In computer networks (including wireless networks), goodput is the application level throughput, i.e. the number of useful bits per unit of time forwarded by the network from a certain source address to a certain destination, excluding protocol overhead, and excluding retransmitted data packets.
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 generally 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 may cause lower goodput than throughput are:
- Protocol overhead.
- Retransmission of lost or corrupt packets, caused by bit errors or packet dropping in congested switches and routers. Automatic repeat request (ARQ), i.e. retransmission of lost or corrupt data packets, is supported by the TCP protocol, some UDP based application layer protocols, and by reliable data link layer protocol used for example in wireless networks.
- Collision detection in the Ethernet CSMA/CD protocol, and collision avoidance in for example wireless local area networks using the CSMA/CA protocol, may cause "backoff" waiting time (i.e. increased interframe gap) and retransmission.
Example
Imagine that a file is being transferred using HTTP over a switched ethernet connection with a total channel capacity of 100 megabits per second. The file cannot be transferred over Ethernet as a single contiguous stream, instead it must be broken down into individual segments, called packets. These packets must be no larger than the maximum transmission unit of Ethernet, which is 1500 bytes. Each packet requires 20 bytes of IP header information and 20 bytes of TCP header information, so only 1460 bytes are available per packet for the file transfer data itself. Furthermore, the packets are transmitted over Ethernet in a frame which imposes a 38 byte overhead per packet. Given these overheads, the maximum goodput is 1460/1538 × 100 Mbit/s which is 94.92 megabits per second or 11.866 megabytes per second.
Note that this example doesn't consider some additional overhead, such as the space required between frames (a minimum of 96 bit times), or collisions (which have a variable impact, depending on the network load).
See also
References
- Goodput calculation
- An Empirical Characterization of Instantaneous Throughput in 802.11b WLANs
- Energy-Efficient Power and Rate Control with QoS Constraints: A Game-Theoretic Approach
- RFC 2647 — Benchmarking Terminology for Firewall Performance
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