Cellular telephone networking has emerged as a strong contender for oil and gas SCADA system communications. 3G cellular networks are widely installed even in remote areas. Data plans for SCADA applications are competitive.
Industrial 3G modems are commonly available, proven and have been integrated in some RTU models. Tight integration of 3G modems in RTU products reduces purchase costs and mitigates risks in terms of start-up, operation and maintenance.
Cellular Networks – A Brief Review
A quick review of cellular evolution shows that earlier “1G” and “2G” technologies presented drawbacks that made them less attractive than competing technologies such as radio, satellite and hard-wired networks.
In retrospect, “1G” designates the first-generation, analog cellular technology that was introduced in the 1980s. While modems with reasonable performance, about 28K bps (bits per second), appeared on the market, carriers were not oriented to serving industrial data users. Setting up a network was often a frustrating experience.
In the 1990s, carriers introduced second-generation (“2G”) digital cellular networks. With 2G came such services as SMS text messaging as well as CDMA and GSM multiplexing technologies, which increased the efficiency of digital transmission. While data performance was initially limited, 2G evolved with packet-switching and encoding technologies including GPRS and EDGE. GPRS networks provide data rates of at least 56K bps. EDGE increases performance by three times over GPRS.
GPRS and EDGE also happened to mark the beginning of potentially confusing jargon for interim generations, respectively, “2.5G” and “2.75G.” But applications for industrial systems including machine-to-machine (M2M) and SCADA substantially expanded.
3G, the third generation of cellular telephone technology, is a set of standards that comply with the International Telecommunications Union’s IMT-2000 (International Mobile Telecommunications) specifications. While 3G networks are required to provide peak data rates of 200K bps, most systems are faster. Higher-performing networks are often dubbed “3.5G” and “3.75G.” Essentially, these networks cater to users of smart phones, tablets and PC’s for services such as social networking, web browsing and mobile web apps.
Carriers are also well into the process of installing 4G networks. 4G networks must meet the IMT-Advanced specification, which requires peak data rates of 100M bps for “high mobility” applications such as trains and cars and 1G bps for “low mobility” applications such as pedestrians and stationary users. As usual, actual performance varies. On any carrier, 4G will be faster than 3G. Typically, it will be at least ten times faster and, in some networks, it could be 100 times faster.
The problem with 4G today is the sparse coverage, especially in remote areas such as pipelines and production fields. However, there is no need to wait for 4G. For SCADA systems, 3G networks meet the needs now. 3G networks offer substantial bandwidth for continual polling, uploading of historical logs such as from gas flow computers and video transmission. Secure, IP networks can use cyber security measures such as encryption and operate as virtual private networks (VPNs).
Data Plans For SCADA Applications
Carriers and third-party service providers offer competitive rates for SCADA systems and include such features as scalability, security and uptime guarantees. Today, monthly data plans range from 1 MB to 10 GB. Lower-usage plans such as 2 MB are priced around $6 per month.
Note that the speed of a network is measured in bits-per-second (bps) but the amount of data transmitted – and stated in the data plan – is in bytes (capital ‘B’). One byte = 8 bits. Prefixes “K,” “M” and “G” multiply by 1,024. 1 KB = 1,024 bytes. 1 MB = 1,024 Kbytes and 1 GB = 1,024Mbytes.
While smart phone data plan shoppers use approximations, which are widely available on carrier websites, SCADA users can precisely calculate their expected monthly usage. Compared to mobile device users who surf the web, send high-resolution photos via e-mail and upload photos on social media websites, an RTU on a SCADA network transmits a modest amount of data.
Typically, an RTU stores process variables in 32-bit (four-byte) floating-point format and transmits the variables via efficient protocols that add very little overhead. Depending on the RTU model, on/off status information uses, at most, one byte. Some models consolidate on/off states on a per-bit basis that allow, for example, 16 discrete inputs to be represented by a single, 16-bit integer.
