Transcript ch03 - Computer Science Department

Chapter 3. Physical Layer
Business Data Communications and
Networking Fitzgerald and Dennis,
7th Edition
Copyright © 2002 John Wiley & Sons, Inc.
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Chapter 3. Learning Objectives
• Be familiar with different types of network
circuits and media
• Understand digital transmission of digital data
• Understand analog transmission of digital data
• Understand digital transmission of analog data
• Be familiar with analog/digital modems
• Be familiar with multiplexing
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Chapter 3. Outline
• Introduction
• Circuits
– Circuit Configuration, Data Flow, Communication Media, Media
Selection
• Digital Transmission of Digital Data
– Coding, Transmission Modes, Dig. Trans, Ethernet Data Transmission
• Analog Transmission of Digital Data
– Modulation, Voice Circuit Capacity, How Modems Transmit Data
• Digital Transmission of Analog Data
– Pulse Amplitude Modulation, How Telephones Transmit Voice Data,
How Instant Messenger Transmits Voice Data
• Analog/Digital Modems
• Multiplexing
– FDM, TDM, STDM, WDM, Inverse Multiplexing, DSL Transmission
• Summary
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Introduction
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The Physical Layer
• The physical layer includes network hardware and
circuits.
• Network circuits include physical media (e.g., cables)
and special purposes devices (e.g., routers and hubs).
Networks are made of both physical and logical
circuits.
– Physical circuits connect devices & include actual wires.
– Logical circuits refer to the transmission characteristics of
the circuit, such as a T-1 connection.
• Sometimes the physical and logical circuits are the
same, but they can be different. For example, in
multiplexing, one wire carries several logical circuits.
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Special Purpose Devices
Router
Hub
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Analog and Digital Data
• Another fundamental physical layer distinction is
between digital and analog forms of data.
• Sounds waves, which vary continuously over time are
analog data.
• Computers produce digital data that is in binary form,
that is, it is represented as a series of ones and zeros.
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Analog vs. Digital Transmission
• Transmissions can also be either analog or digital.
– Analog transmissions, like analog data, vary continuously.
Examples of analog data being sent using analog
transmissions are broadcast TV and radio.
– Digital transmissions are made of square waves with a
clear beginning and ending. Computer networks send digital
data using digital transmissions.
• Data can be converted between analog and digital
formats.
– When digital data is sent as an analog transmission modem
(modulator/demodulator) is used.
– When analog data is sent as a digital transmission, a codec
(coder/decoder) is used.
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Converters
Codec
Modem
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Data Type vs. Transmission Type
Analog
Transmission
Analog
Data
Digital Data
Digital
Transmission
Radio, Broadcast Pulse Code
TV
Modulation &
Video standards
using codecs
Modem-based
LAN Cable
Communications Standards
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Advantages of Digital Transmission
• Digital transmission:
– produces fewer errors than analog transmission. Because
the transmitted data is binary (1s and 0s), it is easier to
detect and correct errors.
– permits higher transmission rates. Optical fiber, for
example, is designed for digital transmission.
– is more efficient. It’s possible to send more data through a
given circuit using digital rather than analog transmission.
– is more secure since it is easier to encrypt.
• Integrating voice, video and data on the same circuit is
also far simpler with digital transmission since signals
made up of digital data are easier to combine.
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Circuits
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Circuit Configuration
• Two basic circuit configurations:
• Point-to-point connects just one sender and
receiver together (Figure 3-1)
• Multipoint (also called a shared circuit)
connects a number of senders and receivers
together (Figure 3-2)
– The advantage of multipoint is that it is cheaper
and simpler to wire.
– The disadvantage is that only one computer can
use the circuit at a time.
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Figure 3-1 Point-to-point configuration
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Figure 3-2 Multipoint configuration
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Data Flow (Figure 3-3)
• Data can move in one direction or both directions.
• Simplex data flows move in one direction only,
such as radio or cable television broadcasts.
• Half Duplex data flows both ways, but only one
direction at a time. As with CB radio, some kind
of control information must also be included so
that sender and receiver don’t send at the same
time.
• Full Duplex data flows in both directions at the
same time.
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Figure 3-3 Simplex, half-duplex and full
duplex transmission
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Communications Media
• Medium: the physical matter that carries the
transmission. Two basic categories of media:
• With Guided media the transmission flows along
a physical guide. The three main types of guided
media: twisted pair wiring, coaxial cable and
optical fiber cable.
