Bandwidth
How much spectrum or bit-rate a channel offers. Not the same thing as throughput.
Level: beginner
What is it?
Bandwidth can refer to the frequency range of a channel in hertz or, in everyday networking, a link's nominal bit-rate capacity. The units and context must be specified to avoid confusion with actual throughput.
Why does it matter?
Capacity limits how much data can be transmitted per unit time, but applications also depend on latency, packet loss, congestion, and protocol overhead. A link can have high bandwidth and still deliver poor interactive performance.
Mental model
Bandwidth is the width of the pipe; throughput is the measured flow through it. Latency is how long a unit takes to travel, so large capacity does not imply a short round trip.
How it works
- Frequency bandwidth is measured in hertz; digital link rate is typically measured in bits per second.
- Throughput is the achieved data rate and is lower than nominal capacity due to framing, contention, loss, and other overhead.
- Goodput counts useful application data and excludes protocol overhead and retransmitted bytes.
- Bandwidth-delay product estimates how much data can be in flight to fill a path: capacity × round-trip time.
- Congestion and competing flows can keep a sender from using all nominal bandwidth.
Steps
- Read the path's link capacities and round-trip latency.
- Calculate the bandwidth-delay product to estimate the in-flight data needed to fill the path.
- Measure achieved throughput and retransmissions under the expected workload.
- Compare the application payload rate with overhead, loss, and queueing rather than judging by the advertised link rate alone.
Real-world usage
- Sizing network interfaces and WAN links.
- Explaining why large transfers and interactive requests respond differently to a path.
- Estimating TCP window and buffering needs.
Common mistakes
- Calling throughput the same as bandwidth.
- Ignoring units such as Mbps versus MB/s.
- Assuming buying a higher-rate link fixes latency caused by distance or queueing.
Interview questions
What is the bandwidth-delay product?
It is path capacity multiplied by round-trip time, estimating how many bits can be in flight to fully use the path.
Why can throughput be below link bandwidth?
Protocol overhead, congestion, competing traffic, packet loss, sender/receiver limits, and insufficient data in flight reduce achieved throughput.