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Physical Layer

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Title: Physical Layer


1
Physical Layer
  • Chapter 3

2
Outline
  • Circuit Media
  • Digital Transmission of Digital Data
  • Analog Transmission of Digital Data
  • Digital Transmission of Analog Data
  • Multiplexing
  • Examples
  • Analog-Digital Modems (56k Modem)
  • Cable Modem
  • ADSL Modem

3
Circuits Media
4
Network Configuration
  • Network configuration is the basic physical
    layout of the network.
  • There are two fundamental network configurations
  • Point-to-point configuration (or two-point) -
    sometimes called dedicated circuits.
  • Multipoint configuration (or multidrop).
  • Most complex computer networks have many
    circuits, some of each type.

5
Network Configuration
6
Multipoint Configuration
7
Data Flow
  • Circuits can be designed to permit data to flow
    in one or both directions. There are three ways
    to transmit
  • Simplex - One way transmission
  • Half-duplex -Two way communications link, but
    only one system can talk at a time.
  • Full duplex -Transmit in both directions
    simultaneously.

8
Data Flow
9
Communication Media
  • The medium is the matter or substance that
    carries the voice or data transmission.
  • There are two basic types of media
  • Guided media - those in which the message flows
    through a physical media.
  • Radiated media (unguided) - Those in which the
    message is broadcast through space.
  • Circuits sold by the common carriers are called
    communications services.

10
Guided Media
  • Twisted Pair Wire - insulated pairs of wire,
    twisted to minimize electromagnetic interference
    between wires.
  • Coaxial Cable - wire with a copper core and an
    outer cylindrical shell for insulation.
  • Fiber Optic Cable - high speed streams of light
    pulses from lasers or light-emitting diodes
    (LEDs) carry information inside hair-thin strands
    of glass or plastic called optic fibers.

11
(No Transcript)
12
Fiber Optic Cable
  • The earliest fiber optic systems were multimode,
    (light could reflect inside the cable at many
    different angles).
  • Single mode fiber optic cables transmit a single
    direct beam of light through a cable that ensures
    the light only reflects in one pattern.
  • Fiber optic technology is a revolutionary
    departure from the traditional message-carrying
    systems of copper wires.

13
Guided Media
14
Optical Fiber Transmission Modes
15
Radiated Media
  • Radio (wireless) data transmission uses the same
    basic principles as standard radio transmission.
  • Infrared Transmission uses low frequency light
    waves to carry data through the air on a direct
    line-of-sight path between two points.

16
Radiated Media
  • A microwave is an extremely high frequency radio
    communication beam that is transmitted over a
    direct line-of-sight path between two points.
  • Transmission via satellite is similar to
    transmission via microwave except, instead of
    transmitting to another nearby microwave dish
    antenna, it transmits to a satellite 22,300 miles
    in space.

17
Radiated Media
18
Radiated Media
  • One disadvantage of satellite transmission is the
    delay that occurs because the signal has to
    travel out into space and back to Earth
    (propagation delay, about 0.5 second, 186,000
    miles/second).
  • One problem associated with some types of
    satellite transmission is raindrop attenuation
    (some waves at the high end of the spectrum are
    so short they can be absorbed by raindrops).

19
Media Selection
Guided Media Radiated Media
Network Transmission Error Media Type Cost Di
stance Security Rates Speed Twisted
Pair LAN Low 100-300M Good Low 1-100Mbps Coaxial
Cable LAN Mod. 200-500M Good Low 1-100Mbps Fiber
Optics any High up to 75Mile V. Good V.Low 10Gbps
Network Transmission Error Media Type Cost Di
stance Security Rates
Speed Radio LAN Low Short Poor Mod
1-4Mbps Infrared LAN, BN Low Short
Poor Mod 1-4Mbps Microwave WAN Mod Long
Poor Low-Mod 50 Mbps Satellite WAN
Mod Long Poor Low-Mod 50 Mbps
20
Data Transmission
21
Data
  • There are two fundamentally different types of
    data
  • Digital -Computer produced signals that are
    binary, either on or off.
  • Analog - Electrical signals which are shaped like
    the sound waves they transfer.

