Improving SCADA Connectivity

Today’s market offers a variety of communications systems ranging from simple to complex. When making these decisions, operators are most interested in the issue of connectivity between the SCADA server (master terminal unit), remote terminal units (RTU), electronic flow meter (EFM) and programmable logic controller (PLC).

Common practice is to remotely locate the SCADA server in a data center or business office. This location is usually located away from any production field or pipeline. This practice can create demands on connectivity options. Recurring cost must be controlled while providing the service that customers demand. Managing this connectivity must be well documented and maintained. This can be a difficult task when many locations are isolated in hard-to-reach locations.

This task may seem impossible, but the reality is that we don’t have to choose a single option for all needs. In fact, the more complex systems usually rely on numerous connectivity methods. Some of the options are described here. When choosing from the available options, SCADA server connectivity must be considered. The options fall into two groups: circuit switched and networked. Broadband is generally desirable between the SCADA server and the production area, but not usually required at the production field or pipeline (Figure 1).

Dial up modems can be very costly and mainly used on small systems when the setup time for the modem/call is not critical. Monthly recurring cost and long distance/toll charges limit the usefulness of this option. Microwave and dedicated leased line are still options but tend to be more expensive. Dedicated T1, DSL, cable modem, satellite internet or wireless ISP and Ethernet radio offer benefits for the SCADA server. The internet connection should have a public IP address or be on a VPN. Internet access for the SCADA server provides the ability to report meter data and share it over a variety of applications such as email, remote desktop or server.

To start planning for basic EFM/RTU sight, you must first perform a site survey, obtaining the geographic coordinates of each site. Make sure your GPS receiver, map software and radio software use the same units for geographic coordinates. If you have other station locations in the general area (even if you are not using them for SCADA), you may need to use them as repeater sites. Topography mapping software can be used to locate and plot the location of sights in reference to the relationship of the equipment in that area. Organize your network by using the (master/slave) or cluster (node/terminal) configuration.

EFM/RTUs in close proximity are well suited for a radio link. You may have sights that are isolated due to mountains or large objects that limit their radio connections. These sights may require a repeater, radio tower or a separate connection to the SCADA server. When EFMs are clustered in the same general area, a radio link is often the most economical and practical solution for connectivity. The most popular choice is unlicensed frequency hopping spread spectrum (FHSS) radio in the 902-928 MHZ band. Licensed equipment is a good choice for some applications where legacy equipment exists or for other technical and business reasons. The focus here is on the use of FHSS radio equipment. This equipment is available from several manufacturers, and has been proven to be both reliable and economical.

Most FHSS radios can perform a dual role as a master/slave or slave/repeater. Diagnostic features and software enable the user to monitor the performance of the network and individual radios. The topography showing the terrain, vegetation and location of EFMs are the primary factors in developing a radio network. The most common network includes a master radio and slave radios at each RTU site. The terrain may not allow the communications from all slave sites to the master. In such a case, repeaters and towers are used to extend the coverage of the network. Several repeaters may be needed to cover the area when trying to communicate over a large network.

Some of the new street mapping software includes 3D topography that may be sufficient in determining the terrain and elevation necessary to design a reliable radio link. If your company does not have the proper hardware or software, there are radio manufacturers that provide the service of performing a radio survey. However, this service can be expensive, and good propagation software is usually less expensive. The software allows the use of 1 ARC Second terrain data (the data can be obtained from the U.S. Geological Service at no cost). The location for the master radio is usually determined by the connectivity options between the master and the SCADA server. The master is not required to be at a central repeater site. The network design is critical to the performance of the system.

Key factors to consider during network design are: 

• Link path fade margin, especially on the backbone, should be at least 26 dB. 
• Selection of antenna mounting structure (the antenna must be stable during high wind and adverse weather conditions). High gain antennas have a narrow beam width. Movement of the antenna changes the pointing angle of the main beam, resulting in intermittent communications. 
• Antenna and feed line. Observe FCC EIRP limits. The use of high gain directional antennas on slaves improves the selectivity, reduces the potential for interference, and provides more power at the receiver antenna port. One can always reduce transmit power, but it is impossible to increase receiver gain. When using long coaxial line length, select low loss coax and high quality connectors (design with the receive signal in mind). 
• To produce better results when setting up your system, it is a good idea to create a table containing the site name, antenna system components, antenna mounting height, antenna pointing angle and fade margin expected. This information will be used by the installer and will save a lot of time. 
• Selection of FHSS equipment and the configuration of that equipment on the network require a good knowledge of the function/features available for each configuration. When in doubt, request assistance from the manufacturer. Most manufacturers are eager to provide customers with design and configuration assistance. 
• When calculating the pointing angle of directional antennas, factor in the magnetic declination. 
• Create standard antenna system configurations for use in designing the network for slave, master and repeaters.

Selection of the location for a master radio depends on the options for connectivity between the SCADA server and the rest of the network. Generally, the master should be at a location where it is easily accessible to service personnel, secure and within radio range of a repeater. When the SCADA server and EFMs are close to each other, the master can be co-located with the server. Most of the time, the server and EFMs are in different locations and a network solution is in order. Most gas production fields are not conveniently located near a city with DSL or cable modems. If they are, this is a natural choice. Some production fields are served by wireless internet, or will eventually have this option. All of the above options require AC power or substantial solar power and battery backup.

The use of satellite internet is becoming a good option. Several service providers have low-cost equipment and reasonable subscription rates. Before choosing this option, consider the availability of service on the system when it fails. It may take a week or more to get replacement equipment. Satellite has two built-in known weaknesses. The first problem stems from the spring and fall solstice, when the sun is behind the satellite, causing high bit error rate (BER). The second is experienced during rain, ice and storms, causing high BER. Both of these conditions tend to be of a short duration with the exception of ice.

Make a close examination of the amount of data you expect to use over a month. Some satellite subscription plans have a low monthly rate but a high rate for increased data usage. Select a satellite provider with a known good product and reputation. Most of the low-cost satellite solutions are intended for home and small business use, and may not be rugged enough for the gas industry. Internet service is usually terminated on a RJ45 using 10/100 Ethernet. A terminal server is required to adapt the network to the RS232 or RS485 required by the master radio. Cellular modems in one form or another have been around since the early days of cellular.

Cellular digital packet data (CDPD) became available in 1997 for the time division multiple access (TDMA) networks, and some networks still support this technology. The trend is to use the general packet radio service/enhanced data GSM environment (GPRS/EDGE) technology. The EDGE modems are supported by the major cellular carriers; they are economical to purchase and are high quality; and have reduced power consumption as compared to the CDPD counterpart. The modems are dual band (the 860 conventional cellular and the PCS bands).

One of the big disadvantages of the current implementation of EDGE modems is that you can’t get a static IP address for them. Without a static IP address, the server must dial the modem telephone number. TCP/IP allows you to address a location by sending a public IP as the address or using a dynamic name server (DNS) containing the IP routing address of the location. A work around was developed to overcome the static IP problem. After the modem gains access to the cellular ISP, it will be assigned a temporary IP address. The modem can then be configured to ping a dynamic DNS server with its name and temporary IP address. The SCADA server can then use the name of the modem and the DNS server to poll the modem over the internet. Temporary IPs are just that (temporary), and the EDGE modem must be configured to ping the dynamic DNS server periodically to maintain the use of the temporary IP. This pinging generates billable data over the cellular network, and must be taken into account when selecting a service plan.

Acknowledgment

Based on a paper presented at the International School of Hydrocarbon Measurement held in Oklahoma City, Oklahoma, May 15-17, 2007.