Computer/Embedded Technology

10 issues to consider before installing industrial Ethernet - Part III

17 July 2002 Computer/Embedded Technology

After having a look at speed and distance issues in Part II, we now turn our attention to a consideration between hubs vs switches.

Some hubs and converters are shown here
Some hubs and converters are shown here

Repeating hubs

Modern Ethernet networks must be wired in a star topology utilising either twisted-pair or fibre-optic cabling. Links, consisting of only two devices, are established between a single Ethernet device and a port on a hub. Hubs are multiport devices usually capable of having 4, 8 or 12 ports. Hubs can be cascaded with a hub-to-hub connection.

Repeating hubs must conform to the requirements for IEEE-802.3 repeater units. These requirements include preamble regeneration, symmetry and amplitude compensation. Repeaters must re-time signals so that jitter, introduced by transceivers and cabling, does not accumulate over multiple segments. These devices detect runt packets and react to collisions by generating a JAM signal. They automatically partition jabbering ports to maintain network operability.

A point to remember: there is a limit to the number of hubs that can be cascaded. Ethernet's contention-based station arbitration method requires that all stations note if a collision has occurred on the network. The limit of this detection is called the collision domain, and it restricts the network's overall size. Exceeding the collision domain by introducing too many repeating hubs creates an unstable network with lost messages and generally poor performance. However, on a properly designed network, repeating hubs are simple to understand and use, not to mention very effective.

Repeating hubs have been criticised because they do not improve the determinism of Ethernet. With contention-based networks, such as Ethernet, it is impossible to predict the amount of time it takes for a station-to-station message when collisions occur since the backoff time is variable. A potential solution to this problem is to avoid collisions altogether.

Industrial automation systems frequently utilise master/slave protocols where a response from a slave only occurs after a command is initiated by the master. This type of protocol tends to limit collisions and thereby improves determinism. Repeating hubs will function quite well in this situation.

Media converters

Another class of physical devices are the media converters. Sometimes called transceivers, these devices convert one type of media to another. The most important conversion is from twisted-pair cable to fibre optics. Since some hubs do not have any fibre-optic ports, media converters are required in order to support fibre-optic cable in a network. Media converters should appear to the network as transparent devices. They are two-port devices that do not store frames or detect collisions. They only convert the signals sent over one medium to compatible signals over another.

Switching hubs

It is possible to replace repeating hubs with switching hubs and achieve higher network performance. Unlike repeating hubs, which are physical layer devices, the switching hub is actually a bridge that connects two data links together. By doing so, collision domains terminate at each switch port. Therefore, adding a switch doubles the possible geographic limit of the network. Switches can be cascaded for an even larger network.

Switches are much more complex than repeating hubs. Each twisted-pair port automatically negotiates with its attached device the data rate for that port, be it 10 or 100 Mbps. The flow control mechanism is also negotiated. For full-duplex segments, the PAUSE scheme is used. For half-duplex segments, the backpressure approach is used. The switch learns the port locations of Ethernet devices by reading complete Ethernet frames and observing source addresses. The switch then creates and maintains a table of source addresses and corresponding port assignments. From that time on, traffic is restricted to only those ports involved in a transmission. This allows for improved throughput since simultaneous transmissions can be initiated on those ports without activity. Table values are aged to automatically accommodate changes to field wiring.

If a broadcast, multicast or unicast transmission to an unknown destination is received on a port, all other ports are flooded with the transmission.

In Figure 1 we have the same identical network as the preceding example except that all the repeating hubs have been replaced by switching hubs. The result is that instead of one overall collision domain we have several collision domains allowing us to have a much greater overall network diameter. Within each collision domain you must follow the same rules as stated earlier. You could add repeating hubs connected to switch ports. You could also make it easy on yourself by only specifying switching hubs and not repeating hubs. If you do that, the maximum twisted-pair segment length remains at 100 m; however, switches can be cascaded with little concern. If you want the same flexibility using fibre optics, we need to address the half-/full-duplex issue first.