22
Data Transmissions
Analog data
Analog Transmission
Modem Codec
Digital data
Digital Transmission
23
Data Transmission Devices
  • Data can be transmitted through a circuit in the
    same form they are produced, or converted from
    one form into the other for transmission over
    network circuits.
  • Modem (Modulator/demodulator) translates digital
    data into analog form for transmission over
    analog circuits.
  • Codec (Coder/decoder) translates analog voice
    data into digital form for transmission over
    digital circuits.

24
Benefits of Digital Transmission
  • Digital transmission offers five key benefits
    over analog transmission.
  • Digital transmission produces fewer errors than
    analog transmission.
  • Digital transmission is more efficient.
  • Digital transmission permits higher maximum
    transmission rates.
  • 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.

25
Technical Focus
  • In an analog system data are represented by
    measurements on a continuous scale. Analog is
    also called broadband.
  • In contrast, digital data can only take on
    specific discrete values. Digital is also called
    baseband.

26
Digital Transmission of Digital Data
27
Digital Transmission of Digital Data
  • All computers produce binary data. For this data
    to be understood by both the sender and receiver,
    both must agree on a standard system for
    representing the letters, numbers, and symbols
    that comprise the messages.

28
Coding
  • Character A symbol that has a common, constant
    meaning.
  • Characters in data communications, as in computer
    systems, are represented by groups of bits 1s
    and 0s.
  • The group of bits representing the set of
    characters in the alphabet of any given system
    are called a coding scheme, or simply a code.

29
Coding
  • A byte consists of 8 bits that is treated as a
    unit or character. (Some Asian languages use 2
    bytes for each of their characters, such as
    Chinese.)
  • (The length of a computer word could be 1, 2, 4
    bytes.)
  • There are two predominant coding schemes in use
    today
  • United States of America Standard Code for
    Information Interchange (USASCII or ASCII)
  • Extended Binary Coded Decimal Interchange Code
    (EBCDIC)

30
Transmission Modes
  • Parallel Mode Data are transferred
    simultaneously in groups of bits. It is the way
    the internal transfer of binary data takes place
    inside a computer.
  • Serial Mode Data are sent one bit after another.
    It is the predominant method of transferring
    information in data communications.

31
Transmission Modes
Serial Mode
32
Baseband Transmission
  • Digital Transmission the transmission of
    electrical pulses.
  • Digital Information It has only two possible
    states 1 or 0, or binary values.
  • Baseband Signals i.e. Digital signals.
  • Data Rate In order to successfully send and
    receive a message, both the sender and receiver
    have to agree how often the sender can transmit
    data.

33
Baseband Transmission
  • With unipolar signaling technique, the voltage is
    always positive or negative (like a dc current).
  • In bipolar signaling, the 1s and 0s vary from a
    plus voltage to a minus voltage (like an ac
    current).
  • In general bipolar signaling experiences fewer
    errors than unipolar signaling because the
    signals are more distinct.

34
Baseband Transmission
35
Baseband Transmission
  • Manchester encoding is a special type of unipolar
    signaling in which the signal is changed from a
    high to low or low to high in the middle of the
    signal.
  • Manchester encoding is less susceptible to having
    errors go undetected, because if there is no
    transition, the receiver knows that an error must
    have occurred.
  • Manchester encoding is commonly used in local
    area networks (Ethernet, token ring).

36
Manchecter Encoding
00001110 in a 10BASE-T circuit
37
Analog Transmission of Digital Data
38
Bandwidth on a Voice Circuit
  • Every sound wave has two parts, half above the
    zero point (positive), and half below (negative)
    and three important characteristics.
  • The height of the wave is called amplitude.
  • The length of the sound wave is expressed as the
    number of waves per second or frequency,
    expressed in Hertz (Hz).
  • The phase is the direction in which the wave
    begins.
  • Bandwidth refers to a range of frequencies.

