Line Rate & Throughput Calculator
What is Line Rate in Data Networking?
Line rate, often referred to as "wire speed," is the maximum raw bit rate that a physical communication medium can transmit. In network engineering, it represents the speed at which bits are pushed onto the cable. This is distinct from Throughput, which is the actual amount of useful data (payload) delivered over the network after accounting for protocol overheads such as IP headers, TCP segments, and Ethernet framing.
The Difference Between Throughput and Line Rate
When you subscribe to a 100 Mbps internet connection, that is typically your line rate. However, you will never see a 100 Mbps download speed for a file. This is because every packet sent contains "extra" data required for routing and error checking. If your payload is 1460 bytes and your overhead (TCP/IP headers) is 40 bytes, the network must actually transmit 1500 bytes to deliver those 1460 bytes of data.
How to Calculate Line Rate
To calculate the required line rate for a specific desired throughput, we use the following formula:
Line Rate = Throughput × ((Payload + Overhead) / Payload)
Real-World Example Calculation
Imagine you need to transfer 1,000 Mbps (1 Gbps) of pure video data using standard Ethernet frames:
- Payload: 1,460 bytes
- Overhead: 40 bytes (TCP/IP) + 20 bytes (Ethernet framing) = 60 bytes total
- Total Frame Size: 1,520 bytes
- Calculation: 1,000 × (1520 / 1460) = 1,041.09 Mbps
In this scenario, to achieve a true 1 Gbps data transfer, your physical link must support at least 1,041.09 Mbps to account for the necessary headers.
Why Packet Size Matters
Smaller packets result in lower efficiency. If you send many small packets (e.g., 64-byte VoIP packets), the ratio of overhead to payload increases significantly. This is why "Jumbo Frames" are often used in data centers to increase efficiency; by increasing the payload size, the percentage of bandwidth lost to overhead is reduced.