Figure 1. Because switches break the network into multiple collision domains, the physical size of the network is virtually unlimited
Figure 1. Because switches break the network into multiple collision domains, the physical size of the network is virtually unlimited

Repeating hub vs switching hub debate

From the above discussion it would seem like switching hubs are an all-round best choice over repeating hubs. However, repeating hubs have their advantages. Repeating hubs are simple to understand and you can connect a network analyser to any transmission. A 'flood' port on the switch is required in order to observe all traffic on the network. Switching hubs are bridges that store and forward complete Ethernet frames, creating a degree of data latency. Cascading switches aggravate the problem. Therefore, you can see that repeating hubs, as well as switching hubs, have their place with industrial Ethernet.

Part IV will continue in a subsequent issue (10 September, 2002) and will look at half-duplex and full-duplex issues.


Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Mini PCIe reference design for MIL-STD-1553
25 March 2020, ASIC Design Services , Computer/Embedded Technology
Holt Integrated Circuits introduced a new dual-channel development kit and complete reference design based on the popular HI-2130LBx MIL-STD-1553 fully integrated terminals. The kit includes a full-size ...

Compact fanless PC for harsh environments
25 March 2020, Brandwagon Distribution , Computer/Embedded Technology
Compulab’s Airtop3 is a ruggedised, small-form-factor fanless IoT edge server with optimised performance, features and cooling. The ruggedised aluminium case is specially designed to generate natural ...

Waterproof touchscreen keyboard console
26 February 2020, Brandwagon Distribution , Computer/Embedded Technology
The industrial IP65-rated keyboard console from Digisign features a high-performance fanless processor and a user-friendly IP65 touch monitor, making it a modern solution for industrial automation and ...

NVIDIA-powered computing for AI at the edge
26 February 2020, Altron Arrow , Computer/Embedded Technology
The AI revolution is transforming industries, reaching products that are smaller and more affordable than ever before. Many companies have been constrained by the challenges of size, power, and AI compute ...

Silicon Labs makes µC/ RTOS open source
26 February 2020, NuVision Electronics , Computer/Embedded Technology
Silicon Labs has announced a new open-source licensing model for the Micrium µC/ family of RTOS (real-time operating system) components. By adopting permissive licence terms for the µC/ components, the ...

Silicon Labs makes µC/ RTOS open source
29 January 2020, NuVision Electronics , Computer/Embedded Technology
Silicon Labs has announced a new open-source licensing model for the Micrium µC/ family of RTOS (real-time operating system) components. By adopting permissive licence terms for the µC/ components, the ...

Rugged single-board computer
25 November 2019, ETION Create , Computer/Embedded Technology
ETION Create released the 2nd in its family of 3U OpenVPX COTS modules. The VF370 3U OpenVPX single-board computer (SBC) is based on the Intel Atom E3900 series of embedded processors and Intel Cyclone ...

Signal recorder for military intelligence
25 November 2019, Rugged Interconnect Technologies , Computer/Embedded Technology
A new addition has been made to the family of Talon signal recording and playback systems from Pentek. The RTR 2654 26,5 GHz RF Sentinel intelligent signal scanning rackmount recorder combines the power ...

3-channel RGB LED lighting shield
25 November 2019, Altron Arrow , Computer/Embedded Technology
The XMC 3-channel RGB LED lighting shield from Infineon Technologies is an evaluation board compatible with Arduino as well as Infineon’s XMC1100 Boot Kit. It is designed to be easily configurable and ...

Solderless robotics kit for university education
25 November 2019, Avnet South Africa , Computer/Embedded Technology
Texas Instruments introduced the newest addition to the TI Robotics System Learning Kit (TI-RSLK) family, the TI-RSLK MAX, a low-cost robotics kit and curriculum that is simple to build, code and test. ...