ケーブル速度の理解：Gbps、MHz、その意味

## Gbps vs GB/s: The 8x Difference

Cable speeds are measured in **gigabits per second (Gbps)**. File sizes are in **gigabytes (GB)**. There are 8 bits in a byte, so:

- 10 Gbps = ~1.25 GB/s theoretical maximum
- 40 Gbps = ~5 GB/s theoretical maximum

## Why Real Speeds Are Lower

You'll never hit the theoretical maximum because of:

- **Protocol overhead**: USB uses ~20-25% of bandwidth for signaling
- **Encoding**: 8b/10b encoding turns 10 Gbps into 8 Gbps usable
- **Drive speed**: Your SSD may be the bottleneck, not the cable
- **CPU/chipset limits**: Especially on older laptops

## Real-World Transfer Times (10 GB File)

| Standard | Theoretical | Realistic |
|---|---|---|
| USB 2.0 (480 Mbps) | 2.8 min | 5-8 min |
| USB 3.2 Gen 1 (5 Gbps) | 16 sec | 25-30 sec |
| USB 3.2 Gen 2 (10 Gbps) | 8 sec | 12-15 sec |
| Thunderbolt 4 (40 Gbps) | 2 sec | 3-4 sec |

## The Practical Rule

Expect **60-75% of theoretical speed** in the real world. If you need 1 GB/s sustained, get a cable rated for at least 10 Gbps.