39
Bandwidth on a Voice Circuit
Frequency 1 Period/Sec 1 Hertz
40
Bandwidth on a Voice Circuit
?
?
Phase
Frequency 1 Period/Sec 1 Hertz
41
Bandwidth on a Voice Circuit
  • Human hearing ranges from about 20 Hz to about
    14,000 Hz (some up to 20,000 Hz). Human voice
    ranges from 20 Hz to about 14,000 Hz.
  • The bandwidth of a voice grade telephone circuit
    is 0 to 4000 Hz or 4000 Hz (4 KHz).
  • Guardbands prevent data transmissions from
    interfering with other transmission when these
    circuits are multiplexed using FDM.

42
Bandwidth on a Voice Circuit
43
(No Transcript)
44
Bandwidth on a Voice Circuit
  • It is important to note that the limit on
    bandwidth is imposed by the equipment used in the
    telephone network.
  • The actual capacity of bandwidth of the wires in
    the local loop depends on what exact type of
    wires were installed, and the number of miles in
    the local loop.
  • Actual bandwidth in North America varies from 300
    KHz to 1 MHz depending on distance.

45
Modulation
  • Modulation is the technique that modifies the
    form of a digital electrical signal so the signal
    can carry information on a communications media.
  • Three fundamental methods
  • Amplitude Modulation (AM) (also called Amplitude
    Shift Keying, or ASK)
  • Frequency Modulation (FM) (also called Frequency
    Shift Keying, or FSK)
  • Phase Modulation(PM) (also called Phase Shift
    Keying, or PSK)

46
Amplitude Modulation and ASK
47
Frequency Modulation and FSK
48
Phase Modulation and PSK
49
Sending Multiple Bits Simultaneously
  • Each of the three modulation techniques can be
    refined to send more than one bit at a time. It
    is possible to send two bits on one wave by
    defining four different amplitudes.
  • This technique could be further refined to send
    three bits at the same time by defining 8
    different amplitude levels or four bits by
    defining 16, etc. The same approach can be used
    for frequency and phase modulation.

50
Sending Multiple Bits Simultaneously
51
Sending Multiple Bits Simultaneously
?/2 ? 01
?? 10
0 00
3?/2 ? 11
52
Sending Multiple Bits Simultaneously
  • In practice, the maximum number of bits that can
    be sent with any one of these techniques is about
    five bits. The solution is to combine modulation
    techniques.
  • One popular technique is quadrature amplitude
    modulation (QAM) involves splitting the signal
    into eight different phases, and two different
    amplitude for a total of 16 different possible
    values.

53
Sending Multiple Bits Simultaneously
  • Trellis coded modulation (TCM) is an enhancement
    of QAM that combines phase modulation and
    amplitude modulation. It can transmits different
    numbers of bits on each symbol (6-10 bits per
    symbol).
  • The problem with high speed modulation techniques
    such as TCM is that they are more sensitive to
    imperfections in the communications circuit.

54
Bits Rate Versus Baud Rate Versus Symbol Rate
  • A bit is a unit of information, a baud is a unit
    of signaling speed, the number of times a signal
    on a communications circuit changes. ITU-T now
    recommends the term baud rate be replaced by the
    term symbol rate.
  • The bit rate and the symbol rate (or baud rate)
    are the same only when one bit is sent on each
    symbol. If we use QAM or TCM, the bit rate would
    be four to eight times the baud rate.

55
Capacity of a Voice Circuit
  • The capacity of a voice circuit (the maximum data
    rate) is the fastest rate at which you can send
    your data over the circuit.
  • The maximum symbol rate in any circuit depends
    upon the bandwidth available and the signal to
    noise ratio.
  • Voice grade lines provide a bandwidth of 3000 Hz.

56
Modems
  • Modem is an acronym for Modulator/ Demodulator,
    and takes digital electrical pulses from a
    computer, terminal, or microcomputer and converts
    them into a continuous analog signal, for
    transmission over an analog voice grade circuit.
  • There are many different types of modems
    available today. Most modems support several
    standards so that they can communicate with a
    variety of different modems.
  • Better modems can change data rates during
    transmission to reduce the rate in case of noisy
    transmission (fast retrain).

