Factors affecting network bandwidth

Bandwidth is defined as the carrying capacity of a circuit. It's usually measured in bits per second for digital circuits, or in hertz for analog circuits. Though a hardwired network is also limited by the speed of its devices and microprocessors, a typical 4-20 mA signal over typical 18-gauge wire usually has about a 10-ms response time.

09/01/2003


Bandwidth is defined as the carrying capacity of a circuit. It's usually measured in bits per second for digital circuits, or in hertz for analog circuits. Though a hardwired network is also limited by the speed of its devices and microprocessors, a typical 4-20 mA signal over typical 18-gauge wire usually has about a 10-ms response time. Fieldbus response times can range from 2-50 ms or more.

"Network bandwidth comes down to two measures: 1) transactions per second—how many communication cycles is the network accomplishing per second, and 2) latency— how long did it take to get the data from the time you wanted it?" says Wally Pratt, chief engineer at the HART Communication Foundation. "When you're talking about throughput, bytes per second, latency means how many times can you measure your primary variable? When you're talking about a multi-drop network, you have to take transactions per second and divide by the number of devices on the network, which will show the number of times you can talk to each one. Then, you have to ask is that time enough for the application?"

Perry Marshall, author and consultant, adds that users need to determine their speed and data requirements. "Does your I/O device need to be scanned every 2 ms? Or is 500 ms okay? Do your devices send a few bits of data, or hundreds of bytes? Are you mixing simple and complex devices on the same network?" asks Marshall. "To answer these questions, determine the 'hard limits' and worst-case requirements of your design. Which networks are available on specific devices you've chosen? This issue may force your hand, or at least limit your choices, if you are required to use a specific brand of PLC or other component. "There's only a loose relationship between bandwidth and speed. The real tradeoff is among bandwidth, response time, determinism, and noise immunity."

"A lot of the improvements in network capacity is due to refinements in the technology used to pump data down a wire at ever higher rates," says Dick Caro, a well-known fieldbus expert." Users need to be sure that the network in their production areas is designed for real-time data transmission, which means about a 1-second response in process applications and 5 ms in applications with factory safety switches.

Jim Montague, news editor, jmontague@reedbusiness.com

Fieldbus Comparison Chart
The following includes sections of the Fieldbus Comparison Chart that deal directly with capacity and bandwidth issues. The full chart, developed by the former Synergetic Micro Systems and now known as Grid Connect Inc., includes other fieldbus protocols and is located at www.synergetic.com/compare.htm .

Name

Max. devices/distance

Transmission properties

Data transfer size

Profibus PA/DP

127 nodes/100 m between segments at 12 Mbps

DP: 9.6,19.2, 93.75, 187.5, 500 kbps and 1.5, 3, 6, 12 Mbps PA: 31.25 kbps

0-244 bytes

DeviceNet

64 nodes/500 m depending on baud rate

500, 250, 125 kbps

8-byte variable message w/ fragmentation for larger packets

Foundation fieldbus (H1)

240 per segment and 65,000 segments/1,900 m at 31.25 kbps wire

31.25 kbps

128 octets

Foundation fieldbus (HSE)

Unlimited with IP addressing/2,000 m at 100 Mbps fiber full-duplex

100 Mbps

Varies; uses standard TCP/IP

Industrial Ethernet

1,024 nodes, expandable via routers

10, 100 Mbps

46-1,500 bytes

Modbus Plus

32 nodes per segment; 64 max./500 m per segment

1 Mbps

Variable





No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Save energy with automation; Process control system upgrades; Dispelling controll myths; Time-sensitive networking; Control system integration; Road to IANA
Additive manufacturing advancements; Machine vision enhances robotics; Fieldbus evolution; Process safety; Advice from System Integrators of the Year; Road to IANA
Salary and career survey: Benchmarks and advice; Designing controls; Remote data collection, historians; Control valve advances; Hannover Messe; Control Engineering International
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
click me