computer science

Goodput

Goodput is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Goodput rather than just read about it. In short: In computer networks, goodput (a portmanteau of good and throughput) is the application-level throughput of a communication. In other words, goodput is the number of useful information bits delivered by the network to a certain destination per unit of time.

Key takeaways

Goodput belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Goodput to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Goodput from memory before moving on to harder problems.

Reference excerpt

In computer networks, goodput (a portmanteau of good and throughput) is the application-level throughput of a communication. In other words, goodput is the number of useful information bits delivered by the network to a certain destination per unit of time. The amount of data considered excludes protocol overhead bits as well as retransmitted data packets. This is related to the amount of time from the first bit of the first packet sent (or delivered) until the last bit of the last packet is delivered. For example, if a file is transferred, the goodput that the user experiences corresponds to the file size in bits divided by the file transfer time. The goodput is always lower than the throughput (the gross bit rate that is transferred physically), which generally is lower than network access connection speed (the channel capacity or bandwidth). Examples of factors that cause lower goodput than throughput include:

Protocol overhead: Typically, transport layer, network layer and sometimes data link layer protocol overhead is included in the throughput, but is excluded from the goodput. Transport layer flow control and congestion avoidance: For example, TCP slow start may cause a lower goodput than the maximum throughput. Retransmission of lost or corrupt packets due to transport layer automatic repeat request (ARQ), caused by bit errors or packet dropping in congested switches and routers, is included in the data link layer or network layer throughput but not in the goodput.

Example For an application using the common Transmission Control Protocol (TCP) over IPv4 over Ethernet, the minimum protocol overhead consists of

Ethernet physical layer: 20 bytes preamble, start frame delimiter and interpacket gap Ethernet data link layer: 18 bytes frame headers and frame check sequence IPv4 network layer: 20 bytes packet headers TCP transport layer: 20 bytes segment headers for a total of 78 bytes: 38 bytes for Ethernet, and 40 bytes for IPv4 and TCP. An IP packet over Ethernet may have a size of up to 1500 bytes – the maximum transmission unit for Ethernet. That means that a TCP segment can transport up 1500 - 40 = 1460 bytes, while Ethernet needs to transport 1500 + 38 = 1538 bytes. The overall efficiency is 1460 / 1538 = 94.9%. Accordingly, the maximum goodput for any application running over 100 Mbit/s Ethernet with TCP/IP is 94.9 Mbit/s or 11.9 MB/s. For Gigabit Ethernet, the same calculation results in 949 Mbit/s, or 119 MB/s. When large objects or files (megabytes) are transmitted, the overhead of the application itself may be ignored since most protocols only add a single header of limited size for each object or file. When only small objects are transmitted, the overhead increases in proportion and becomes more significant.

Data delivery time The goodput is a ratio between delivered amount of information, and the total delivery time. This delivery time includes:

Inter-packet time gaps caused by packet generation processing time (a source that does not use the full network capacity), or by protocol timing (for example collision avoidance) Data and overhead transmission delay (amount of data divided by bit rate) Propagation delay (distance divided by wave propagation speed) Packet queuing delay NAT translation delay Intermediate node store-and-forward processing delay Packet retransmission time (in case of deleted packets in congested routers, or detected bit errors) Delayed acknowledge due to flow control, congestion avoidance and processing delay

See also Measuring network throughput Spectral efficiency

References

Sources Energy-Efficient Power and Rate Control with QoS Constraints: A Game-Theoretic Approach

Worked examples

Example 1 — a first encounter with Goodput

Start with the simplest possible case. Write down what Goodput claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Goodput before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Goodput ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Goodput

In research: Goodput appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Goodput in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Goodput is common in secondary-school and first-year university syllabi. It links to neighbouring topics Network performance, so understanding it makes those chapters shorter.
In everyday life: Look for Goodput outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Goodput in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Goodput means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Goodput out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Goodput in simple terms?

Why does Goodput matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Goodput?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Goodput.