• With Wireless media there is no wave guide and
the transmission just flows through the air (or
space). The main forms of wireless
communications are radio, infrared, microwave,
and satellite communications.
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Guided Media: Twisted Pair Wires
• Twisted pair wire cables are commonly used for
telephones and local area networks.
• Twisting two wires together reduces
electromagnetic interference.
• TP cables have a number of pairs of wires.
– Telephone lines have two pairs (4 wires, usually only
one pair is used by the telephone)
– LAN cables have 4 pairs (8 wires)
• Shielded twisted pair (STP) also exists, but is
more expensive.
• TP cables are also used in telephone trunk lines
and can have up to several thousand pairs.
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Guided Media: Coaxial Cable
• Formerly common on LANs, but now
disappearing (but still used on other comm.
equipment, e.g., CATV – cable television).
• More expensive than twisted pair, but coax is
shielded, so it’s less prone to interference than
twisted pair.
• Coaxial Cable Structure (Figure 3-5):
–
–
–
–
Inner conductor
Insulator
Wire mesh ground
Outer protective jacket or shell
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Figure 3-5 Coaxial Cable
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Guided Media: Fiber Optic Cable
• Widely used and has extremely high capacity.
• Light created by an LED (light-emitting diode) or
laser is sent down a thin glass or plastic fiber.
• Ideal for broadband. Most observers feel that fiber
will be used more and more extensively in the
future.
• Fiber optic cable structure (from center):
– Core (v. small, 5-50 microns, ~ the size of a
single hair)
– Cladding, which reflects the signal
– Protective outer jacket
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Optical Fiber
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Guided Media: Fiber Optic Cable
• Types of Optical Fiber:
– Multimode is cheap, but the signal spreads out
over short distances (up to ~500m). Multimode:
light can reflect inside the cable at many angles.
– Graded index multimode reduces the spreading
problem by changing the refractive properties of
the fiber to refocus the signal can be used over
distances of up to about 1000 meters.
– Single mode is expensive because difficult to
manufacture, but signal can be sent over many
kilometers without spreading.
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Figure 3-6 Optical Fiber
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Wireless Media
• Wireless media signals are becoming popular for
LAN use. The main forms are:
– Radio: wireless transmission of electrical waves.
Includes AM and FM radio bands. Microwave is also a
form of radio transmission.
– Infrared: “invisible” light waves whose frequency is
below that of red light. Requires line of sight and are
generally subject to interference from heavy rain. Used
in remote control units (e.g., TV).
– Microwave: high frequency form of radio with
extremely short wavelength (1 cm to 1 m). Often used
for long distance, terrestrial transmissions and cellular
telephones. Requires line-of-sight.
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Guided Media: Satellite Communications
• Satellite communications are a special form of
microwave communications.
• Instead of transmitting from one terrestrial
microwave dish to another, satellite
communications are sent from the ground to a
satellite, usually in geosynchronous orbit, about
23,000 miles above the earth. The satellite then
relays the signal to its destination ground station.
• Even with signals traveling at light speed, the
great distance between ground station and satellite
means a relatively long propagation delay occurs
between sending and receiving a signal.
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Figure 3-8 Satellites in operation
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Media Selection depends on many
factors including:
•
•
•
•
•
•
•
Type of network
Cost
Transmission distance
Security
Error rates
Transmission speeds
See Figure 3-9.
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Digital Transmission of Digital
Data
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Coding
• Any written language uses symbols, but computers
send signals in 1s and 0s (bits).
• Each written character needs a bit code in order to be
used by a computer. A set of these codes for a
language is called a coding scheme.
• The two main character codes in use in North
America are ASCII and EBCDIC.
– ASCII: American Standard Code for Information
Interchange, originally used a 7-bit code (128
combinations), but an 8-bit version is now in use.
– EBCDIC: Extended Binary Coded Decimal Interchange
Code, an 8-bit code developed by IBM.
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Transmission Modes
• Data can be sent either in serial or in parallel
• Parallel mode (Figure 3-10): uses several wires,
each wire sending one bit at the same time as the
others.
– A parallel printer cable sends 8 bits together.
– Your computer’s processor and motherboard also use
parallel busses to move data around.
• Serial Mode (Figure 3-11): sends bit by bit over a
single line. Serial mode is slower than parallel, but
can be used over longer distances because the bits
stay in the order they were sent, while bits sent in
parallel mode tend to spread out over long
distances.
