SCADA Network Components
A SCADA network consists of one of more Master Terminal Units (MTUs) which the operators utilize to monitor and control a large number of Remote Terminal Units (RTUs). The MTU is often a general purpose computing platform, like a PC, running SCADA management software.
The RTUs are generally small dedicated devices which are hardened for outdoor use and industrial environments. Fiber optic data transceivers are ideal in SCADA networks because they offer EMI immunity. When transceivers are used for the master and remote terminal units, a fault tolerant self-healing ring network is easy to configure. Figure 1 illustrates a self-healing ring network topology.
Reliable operations of SCADA systems depends on proper configuration, cyber security measures, and other factors.
Important components for a DWDM systems are transmitters, receivers, fiber amplifiers, DWDM multiplexers, and DWDM demultiplexer. These components, along with conforming to ITU channel standards, allow a DWDM system to interface with other equipment and to implement optical solutions throughout the network.
These designs require narrow passbands, usually 0.4 nm wide, steep roll-off to reject adjacent channels, and stable operation over increased temperature. Recently, multiplexers have gained versatility, moving beyond the "wideband" wavelengths and into densely packed wavelengths that can be integrated into a multiple high frequency, 192 to 200 THz, transmission system. This type of system can maintain up to 16 channels, acting as a 16 fiber channel cable with each frequency channel operating to serve a STM-16/OC-48 carrier. Demultiplexers need to eliminate crosstalk and channel interference. Couplers and dichroic filter, both passive devices, are the most favorable demultiplexers today. The first DWDM coupler design is based on fiber Bragg grating (FBG) filters illustrated in Figure 2. Bragg gratings are comprised of a length of optical fiber with the index of the core permanently modified periodically usually when exposed to an ultraviolet interference pattern. As a result, the fiber grating behaves as a wavelength dependent
reflector and lends itself to precise wavelength separation.
Erbium-doped fiber amplifiers (EDFAs) counteract these problems. EDFAs are silica based optical fibers that are doped with erbium. This rare earth element has the appropriate energy levels in its atomic structure for amplifying light at 1550 nm. A 980 nm "pump" laser is used to inject energy into the doped fiber. When a weak signal at 1310 nm or 1550 nm enters the fiber, the light stimulates the rare earth atoms to release their stored energy as additional 1310 nm or 1550 nm light. This process continues as the signal passes down the fiber, continually growing stronger. Figure 4 illustrates an erbium-doped fiber.
However, channel spacing depends on the system's components. Channel spacing is the minimum frequency separation between two multiplexed signals. An inverse proportion of frequency versus wavelength of operation calls for different wavelengths to be introduced at each signal. The optical amplifiers bandwidth and receivers ability to identify two close wavelength, sets the channel spacing. Figure 5 illustrates the typical DWDM specifications.
The center of the DWDM band lies at 193.1 THz with standard channel spacing of 200 GHz and 100 GHz. The closest "standard" spacing (100 GHz) allows transmission of 45 channels on one fiber. A 45 channel system spaced at 100 GHz would cover a optical span of 35 nm and require a costly wide bandwidth, gain-flattened EDFA. As system designers looked to pack more than the 45 channels at 100 GHz spacing, they started to use closer spaced optical channels. The channel spacing, in GHz, relates to the optical wavelength as follows: A spacing of 200 GHz corresponds to about 1.6 nm, 100 GHz corresponds to about 0.8 nm, and 50 GHz corresponds to about 0.4 nm channels spacing. Most commonly 50 GHz follows 100 GHz, although attempts at 75 GHz and 37.5 GHz show up in literature. While there is nothing magical about any of these numbers, it seems likely that 50 GHz will be the next logical step below 100 GHz. Using a channel spacing of 50 GHz (0.4 nm) allows 45 channels to occupy only 17.5 nm of optical bandwidth. This greatly simplifies the requirement for optical amplifiers in the system. Fiber increases in channels per fiber would likely lead to the use of 25 GHz spacing. Designing the optical demultiplexer to separate the signals at the receive end defines the greatest challenge in closely spaced optical channels. Because of subtle color differences in each of the optical channels, high performance DWDM optical demultiplexers must have three characteristics. First, it must be very stable over time and temperature. Second, it needs to have a relatively flat passband or region of frequencies. Third, it must reject adjacent optical channels so that they do not interfere. Several basic types of designs can be used in optical demultiplexers to separate the optical channels. Many of these designs have an increasingly difficult time separating the optical channels as the spacing becomes very close. Some, however, such as fiber Bragg gratings actually appear better suited for closer channel spacing. The need for close optic channel spacing is a trade-off between the performance required of the optical amplifiers used in the system and the number of channels to be transmitted per fiber. Figure 6 illustrates the transmission spectra of 0.4 nm spacing DWDM FBGs.
