Wide-Area Networks overview

Wide-Area Networks (WANs) consist of computers located across large distances. Communication between the computers is typically handled by modems, T1 lines, or satellite links. Data transmitted in this environment must travel through a large number of network components (routers, satellites, modems). By doing so, latency (delay from when the data was sent to when it was received) is increased.

Further, the underlying technologies used for communication is often limited to low bandwidth. As a result, these distributed networks may experience delays or breaks in communication due to relatively high amounts of network traffic, or interference by external conditions such as severe weather.

WANs are used in industries such as water/waste-waster, telecommunications, natural gas production, and oil production/distribution, where they are implemented as part of a SCADA system. The SCADA system is usually a central computer that communicates over the WAN to remote PLCs or RTUs.

Note: In this context, a Remote Terminal Unit (RTU) is defined as an industrial data collection device typically located at a remote location, which communicates data to a host system by using telemetry (such as radio, dial-up telephone, or leased lines).

A SCADA system gathers real-time data, transfers the data to a central site, performs the necessary analysis and control, and displays the information visually in an appropriately organized fashion. SCADA topologies can easily be expanded to handle additional remote sites and I/O points.

The SCADA system collects and records data events and alarms. A SCADA host performs centralized alarm management, data trending, and operator display and control.

Current status and commands are handled by remote controllers consisting of RTUs and PLCs. SCADA systems employ RTU or PLC protocols including Modbus, AB-DF1, and DNP3.0. SCADA communications can use a range of wired (lease line, dialup line, fiber, ADSL, cable) and wireless media (licensed radio, spread spectrum, cellular, CDPD, satellite). Communication drivers collect data from remote units, then send this data to the Application Server using OPC or SuiteLink protocols.

Network terminology

When metric prefixes (k for kilo, M for Mega) are used in a network context, they retain their original definitions. That is, k = 1,000 and M = 1,000,000. This usage differs from disk-storage terminology, where KB = 1024 Bytes and MB = 1,048,576.

The following table summarizes the conventions used in this section: