**6. Communication infrastructure of WAMS**

The communication system of WAMS is responsible for data delivery from data resources to the control center(s) and from control center(s) to the system actuators. Indeed, the communication network of WAMS is similar to the neural network of humans. As in case of failure or mal-functioning of neural network paralyzed may happen, failure of

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On the other hand, unguided media provides a means to transmit electromagnetic waves. However, this media does not guide the signals. The atmosphere and outer space are some examples of this case and usually referred to as wireless communication. Unlike guided media, in the case of unguided media, the signal strength provided by wireless antenna is more important than the media itself (Stallings, 1997). The next sub-sections review these

Power line carrier (PLC) has used transmission lines as a medium for communication. This type of transmission media has been one of the first reliable media utilized in power systems for critical communications (Marihart, 2001). This media is also the first guided media commonly utilized in power systems and is a part of power system infrastructure. As a result, failure in power system infrastructure such as line outage causes communication difficulty. PLC systems may be classified as two groups in common, narrow band and broad

Narrow band PLC usually has low data rates (up to 100kbps). It is used for automation and control applications or few voice channels (Hrasnica et al., 2004). However, due to the fact that the narrowband PLC works in low data rates, this system is very reliable and PLC

Unlike narrowband PLC, broadband PLC establishes a high data rate (beyond 2 Mbps) between two modems (Hrasnica et al., 2004). This kind of communication can be used for multi services such as automation, internet access and telephony at the same time. Broadband PLCs work in high data rates; therefore, distance between two modems is short and modems require more maintenance. This type of communication is not recommended

When power lines are used for broadband internet access, power line communication is known as broadband over power line (BPL). BPLs use spread spectrum techniques to deliver data rates previously inaccessible. But because of the fundamental physical constraints, successful data rates will be achieved much above several megabits per second

Optical fiber can be used as a medium for communication. Because of its flexibility, fiber optic can be bundled as a cable. As mentioned at the beginning of this section, signals are transmitted through the media by a type of waveform. In fiber cables, the signal is a light wave; either visible or infrared light. Essentially, two types of fiber optic cables including optical power ground wire (OPGW) and all-dielectric self supporting (ADSS) are used in

OPGW cable combines the function of grounding and communication. This kind of cable can be used in transmission or distribution lines. In transmission lines, OPGW is replaced

with shield wire and is suspended above the lines (Marihart, 2001).

modems can be installed far from each other (Shahraeini et al., 2010).

two groups of media.

**6.1.1 Power Line Carrier (PLC)** 

band PLCs (Shahraeini et al., 2010).

for noisy power lines (Marihart, 2001).

power industries (Shahraeini et al., 2010).

(Nordell, 2008).

**6.1.2 Optical fiber** 

**6.1 Guided media** 

communication network may cause huge problems in system operation and control, especially in operation of WAMS applications. Consequently, especial attention should be paid to communication infrastructure, which is as important as electrical infrastructure itself. These two infrastructures (communication and electrical) have become increasingly interdependent so that in the case of failure for each of them, another one may also become out of service (Shahidehpour & Wang, 2003; Lukszo et al., 2010).

New communication systems are designed based on open system interconnection (OSI) layer model. In this architecture, upper layers relay data, assuming that the lower layers work perfectly. In fact, this model is an effective architecture for explanation, designing, implementation, standardization and use of communications networks. The OSI reference model consists of seven layers: physical, data link, network, transport, session, presentation, and application (Shahraeini et al., 2010).

In wide area measurement systems, data resources and WAMS applications normally work at the upper layers of network models. Fig. 4 shows the map between OSI layers and three major blocks of WAMS shown in Fig. 1 i.e. data resources, WAMS applications and communication system (Shahraeini et al., 2010).

Fig. 4. Layering in WAMS based on OSI reference model (Shahraeini et al., 2010).

The first layer of OSI model, referred to the physical layer, is a kind of medium that establishes the physical connection between transmitter and receiver. The characteristics of the communication systems will become seriously influenced by the characteristics of their media. Therefore, it can be concluded that the characteristics of the transmission media play an important role in communication infrastructure of WAMS. Some major characteristics of a medium are as follows: cost, bandwidth, propagation delay, security and reliability (Shahraeini et al., 2010).

Transmission media, as described below, can be classified as guided and unguided ones (Stallings, 1997). Guided media guides the waves through a solid medium. Twisted pair, coaxial cable, power transmission/distribution line and optical fiber are some examples of guided media. In the case of guided media, the media itself has the most important role in characterizing the limitations of transmission (Stallings, 1997).

On the other hand, unguided media provides a means to transmit electromagnetic waves. However, this media does not guide the signals. The atmosphere and outer space are some examples of this case and usually referred to as wireless communication. Unlike guided media, in the case of unguided media, the signal strength provided by wireless antenna is more important than the media itself (Stallings, 1997). The next sub-sections review these two groups of media.