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Figure 3-10 Parallel transmission
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Figure 3-11 Serial transmission
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Digital Transmission (Figure 3-12)
• Digital signals are sent as a series of “square waves”
of either positive or negative voltage. Voltages vary
between +3/-3 and +24/-24 depending on the circuit.
• Each digital transmission standard defines what
voltage levels correspond to a bit value of 0 or 1.
• Unipolar signal voltages either vary between 0 and a
positive value or between 0 and some negative value.
• Figure 3-12 shows a unipolar signal for which 5 volts
mean a binary 1, and 0 volts mean a binary 0.
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Digital Transmission (cont.)
• With bipolar signals, signals are sent using both
positive and negative voltages.
• A second digital transmission factor, called return
to zero (RZ) means the signal returns to the 0
voltage level after sending a bit. In non return to
zero (NRZ), the signals maintains its voltage at
the end of a bit.
• Ethernet uses Manchester encoding in which the
bit value is defined by a mid-bit transition. A
high to low voltage transition is a binary 0 and a
low-high mid-bit transition defines a binary 1.
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Figure 3-12 Digital transmission types
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Analog Transmission of Digital
Data
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The Telephone Network
• Originally designed for analog
communications only.
• Today, standard analog telephone service is
called POTS (Plain Old Telephone Service).
• Modem communications use the telephone
network to send digital data that has been
converted into an analog format.
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Carrier Waves
• Modems use carrier waves to send information
(Figure 3-13).
• Each wave has three fundamental characteristics:
– Amplitude, meaning the height (intensity) of the wave
– Frequency, which is the number of waves that pass in a
single second and is measured in Hertz (cycles/second)
(wavelength, the length of the wave from crest to crest,
is related to frequency.).
– Phase is a third characteristic that describes the point in
the wave’s cycle at which a wave begins and is
measured in degrees. (From example, changing a
wave’s cycle from crest to trough corresponds to a 180
degree phase shift).
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Figure 3-13 A Carrier Wave
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Modulation
• Modulation is the modification of a carrier
wave’s fundamental characteristics in order to
encode information.
• There are three basic ways to modulate a
carrier wave:
– Amplitude Modulation
– Frequency Modulation
– Phase Modulation
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Amplitude Modulation
• Amplitude Modulation (AM), also called
Amplitude Shift Keying (ASK), means changing
the height of the wave to encode data.
• The AM dial on the radio uses amplitude
modulation to encode analog information.
• Figure 3-14 shows a simple case of amplitude
modulation in which one bit is encoded for each
carrier wave change.
– A high amplitude means a bit value of 1
– Zero amplitude means a bit value of 0
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Figure 3-14 Amplitude Modulation
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Frequency Modulation
• Frequency Modulation (FM), also called
Frequency Shift Keying (FSK), means changing
the frequency of the carrier wave to encode data.
• The FM dial on the radio uses frequency
modulation to encode analog information.
• Figure 3-15 shows a simple case of frequency
modulation in which one bit is encoded for each
carrier wave change.
– Changing the carrier wave to a higher frequency
encodes a bit value of 1
– No change in the carrier wave frequency means a bit
value of 0
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Figure 3-15 Frequency Modulation
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Phase Modulation
• Phase refers to the point in each wave cycle at
which the wave begins.
• Phase Modulation (PM), also called Phase Shift
Keying (PSK) means changing the phase of the
carrier wave to encode data.
• Figure 3-16 shows a simple case of phase
modulation in which one bit is encoded for each
carrier wave change.
– Changing the carrier wave’s phase by 180o corresponds
to a bit value of 1
– No change in the carrier wave’s phase means a bit value
of 0
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Figure 3-16 Phase Modulation
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Sending Multiple Bits Simultaneously
• Each modification of the carrier wave to encode
information is called a symbol.
• By using a more complicated information coding system, it
is possible to encode more than 1 bit/symbol.
• Figure 3-17 gives an example of amplitude modulation
using 4 amplitude levels, corresponding to 2 bits/symbol.
• Increasing the possible number of symbols from 4 to 8
corresponds with encoding 3 bits/symbol, 16 levels to 4
bits, and so on.
• Likewise, multiple bits per symbol might be encoded using
phase modulation, say using phase shifts of 0o, 90o, 180o,
and 270o.
