Leased Line Replacement for SCADA Communications with Wireless IP
Recently industrial operations have had the need to replace Legacy Leased Line (Wireline) SCADA Communications networks for the following reasons:
- Telcos are obsoleting leased line service.
- Telcos taking back leased lines for use in digital ADSL service
- Leased line failures affecting availability and high MTBR
- Telcos increase leased line cost significantly.
Replacement of leased line with IP Networks has the following benefits:
- Higher Bandwidth allows faster SCADA update rate
- IP networks with multiple independent sockets allow remote maintenance and configuration of remote PLC/RTUs.
Leased Line SCADA Communications
Legacy Leased Line SCADA Communications were typically 1200 Baud BEL202 Multidrop networks operating on 2 or 4 wire Telco Voice grade circuits. Leased line communication circuit costs are typically charged based on the line distance to the telco central office (CO). Point to multi-point SCADA architectures were also implemented on private cable networks – buried, overhead or in conduit.
Leased Line SCADA Communication Architectures are Point to Multipoint architectures where a SCADA host or Master PLC communicates to a network of PLC/RTUs over the multi-drop BEL202 modem network using a serial protocol such as Modbus RTU. In these system the SCADA host or master PLC sequentially polls remote RTU/PLCs on a round robin basis.
Leased Line SCADA Communications Architecture with Master PLC
Leased Line SCADA Communications Architecture with SCADA Host
Wireless IP Network Options for Leased Line Replacement
Options for Wireless Connectivity for Lease Line Replacement include Cellular, Satellite, and Private Radio Systems. The choice of wireless technology depends the available infrastructure and the terrain.
Challenges in Replacement of Multi-drop Polling Communications Networks with IP Networks
Cellular, Satellite, and Ethernet Radio-modem Systems are IP based. Upgrading polling networks to IP based networks poses integration challenges if minimal or no changes to the SCADA Host or existing RTU/PLC equipment is to be made.
This is particular true where the SCADA architecture has a master polling PLC. In this case, the master polling PLC would have to able to updated and reprogrammed to communicate via IP. For SCADA systems with computer based host systems, the SCADA host or SCADA communications server would have to be reconfigured for IP based communications. For many legacy SCADA systems, the SCADA host software may not have IP communications capability.
Use of IP100 Serial to Ethernet Gateway for Lease Line Replacement
Serial to Ethernet Gateways that allow emulation of multi-drop polling communication can used to enable integration of IP networks to existing serial polling SCADA architectures. Specialized serial server devices such as the SCADALink IP100 can be used with IP based network to emulate the point to multi-point broadcast architecture of the original leased line SCADA communication network. The SCADALink IP100 Serial Ethernet Gateway employs UDP Broadcast or RTU Address Based Routing to perform multi-drop network emulation. UDP Broadcast – All messages from the SCADA host or Master PLC are broadcast to list of UDP addresses associated with PLC/RTU in the network. In an IP Address RTU Address Based Routing mechanism, messages are transferred to IP addresses based on a RTU Address Lookup in the Serial to Ethernet Gateway.
Multi-drop Polling SCADA Communication Architecture over Wireless Cellular IP Communications
Multi-drop Polling SCADA Communication Architecture over Wireless Satellite IP Communications
What serial protocols can be supported by Serial to Ethernet Gateways?
With IP broadcast mechanism, all point to multi-point serial protocols can be supported, including:
- Modbus RTU/ASCII
- AB DF1
With RTU address based routing mechanisms – the serial to ethernet devices needs to be protocol aware. The SCADALink IP100 currently supports Modbus RTU address based routing.
My legacy RTU equipment has a built in wireline modem. Wireless IP Modems do not have this wireline interface. What can I do?
You may want to utilize another serial port on the legacy RTU equipment for SCADA communications if it is available. In this is not available, and the RTU is to be kept, you will have to interface to the wireline modem line interface.
If the legacy RTU has a BEL202 internal modem you may want to connect it with back to back BEL202 Modems as shown below. The Wireless IP Modem is then connected to a new external BEL202 modem by the RS232 serial interface. The telco line interface of the BEL202 modem is connected to the telco line interface of integrated modem in the legacy RTU device. This method can be a deployed if there is a limited number of sites with this configuration. If you have large number of sites with this configuration, please contact us to discuss an implementation of an OEM BEL202 to serial interface.
If the legacy RTU has an internal modem that is not BEL202, you can interface to it with wireline modem with the same modulation scheme or you can replace the legacy RTU.
When converting Wire Line SCADA communication to IP communications do I need to convert my SCADA Host to IP Addressed Communications?
You do not need to do this immediately if you utilize a Serial to Ethernet Gateway (i.e. SCADALink IP100) mechanism to route serial traffic to the appropriate wireless IP modem. In fact you may not ever need to do this if there are no need for system upgrades. Or you can delay this upgrade to a latter phase of the project. It is a good idea to convert your SCADA host to IP communications on any SCADA infrastructure that needs to be current.