Many switches support two types of negotiation. The first is called "auto-negotiation." This protocol allows two interconnected Ethernet entities (for example a switch and a computer) to come to some agreement over several parameters used in their communications. One parameter they negotiate is the data rate to be used. This is generally 10 Mbps or 100 Mbps. They negotiate the use of half- or full-duplex. Full-duplex allows communications to exist in both directions at the same time, while half-duplex only allows communications in one direction at a time. They also negotiate the use of flow control in their communications. If flow control is utilized, then either device can request that communications be halted if the device needs time to process received frames.

If supported, Auto-MDIX or Auto-crossover is another negotiation that can occur between two Ethernet entities. This allows these entities to decide which wire pair to use for transmitting frames and which wire pair to use for receiving frames. This feature is attractive when connecting two switches as this would normally require the use of a special cross-over cable. With Auto-MDIX the two switches negotiate which wire pairs to use when communicating and this allows the use of standard (straight-through) cables when connecting two switches or two end devices.

Fiber or Twisted-Pair

Most Ethernet communications occur over twisted-pair cabling. However, there are times when communications should occur over fiber-optic cabling. These cables are used when signals need to travel over greater distances than the 100 m supported by twisted-pair cabling. Most fiber-optic devices can communicate up to 2 km when using a full-duplex setting. Fiber-optic cables are also used in high noise environments because their communications are unaffected by electrical or magnetic fields.

Managed vs.Unmanaged

One of the key questions in choosing an Industrial Ethernet switch is whether to select a managed switch or an unmanaged switch. A managed switch is generally more expensive than an unmanaged switch, however, with this additional cost, extra features are provided. Also, a managed switch will contain the features of its unmanaged siblings.

A managed switch is basically a switch that supports SNMP (Simple Network Management Protocol). Of course, most managed switches provide features beyond SNMP.

Basically, a managed switch allows you to take control of your network. An unmanaged switch will simply allow Ethernet devices to communicate. You connect your Ethernet devices to the unmanaged switch and they usually communicate automatically. There will be status LEDs to give you some feedback regarding link and activity; but this is generally all you get. With a managed switch you will have the same status LEDs, but the managed switch will let you adjust your communication parameters to any setting you desire and let you monitor the network behavior in a number of different ways.

For example, in systems that communicate in high noise environments, it is sometimes advantageous to force the data rate to 10 Mbps because noise coupled into the cables may confuse the auto-negotiation process. Most managed switches will allow you to set the data rate of each port. These environments can also benefit from disabling Auto-MDIX support since this negotiation can become confused by noise. Again, a managed switch is normally required to enable or disable this feature on a port-by-port basis.

With a managed switch you can also monitor the network. Through SNMP you can view a multitude of network statistics. This includes the number of bytes transmitted, received; number of frames transmitted, received; number of errors and port status. All of this can be viewed on a port basis. Some managed switches also make this data available via a web server so that you can use a standard browser to view the network status.

Most managed switches also offer advanced features that enhance your control of the network.