Consider an RTU or flow computer at a natural gas well site. Typically, the data collected from the RTU includes the following:
• Hourly report
• Daily report
• Monthly report
• Audit trail (Alarm/Event log)
• Meter parameters report
• Optionally, a gas quality report
The hourly report includes values such as totalized gas flow over the hour, average line pressure over the hour, site i.d., time/date stamp, etc. Typically, the report includes a dozen four-byte floating-point numbers. That’s only 48 bytes! An efficient protocol such as Modbus RTU adds only two bytes each for a request and response. For a 31-day month, transmitting the hourly report uses 52 bytes/hour x 24 hours/day x 31 days/month = 38,688 bytes or about 38KB. Even a conservative usage calculation that considers communication problems and re-transmissions, even to the point of doubling the original figure, won’t add a major amount.
The daily and monthly reports usually include information that is similar to that in the hourly report. The daily report usage is 52 bytes/day x 31 days/month = 1,612 bytes. The monthly report is simply a one-time, 52 bytes for the month. We can round these up to 2 KB.
Alarm/event usage estimates should consider a worst-case scenario. An alarm request is 8 bytes and a response is 25 bytes for a total of 33 bytes. Twenty alarms and events per day would give us 660 bytes. Over a month, that is 20,460 bytes or about 20 KB.
The meter parameters report includes the latest values of configurable settings such as orifice plate size and pipe diameter. Without gas quality information from a chromatograph, the meter parameters report could still easily include 80 values. Four bytes each makes 320 bytes per report. Transmitting that in Modbus would take two messages; therefore, add eight bytes. If that is done each day, information transferred in a 31-day month would add up to about 10 KB.
A typical gas quality report would include 38, four-byte values. If that is transmitted each day, it would add up to about 5 KB for a month.
The monthly “data usage” for this site would be 38 KB + 2 KB + 20 KB + 10 KB + 5 KB = 75 KB. That’s very modest compared with monthly plans that are measured in MB. This is a minimum flow computer and it’s easy to see that accounting for conservative data calculations and more data would still be within a 1 MB data plan:
• Accounting for message re-transmission, worst-case add 75KB
• Transmitting more information such as transmitter calibration data and multiple meter run data, add 100 KB
• Using a protocol with considerably more overhead, add 50 KB
• Adding cyber security measures such as encryption, add 200 KB
• Total: 500 KB
Collecting information more often than once-per-hour would add significantly to the data plan. Transmitting certain parameters such as live values and alarms every six minutes multiplies the number of messages by a factor of ten. Depending on the number of values transmitted, this could add between 200 KB and 800 KB. But the total usage would still be well within a 2 MB monthly data plan.
Cellular Data And The Internet RTU
Unlike traditional RTUs, which respond to polls using protocols such as Modbus, today’s “Internet RTUs” serve web pages, transmit e-mail and FTP messages using push technology, and transfer images from IP video cameras. Selecting a data plan seems to share more in common with a smart phone.
While carrier websites provide data plan estimates for e-mails, e-mails with attached files, web pages and photos, RTU users can closely estimate usage with assistance from manufacturers. Internet RTU data is modest compared with smart phones, tablets and PCs. Files transferred and web pages served are typically smaller in the case of the RTU. For example, an e-mail could be less than 1 KB vs. the 3 KB estimate on most carrier web sites. A web page could similarly be between 1 KB and 50 KB vs. the typical 300 KB estimate. This is also true for video frames from IP cameras. Compacted files could be as small as 4 KB. Compare that with a single 4 MB high-resolution, digital photograph!
In terminology that is familiar to smart phone data plan shoppers, a 2 MB monthly data plan allows an Internet RTU to send 200 e-mail messages at 1 KB each, 30 e-mail messages with attached files at 20 KB each, serve 30 web pages at 30 KB each and send 60 video frames at 5 KB each.
Cellular telephone networking has arrived as a competitive technology for SCADA systems. Today’s 3G networks provide the bandwidth to readily accommodate SCADA protocol messaging as well as the needs of modern, “Internet RTU” products, which serve web pages, send e-mails and send files including video frames. Industrial 3G modems are widely available and have been tightly integrated in some RTU products. These advancements provide for cost-competitive, reliable networks with the performance that satisfies SCADA system requirements, now and in the future.