There are many misconceptions about industrial Ethernet switches. Some vendors provide misleading information indicating that their switches have some special features required for Ethernet/IP or ProfiNet. Protocols such as LLDP and IEEE 1588 are 'open' protocols and not proprietary 'clubs' that limit membership. What limits the ability of a switch to support these standards is the capability of the device. Switches require sufficient processor power and memory to support the evolving standards.
Industrial Ethernet switches vs. commercial grade switches
The following are some of the benefits offered by industrial network devices:
* Greater permissible temperature range - typically -10°C to +70°C.
* Ability to handle large fluctuations in power supply.
* Incorporation of redundant power sources.
* DIN rail mounting with industrial equipment.
* Longer MTBF times - typically 215 000 hours+.
* Better shock, vibration, RFI and EMI resistance.
* No moving parts - no fans.
Requirements for industrial applications
These requirements include:
* IGMP (Internet Group Multicast Protocol) snooping for multicast traffic control (Ethernet/IP implicit messaging).
* Port mirroring - to help in diagnostics.
* RMON (remote monitoring) - to help in diagnostics.
* Redundant power sources.
* DIN rail mounting with industrial equipment.
* Metal cases for heat dissipation.
* Redundancy schemes (ring topology preferred).
* Install it, program it and have the ability to monitor the switch and use diagnostics to troubleshoot the network devices.
SNMP features
Managed switches should be a major consideration for industrial networks where the health of the network is mission critical. A managed switch supports SNMP (Simple Network Management Protocol) v1, v2, or v3 and allows communication with network management packages. This requires greater intelligence at the switch, and there is a wide variation in the performance of SNMP in industrial switches. Users should not be misled by companies that label switches that have a web-server or answer to specific SNMP queries as 'smart' or 'managed'. Some vendors state that SNMP is a security risk, implying that, although this is a standard method for all applications, it is a problem for automation applications. For industrial applications comprehensive SNMP support is required.
IGMP snooping
With unicast traffic, the switch learns MAC address by looking into the source address field of every frame. With multicast packets, the switch must deal with a multicast MAC header, which may or not appear in its bridging table. As a result, multicast packets are copied and transmitted ('flooded') to every port of the switch. During multicast floods devices are unable to use the network, preventing control data from being sent. The effect of these floods is particularly serious with full duplex links, because the bandwidth used is proportional to the number of attached nodes - each of which invites a multicast packet.
IGMP snooping prevents packets from 'flooding' a network segment where a node is not interested in receiving the packets. IGMP is an integral part of IP and is used by Layer 3 switches (routers) to report their multicast status to nearby routers. Because a router must look into the MAC header and 'snoop' into the IP header before handling the packet, this capability is called 'IGMP Snooping'. The multicast packet is then directed only to those nodes listed in the router's table of learned multicast addresses said to be interested in receiving the traffic.
This capability is absolutely required for I/O over Ethernet/IP, unless using ProfiNet I/O, which does not use multicasting. IGMP reduces flooding caused by multicasting PLCs. A multicasting PLC can reduce switch performance to the performance level of hubs and severely compromise network bandwidth.
VLAN (virtual local area network)
A VLAN allows switches to create a single logical Collision Domain even though the nodes are on physically separate network segments. The benefit is that instead of physically installing hardware to segment a network, VLANs can achieve the same through software. VLAN configuration can be based on port ID, MAC or IP addresses. The port-based VLAN standard is IEEE 802.1q. Each switch in the VLAN must be able to implement the port-based VLAN policy.
MIB (management information base)
MIBs are databases maintained by the SNMP package that logs the device's condition and the traffic through the device. The IETF (Internet Engineering Task Force) has defined MIBs for Ethernet switches. MIBs are critical to interoperability. The NMS (network management system) talks to devices through SNMP, but pulls information from the switches' MIBs. A common SNMP and MIB structure make it possible to operate different vendors' switches on the same network because the NMS works with a common interface.
Ring redundancy
The use of a ring for redundancy or failure recovery is common among suppliers. Since there is no IEEE or IETF standard, each supplier has a slightly different scheme and thus, proprietary method. For this reason, many customers do not select ring redundancy as it would lock them into a particular supplier. Those customers choose Rapid Spanning Tree - an IEEE standard redundancy scheme that can be used for most applications. Ring redundancy must be configured in managed switches using, according to the vendor, pre-defined ports, and one switch must be defined as the 'ring manager'. Switches should allow any ports to be used as 'ring' ports.
Rapid Spanning Tree Protocol redundancy (RSTP)
This redundancy scheme allows a back-up path to be put in a standby mode and activated upon failure of the primary path. The network topology can be in a ring - similar to ring redundancy. Different switch vendors that support RSTP in their managed switches can be used in the same network segments as RSTP is an IEEE standard. Users must configure RSTP in managed switches - defining various aspects of the configuration.
Setting up managed switches
Most parameters are easily configured:
* Setting the IP address and optionally giving the switch a name, location and description.
* Enabling IGMP snooping.
* Port Control - turning off unused ports.
* Security - entering the IP addresses (of laptops, desktops) allowed for management changes.
* Ring redundancy - entering the ports used for the ring.
Parameters that are more difficult: you should read the manual:
* VLANs.
* RSTP Redundancy - setting all the necessary parameters.
For more information contact Mark Dilchert, Lapp Cable, 0861 CABLES, mark@lappcable.co.za, www.lappcable.co.za
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