What You Need to Know About Jitter in Industrial Automation

Industrial Automation Overview

Industrial Automation is the use of digital control systems, robotics, and information technologies to control different processes and machinery in an industrial environment to replace human involvement in performing certain functions. These functions primarily focus on material handling, manufacturing, and quality control processes. Concisely, industrial automation makes use of logical programming commands and powerful computer-controlled machinery such as CNC machines to replace human workers on factory floors.

At the core of industrial automation are three significant control techniques, namely: Distributed Control Systems (DCS), Programmable Logic Controllers (PLCs), and Supervisory Control and Data Acquisition (SCADA) systems. These control mechanisms consist of smart and interconnected field input and output devices, field instruments, distributed I/O controllers, supervisory control PCs, and Human-Machine Interfaces (HMIs). To provide flexible connectivity between the aforementioned automation devices and to also create an effective communication path among them, powerful and efficient communication networks are required.

Industrial networks provide a powerful means of data exchange, data control, and flexible connections in any industrial automation system. To transmit data and control signals, these networks utilize two types of transmission media–Wired and Wireless. In wired transmission, various network cables are used including twisted-pair cabling, fiber optics, and coaxial cables. In contrast, wireless transmission does not involve any physical link between the connected automation devices, instead, communication is carried out through radio waves, infrared, cellular radio, microwaves, broadcast radio, and communication satellites.

In addition, industrial networking enables the implementation of different communication protocols between digital logic controllers, field I/O devices, industrial PCs, automation-related software tools, and other external systems. Common industrial communication protocols include ControlNet, RS-232/485, DeviceNet, Modbus TCP/IP, EtherNet/IP, etc.

Current industrial automation trends such as increased Industrial Internet of Things (IIoT) connectivity, use of centralized cloud platforms, and the need to leverage Ethernet benefits at production floors/field level are increasing the need for real-time, high-speed, and high-bandwidth Ethernet-based industrial networks with faster data transfer rates.

Additionally, as data transmission speeds increase, there’s a high demand for Real-Time Clocks and other timing components that support such speeds in industrial Ethernet networks. This has led to the emergence of Real-Time Ethernet (RTE) network solutions such as EtherNet/IP, PROFINET IRT, EtherCAT (Ethernet for Control Automation Technology), PROFINET IO, CC-Link IE (Industrial Ethernet), SERCOS III, Ethernet POWERLINK, and MODBUS-RTPS (Modbus Real-Time Publish-Subscribe).

With the emerging Ethernet technologies, users can now incorporate both wired and wireless automation devices in a given industrial Ethernet network for enhanced transparency, monitoring, and control, as well as streamlined industrial operations. Also, these networking technologies allow faster, more accurate, and highly-secure real-time communications among the interconnected industrial automation devices. However, any form of real-time communication, be it across an industrial or enterprise network, is prone to communication issues caused by network jitter. That brings about a common question among industrial automation stakeholders –“what’s network jitter”? This article will provide an overview of jitter in real-time, Ethernet-based industrial networks used in industrial automation.

What is Jitter?

Jitter is simply the variation in the time delay between when a high-frequency digital signal is transmitted over a network connection and when it is received, thereby measuring the variability in network latency. This time delay variance is measured in milliseconds (ms) and is often described as the disruption in the usual sequence of transmitting data packets over a digital network.

Hence, in other words, jitter can be defined as the deviation between the time when a network device is required to send a message and the time when the message is sent. For example, if an EtherNet/IP or a PROFINET IO device is expected to send cyclic messages every 10 milliseconds but it issues them between 11 and 9 milliseconds, the result is a network jitter of 1 millisecond.

Industrial automation systems use closed-loop control that often requires high-performance industrial communication networks providing real-time and deterministic control of data transmission. That means the network to be used in an industrial automation system should guarantee that a message will be transmitted or responded to in a specific, foreseeable period–not slower or faster but deterministic–and that the transmission of the message as well as responding to it will take place in real-time. To satisfy these requirements, the Ethernet-based Fieldbus networks used in industrial automation applications must have an acceptable amount of jitter and low latency.

