When designing EtherNet/IP networks for control systems, its important to understand what speed and bandwidth are and how they might effect the media that you use in a network.

In this post, I'll explain what the terms speed, bandwidth, and packet rate mean in an EtherNet/IP network.

Let's start by talking about speed.

Speed

A network's speed is a measurement of the amount of data being transmitted on a channel at any given time.

A network's speed is expressed in bits per second or bps. When we talk about higher speeds, they may be expressed in Kbps, Mbps, and Gbps which are 1,000, 1,000,000, 1,000,000,000 bits per second respectively.

Now that we know what network speed is, let's talk about bandwidth.

Bandwidth

Bandwidth is the maximum speed supported by a channel. It is also measured in bits per second or bps.

A channel's bandwidth has to be greater than the required speed for the network to work properly. This is how speed and bandwidth affect media selection during the design of a network.

Let's look at an analogy to make the relationship between bandwidth and speed more clear.

Analogy

A network channel is like a pipe.

The diameter of the pipe is the channel's bandwidth. This diameter determines the maximum amount of liquid that can flow through the pipe.

The flow through the pipe is the network's speed. If the pipe is not big enough, then the flow is constrained.

Packet Rate

A final term that I'd like to explain is Packet Rate which is often used in Rockwell Automation manuals and device specifications. Packet rate is a way to express the bandwidth of a channel in packets per second, or pps.

To work out the bandwidth in bps, we have to convert this value from packets per second to bits per second. Here's how to do that conversion:

From the ODVA specifications, we know that CIP I/O packets have a maximum size of about 500 bytes. For safety, we can add 100 bytes of overhead to that size giving us a size per packet of 600 bytes

We can use this information to calculate the maximum bandwidth required for a device:

600 (bytes per packet)

x 8 (bits per byte)

x 5000 (packets per second

= 24,000,000 bps or 24 Mbps

Since CIP I/O packets are generally much smaller than the maximum size, this calculation includes a built-in margin for safety and can be used to network media selection.

Synonyms

Finally, I'd like to introduce some commonly used synonyms of speed and bandwidth.

Data rate and data transfer rate are synonyms of speed.

Maximum data rate is a synonym of bandwidth.

Wrap Up

This post introduced the concepts of speed and bandwidth in an EtherNet/IP network.

Bandwidth is the maximum bit rate supported by a channel whereas speed is the actual amount of information that moves over a network at a period of time.

We also learned what the packet rate of a device is and how to convert a value expressed in packets per second to bits per second.