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Fig. 3-17 Two-bit Amplitude Modulation
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Quadrature Amplitude Modulation (QAM)
• QAM is a family of encoding schemes that are
widely used for encoding multiple bits per symbol
that combine Amplitude and Phase Modulation.
• 16QAM is a common form of that uses 8 different
phase shifts and 2 different amplitude levels. Since
there are 16 possible symbols, each symbol
encodes 4 bits.
• QAM and related techniques are commonly used
for voice modems with a data rate of up to about
28 kilobits/second.
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Bit Rate vs. Baud Rate (Symbol Rate)
• Bit rate (or data rate) is the number of bits
transmitted per second.
• Baud rate (same as symbol rate) refers to the
number of symbols transmitted per second.
• As we have just seen, since multiple bits can be
encoded per symbol, the two terms are not the same.
• For example, in Figure 3-17, the bit rate is twice the
baud rate. The general formula is:
Data Rate (bits/second)=
Symbol Rate (symbols/sec.) x No. of bits/symbol
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Capacity of a Voice Circuit
• The capacity of the telephone network is
constrained by the limitations of the telephone
lines and equipment used to transmit voice.
• Human hearing has a freq. range of 20 Hz to 1420 kHz, but the voice circuit range is 0-4000 Hz.
• QAM using 6 bits per symbol and the maximum
voice channel carrier wave frequency, corresponds
to a data rate of 6 * 4000 = 24 kbps.
• Phone lines have a far higher transmission
capacity: 1 MHz for lines up to 2 miles (3 km)
from a telephone exchange and 300 kHz for lines
2-3 miles (3 km) away.
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How Modems Transmit Data
• Modem means modulator/demodulator. It is the device that
encodes and decodes data by manipulating the carrier wave.
• The V-series of modem standards are those approved by the
ITU-T standards group.
– V.22, an early standard, had a 2400 bps bit rate
– V.34, one of the robust V standards, includes multiple data
rates (up to 28.8 kbps) and a handshaking sequence that
tests the circuit and determines the optimum data rate.
V.34+ increases the max. to 33.6 kbps
• Modems also use data compression which looks for more
efficient ways to encode redundant data strings.
• For example, Lempel-Ziv encoding, compresses data by
creating a dictionary of character patterns and then
transmitting compressed versions of those patterns.
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Digital Transmission of Analog
Data
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Pulse Amplitude Modulation (PAM)
• An analog voice signal can be converted
into digital form using a device called a
codec (coder/decoder) which also converts
it back to analog data at the receiving end.
• The codecs used by the phone system use
Pulse Amplitude Modulation. PAM
involves 3 steps (see Figures 3-19a-c):
– Measuring the signal
– Taking samples of the signal
– Encoding the signal as a binary data sample
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Figure 3-19a Pulse Amplitude Modulation
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Figure 3-19b Pulse Amplitude Modulation
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Figure 3-19c Pulse Amplitude Modulation
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How Telephones Transmit Voice
• The line from your phone to the first phone switch
(called the local loop), still uses the analog
techniques developed over a century ago by Bell.
• Today’s switches are now almost completely
digital and convert these analog signals to digital
data using a technique called Pulse Code
Modulation (PCM).
• PCM (similar to PAM) specifies a sample rate of
8000 samples/second and 8 bits/sample. The basic
digital communications unit used by the phone
network, the DS-0, has a data rate of 64 kbps,
corresponding to 1 digital voice signal.
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How Instant Messenger Transmits Voice
• Instead of PCM, Instant Messaging uses an
alternative technique called ADPCM, adaptive
differential pulse code modulation.
• ADPCM encodes the differences between
samples. Instead of 8 bits/sample, ADPCM uses
only 4 bits/sample, generally at 8000
samples/second. This allows a voice signal to be
sent at 32 kbps, which makes it possible for IM to
send voice signals as digital signals using POTSbased analog phone lines.
• ADPCM can also use lower sampling rates, at 8 or
16 kbps, but these produce lower quality voice
signals.
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Analog/Digital Modems
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56k Modems
• 56k modems, the fastest possible on voice grade
lines, are based on the V.90 and V.92 standards.
• Downstream transmissions (from phone switch to
the user’s computer) use a technique based on
recognizing PCM’s 8-bit digital symbols.
• With the V.90 standard, upstream transmissions
are still based on the V.34+ standard. The V.92
standard uses this PCM symbol recognition
technique for both up and downstream channels.
