Chapter 3: Physical Layer -- N&DP

Digital transmission produces more errors than analog transmission.

False

In a multipoint circuit configuration, each computer has a dedicated circuit between itself and each of the other computers in the network.

False

Radio data transmission requires that each device has a radio receiver/transmitter and uses a specific frequency.

True

The representation of the character A by the group of 8 bits, 01000001, is an example of:

Digital coding

_____ refers to the transmission characteristics of the circuit.

Logical circuit

____ is one of the most commonly used coding schemes used in data transmission.

ASCII

Which multiplexing technique involves dividing the communication channel into multiple frequency bands to transmit multiple signals simultaneously?

Frequency-division multiplexing

What is the process called where analog voice data is converted to digital form for transmission over digital computer networks?

Analog-to-digital conversion

Which wireless networking technology is commonly used for short-range wireless LAN connections?

Radio

Which statement best describes a key difference between fiber-optic cable and wireless communication technologies?

Fiber-optic cables can transmit data over longer distances than wireless.

The physical layer is the…

network hardware including servers, clients, and circuits

Circuits are a combination of both…

physical media and special-purpose devices

Physical circuit

is the actual wire used to connect two devices

logical circuit

refers to the transmission characteristics of the connection between two devices

There are fundamentally two different types of data that can flow through the circuit

1) digital (on or off / 0 or 1)

2) analog

Modem

translates data to/from digital and analog

What are the five key benefits of digital transmission over analog

Digital Transmission…

1) has fewer errors

2) has higher maximum transmission rates

3) is more efficient

4) is more secure because it’s easier to encrypt

5) can more simply integrate voice, video, and data on the same circuit

Regardless of analog/digital transmission, the sender and receiver are required to agree on two parameters:

1) The symbols they will use

2) The symbol rate

Circuit configuration

The basic layout of the circuit

Two fundamental circuit configurations:

1) Point-to-point

2) Multipoint

Usually, complex computer networks contain both.

Point-to-point circuit is also referred to as a…

Dedicated circuit; the circuit goes from one point (computer) to another point (computer)

Expensive!!! Each computer has its own circuit running from itself to other computers

Multipoint circuit is also referred to as a…

Shared circuit; many computers are connected to the same circuit

Advantages and disadvantages of a multipoint circuit:

Advantage: more efficient and reduce the amount of cable

Disadvantage: only one computer can use the circuit at a time

Simplex transmission

one-way transmission (radios & TVs)

Half-duplex transmission

two-way transmission; can only transmit one way at a time (walkie-talkie)

Turnaround time

The time half-duplex communication takes to switch between sending and receiving

Full-duplex transmission

Transmission in both directions simultaneously

Multiplexing

breaking one high-speed physical communication circuit into several lower-speed logical circuits

Many devices can simultaneously use and still have “separate” circuits

Often done in multiples of 4

What are the four types of multiplexing?

1) Frequency division multiplexing (FDM)

2) Time-divison multiplexing (TDM)

3) Statistical time-division multiplexing (STDM)

4) Wavelength division multiplexing (WDM)

What is Frequency division multiplexing (FDM)?

Divides circuit “horizontally” so many signals can travel at the same time (like a road)

What is Time-divison multiplexing (TDM)?

Shares communication circuit among two+ computers by taking turns

What is Statistical time-division multiplexing (STDM)?

Transmission speed for the multiplexed circuit is based on statistics of usage requirements for each computer

What is Wavelength division multiplexing (WDM)?

A version of frequency division multiplexing in fiber-optic cables.

medium/media

The physical matter or substance that carries the voice or data transmission

What are the two basic types of media?

1) Guided media: message flows through physical medium (twisted-pair wire)

2) Wireless media: message broadcasted through air (microwave/satellite)

Twisted-pair cable

Twisted insulated pairs of wires that are packed tightly together.

Twisting them minimizes electromagnetic interference.

Usually packaged as four sets of pairs.

Coaxial cable

Has a copper core with an outer cylindrical shell for insulation

Outer shield (just under the shell) is the second conductor

Is a coaxial cable or twisted-pair cable better?

Neither; Though coaxial cables are less prone to interference and errors than twisted-pair wires, few companies are installing coaxial cables today

fiber-optic cable

Uses high-speed streams of light pulses from lasers or LEDs to carry info inside hair-thin strands of glass called optical fibers.

How far can fiber-optic cables send information?