What’s the Difference Between Latency and Jitter?

When assessing or describing the performance of a network, many people often use the terms Latency and Jitter interchangeably, but the two have distinct meanings.

Latency: This is a measure of the time it takes for a message to travel between two devices across a network. It’s simply the actual or perceived time delay via a network. With network latency, the entire message being transmitted is received later than intended, not just bits of data packets.

Transmission delays are either introduced by the hardware components of the industrial network in use, or they can be caused by routing network traffic via electronic and electrical communication pathways. On a VoIP (Voice over Internet Protocol) call, network latency may sound like an echo or a delay in voice communication.

Jitter: This is a variation in network latency above the upper limit. Jitter causes time delay variability–an inconsistent change in the time it takes a data packet or message to travel from the source to the destination across a network. This means the data packets or messages being transmitted can arrive at the destination in irregular intervals or out of order/in wrong “groups”, causing delays in processing. Jitter is known to cause glitchy/stuttering audio messages in VoIP communications.

Acceptable Jitter Level

Jitter is a measure of a network’s timing performance. In most cases, a high jitter score means that the network in question has poor timing performance. While a low jitter score translates to good network connections with reliable and consistent response times.

Ideally, allowable network jitter should be less than 30 milliseconds with a loss of data packets of no more than 1%. And acceptable network latency should not exceed 150 milliseconds one-way and 300 milliseconds Round-Trip Time (RTT). However, it’s worth noting that the amount of acceptable jitter highly depends on the type of application, because some applications have higher tolerance levels for network jitter than others. For example, jitter affects VoIP audio communications more than it affects sending emails.

Demanding industrial automation applications require synchronized, high-performance industrial network protocols with millisecond network updates and microsecond (µs) jitter levels or less. Most high-performance industrial Ethernet networks such as CC-Link IE, Ethernet POWERLINK, EtherNet/IP (through CIP Sync extension), SERCOS III, and EtherCAT claim a network jitter of less than 1 microsecond. Although the real-world performance of such networks may be affected by various factors including the amount of inherent jitter in other network devices and the number of cascaded network devices a data packet must flow through.  

To achieve a significantly low jitter score, EtherNet/IP and EtherCAT networks utilize time synchronization techniques that are compliant with IEEE 1588 standards. While SERCOS III, PROFINET IRT, and Ethernet POWERLINK networks use ISOCHRONOUS (ISOC) data transmission principles for the same. Thus, by using different methods of time synchronization and data transmission, the aforementioned industrial Ethernet networks can provide real-time, deterministic communications with an acceptable jitter score and low latency.

Types of Jitter

A network jitter can fall into either of the following categories:

• Constant Jitter: This is a nearly steady level of packet-to-packet time delay variation.
• Transient Jitter: This type of network jitter is characterized by a considerable incremental time delay variation, which may be experienced by a single data packet.
• Short-term Jitter: This type of jitter is normally identified by an increase in time delay that continues for several data packets, and in some cases, it may be accompanied by an increased packet-to-packet time delay variation. It’s often caused by changes in data packet routing and network congestion.

Effects of Jitter

Network jitter can lead to loss of the data packets being transmitted between network devices, it can affect the ability of a PLC processor to perform as intended, it can cause an HMI screen display to flicker, and in other cases, jitter can introduce undesired effects such as clicks in audio signals. Jitter is particularly problematic in real-time network communications; everyday examples of this would be choppy audio in IP telephony or buffering delays in video conferencing, due to irregular transmission of the packets containing the audio and video data, respectively.