57
Modem Standards
  • V.22
  • 1200-2400 baud/bps (FM)
  • V.32 and V.32bis
  • full duplex at 9600 bps (2400 baud at QAM)
  • bis uses TCM to achieve 14,400 bps.
  • V.34
  • for phone networks using digital transmission
    beyond the local loop.
  • 59 combinations of symbol rate and modulation
    technique
  • symbol rates 3429 baud. Its bit rate is up to
    28,800 bps (TCM-8.4)
  • V.34
  • up to 33.6 kbps with TCM-9.8

58
Data Compression
  • V.42bis
  • data compression modems, accomplished by run
    length encoding, code book compression, Huffman
    encoding and adaptive Huffman encoding
  • MNP5 - uses Huffman encoding to attain 1.31 to
    21 compression.
  • it uses Lempel-Ziv encoding and attains 3.51 to
    41.
  • V.42bis compression can be added to almost any
    modem standard effectively tripling the data rate.

59
Data Compression
  • How fast if using V.42bis
  • V.32 - 57.6kbps
  • V.34 - 115.2 kbps
  • V.34 - 133.4 kbps
  • V.90 ?

60
Data Compression
  • There are two drawbacks to the use of data
    compression
  • Compressing already compressed data provides
    little gain.
  • Data rates over 100 Kbps place considerable
    pressure on the traditional microcomputer serial
    port controller that controls the communications
    between the serial port and the modem.

61
Digital Transmission of Analog Data
62
Digital Transmission of Analog Data
  • Analog voice data can be sent over digital
    networks using a pair of special devices called
    CODECs (Coder/Decoder).
  • Operation is very similar to how modems function.

63
Codec vs. Modem
  • Codec is for coding analog data into digital form
    and decoding it back. The digital data coded by
    Codec are samples of analog waves.
  • Modem is for modulating digital data into analog
    form and demodulating it back. The analog symbols
    carry digital data.

64
Pulse Code Modulation (PCM)
  • Analog voice data must be translated into a
    series of binary digits before they can be
    transmitted.
  • With Pulse Code Modulation (PCM), the amplitude
    of the sound wave is sampled at regular intervals
    and translated into a binary number.
  • The difference between the original analog signal
    and the translated digital signal is called
    quantizing error.

65
PCM
66
PCM
67
PCM
68
PCM
  • PCM uses a sampling rate of 8000 samples per
    second.
  • Each sample is an 8 bit sample resulting in a
    digital rate of 64,000 bps (8 x 8000).

69
Multiplexing
70
Multiplexers
  • A multiplexer puts two or more simultaneous
    transmissions on a single communications circuit.
  • Multiplexing usually is done in multiples of 4,
    8, 16, or 32.
  • Generally speaking, the multiplexed circuit must
    have the same capacity as the sum of the circuits
    it combines.
  • The primary benefit of multiplexing is to save
    money.

71
Multiplexed Circuit
72
Multiplexing
  • There are three major types of multiplexers
  • Frequency division multiplexers (FDM)
  • Time division multiplexers (TDM)
  • Statistical time division multiplexers (STDM)

73
Frequency Division Multiplexing (FDM)
74
Frequency Division Multiplexing (FDM)
  • Frequency division multiplexers are somewhat
    inflexible because once you determine how many
    channels are required, it may be difficult to add
    more channels without purchasing an entirely new
    multiplexer.
  • Wavelength division multiplexing (WDM) is a
    version of FDM used in fiber optic cables. WDM
    permits up to 40 circuits, each capable of 10
    Gbps. It should reach 25 terabits per second
    within five years.

75
Time Division Multiplexing (TDM)
  • Time division multiplexing shares a circuit among
    two or more terminals by having them take turns,
    dividing the circuit vertically.
  • Time on the circuit is allocated even when data
    are not transmitted, so that some capacity is
    wasted when a terminal is idle.
  • Time division multiplexing is generally more
    efficient and less expensive to maintain than
    frequency division multiplexing, because it does
    not need guardbands.

76
Time Division Multiplexing (TDM)
77
Statistical Time Division Multiplexing (STDM)
  • Statistical time division multiplexing is the
    exception to the rule that the capacity of the
    multiplexed circuit must equal the sum of the
    circuits it combines.
  • STDM is called statistical because selecting the
    transmission speed for the multiplexed circuit is
    based on statistical analysis of the usage
    requirements of the circuits to be multiplexed.