• The technique is very sensitive to noise and both
V.90 and V.92 modems often must use lower data
rates. The max. V.92 upstream rate is 48 kbps.
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Multiplexing
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Multiplexing
• Multiplexing means breaking up a higher speed
circuit into several slower circuits.
• The main advantage of multiplexing is cost;
multiplexing is cheaper because fewer network
circuits are needed.
• There are four categories of multiplexing:
–
–
–
–
Frequency division multiplexing (FDM)
Time division multiplexing (TDM)
Statistical time division multiplexing (STDM)
Wavelength division multiplexing (WDM)
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Frequency Division Multiplexing (FDM)
• FDM works by making a number of smaller channels from
a larger frequency band (Figure 3-21). FDM is sometimes
referred to as dividing the circuit “horizontally”.
• In order to prevent interference between channels, unused
frequency bands called guardbands are used to separate the
channels. Because of the guardbands, there is some wasted
capacity on an FDM circuit.
• CATV uses FDM. FDM was also commonly used to
multiplex telephone signals before digital transmission
became common and is still used on some older
transmission lines.
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Figure 3-21 Frequency Division Multiplexing
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Time Division Multiplexing (TDM)
• TDM allows multiple channels to be used by allowing the
channels to send data by taking turns. TDM is sometimes
referred to as dividing the circuit “vertically”
• Figure 3-22 shows an example of 4 terminals sharing a
circuit, with each terminal sending one character at a time.
• With TDM, time on the circuit is shared equally with each
channel getting a specified time slot, whether or not it has
any data to send.
• TDM is more efficient than FDM, since TDM doesn’t use
guardbands, so the entire capacity can be divided up
between the data channels.
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Figure 3-22 Time Division Multiplexing
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Statistical Time Division Multiplexing
(STDM)
• STDM is designed to make use of the idle time created when
terminals are not using the multiplexed circuit.
• Like regular TDM, STDM uses time slots, but the time slots
are not fixed. Instead, they are used as needed by the different
terminals on the multiplexed circuit.
• Since the source of a data sample is not identified by the time
slot it occupies, additional addressing information must be
added to each sample.
• If all terminals try to use the multiplexed circuit intensively,
response time delays can occur. The multiplexer also needs to
contain memory to store data in case more data samples come
in than its outgoing circuit capacity can handle.
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Wavelength Division Multiplexing (WDM)
• Optical fiber uses lasers or LEDs which previously
transmitted at only a single frequency, with a typical
transmission rate being around 622 Mbps.
• With WDM, data is transmitted at several different
frequencies over the same fiber.
• The data transmission capacity of optical continues to
increase dramatically. A new version of WDM, Dense
WDM or DWDM promises data rates in the terabits, with
over a hundred channels per fiber, each transmitting at a
rate of 10 Gbps, making aggregate data rates in the low
terabit range possible.
• Future versions of DWDM will make petabit aggregate
transmission rates possible as per channel data rates and
the total number of channels both continue to rise.
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Inverse Multiplexing (Figure 3-24)
• Instead of using a single line, an inverse
multiplexer (IMUX) shares the load by sending
multiplexed data over two or more lines.
• For example, two T-1 lines can be used to send
data, creating a combined multiplexed capacity of
2 x 1.544 = 3.088 Mbps.
• A recent IMUX standard, the Bandwidth ON
Demand Network Interoperability Group
(BONDING) standard, is often used for
videoconferencing applications. With the
BONDING standard, six 64 kbps lines can be
combined to create an aggregate line of 384 kbps
for transmitting video.
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Figure 3-24. Inverse Multiplexing
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How DSL Transmits Data
• Digital Subscriber Line is becoming popular as a way to
increase data rates in the local loop.
• Instead of using the 0-4000 KHz voice channel, DSL uses the
physical capacity of the copper phone lines of up to 1 MHz.
• The 1 MHz capacity is split into: 1) a 4 KHz voice channel,
2) an upstream channel and 3) a downstream channel.
• There are several versions of DSL, with the main differences
being how much of the bandwidth is allocated between the
upstream and downstream channels.
• One form of DSL, G.Lite provides a 4 Khz voice channel,
384 kbps upstream and 1.5 Mbps downstream (provided line
conditions are optimal).
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Next Day Air Service Case Study
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Figure 3-25 NDAS Case Study
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End of Chapter 3
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