Advanced Switch Features
Features such as Quality of Service (QoS), Trunking, Virtual Local Area Network (VLAN), port mirroring, fault relay, IGMP snooping, redundancy and SNMP are normally only found on managed switches.
QoS
QoS is the ability of the switch to apply a higher priority to certain frames. A switch can use the port on which the frame arrived to determine the frame priority (port QoS) or it can use a tag within the frame to determine its priority (IEEE 802.1p and 802.1Q). These features are useful in improving determinism.
Trunking
Trunking is two or more ports grouped together and acting as one logical path. This can be used to increase the bandwidth between two switches. Also, in some cases, these paths can provide some redundancy. For example, if two 100 Mbps switches are interconnected by two cables, then the bandwidth between these two switches can be 200 Mbps.
VLAN
VLANs allow a switch to logically group devices and to isolate traffic between these groups even if the devices all share the same physical switch. For example, if the switch was being used for both office communications and factory communications, two VLANs could be created to isolate the office communications from the factory communications. Some switches also allow devices to be located on multiple VLANs. This is sometimes called overlapping VLANs. This can be used if one device, a SCADA system for example, needs to communicate to both the office and the factory then this device would exist in both the office VLAN and the factory VLAN. This would isolate traffic between the remaining office and factory devices, but allow the SCADA system to communicate on both networks.
Port Mirroring
Traffic isolation — one of the advantages of using a switch instead of a hub — also becomes a disadvantage when trying to debug a communications problem. A switch will only send frames to those devices in the conversation. This helps lessen network congestion. However, if you want to view all frames transmitted on the network, this feature becomes an issue. Many managed switches offer a feature called port mirroring. Port mirroring allows one port of the switch to monitor the traffic sent/received by one or more ports of the switch. With this feature a PC running a protocol analyzer program can capture traffic from one or many ports after port monitoring has been enabled. Protocol analyzers are popular problem-solving tools for Ethernet networks.
Fault Relay
Many switches offer a fault relay to monitor link status of specific ports or the power status of the switch. These dry contacts can then be connected to a PLC or other such control device and used as an input to the control system. This is useful if you want to alert the control system when communication to one or more Ethernet devices has failed.
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