Chapter 3 - Physical Layer

Question 1

Physical circuit

Answer 1

The actual wire used to connect two devices.

Question 2

Logical circuit

Answer 2

Transmission characteristics of the connection

Question 3

What is the difference between digital and analog data?

Answer 3

Digital data is binary, meaining values are 0 or 1; analog data can take any value in a wide range of possibilities.

Question 4

Why is digital transmission better than analog?

Answer 4

1) Digital transmission produces fewer errors than analog transmission. Because the transmitted data are binary (only two distinct values), it is easier to detect and correct errors.

2) Digital transmission permits higher maximum transmission rates. Fiber-optic cable, for example, is designed for digital transmission.

3) Digital transmission is more efficient. It is possible to send more data through a given circuit using digital rather than analog transmission.

4 )Digital transmission is more secure because it is easier to encrypt.

5) Finally, and most importantly, integrating voice, video, and data on the same circuit is far simpler with digital transmission.

Question 5

Circuit configuration

Answer 5

The basic physical layout of the circuit

Question 6

Point-to-point circuit

Answer 6

Goes from one point to another. Sometimes called dedicated circuits because they are dedicated to the use of two computers. This type is used when computers generate enough data to fill the capacity of the communication circuit.

Question 7

Multipoint circuit

Answer 7

In this configuration, many computers are connected to the same circuit. Only one computer can use the circuit at a time, but they reduce the amount of cable required and use the communication circuit more efficiently. Also called a shared circuit.

Question 8

Simplex transmission

Answer 8

One-way transmission, such as with radios and TVs

Question 9

Half-duplex transmission

Answer 9

Two-way transmission, but you can only transmit in one direction at a time. Similar to a walkie-talkie link.

Question 10

Full-duplex transmission

Answer 10

Allows you to transmit in both directions simultaneously, with no turnaround time.

Question 11

Multiplexing

Answer 11

To break one high-speed physical communication circuit into several lower-speed logical circuits so that many different devices can simultaneously use it but still “think” that they have their own separate circuits

Question 12

What are the four types of multiplexing?

Answer 12

frequency division multiplexing (FDM), time-division multiplexing (TDM), statistical time-division multiplexing (STDM), and wavelength division multiplexing (WDM).

Question 13

Frequency Division Multiplexing (FDM)

Answer 13

Divides the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously. The circuit is divided into a series of separate channels, each transmitting on a different frequency, much like a series of different radio or TV stations. All signals exist in the media at the same time, but because they are on different frequencies, they do not interfere with each other.

Question 14

Time Division Multiplexing (TDM)

Answer 14

shares a communication circuit among two or more computers by having them take turns, dividing the circuit vertically, so to speak.

Question 15

Statistical Time-Division Multiplexing (STDM)

Answer 15

Called statistical because the selection of transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed.

Question 16

Wavelength Division Multiplexing (WDM)

Answer 16

A version of FDM used in fiber-optic cables. When fiber-optic cables were first developed, the devices attached to them were designed to use only one color of light generated by a laser or LED.

Question 17

Digital Subscriber Line (DSL)

Answer 17

Allows for simultaneous transmission of voice (phone calls), data going to the Internet (called upstream data), and data coming to your house from the Internet (called downstream data).

Question 18

Guided Media

Answer 18

Those in which the message flows through a physical medium such as a twisted-pair wire, coaxial cable, or fiber-optic cable; the medium “guides” the signal.

Question 19

Wireless media

Answer 19

Those in which the message is broadcast through the air, such as microwave or satellite.

Question 20

Twisted-pair cable

Answer 20

An insulated pair of wires that can be packed quite close together to minimize the electromagnetic interference between one pair and any other pair in the bundle.

Question 21

Coaxial cable

Answer 21

Copper core with an outer shell. Less prone to interference and errors than basic low-cost twisted-pair wires, but three times as expensive for few benefits.

Question 22

Fiber-optic cable

Answer 22

Uses high-speed streams of light pulses from lasers or LEDs (light-emitting diodes) that carry information inside hair-thin strands of glass called optical fibers.

Question 23

Radio transmission

Answer 23

One of the most commonly used forms of wireless media. Each device or computer on a network has a radio receiver/transmitter with a specific frequency range.

Question 24

Microwave Transmission

Answer 24

High-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points.

Question 25

Satellite transmission

Answer 25

Similar to microwave transmission, except it involves a satellite many miles up in space.

Question 26

What are the three predominant coding schemes in use today?

Answer 26

ASCII, ISO 8859, and Unicode.

Question 27

Serial transmission

Answer 27

A stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion

Question 28

Digital transmission

Answer 28

Transmission of binary electrical or light pulses. Can be unipolar or bipolar.

Question 29

Manchester encoding

Answer 29

A special type of bipolar signaling in which the signal is changed from high to low or low to high in the middle of the signal. Ethernet uses this.

Question 30

Explain analog transmission of digital data

Answer 30

Modems are used to translate the digital data produced by computers into the analog signals for transmission in today’s voice communication circuits. Both the sender and receiver need to have a modem. Data are transmitted by changing (or modulating) a carrier sound wave’s amplitude (height), frequency (length), or phase (shape) to indicate a binary 1 or 0.

Question 31

Explain digital transmission of analog data.

Answer 31

Because digital transmission is better, analog voice data are sometimes converted to digital transmission. Pulse Code Modulation (PCM) is the most commonly used technique. PCM samples the amplitude of the incoming voice signal 8,000 times per second and uses 8 bits to represent the signal. PCM produces a reasonable approximation of the human voice, but more sophisticated techniques are needed to adequately reproduce more complex sounds such as music.