Introduction to Latency on Computer Networks

About Latency

Laptop computer streaming data.
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What is Network Latency?

Bandwidth is just one element of what a person perceives as the speed of a network. Latency is another element that contributes to network speed. The term latency refers to any of several kinds of delays typically incurred in processing of network data. A so-called low latency network connection is one that experiences small delay times, while a high latency connection suffers from long delays.

Besides propagation delays, latency also may also involve transmission delays (properties of the physical medium) and processing delays (such as passing through proxy servers or making network hops on the Internet).

Latency vs. Throughput

Although the theoretical peak bandwidth of a network connection is fixed according to the technology used, the actual amount of data that flows over a connection (called throughput) varies over time and is affected by higher and lower latencies. Excessive latency creates bottlenecks that prevent data from filling the network pipe, thus decreasing throughput and limiting the maximum effective bandwidth of a connection. The impact of latency on network throughput can be temporary (lasting a few seconds) or persistent (constant) depending on the source of the delays.

Latency of Broadband Internet Services

On DSL or cable Internet connections, latencies of less than 100 milliseconds (ms) are typical and less than 25 ms are often possible.

Satellite Internet connections, on the other hand, typical latencies can be 500 ms or higher. An Internet service rated at 20 Mbps can perform noticeably worse than a service rated at 5 Mbps if it is running with high latency. 

Satellite Internet service illustrates the difference between latency and bandwidth on computer networks.

Satellite Internet connections possess both high bandwidth and high latency. When loading a Web page, for example, most satellite users can observe a noticeable delay from the time they enter a Web address to the time the page begins loading. This high latency is due primarily to propagation delay as the request message travels at the speed of light to the distant satellite station and back to the home network. Once the messages arrive on Earth, however, the page loads quickly like on other high-bandwidth Internet connections (such as DSL or cable Internet).

Measuring Network Latency

Network tools like ping tests and traceroute measure latency by determining the time it takes a given network packet to travel from source to destination and back, the so-called round-trip time. Round-trip time is not the only way to measure latency, but it is the most common.

Summary

Two key elements of network performance are bandwidth and latency. The average person is more familiar with the concept of bandwidth as that is the one advertised by manufacturers of network equipment. However, latency matters equally to the end user experience as the behavior of satellite Internet connections illustrates. Quality of Service (QoS) features of home and business networks are designed to help manage both bandwidth and latency together to provide more consistent performance.

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