1. Topic 3
Physical Layer
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2. Outline Circuit & Media Digital Transmission of Digital Data
Analog Transmission of Digital Data
Digital Transmission of Analog Data
Multiplexing
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3. Circuits & Media
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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.
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5. Network Configuration
Point-to-point configuration
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6. Multipoint Configuration
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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.
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8. Data Flow
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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.
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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.
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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.
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13. Guided Media
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14. Optical Fiber Transmission Modes
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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.
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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.
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17. Radiated Media
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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).
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19. Media Selection Guided Media Radiated Media Network Transmission Error
Media Type Cost Distance Security Rates Speed
Twisted Pair LAN Low M Good Low 1-100Mbps
Coaxial Cable LAN Mod M Good Low 1-100Mbps
Fiber Optics any High up to 75Mile V. Good V.Low 10Gbps
Network Transmission Error
Media Type Cost Distance Security Rates Speed
Radio LAN Low Short Poor Mod Mbps
Infrared LAN, BN Low Short Poor Mod Mbps
Microwave WAN Mod Long Poor Low-Mod Mbps
Satellite WAN Mod Long Poor Low-Mod Mbps
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20. Data Transmission
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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.
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22. 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.
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23. Bandwidth on a Voice Circuit
Frequency: 1 Period/Sec = 1 Hertz
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24. Bandwidth on a Voice Circuit 
Phase
Frequency: 1 Period/Sec = 1 Hertz
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25. 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.
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26. Bandwidth on a Voice Circuit
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28. 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.
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29. 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.
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30. 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.
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31. Broadband and Baseband
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.
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32. Data Transmissions Analog data Analog Transmission Digital data
Modem
Codec
Digital data
Digital
Transmission
We use signals to send data in different transmission methods.
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33. Digital Transmission of Digital Data
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34. 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.
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35. Coding Character: A symbol that has a common, constant meaning.
Characters in data communications, as in computer systems, are represented by groups of bits [1’s and 0’s].
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.
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36. 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)
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37. 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.
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38. Transmission Modes Serial Mode
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39. 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.
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40. Baseband Transmission
With unipolar signaling technique, the voltage is always positive or negative (like a dc current).
In bipolar signaling, the 1’s and 0’s 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.
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41. Baseband Transmission
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42. 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).
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43. Manchester Encoding “00001110” in a 10BASE-T circuit
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44. *Differential Manchester
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential encoding
Used in token-ring
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45. *Digital Encoding Illustration
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46. Analog Transmission of Digital Data
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47. 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)
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48. Amplitude Modulation and ASK
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49. Frequency Modulation and FSK
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50. Phase Modulation and PSK
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51. 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.
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52. Sending Multiple Bits Simultaneously
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53. Sending Multiple Bits Simultaneously
/2  01
 10 00
3/2  11
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54. 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.
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55. 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.
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56. 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.
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57. 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.
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58. 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).
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59. Modem Standards V.22 V.32 and V.32bis V.34 V.34+ V.90
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
V.90
download up o 56 kbps, upload 33.6 kbps
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60. 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 1.3:1 to 2:1 compression.
it uses Lempel-Ziv encoding and attains 3.5:1 to 4:1.
V.42bis compression can be added to almost any modem standard effectively tripling the data rate.
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61. Data Compression How fast if using V.42bis V.32 - 57.6kbps
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62. 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.
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63. Digital Transmission of Analog Data
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64. 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.
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65. 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.
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66. 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.
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67. PCM
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68. PCM
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69. PCM
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70. 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).
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71. Multiplexing
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72. 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.
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73. Multiplexed Circuit
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74. Multiplexing There are three major types of multiplexers
Frequency division multiplexers (FDM)
Time division multiplexers (TDM)
Statistical time division multiplexers (STDM)
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75. Frequency Division Multiplexing (FDM)
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76. 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.
