comparison of latency and throughput
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A comparison of latency and throughput in telecommunications can address a common misunderstanding that having greater throughput means a "faster" connection. However, throughput, latency, the type of information transmitted, and the way that information is applied all affect the perceived speed of a connection.
Terms
Latency is the delay between the initiation of a network transmission by a sender and the initial receipt of that transmission by a receiver. It is typically commensurate with the distance the signal must travel, but is also affected by delays introduced in network routing, including queues, multiple routes, packet loss, etc. and also
Throughput is the rate at which the transmission occurs. It is typically commensurate with the channel capacity of the lowest-bandwidth portion of the transmission conduit.
Interplay of factors
Latency and throughput together affect the perceived speed of a connection. However, the perceived speed of a connection can still vary widely, depending in part on the type of information transmitted and how it is used.
For example, to view a web page over a 56 kbit/s modem transmitted from a server 4,800 km (~3,000 mi.) away, latency over the Internet is fairly low – typically about a quarter of a second – and an average web page of 30–100 kilobytes will transfer in 10–30 seconds.
However, to transfer the contents of a DVD over a modem could take a week or more at this rate. Simply packing the DVD into an envelope and mailing it could be faster.
Using a T1 line with similar latencies, one could download the same web page in under a second. To download a 5 GB DVD over this 1.5 Mbit/s connection would take about 7.4 hours.
Latency can also directly affect throughput. In TCP connections, the large Bandwidth-delay product of high latency connections, combined with relatively small TCP window sizes on many devices, effectively causes the throughput of a high latency connection to drop sharply with latency. This can be remedied with various techniques, such as increasing the TCP congestion window size, or more drastic solutions, such as packet coalescing, TCP acceleration, and forward error correction, all of which are commonly used for high latency satellite links.
TCP acceleration converts the TCP packets into a stream that is similar to UDP. Because of this, the TCP acceleration software must provide its own mechanisms to ensure the reliability of the link, taking the latency and bandwidth of the link into account, and both ends of the high latency link must support the method used.
External links
- It's the Latency, Stupid
- more formal paper by same author
- A technical article
from Infocom 2001 on techniques for reducing web latency
"As network technology becomes less dominated by bandwidth limitations, the round-trip times spent for protocol handshakes will become a dominant component in the overall transfer time" for many web pages.
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