The success of Ethernet technology came about because of its cost, effectiveness and simplicity compared with other network schemes. The lure of linking large numbers of systems was powerful.
All this connectivity came at a price. As networks grew, data collisions and delays increased. These delays slowed Ethernet’s progress with industrial control networks, where guaranteed response times are so important.
Then switch technology entered the scene. With unique pathways to each port, switches reduce collisions. Switch costs came down and they became widespread in non-industrial Ethernet networks. Then the development of industrial-hardened switches allowed these to be used more widely in industry.
Switches by themselves don’t limit traffic, but facilitate it. This traffic, if left uncontrolled, could lead to congested bandwidth and “clogged arteries.”
Virtual Local Area Networks or VLANs use managed switches to balance this expanded connectivity with useful boundaries on data traffic. VLANs make networks more reliable, and this reliability is what makes VLANs so useful in industrial settings.
Industrial VLANs
A VLAN is a single broadcast domain, not limited by physical location. VLANs create logical separation of control networks within a physical network, dividing physical interfaces into useful groups.
VLANs are created using parts of a switch, full switches or multiple switches linked together. An administrator assigns ports to each VLAN. Each VLAN then functions as an independent switch, broadcast domain and network. Each device on a VLAN has its own Media Access Control (MAC) address. MAC addresses are assigned by the manufacturer, and the VLAN uses these to tell where data goes.
Each VLAN is in effect a separate network. The only way to get data to a device on another VLAN is to send it outside the VLAN through a router – even if the receiving device is on a port on the same switch but on another VLAN.
Figure 1. Examples, Industrial VLAN Components
No comments:
Post a Comment