1000 meters

A single-mode fiber-optic cable transmits…

a single direct beam of light through a cable that ensures the light reflects only in that one pattern

THE LIGHT SOURCE MUST BE PERFLECTLY ALIGNED WITH THE CABLE

What is the major advantages of fiber optics?

1) Carries huge amounts of information at extremely fast data rates

2) Works better under harsh environmental conditions

Radio is the…

most common used form of wireless media. When people use the term wireless they usually mean radio transmission.

When operating with radio each device or computer on the network has a…

radio receiver/transmitter. The transmitters are very low power

Microwave transmission

is an extremely high-frequency radio communication bream that is transmitted over a direct line-of-site path between any two points

Used for long-distance data and voice transmisson

NO CABLE!

Satellite transmission

Involves a geosynchronous satellite in space

What are the disadvantages of satellite transmission

Propagation delay (takes time) and raindrop attenuation(scatters RF)

Factors in media selection

1) Type of network

2) Cost

3) Transmission distance

4) Security

5) Error rates

6) Transmission speed

There are 3 predominant coding schemes in use today:

1) ASCII

2) ISO 8859

3) Unicode

• UTF-8

• UTF-16(Windows)

Parallel transmission

the way the internal transfer of binary data takes place inside a computer. 8-bits transferred simultaneously from MM to CPU over 8 separate connects.

Serial transmission

Data is sent over circuit bit-by-bit because there is only one physical wire.

Much slower than parallel transmission

Digital transmission

Transmission of binary electrical or light pulses (1 or 0)

Require a set of symbols and symbol rate that the sender and receiver both agree on

Unipolar signaling

The voltage is always positive (bit value of 1) or negative (bit value of 0)

Bipolar Signaling

The ones and zeros vary from plus voltage to a minus voltage.

NRZ (nonreturn to zero) Voltage is +5 volts and -5 volts w/o returning to zero (+5 is 1; -5 = 0)

RZ (return to zero) Voltage is +5 volts and -5 volts with returning to zero (+5 is 1; -5 = 0)

AMI (alternate mark inversion) a 0 is always sent using 0 volts but 1s alternate between +5 and -5

The most common technology used in LAN is…

Ethernet

Ethernet

Uses digital transmission over serial or parallel circuits.

Uses Manchester encoding

Bipolar signaling changes from low to high and vice versa in the middle of the signal

How to read Ethernet signaling

high to low = 0

low to high = 1

Analog transmission occurs when…

the signal sent over the transmission media continuously varies like a wave (POTS plain old telephone service)

Modems translate the digital binary data produced by computers into the analog signals required by voice transmission circuits

True

Amplitude

The height of the wave measured in decibels

Frequency

The length of the wave measured in Hz

Phase

The direction in which the wave began(measured in degrees)

When data is transmitted through telephone lines, the shape of the sound waves represents different data values. This is done with a…

Carrier wave: a circuit that sound waves are sent through that interpret its shape into zeros and ones

There are three fundamental modulation techniques:

1) Amplitude modulation (AM)

2) Frequency modulation (FM)

3) Phase modulation (PM)

With amplitude modulation(AM) the ___ of the wave is changed

height; AM is more susceptible to noise than other methods

In frequency modulation (FM) 0s and 1s are represented by ______

the number of waves per second

In phase modulation (PM) 1s and 0s are represented by _____

the direction in which the wave begins

Each of the three modulation techniques can only send one bit at a time

False

Bit rate

The number of bits per second transmitted

baud

a unit of signaling speed which indicates the number of times per second the signal on the circuit changes

Data capacity of a circuit

the fastest rate at which data travels over the circuit (bits/sec)

data rate equation

number of bits sent on each symbol * maximum symbol rate

bandwidth

The difference between the highest and lowest frequencies. (telephone is 4000Hz)

A modem’s data transmission rate is the primary factor that determines the throughput rate of data

True

Data compression can decrease the throughput of data over the communication link

False

Codecs

Special devices used like modems to translate data sent across a circuit (analog)

Analog voice data needs to be translated into binary digits beofre transmitted

True

Quantizing error and the two ways to reduce is…

The difference between analog and digital signals

1) Increase the number of amplitude levels(mimizes the difference between levels = smoother signal)

2) Sample more frequently (reduces the length of each step = smoother signal)

local loop

The wire at your home connects to a switch that connects you to the telephone network

Adaptive differential pulse code modulation (ADPCM)

The alternative used by IM that provides voice services over lower-speed digital circuits

Voice over Internet Protocol (VoIP)

commonly used to transmit phone conversations over digital network because a codec is built into the telephone