**6.1.3 Leased line**

Historically, leased telephone circuits have been widely used in electric utilities to create a point-to-point or point-to-multipoint communications (Marihart, 2001). The leased lines only provide a share medium for communication and some technologies should be implemented in order to transmit signals through this media. Digital Subscriber Line (DSL) is a group of technologies, which provides digital data transmission over leased telephone circuits. The first version of DSL was defined in 1988 and called ISDN (Integrated Services Digital Network). ISDN provides a maximum of 128 Kbps in both uplink and downlink directions (Hrasnica et al., 2004). Other DSL versions have appeared in different forms, such as high-data-rate DSL (HDSL), single-line DSL (SDSL), asymmetric DSL (ADSL), rateadaptive DSL (RADSL), and very high-data-rate DSL (VDSL), all of which utilize copper lines. The differences between xDSL technologies are their data rates and directionality of transmission, distances to which those rates can be supported, and the size of the wire.

### **6.2 Unguided media**

Wireless transmission is used when we have several challenges such as environmental or financial limitations for utilization of guided media. However, as transmitted signals using wireless communication can be accessed by anyone, the security of wireless communication is naturally low. On the other hand, various signals, which are transmitted by different sources, may be broadcast on the same frequency and thereby, collision may happen. Thus, it can be concluded that the reliability of wireless communication is less than the reliability of transmission through a guided media. In wireless transmission, signal can take the form of waves in the radio spectrum, including very high frequency (VHF) and microwaves, or it can be light waves including infrared or visible lights such as laser (Shahraeini et al., 2010).

The first important parameter in wireless communication is its range. In accordance with wireless ranges, four wireless types may be defined (Fourty et al., 2005):