78
Statistical Time Division Multiplexing (STDM)
  • STDM provides more efficient use of the circuit
    and saves money. However, STDM introduces two
    complexities
  • 1. STDM can cause time delays, if all terminals
    decide to transmit simultaneously.
  • 2. All data must be identified by an address
    that specifies the device to which terminal it
    belongs.
  • Most STDM multipexers do not send one character
    at a time from each terminal, but send groups of
    characters at one time.

79
(STDM)
80
Inverse Multiplexing
  • Inverse multiplexing (IMUX) combines several low
    speed circuits to appear as one high speed
    circuit.
  • One of the most common uses is to provide T-1
    (1.544 Mbps) circuits for wide area networks, by
    combining 24 slower (64 Kbps) circuits.
  • Until recently, there were no standards for
    inverse multiplexing.

81
Inverse Multiplexing
82
Examples of Modem
83
Analog/Digital Modems (56k Modems)
  • The basic idea behind 56K modems (V.90) is
    simple. 56K modems take the basic concepts of
    PCM and turn them backwards. They are designed to
    recognize an 8-bit digital signal 8000 times per
    second.
  • It is impractical to use all 256 discrete codes,
    because the corresponding DAC output voltage
    levels near zero are just too closely spaced to
    accurately represent data on a noisy loop.
    Therefore, the V.90 encoder uses various subsets
    of the 256 codes that eliminate DAC output
    signals most susceptible to noise. For example,
    the most robust 128 levels are used for 56 Kbps,
    92 levels to send 52 Kbps, and so on. Using fewer
    levels provides more robust operation, but at a
    lower data rate.

84
Analog/Digital Modems (56k Modems)
  • Noise is a critical issue. Recent tests found
    56K modems to connect at less than 40 Kbps 18 of
    the time, 40-50 Kbps 80 of the time, and 50
    Kbps only 2 of the time.
  • It is easier to control noise in the channel
    transmitting from the server to the client than
    in the opposite direction.
  • Because the current 56K technology is based on
    the PCM standard, it cannot be used on services
    that do not use this standard.

85
Downstream vs. Upstream
86
Downstream vs. Upstream
87
STDM Cable Modems
  • Cable TV provider dedicates two channels, one for
    each direction.
  • Channels are shared by subscribers, so some
    method for allocating capacity is
    needed--typically statistical TDM

88
Cable Modem Scheme
89
Cable Company Fiber Node
Customer Premises
Cable Company Distribution Hub
TV Video Network
Cable Splitter
Cable Modem
Combiner
Downstream
Optical/Electrical Converter
Upstream
Hub
TV
Router
Shared Coax Cable System
Cable Company Fiber Node
Cable Modem Termination System
Computer
Computer
ISP POP
Customer Premises
Customer Premises
Figure 9-8 Cable Modem Architecture
90
FDM Example ADSL
  • ADSL uses frequency-division modulation (FDM) to
    exploit the 1-MHz capacity of twisted pair.
  • There are three elements of the ADSL strategy
  • Reserve lowest 25 kHz for voice, known as POTS
    (Plain old telephone service)
  • Use echo cancellation or FDM to allocate a small
    upstream band and a larger downstream band
  • Use FDM within the upstream and downstream bands,
    using discrete multitone

Upstream
Downstream
POTS
0 20 25 200 250
91
Discrete Multitone (DMT)
  • Uses multiple carrier signals at different
    frequencies, sending some of the bits on each
    channel.
  • Transmission band (upstream or downstream) is
    divided into a number of 4-kHz subchannels.
  • Modem sends out test signals on each subchannel
    to determine the signal to noise ratio it then
    assigns more bits to better quality channels and
    fewer bits to poorer quality channels.

Bits/Hertz
Frequency
92
Customer Premises
Local Carrier End Office
Line Splitter
DSL Modem
Main Distribution Frame
Voice Telephone Network
Local Loop
Hub
Telephone
ISP POP
ATM Switch
Computer
DSL Access Multiplexer
Computer
ISP POP
Customer Premises
ISP POP
ISP POP
Customer Premises
Figure 9-5 DSL Architecture
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