What Is Network Latency?
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 generally experiences small delay times, while a high latency connection generally suffers from long delays.
The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include:
• Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light.
• Transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.
• Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the time-to-live field).
• Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers.
Within a computer, latency can be removed or "hidden" by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads.
Latency vs. Bandwidth
Although the theoretical peak bandwidth of a network connection is fixed according to the technology used, the actual bandwidth you will obtain varies over time and is affected by high latencies. Excessive latency creates bottlenecks that prevent data from filling the network pipe, thus decreasing effective bandwidth. The impact of latency on network bandwidth can be temporary (lasting a few seconds) or persistent (constant) depending on the source of the delays.
Latency of Satellite Internet Service
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 (DSL or cable).
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).
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 specify latency, but it is the most common.
On DSL or cable Internet connections, latencies of less than 100 milliseconds (ms) are typical and less than 25 ms desired. Satellite Internet connections, on the other hand, average 500 ms or higher latency.
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. Businesses use the term Quality of Service (QoS) to refer to measuring and maintaining consistent performance on a network by managing both bandwidth and latency in a coordinated fashion.
Bandwidth and Speed
In computer networking, the term bandwidth refers to the rate of data transfer. Very often the bandwidth numbers one sees and hears represent theoretical or peak performance of a device. A 100 Mbps Ethernet adapter installed in a computer with a slow processor, for example, rarely exceeds 10 Mbps in actual performance. Traditional dial-up modems, advertised as capable of 56 Kbps, never actually perform at that rate in practice.
The term speed is often used synonymously with data rate in networking. Technically speaking, speed refers to the user-perceived performance of the network application. Speed correlates positively with bandwidth in many cases, but not always.
One's Web browser, for example, may perform "slowly" on a high-bandwidth network for several reasons, such as a bottleneck at the Web server or at one's ISP. Those who have upgraded from traditional dial-up to broadband may already be familiar with the phenomenon. It's common to attribute performance problems to a "slow network" even though the local area data transferates (when transfers occur) remain high.
Tips to solve network latency issues
The network is running all kinds of applications, from single-tier applications like e-mail to multitier Web-based databases or complex voice-over-Internet-protocol (VoIP) applications. The enterprise IT department does not necessarily have control over all of the applications, because some of these applications are running on networks outside of the enterprise and some may be outsourced. Pinpointing and correcting slowdowns, therefore, can be a challenge.
Monitoring tools can help locate and solve problems with latency. A quality-of-service (QoS) monitoring solution can provide the data needed to prove that the network is performing poorly or applications are slow. The monitoring solution can collect data 24/7 and send alarms automatically to notify administrators when something on the network is not performing as expected. The first step is to determine if the latency is coming from the network or the application.
Network latency is the amount of time it takes for an application to make a request and the server to respond with an acknowledgement (a packet message used in the transmission control protocol to acknowledge receipt of a packet). Application latency is the amount of time needed for the application to process the request and send a response containing real data.
Most network-monitoring products provide some sort of latency-monitoring features, typically either one or the other, not both. Ideally, these measurements can be graphed together over time, making clear whether the problem lies with the network or the application. Comparing the measurements of the two types of latency over time and seeing the differences can provide information that might have otherwise been overlooked.
Graphing latency over time helps to identify patterns and anomalies that deserve closer attention. Latency monitoring can help correlate areas of latency with other relevant statistics, as well as the actual packets that occurred during that time. This type of high-resolution forensic analysis can help to detect latency problems at the highest level and drill down quickly for closer inspection.
Being able to see graphs over time and correlate the latency is important, as latency patterns allow the administrator to compare the patterns to other events on the network, and determine the root cause of the problem. Latency monitors can include a feature that sets thresholds on latency, so alarms will go off when normal conditions are exceeded. The administrator can be made aware of excessive latency before applications on the network start dropping packets, allowing him to make necessary adjustments to the network proactively. This type of proactive latency monitoring allows the administrator to detect and correct problems in the network and applications before users notice a slowdown.
