Ch 3: Physical Layer Flashcards
(49 cards)
1
Q
Five Key Benefits of Digital Transmission over Analog
A
- Digital transmission produces fewer errors than analog transmission
- Digital transmission permits higher maximum transmission rates
- Digital transmission is more efficient
- Digital transmission is more secure because it is easier to encrypt
- Integrating voice, video, and data on the same circuit is far simpler with digital transmission
2
Q
Parameter Agreement
A
- Regardless of whether digital or analog transmission is used, transmission requires the sender and receiver to agree on two key parameters:
1. They have to agree on the symbols that will be used
2. Once these symbols are set, the sender and receiver have to agree on the symbol rate - Analog and digital transmissions are different, but both require a
commonly agreed on a set of symbols and a symbol rate
3
Q
Circuits
A
- Circuit configuration is the basic physical layout of the circuit
- There are two fundamental circuit configurations:
1. Point-to-point
2. Multipoint
4
Q
Point-to-Point Circuit
A
- Circuit goes from one point (say
a computer) to another point
(also likely a computer) - These circuits sometimes are
called dedicated circuits - Each computer has its own
circuit running from itself to the
other computers
5
Q
Multipoint Circuit
A
- In this configuration, many computers are connected to the
same circuit - Also called a shared circuit
- The disadvantage is that only one computer can use the circuit
at a time - Advantage is they are more efficient and reduce the amount
of cable required
6
Q
Data Flow
A
- Simplex transmission is one-way transmission, such as that with
radios and TVs - Half-duplex transmission is a two- way transmission, but you can
transmit in only one direction at a time, such as a walkie-talkie - The time half-duplex communication takes to switch between sending and receiving is called turnaround time
- Full-duplex transmission is when you can transmit in both directions simultaneously
7
Q
Multiplexing
A
- Multiplexing means to break one high-speed physical communication circuit into several lower-speed logical circuits
- That way, many different devices can simultaneously use it but
still “think” that they have their own separate circuits - Multiplexing often is done in multiples of 4
8
Q
Four Types of Multiplexing
A
- Frequency Division Multiplexing (FDM)
- Time-division Multiplexing (TDM)
- Statistical Time-division Multiplexing (STDM)
- Wavelength Division Multiplexing (WDM)
9
Q
Frequency Division Multiplexing (FDM)
A
Divides the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously
10
Q
Time-division Multiplexing (TDM)
A
Shares a communication circuit among two or more computers by having them take turns
11
Q
Statistical Time-division Multiplexing (STDM)
A
Selection of transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed
12
Q
Wavelength Division Multiplexing (WDM)
A
A version of FDM used in fiber-optic cables
13
Q
Communication Media
A
- The medium/media is the physical matter or substance that carries the voice or data transmission
- There are two basic types of media:
1. Guided media are those in which the message flows through a physical medium such as a twisted-pair wire
2. Wireless media are those in which the message is broadcast through the air, such as microwave or satellite
14
Q
Types of Media To Review
A
- Twisted-pair cable
- Coaxial cable
- Fiber-optic cable
- Radio
- Microwave
- Satellite
15
Q
Twisted-Pair Cable
A
- With twisted-pair cable, insulated pairs of wires that are packed quite close together
- The wires usually are twisted to minimize the electromagnetic interference
- The twisted-pair cable used in LANs are usually packaged as four sets of pairs
16
Q
Coaxial Cable
A
- Coaxial cable has a copper core with an outer cylindrical shell for insulation
- The outer shield, just under the shell, is the second conductor
- Less prone to interference and errors than basic low-cost twisted-pair wires
- Few companies are installing coaxial cable today
17
Q
Fiber-Optic Cable
A
- Fiber-optic cable uses high-speed streams of light pulses from lasers or LEDs
- They carry information inside hair-thin strands of glass called
optical fibers - The effective distance is just under 1,000 meters
18
Q
Radio
A
- One of the most commonly used forms of wireless media is radio
- When people used the term wireless, they usually mean radio
transmission - Each device or computer on the network has a radio receiver/transmitter
- The transmitters are very low power
19
Q
Microwave
A
- Microwave transmission is an extremely high-frequency radio
communication beam that is transmitted over a direct line-of-sight path between any two points - Typically used for long-distance data or voice transmission
- Does not require the laying of any cable
20
Q
Satellite
A
- Satellite transmission involves a satellite many miles up in space
- The satellite is geosynchronous
- One disadvantage of satellite transmission is the propagation
delay - Sometimes also affected by raindrop attenuation
21
Q
Factors in Media Selection
A
- Type of network
- Cost
- Transmission distance
- Security
- Error rates
- Transmission speed
22
Q
Digital Transmission of Digital Data
A
- All computer systems produce binary data
- Sender and receiver agree on a standard system for representing the message
- The coding scheme is the language that computers use to represent data
23
Q
Coding
A
- A character is a symbol that has a common, constant meaning
- Characters in data communications are represented by groups of bits that are binary zeros and ones
- The groups of bits representing the set of characters that are the
“alphabet” of any given system are called a coding scheme, or simply a code
24
Q
Parallel Transmission
A
- This is the way the internal transfer of binary data takes place inside a computer
- If the internal structure of the computer is 8bit, then all 8 bits of
the data element are transferred between the main memory and the central processing unit simultaneously on 8 separate connections - Each physical wire is used to send 1 bit
25
Serial Transmission
* Serial transmission means that a stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion
* There is only one physical wire inside the bundle, and all data
must be transmitted over that one physical wire
* Serial transmission is much slower
26
Digital Transmission
* Digital transmission is the transmission of binary electrical or light pulses in that it only has two possible states, a 1 or a 0
* All digital transmission techniques require a set of symbols and the symbol rate
* Both the sender and receiver have to agree on how often the
sender can transmit data—that is, on the symbol rate
27
Unipolar Signaling
