer

The **transfer rate** (useful information rate) is the fraction of the total bit rate that actually carries information bits, excluding framing/overhead bits.

**T1 (Asynchronous transmission)**

Each character frame = 2 start bits + 8 information bits + 1 parity bit + 1 stop bit = **12 bits total**, of which 8 are useful information bits.

```
Useful fraction = 8 / 12

Transfer rate (T1) = Bit rate x (useful bits / total bits per frame)
                    = 1200 x (8/12)
                    = 1200 x 0.667
                    = 800 bits/sec
```

**T2 (Synchronous transmission)**

Each frame = 3 sync characters (8 bits each) + 30 information characters (8 bits each)

```
Overhead bits = 3 x 8 = 24 bits
Information bits = 30 x 8 = 240 bits
Total frame size = 24 + 240 = 264 bits

Useful fraction = 240 / 264 = 0.9091

Transfer rate (T2) = 1200 x (240/264)
                    = 1200 x 0.9091
                    ≈ 1090.9 bits/sec
```

**Result:** T1 ≈ **800 bps**, T2 ≈ **1090.9 bps**. Synchronous transmission (T2) achieves a higher effective transfer rate because its fixed per-frame overhead (3 sync bytes) is amortized over more information bytes (30), unlike asynchronous transmission where every single character carries its own start/stop/parity overhead.

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