3.1 INTRODUCTIONThis chapter examin

3.1 INTRODUCTION
This chapter examines how the physical layer operates. The physical layer is the network hardware including servers, clients, and circuits, but in this chapter we focus on
the circuits and on how clients and servers transmit data through them. The circuits
are usually a combination of both physical media (e.g., cables, wireless transmissions)
and special-purpose devices that enable the transmissions to travel through the media.
Special-purpose devices such as hubs, switches, and routers are discussed in Chapter 6
and 8.
The word circuit has two very different meanings in networking, and sometimes it
is hard to understand which meaning is intended. Sometimes, we use the word circuit to
refer to the physical circuit—the actual wire—used to connect two devices. In this case,
we are referring to the physical media that carry the message we transmit, such as the
twisted pair wire used to connect a computer to the LAN in an office. In other cases, we
are referring to a logical circuit used to connect two devices, which refers to the transmission characteristics of the connection, such as when we say a company has a T1 connection into the Internet. In this case, T1 refers not to the physical media (i.e., what type of
wire is used) but rather to how fast data can be sent through the connection.1 Often, each
physical circuit is also a logical circuit, but as you will see in the section on multiplexing,
sometimes it is possible to have one physical circuit—one wire—carry several separate
logical circuits, or to have one logical circuit travel over several physical circuits.
There are two fundamentally different types of data that can flow through the
circuit: digital and analog. Computers produce digital data that are binary, either on
or off, 0 or 1. In contrast, telephones produce analog data whose electrical signals are
shaped like the sound waves they transfer; they can take on any value in a wide range
of possibilities, not just 0 or 1.
Data can be transmitted through a circuit in the same form they are produced.
Most computers, for example, transmit their digital data through digital circuits to printers and other attached devices. Likewise, analog voice data can be transmitted through
telephone networks in analog form. In general, networks designed primarily to transmit
digital computer data tend to use digital transmission, and networks designed primarily
to transmit analog voice data tend to use analog transmission (at least for some parts of
the transmission).
Data can be converted from one form into the other for transmission over network
circuits. For example, digital computer data can be transmitted over an analog telephone
circuit by using a modem. A modem at the sender’s computer translates the computer’s
digital data into analog data that can be transmitted through the voice communication
circuits, and a second modem at the receiver’s end translates the analog transmission
back into digital data for use by the receiver’s computer.
Likewise, it is possible to translate analog voice data into digital form for transmission over digital computer circuits using a device called a codec. Once again, there are two codecs, one at the sender’s end and one at the receiver’s end. Why bother to
translate voice into digital? The answer is that digital transmission is “better” than ana
log transmission. Specifically, digital transmission offers five key benefits over analog
transmission:
• Digital transmission produces fewer errors than analog transmission. Because the
transmitted data is binary (only two distinct values), it is easier to detect and correct
errors.
• Digital transmission permits higher maximum transmission rates. Fiber-optic cable,
for example, is designed for digital transmission.
• Digital transmission is more efficient. It is possible to send more data through a
given circuit using digital rather than analog transmission.
• Digital transmission is more secure because it is easier to encrypt.
• Finally, and most importantly, integrating voice, video, and data on the same circuit
is far simpler with digital transmission.
For these reasons, most long-distance telephone circuits built by the telephone companies
and other common carriers over the past decades use digital transmission. In the future,
most transmissions (voice, data, and video) will be sent digitally.
Regardless of whether digital or analog transmission is used, transmission requires
the sender and receiver to agree on two key parameters. First, they have to agree on the
symbols that will be used: what pattern of electricity, light, or radio wave will be used
to represent a 0 and a 1. Once these symbols are set, the sender and receiver have to
agree on the symbol rate: How many symbols will be sent over the circuit per second?
Analog and digital transmissions are different, but both require a commonly agreed on
set of symbols, and a symbol rate.
In this chapter, we first describe the basic types of circuits and examine the different
media used to build circuits. Then we explain how data are actually sent through these
media using digital and analog transmission.