Jitter also affects the functioning of industrial networks in a variety of ways, including:

• Poor Communication: Network jitter can be a huge hindrance to reliable communications between automation devices in an industry setting. This is particularly true when the data packets being transmitted are required to arrive intact at the destination for the communicated data to make sense. However, jitter is known to transmit data packets out of order, leading to incomprehensible communications.
• Timeouts: Server/Network connection timeouts occur when a server takes too long to respond to a data request from another network device. Some industrial automation applications poll a destination or connection host for a predetermined length of time before the network connection is disconnected and a “timeout” message is displayed.
• Network Bottlenecks: Network jitter caused by the transmission of data packets at irregular intervals, makes the buffers in the connected hardware fill up while waiting for the arrival of the entire message. This results in network latency, or overall delay in data transmission, due to slowed-down traffic for data packets that don’t even need buffering.

What Causes Network Jitter?

Here are some of the common causes of jitter in industrial networks:

A) Network Congestion

Network jitter is generally caused by congestion in a network, which occurs when the network is overcrowded with traffic on receiving an excessive amount of data. This especially happens when the available network bandwidth is limited and too many active devices are trying to send and receive data through it concurrently.

B) Hardware Issues

A high network jitter score can result from using older industrial networks with outdated equipment, such as outdated Ethernet switches, cables, and industrial routers that were not designed to handle huge amounts of data transfers.

C) Wireless Connections

Jitter can result from poorly designed wireless industrial automation systems and the use of routers with low signal strength. Also, placing wireless routers at a far-away distance can cause jitter, leading to low-quality network connections. Hence, wired automation systems are highly recommended for key industrial applications, because wired connections provide a much better user experience.

D) Failure to Implement Packet Prioritization

In some applications, like Voice over IP (VoIP) systems, network jitter occurs when packets containing audio data are not prioritized for delivery before other types of data traffic.  Implementing proper packet queuing ensures that critical data packets are not impacted by network congestion.

How is Network Jitter Resolved?

Now that we’re familiar with the common causes of network jitter, let’s look at various strategies for resolving it and keeping it within acceptable levels.

A) Increase Network Bandwidth

Oftentimes, congestion within a network is reduced by simply increasing the available bandwidth. With increased bandwidth, the network is then capable of handling more data traffic and multiple automation devices at the same time.  This in turn significantly reduces the jitter level in the network, allowing faster data transfer speeds and fewer connection interruptions.

B) Jitter Buffering

Jitter buffers can be used to mitigate the effects of network jitter, either in the network’s host computer, router, or Ethernet switch. Essentially, the automation system consuming the network packets receives them from a jitter buffer instead of directly from the connection host. The data packets are transmitted out of the buffer at a regular delivery rate; this smooths out any time delay variations of the data packets flowing into the jitter buffer.

Note: If the jitter buffers used are too small then very many data packets will be lost, which translates to poor network connections. And if the jitter buffers in use are too large then there will be additional transmission delays, leading to communication problems. Generally, jitter buffers are configured at 30 milliseconds to 50 milliseconds in size, though you can increase the size to a point. But normally, jitter buffers are only effective for time delay variations of less than 100 milliseconds.

C) Prioritize Network Traffic

Prioritizing network traffic means reserving sufficient bandwidth for essential automation devices and high-priority tasks. This is possible by slowing down traffic for low-priority functions or non-essential automation devices. Thus, important processes can run smoothly over the network connection.

Routers with a Quality of Service (QoS) setting allow users to prioritize critical data packets over other types of network traffic, thereby eliminating jitter caused by traffic congestion. Also, where multiple traffic pathways are available, network jitter can be mitigated by selectively routing traffic along the most stable pathways or by always selecting the path that’s closest to the targeted packet transmission rate.

D) Upgrade Existing Network Equipment

As mentioned earlier, outdated network equipment such as Ethernet cables, switches, and routers can cause network jitter. Therefore, for industrial Ethernet networks, you can replace older, low-bandwidth (125 MHz) Ethernet cables with new Cat6 (Category 6) Ethernet cables that provide 250 MHz (Megahertz) of bandwidth while supporting low-latency data transmission speeds of up to 10 Gbps (Gigabits per second) for a maximum distance of approximately 50 meters. This can potentially solve the problems associated with Ethernet jitter in wired industrial automation systems. Also, the four twisted pairs of copper wire in Cat6 Ethernet cables can resist network interference.