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 generally experiences small delay times, while a high latency connection generally suffers from long delays.
The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include:
• Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light.
• Transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.
• Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the time-to-live field).
• Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers.
Within a computer, latency can be removed or "hidden" by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads.
Latency vs. Bandwidth
Although the theoretical peak bandwidth of a network connection is fixed according to the technology used, the actual bandwidth you will obtain varies over time and is affected by high latencies. Excessive latency creates bottlenecks that prevent data from filling the network pipe, thus decreasing effective bandwidth. The impact of latency on network bandwidth can be temporary (lasting a few seconds) or persistent (constant) depending on the source of the delays.
Latency of Satellite Internet Service
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 (DSL or cable).
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).
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 specify latency, but it is the most common.
On DSL or cable Internet connections, latencies of less than 100 milliseconds (ms) are typical and less than 25 ms desired. Satellite Internet connections, on the other hand, average 500 ms or higher latency.
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. Businesses use the term Quality of Service (QoS) to refer to measuring and maintaining consistent performance on a network by managing both bandwidth and latency in a coordinated fashion.
Bandwidth and Speed
In computer networking, the term bandwidth refers to the rate of data transfer. Very often the bandwidth numbers one sees and hears represent theoretical or peak performance of a device. A 100 Mbps Ethernet adapter installed in a computer with a slow processor, for example, rarely exceeds 10 Mbps in actual performance. Traditional dial-up modems, advertised as capable of 56 Kbps, never actually perform at that rate in practice.
The term speed is often used synonymously with data rate in networking. Technically speaking, speed refers to the user-perceived performance of the network application. Speed correlates positively with bandwidth in many cases, but not always.
One's Web browser, for example, may perform "slowly" on a high-bandwidth network for several reasons, such as a bottleneck at the Web server or at one's ISP. Those who have upgraded from traditional dial-up to broadband may already be familiar with the phenomenon. It's common to attribute performance problems to a "slow network" even though the local area data transferates (when transfers occur) remain high.
Tips to solve network latency issues
The network is running all kinds of applications, from single-tier applications like e-mail to multitier Web-based databases or complex voice-over-Internet-protocol (VoIP) applications. The enterprise IT department does not necessarily have control over all of the applications, because some of these applications are running on networks outside of the enterprise and some may be outsourced. Pinpointing and correcting slowdowns, therefore, can be a challenge.
Monitoring tools can help locate and solve problems with latency. A quality-of-service (QoS) monitoring solution can provide the data needed to prove that the network is performing poorly or applications are slow. The monitoring solution can collect data 24/7 and send alarms automatically to notify administrators when something on the network is not performing as expected. The first step is to determine if the latency is coming from the network or the application.
Network latency is the amount of time it takes for an application to make a request and the server to respond with an acknowledgement (a packet message used in the transmission control protocol to acknowledge receipt of a packet). Application latency is the amount of time needed for the application to process the request and send a response containing real data.
Most network-monitoring products provide some sort of latency-monitoring features, typically either one or the other, not both. Ideally, these measurements can be graphed together over time, making clear whether the problem lies with the network or the application. Comparing the measurements of the two types of latency over time and seeing the differences can provide information that might have otherwise been overlooked.
Graphing latency over time helps to identify patterns and anomalies that deserve closer attention. Latency monitoring can help correlate areas of latency with other relevant statistics, as well as the actual packets that occurred during that time. This type of high-resolution forensic analysis can help to detect latency problems at the highest level and drill down quickly for closer inspection.
Being able to see graphs over time and correlate the latency is important, as latency patterns allow the administrator to compare the patterns to other events on the network, and determine the root cause of the problem. Latency monitors can include a feature that sets thresholds on latency, so alarms will go off when normal conditions are exceeded. The administrator can be made aware of excessive latency before applications on the network start dropping packets, allowing him to make necessary adjustments to the network proactively. This type of proactive latency monitoring allows the administrator to detect and correct problems in the network and applications before users notice a slowdown.