Data Transmission
At Layer 1, data is just physics. It’s voltage on copper, light in fiber, or waves in the air. Understanding these physical constraints is critical for debugging performance issues.
1. Duplexing (Direction of Flow)
| Type | Direction | Example |
|---|---|---|
| Simplex | One way only. | TV Broadcast. |
| Half-Duplex | Both ways, but one at a time. | Walkie-Talkie, Wi-Fi (mostly). |
| Full-Duplex | Both ways, simultaneously. | Modern Ethernet, Telephone. |
2. Bandwidth vs Throughput
There is a critical difference between the “Pipe Size” and the “Water Flow”.
- Bandwidth: The Maximum possible data rate (e.g., 1 Gbps link). The width of the highway.
- Throughput: The Actual rate of successful delivery. The speed of traffic.
- Throughput < Bandwidth due to protocol overhead (Headers), packet loss, and flow control.
- Goodput: The amount of useful Application Data delivered per second (excluding headers/retransmissions).
[!NOTE] Bandwidth-Delay Product (BDP)
BDP = Bandwidth * RTT. This is the amount of data “in flight” on the wire. If you have a 1Gbps link with 100ms latency, you need100,000,000 bitsof buffer space to fill the pipe.
3. The 4 Types of Latency
“Lag” isn’t one number. It’s the sum of 4 distinct delays:
- Transmission Delay: Time to push bits onto the wire.
Delay = Size / Bandwidth- Faster Link = Lower Transmission Delay.
- Propagation Delay: Time to travel across the wire.
Delay = Distance / Speed of Light- Shorter Cable = Lower Propagation Delay. Bandwidth doesn’t help here!
- Queuing Delay: Time waiting in a router buffer.
- Variable. Depends on congestion.
- Processing Delay: Time for the Router CPU to read the header.
- Usually negligible (microseconds).
4. Interactive: Latency Calculator
Experiment with the physics.
- Packet Size: 1500 Bytes (Standard Ethernet).
- Bandwidth: 1 Mbps (Slow) vs 1 Gbps (Fast).
- Distance: 100km (City) vs 5000km (Trans-Atlantic).
1500 B
10 Mbps
100 km
+
=
Assumes Speed of Light in Fiber ≈ 200,000 km/s.