**6.1.1 Power Line Carrier (PLC)**

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 band PLCs (Shahraeini et al., 2010).

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

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 for noisy power lines (Marihart, 2001).

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 (Nordell, 2008).
