Introducing Bandwidth Monitoring

You can use the Bandwidth Monitoring service to monitor the 171 CBU 98090 and 171 CBU 98091 processor CPU allocation for each of the following services:

Accessing and Configuring the Bandwidth Tab

The following procedure shows how to access the Bandwidth tab:

Step

Action

1

In the Project Browser, navigate to the Communication > Networks subdirectory.

2

Double-click the appropriate Ethernet network to open its configuration dialog.

3

Select the Bandwidth tab to open the Bandwidth Monitoring screen:

4

Configure the Global Data, I/O Scanning, and Modbus Messaging services before configuring the Bandwidth Monitoring parameters.

5

Enter the parameter settings for the Bandwidth Monitoring. Refer to the Bandwidth Monitoring Properties section, below, for a description of Bandwidth Monitoring settings.

Bandwidth Monitoring Properties

The Bandwidth tab includes the following properties:

Property

Description

Ethernet Environment - Potential CPU Load

Select the environment that describes your Ethernet network:

  • Isolated: Limits the impact of Ethernet communication on the scan by limiting the total number of Ethernet messages (I/O scanner included) polled every second to 500.

  • Mastered: Increases the total number of Ethernet messages (I/O scanner included) polled every second to 1000.

  • Open: Increases the total number of Ethernet messages (I/O scanner included) polled every second to 1600.

    • 1600 for the 171 CBU 98090

    • 2200 for the 171 CBU 98091

Global Data information

Enter an estimate of the number of publication periods. The value entered is the estimated average publication frequency of the distribution (local and remote) group stations.

Messaging information

Enter an estimate of the number of transactions per second.

Controlling Ethernet Messaging

During runtime, Control Expert executes both master (MAST) tasks and Ethernet messaging (ETH) tasks. Control Expert prioritizes ETH tasks, and executes ETH tasks before it executes MAST tasks. Because Control Expert prioritizes ETH tasks, you need to configure Ethernet communications so that sufficient bandwidth remains for the execution of the MAST tasks.

To accomplish this, you need to:

  • separately configure each component of Ethernet communications (Global Data, I/O Scanning, and Modbus Messaging) to control the bandwidth used by each service, and

  • limit the number of Ethernet messages per second that the CPU may handle. As indicated above, set the Potential CPU Load value to one of the following limits:

    • Isolated: 500 messages per second

    • Mastered: 1000 messages per second

    • Open: Either:

      ⇒ 1600 messages per second for the 171 CBU 98090

      ⇒ 2200 messages per second for the 171 CBU 98091

To decrease the message load contributed by the Global Data service, increase the Distribution period setting. Each device on the network can publish 1 variable, and subscribe to up to 63 variables. The estimated loads, below, are presented in messages per second:

Distribution Period (ms)

Global Data Variables

8

16

32

64

20

400

800

1600

–

30

267

533

1067

–

40

200

400

800

1600

50

160

320

640

1280

60

133

267

533

1067

70

114

229

457

914

80

100

200

400

800

90

89

178

356

711

100

80

160

320

640

– indicates a value beyond the upper limit of 1600 messages per second
NOTE: When compiling a program, Control Expert considers only subscribed variables, not published variables. As a result, Control Expert may validate a program that causes bandwidth overload during runtime. When computing the bandwidth load required for Global Data, consider both published and subscribed variable messaging.

To decrease the message load contributed by the I/O Scanning service, increase the Repetitive rate setting. The estimated loads, below, are presented in messages per second:

Repetitive rate (ms)

I/O Scanned Devices

8

16

32

64

10

800

1600

–

–

20

400

800

1600

–

30

267

533

1067

–

40

200

400

800

1600

50

160

320

640

1280

60

133

267

533

1067

70

114

229

457

914

80

100

200

400

800

90

89

178

356

711

100

80

160

320

640

– indicates a value beyond the upper limit of 1600 messages per second

To decrease the message load contributed by the Modbus Messaging service, increase the Message Sampling period of the several Modbus clients operating on the network. The messaging rate can be calculated as follows:

Messaging Rate = ((1/ Message Sampling period) x Objects

This formula includes the following elements:

Element

Description

Messaging Sampling period

The interval at which messages are processed

Objects

The number of devices, either client or server, that are actively communicating
NOTE: Message rates are solved in 10 ms increments. Messages that exceed the message rate per 10 ms increment are buffered. Buffered messages are then added to the next 10 ms time slice.