This chapter focuses on three different types of individual physical links, with one section for each type:
■ The first section, “Sending Bits with Electricity and Copper Wires,” focuses on links that use electricity and copper wires, while also discussing matter to the other types of links as well.
■ The second section, “Sending Bits with Light and Fiber-Optic Cables,” focuses on optical links, which send data by shining light down thin strands of glass inside a cable.
■ The third and final major section of this chapter, “Sending Bits with Radio Waves and No Cables,” looks at wireless links, which use radio waves to transmit bits without using a cable.
Sending Bits with electricity and Copper Wires most Ethernet LAN links use copper cables instead of fiber-optic cable
WANs also use a mix of copper and fiber cables, but many links between telcos and their customers use copper cabling. For example, a leased line acts like an electrical circuit between two endpoints, but physically it exists as a shorter cable from the telco central office (CO) near each site connected to each customer site.
Fundamentals of electrical Circuits
To send data over a link using electricity, nodes vary the electrical signal sent over a link over time
In particular, this topic looks at the basics of two types of electrical circuits: direct current (DC) circuits and alternating current (AC) circuits.
Direct Current electrical Circuits
An electrical circuit must first exist as a complete loop of material over which electricity can flow.
Electrical current is the amount of electricity that flows past a single point on the circuit. electromotive force the voltage represents the difference in electrical force between two points the negative (–) lead on the battery pushes the electrons (which are negative). So the negative lead pushes current flow away from the negative lead. The