CITS3002 Computer Networks  
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Metrics Of Network Measurement

There are two components to performance - latency and bandwidth. We always want high bandwidth and low latency, but can't always get both together.

  • Latency or propagation delay is the amount of time it takes for the first bit to reach its destination. Round-trip time (RTT) is (naively) twice the latency - the time for one bit to travel to the destination, and the first bit in reply to return.
  • Bandwidth is the number of bits that can be fit through a network connection, per unit time. Historically, i.e. without any data compression, bandwidth has been related to frequency within the medium.
  • Throughput is usually the measured number of bits per second (achieved), while bandwidth is the nominal (peak) number of bits per second possible. High bandwidth does not imply low latency: you can broadcast many Mbps over a satellite connection, but it will take hundreds of milliseconds to arrive.

Example: a car can carry 4 people to Bunbury in 2 hours.
Thus the bandwidth is 2 people/hour.

A bus can take 60 passengers and arrive in 2 hours.
Thus the bandwidth is 30 people/hour, but the latency is still 2 hours.

The latency of a message is the total time for the whole message to arrive: 

    TLatency     = TPropagation + TTransmit + TQueue

    TPropagation = distance / speed-of-propagation-in-medium
    TTransmit    = message-size / bandwidth
    TQueue      = time-spent-in-local-and-remote-operating-system-and-switch-queues

On prefixes

In networking and data storage (devices), the prefix Mega means 106, at least when referring to throughput and bandwidth. We always use powers of 10 when referring to frequencies, measured in Hertz. When talking about the size of a message, buffer, file, or other computer-storage item, we use Mega to mean 220. Similar rules apply for the prefixes Kilo and Giga.

See: Transmission time [Wikipedia], Binary prefix [Wikipedia], and Grace Hopper Explains a Nanosecond.



CITS3002 Computer Networks, Lecture 2, The Physical Layer, Errors detection and correction, p2, 6th March 2024.