* With unipolar signaling, the voltage is always positive or negative
* 0 Volts is used to transmit a zero and a signal of +5 volts is used to
transmit a 1
28
Bipolar Signaling
* In bipolar signaling, the ones and zeros vary from a plus voltage to a minus voltage
* With nonreturn to zero (NRZ), the voltage alternates from+5 volts to −5 volts without ever returning to 0 volts
* With return to zero (RZ), because it always returns to 0 volts after each bit before going to +5 volts or −5 volts
* With alternate mark inversion (AMI), a 0 is always sent using 0 volts, but 1s alternate between +5 volts and −5 volts
29
How Ethernet Transmits Data
* The most common technology used in LANs is Ethernet
* Ethernet uses digital transmission over either serial or parallel circuits
* Ethernet uses Manchester encoding
* This is a special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the
signal
30
How Ethernet Transmits Data Continued
* A change from high to low is used to represent a 0
* A change from lot to high is used to represent a 1
* Manchester encoding is less susceptible to having errors go undetected because if there is no transition in midsignal, the
receiver knows that an error must have occurred
31
Analog Transmission of Digital Data
* The telephone system enables voice communication between any two telephones within its network
* Analog transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern
* Modems translate the digital binary data produced by computers into the analog signals required by voice transmission circuits
32
Three Important Characteristics of Sound Waves
1. Amplitude: the height of the wave - decibels (dB)
2. Frequency: the length of the
wave - hertz (Hz)
3. Phase: the direction in which
the wave begins - degrees ∘
33
Modulation
* When we transmit data through the telephone lines, we use the shape of the sound waves we transmit to represent different data values
* We do this by transmitting a simple sound wave through the circuit (called the carrier wave) and then changing its shape in different ways to represent a 1 or a 0
34
Three Fundamental Modulation Techniques
1. Amplitude modulation (AM)
2. Frequency modulation (FM)
3. Phase modulation (PM)
35
Amplitude Modulation (AM)
* With AM the amplitude or height of the wave is changed
* Also called amplitude shift keying [ASK]
* One amplitude is 0 and the other is 1
* AM is more susceptible to noise than are the other methods
36
Frequency Modulation (FM)
* FM is a modulation technique whereby each 0 or 1 is
represented by a number of waves per second
* Amplitude does not vary
* Also called frequency shift keying [FSK]
* One frequency is the symbol defined to be a 1, and a different frequency is 0
37
Phase Modulation (PM)
* Phase refers to the direction in which the wave begins
* Also called phase-shift keying [PSK]
* With PM, one phase symbol is defined to be a 0 and the other
phase symbol is defined to be a 1
38
Sending Multiple Bits Simultaneously
* Each of the three basic modulation techniques can be refined to send more than one bit at one time
* For example, with basic AM, it is possible to send 2 bits on one
wave or symbol by defining four different amplitudes
* This technique could be further refined to send more bits at the
same time
* At some point, however, it becomes very difficult to differentiate between the different amplitudes
39
Bit Rate versus Baud Rate versus Symbol Rate
* The terms bit rate and baud rate are used incorrectly much of the time
* A bit is a unit of information
* Bit rate is the number bits per second transmitted
* A baud is a unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes
* The bit rate and the baud rate are the same only when 1 bit is sent on each symbol
40
Capacity of a Circuit
* The data capacity of a circuit is the fastest rate at which you can send your data over the circuit in terms of the number of bits per second
* The data rate (or bit rate) is calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate
* The bandwidth is the difference between the highest and the lowest
frequencies in a band or set of frequencies
41
How Modems Transmit Data
* Modem is an acronym for modulator/demodulator
* It the digital data from a computer in the form of electrical pulses and converts them into the analog signal
* A modem’s data transmission rate is the primary factor that determines the throughput rate of data
* Data compression can increase the throughput of data over a
communication link by compressing the data
* V.44, the ISO standard for data compression, uses Lempel–Ziv encoding
42
Digital Transmission of Analog Data
* Analog voice data can be sent over digital networks using digital
transmission
* A pair of special devices called codecs (code/decode) is used in the same way that a pair of modems are used to translate the data to send across the circuit
43
Translating from Analog to Digital
* Analog voice data must first be translated into a series of binary
digits before they can be transmitted over a digital circuit
* This is done by sampling the amplitude of the sound wave at
regular intervals and translating it into a binary number
* The digitized signal is only a rough approximation of the original signal
44
Quantizing Error
The difference between the analog and digital signals
45
Two ways to reduce quantizing error and improve the quality of
the digitized signal
1. Increase the number of amplitude levels
2. Sample more frequently
46
How Telephones Transmit Voice Data
* Today, all of the common carrier networks use digital
transmission, except in the local loop
* Also known as the last mile
* The wire at your home connects to a switch that connects you to
the telephone network
* This switch contains a codec that converts the analog signal from your phone into a digital signal
47
How Instant Messenger Transmits Voice Data
* A 64 kbps digital circuit requires a lot of capacity
* Adaptive differential pulse code modulation (ADPCM) is the alternative used by IM and many other applications that provide voice services over lower-speed digital circuits
* It samples incoming voice signals 8,000 times per second and calculates the same 8-bit amplitude value as PCM
* Instead of transmitting the 8-bit value, it transmits the difference between the 8-bit value in the last time interval and the current 8-bit value
48
Voice over Internet Protocol (VoIP)
* Voice over Internet Protocol (VoIP) is commonly used to transmit
phone conversations over digital networks
* Because the codec is built into the telephone, it digital data and
therefore can be connected directly into a local area network
* VoIP phones operate on the same networks as computers
* Must be designed to handle 911 calls
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