Conventionally, remote unmanned facility monitoring systems were constrained by the limitations of the long range communication interfaces available to the system integrators.
Typically, these long-haul communications interfaces were serial-based and used a polling architecture. Simply receiving I/O data over the air was a formidable challenge, because aerial communications imposes a long response time. When the system covers a wide area, polling these remote stations takes even longer and data will often be lost.
The limited amount of bandwidth, or data throughput, available using older technology also imposed significant limitation on remote monitoring systems. Often, throughput is so low that the application can only deliver alarm point data acquisition. Instead of transmitting all the serial and analog data from the remote site, onsite intelligence was used process this data and condense it into simple alarm points that are sent to the central monitoring site.
The advent of advanced cellular communications technology has freed system integrators from these limitations and unleashed new potential in remote monitoring systems. This white paper explores how system integrators can close the performance gap between cellular and wired communications even further so that cellular networks can deliver advanced, bandwidth-heavy features such as live video at the central monitoring station.
IP-based Cellular Technology Grows Up
The beauty of cellular technology is that it is an IP-based technology. The vast majority of field monitoring devices are now IP-enabled, so it is possible to get all the data from field devices over a cellular network. But simply using IP-based communication media alone is not enough to create an ideal remote monitoring system. Bandwidth and latency are also important.
Fortunately, cellular technology has seen dramatic recent advances in performance. The transition from "2.5G" GPRS technology to "3.5G" HSDPA technology has unlocked substantial improvements in bandwidth and network latency. Now, cellular uplink bandwidth can reach 384 Kbps and downlink bandwidth can reach as high as 7.2 Mbps. Cellular latency has also been improved dramatically, now reaching as low as 100 ms. The bottom line is that cellular performance now exceeds most of the other long-range communications technologies available today.
Active Data Transmission Optimises Bandwidth Usage and Avoids Communication Timeouts
Cellular technology is clearly at the head of the pack when it comes to long-range wireless communications, but it obviously still can not compare to hard-wired LAN or WAN communications technologies. Most Ethernet devices are designed to communicate over LAN or high-speed WAN networks with more bandwidth and response time well under 100 ms. This disparity creates some potential problems when deploying Ethernet devices on cellular networks. One such problem is data communication timeouts.
Field devices, Ethernet-based and serial-based alike, often use remote polling to acquire data. Polling, however, needs to take communication timeouts into account. A device whose communication timeout value is set to accommodate LAN communication speeds will face communication timeout issues when it is deployed in a cellular network. Repeated communication timeouts will simply crash the system and often incur additional fees for each reconnection attempt.
Active data reports are the solution to this problem. By replacing constant data polling with active reports, the system can overcome communications timeouts. With active reporting, the central monitoring server does not need to constantly interrogate field devices for data. Instead, the central server can just wait for incoming data. This not only reduces bandwidth usage, it also makes real-time alarms possible. Most importantly, with active reporting it’s possible to dynamically adjust communications margins to accommodate the network’s timeout tolerance and prevent timeouts from occurring at all.
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