Data communications is a major design consideration in the implementation of any measurement or control system. At its most basic, system communications breaks down into two major elements; Communications with field equipment and communications with the system's user. Each of these major communication elements may be addressed using a variety of different technologies or by using a combination of different technologies, depending on the specific needs of the customer.
Field Equipment Communications
The data logger or RTU located remotely in the field typically acts as the initial repository for all data. The data logger or RTU reads measurement data from sensors and 'buffers' the data, storing it until it can be read by the MeasureTek Server. It is field equipment communications that permit the MeasureTek Server to communicate with one or more data logger or RTU, thereby acquiring measurement data or issuing control commands.
Field equipment communications are generally accomplished using one of three technologies. In some cases a combination of technologies are utilized. These communications technologies are:
1. Radio-based Communications
Radio-based communications have long been used for data logger and RTU communications, and the technology is well proven, relatively simple and inexpensive to operate. Depending on the customer's requirements, radio-based networks can utilize VHF or UHF licensed radios, unlicensed UHF spread spectrum radios as well as other wireless networking technologies.
At its most basic, a system would consist of a "field station", where the data logger or RTU is located, and a "base station", where the MeasureTek Server is located. Both sites are equipped with radio modems and antennae's. Communicating via the radio modems, the MeasureTek server "polls" the data logger or RTU for data or sends it control commands. Data acquired from the data logger or RTU is then stored in the SCADA Repository where it can be accessed upon demand by system users.
A single base station can communicate to multiple field stations, and additional field stations can be added as customer circumstances and requirements change over time. Major design considerations for radio-based networks are the distances between stations and the need for clear "line-of-sight" between stations. With clear line of sight (unobstructed by hills, mountains, large buildings, etc.), distances between stations may reach up to 45 miles. For longer distances or for rough terrain, the use of repeaters can be required.
The use of dedicated radio-based or wireless networks offers many advantages to the user. They offer well proven and highly effective communications and are versatile, flexible and easily customizable to meet specific customer requirements. Once installed, they require little maintenance and provide reliable communications, particularly during local or regional emergencies that tend to overload and render most public communications networks virtually useless. Radio and wireless networks can be installed virtually anywhere, so they are a good choice for remote locations where cellular access or landlines are not available. Private radio and wireless networks are also free of the monthly services charges required by most other network service providers.
2. Cellular-based Communications
Working almost identically to radio-based communications, cellular-based communications are accomplished in largely the same way using different technology. Cellular-based communications systems utilize cellular modems as opposed to the VHF or UHF radio modems used in radio-based networks. Communications to Field Stations is established through the use of a cellular service provider (such as Verizon Wireless), and data is accessed via a standard Internet connection at the Base Station.
A single base station can communicate to multiple field stations, and additional field stations can be added as customer circumstances and requirements change over time. Obviously, a major design consideration for cellular-based networks is access to reliable cellular service.
The use of cellular-based communications offers some unique advantages. Setup of communications is relatively simple with less customer-owned hardware than in radio-based systems. They offer well proven and highly effective communications and are easily expandable to accommodate system growth and expansion. Distance or separation between Field Stations becomes less of a concern, since the only real consideration becomes whether or not a site has cellular coverage. This permits the distribution or location of sites over virtually unlimited distances with little or no investment in customer-owned hardware. Once installed, cellular-based communications require little maintenance since most of the network is supplied by the cellular service provider.
Because they utilize a "public" or "shared" network, cellular-based communications are susceptible to outages or performance degradation based on total network demand. This is particularly true during times of local or regional emergencies that tend to overload most public communications networks. Cellular-based communications will also require monthly service or network fees that are a factor of the total number of field stations.
It is common to utilize a combination of cellular and radio communications. This typically occurs when there are several stations in close proximity to one another, permitting reliable radio communications, but the cluster of stations is remote to the computer base. One of the stations will be equipped with a cellular modem, but all will be equipped with radio modems as well. The cellular modem is used to provide the "primary" connection to the Base Station, with the radio links being used to access each of the other remote field stations. This configuration allows each individual station to be accessed by the internet via a combination of cellular and radio communications if just one of the individual stations is connected via a cellular modem.
3. Satellite-based Communications
Satellite-based communications also work in a similar fashion to radio-based communications and are accomplished in largely the same way using different technology. Satellite-based communications systems utilize satellite modems and special antennae to send radio signals into space where they are picked up be dedicated communications satellites. These signals are in turn relayed (down-linked) to earth-based ground stations where the data is made available via a network connection at the Base Station. As with cellular-based communications, satellite-based communications are typically provided by specialized service providers.
While always an option, satellite-based communications are generally reserved for extremely remote applications where all other communications options are impractical. Satellite-based communications services are similar to cellular-based systems in that monthly service or network charges are involved, and cost for satellite-based services are generally more costly.
User communications deals with how the data from the system actually reaches the end user of that data. With recent technology and networking advances, there are many different ways that a user can access and interact with their monitoring and control system. Some are quite simple and inexpensive while others utilize the latest in computer and communication technology to make data available virtually anywhere and at any time.
There are three primary user communications mechanisms in general use today:
1. Local Data Download
Local or "hardwired" data download involves the use of a laptop computer or other portable device to connect directly to the field station data logger or RTU and download the data stored there. Downloaded data can then analyzed using spreadsheets and other similar types of software to create data reports, presentation, etc.
While not particularly "high tech", this approach is a very valid solution where data collection occurs relatively infrequently, where electronic communications with the site are impractical or extremely costly, and where project budgets are limited.
2. Alphanumeric Text, Audible Paging & Audible "Call-In" Messaging
Text and audible paging have long been used as a data delivery mechanism for data monitoring systems to rapidly provide the user with system status and alarm data. Paging is generally handled directly from the data logger or RTU through the use of an autodialer or landline/cellular modem. Paging can be used by itself or it can be combined with other user communication methods to help assure that users receive critical system and alarm condition data.
A related option is to utilize audible "call-in" messaging. With this solution, the user may place a call to the field unit via a landline or cellular phone. The field system will respond with audible messaging that details current system data. Audible messaging is completely configurable, permitting everything from straightforward, unprompted updates of system data to structured, hierarchical responses that respond to user phone input.
3. Computer-based Access
Computer-based access to monitoring and control system data is not only reliable and economical, but offers the user an almost infinite number of ways to visualize and use the data. Using web-based delivery of data content, MeasureTek supports the delivery of data to virtually any device…anytime and anywhere. Web-based delivery allows multiple users to access the same data and eliminates the need for custom or proprietary software to access and display the data. This allows the use of virtually any commercially available computer hardware to access the data, regardless of operating system, platform type, etc. Whether you want to use a PC, a Mac, a tablet such as an iPad or Android, or a smart phone, MeasureTek can meet your needs!