Dense wavelength division multiplexing (DWDM), sometimes called WDM, is a version of FDM used in fiber optic cables. WDM permits up to 80 circuits, each capable of 2.5 Gbps.
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77. 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.
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78. Time Division Multiplexing (TDM)
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79. 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.
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80. 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.
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81. (STDM)
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82. Orthogonal Frequency Division Multiplexing (OFDM)
Distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
Uses:
Digital Audio Broadcasting (DAB) in the European market.
ADSL (asymmetric digital subscriber line) standard.
Wireless Local Area Networks. Such as, IEEE a, which outlines the use of OFDM in the 5.8-GHz band.
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83. 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.
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84. Inverse Multiplexing
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85. Examples of Modem
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86. 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.
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87. 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, 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.
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88. Downstream vs. Upstream
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89. Downstream vs. Upstream
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90. 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
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91. Cable Modem Scheme
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92. Customer Premises Cable Company Fiber Node
Cable Company Distribution Hub
TV Video
Network
Cable Modem
Cable Splitter
Downstream
Combiner
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
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Figure 9-8 Cable Modem Architecture

93. FDM Example: ADSL
Asynchronous Digital Subscriber Line (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
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94. 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
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95. Local Carrier End Office
Customer Premises
Local Carrier End Office
DSL Modem
Line Splitter
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
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Figure 9-5 DSL Architecture

96. *Some Hardware Standards
Digital Interfaces
RS-232C
USB
IEEE 1394
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97. Digital Interfaces The point at which one device connects to another
Standards define what signals are sent, and how
Some standards also define physical connector to be used
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98. Data Terminal Equipment and Data Circuit-terminating Equipment
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99. Generic Communications Interface Illustration
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100. RS-232C (EIA 232C) EIA’s “Recommended Standard” (RS)
Specifies mechanical, electrical, functional, and procedural aspects of the interface
Used for connections between DTEs and voice-grade modems, and many other applications
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101. *RS-232 DB-25 Connectors DB-25 Female DB-25 Male
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102. *RS-232 DB-25 Pinouts
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103. *RS-232 DB-9 Connectors Limited RS-232
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104. *EIA-232-D new version of RS-232-C adopted in 1987
improvements in grounding shield, test and loop-back signals
the prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace
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105. *RS-449 EIA standard improving on capabilities of RS-232-C
provides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps
covers functional/procedural portions of R-232-C
electrical/mechanical specs covered by RS-422 & RS-423
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106. *RS-422 DIN-8 Found on Macs DIN-8 Male DIN-8 Female
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107. Universal Serial Bus (USB)
The USB peripheral bus standard was developed by Compaq, IBM, DEC, Intel, Microsoft, NEC, and Northern Telecom and the technology is available without charge for all computer and device vendors. USB 1.1 supports a data speed of 12 Mbps and new USB 2.0 supports a data rate up to 480 Mbps.
Since October, 1996, the Windows operating systems have been equipped with USB drivers or special software designed to work with specific I/O device types.
A different plug-and-play standard, IEEE 1394, supports much higher data rates and devices such as video camcorders and digital video disk (DVD) players. However, USB and IEEE 1394 serve different device types.
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108. IEEE 1394
IEEE 1394 provides a single plug-and-socket connection on which up to 63 devices can be attached with data transfer speeds up to 400 Mbps. Two popular implementations of IEEE 1394 are Apple's FireWire and Sony's i.LINK.
IEEE 1394 implementations are expected to replace and consolidate today's serial and parallel interfaces, including Centronics parallel, RS-232C, and Small Computer System Interface (SCSI). The first products to be introduced with FireWire include digital cameras, digital video disks (DVDs), digital video tapes, digital camcorders, and music systems. Because IEEE 1394 is a peer-to-peer interface, one camcorder can dub to another without being plugged into a computer. With a computer equipped with the socket and bus capability, any device (for example, a video camera) can be plugged in while the computer is running.
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