The Physical Layer
Document Sample
Chapter 2
The Physical Layer
1
The Theoretical Basis for Data
Communication
• Fourier Analysis
– Any reasonably behaved periodic function can be written as
Fourier series.
• Bandwidth-Limited Signals
– How fast a signal can be transmitted depends on the
bandwidth, general meaning of how much information can
be carried in a given time period (usually a second) over a
communication link, measured mostly by frequency range.
• Maximum Data Rate of a Channel
2
Theory of Data Communications
• The signal (for example, measured in volts) can be
viewed as
a) a function of time, g(t), or
b) a function of frequency, G(f).
• Time-Domain
– Let g(t) denote the voltage on a wire at time t.
– A signal, g(t), is periodic with period T if g(t+T)=g(t) for all
t.
– A signal is discrete if it only takes on a finite number of
values.
– The fundamental frequency is the inverse of the period, f =
1/T, and is measured in cycles per second (Hz).
3
Frequency-Domain Analysis
• Any "reasonably-behaved" periodic function, g(t), can
be written as a Fourier Series - that is broken up into
components with different frequencies.
• The time, T, required to transmit a character depends
on:
a) the encoding method
b) the signalling speed or baud rate; that is, how many times
per second the signal changes its value (voltage).
• Baud rate is not necessarily the same as bit rate. For
example, if the values 0, 1, 2, 3, 4, 5, 6, 7 are used in a
signal, then each signal value can represent 3 bits.
That is 1 baud = 3 bps.
4
Bandwidth-Limited Signals
• A binary signal (‘b’ = 01100010) and its root-mean-square
Fourier amplitudes.
5
(b) – (c) Successive approximations to the original signal.
Bandwidth-Limited Signals
(d) – (e) Successive approximations to the original signal.
6
Frequency-Domain Analysis
• Below we will only consider 2 voltage levels, so the
bit rate is the same as the baud rate.
– Let b = bit rate (measured in bits per second (bps)).
– Then, it takes 8/b seconds to send 8 bits (one character).
– So, T = 8/b, and the fundamental frequency is b/8 Hz.
• A voice grade line is an ordinary telephone line and
has an artificial cutoff frequency, fc, of about 3000Hz.
So, the number of the highest harmonic that can be
passed through is 3000/(b/8) = 24000/b. Note the
highest harmonic has a frequency that is a multiple of
the fundamental frequency (b/8) and the highest
harmonic can have a frequency no more than 3000Hz. 7
Bandwidth-Limited Signals
Relation between data rate and harmonics.
8
Maximum Data Rate of a Channel
• Noiseless channel: Nyquist’s Theorem – If the signal
has V discrete levels over a transmission medium of
bandwidth H , the maximum data rate = 2H log2 V
bits/sec
– Example: a noiseless 3-kHz channel cannot transmit binary
signals at a rate exceeding 6000 bps (= 2 x 3000 log2 2).
• Noisy Channel: Shannon’s Theorem
maximum data rate = H log2 (1 + S/N) bits/sec H:
bandwdith, S: signal power, N: noise power
– S/N (Signal-to-noise ratio), usually measured as 10 log10S/N
in db = decibels, is called thermal noise ratio.
9
Physical Interfaces
• Physical layer is responsible for the generation,
transmission, and receipt of binary data
• Generation and Receipt
– Conversion of data between binary and analog
– E.g. wire: voltage is applied
• +V means a 1
• -V means a 0
• 0V means no data
10
Physical Interfaces
+V
t
-V
0 1 1 0 1 0 0 1
11
Physical Interfaces
• Errors in physical layer:
– Attenuation (reduced signal)
– Distortion (wrong signal)
• Influences to error:
– Type of Media
– Bit Rate
– Distance
• Finally, binary values are passed to Data Link
12
Guided Transmission Data
• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics
13
Guided Transmission Data
• Magnetic Media: magnetic tape or removable
media
• Consider an industry standard Ultrium tape
– It can hold 200 gigabytes.
– A box 60x60x60 cm can hold about 1000 of these
tapes. Total capacity is 200 terabytes or 1600
terabits (1.6 petabits).
– The box can be sent to anywhere in US in 24 hours.
The effect bandwidth is 1600 terabits/86,400 sec, or
19 Gbps.
– If it is sent within an hour drive, the bandwidth is
increased to over 400 Gbps. No computer network
can even approach this.
14
Twisted Pair
• Properties
– A twisted pair consists of two insulated copper wires.
– Why twisted? Countervail the magnetic field
– Used in telephone and local area networking
– Run several kilometers
– The bandwidth depends on the thickness of the wire and
distance travelled.
• Common types: UTP (Unshielded Twisted Pair)
– Category 3: bandwidth of 16 MHz
– Category 5: more twists per centimeter, which results in less
crosstalk and better-quality signal over longer distance,
bandwidth of 100 MHz
– Category 6 and 7: 250 MHz and 600 MHz
15
Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.
16
Coaxial Cable
• 50-ohm cable for digital transmission
• 75-ohm cable for analog transmission and cable
television
• 1 GHz
• Local area networking and CATV
17
Coaxial Cable
A coaxial cable.
18
Fiber Optics
• Glass is used instead of copper wires
• Light is transmitted instead of electrical current
• Components:
– Light source
– Transmission medium
– Detector: convert light plus and electronic signal
• Single-mode fiber
– Different rays bouncing around at different angle are said to
be a multimode fiber.
– If the fiber’s diameter is reduced to a few wavelength of
light, the light can propagate in a straight line without
bouncing, yielding a single-mode fiber.
– 50 Gbps for 100 km 19
Fiber Optics
(a) Three examples of a light ray from inside a silica fiber impinging
on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
20
Transmission of Light through Fiber
• Three bands are used: 0.85, 1.3, 1.55 μm
Attenuation of light through fiber in the infrared region. 21
Fiber Optics
• Ways to connect fibers:
– Terminate in connectors and plugged into fiber sockets: 10 ~
20% light lose
– Spliced mechanically: 10% light lose
– fused
• Comparison of fiber optics and copper wire
– Advantages:
• Higher bit-rates, immune to interference, hard to tap
– Disadvantages:
• Less familiar technology, unidirectional, easily damaged,
expensive interfaces
22
Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers. 23
Fiber Cables
• Light sources: LED (Light Emitting Diodes) and
semiconductor lasers.
• The receiving end consists of a photodiode.
A comparison of semiconductor diodes and LEDs as light sources.
24
Fiber Optic Networks
A fiber optic ring with active repeaters.
25
Fiber Optic Networks
A passive star connection in a fiber optics network.
26
Wireless Transmission
• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission
27
Wireless Transmission
• λf = c where λ is the wavelength, f is the frequency,
and c is the speed of light, 3 x 108 m/s
• Two basic modulation techniques used in spread
spectrum signal transmission:
– Frequency hopping: The transmitter hops from frequency to
frequency.
– Direct sequence: The signal is spread over a wide frequency
band with specific coding for each channel.
• The stream of information to be transmitted is divided
into small pieces, each of which is allocated across to a
frequency channel across the spectrum.
• A data signal at the point of transmission is combined
with a higher data-rate bit sequence (also known as a
chipping code). 28
Electromagnetic spectrum
• LF (Low Frequency, 105 Hz): maritime
• MF (Medium Frequency, 106 Hz): AM radio
• HF (High Frequency, 107 Hz): radio
• VHF (Very High Frequency, 108 Hz): FM radio, TV
• UHF (Ultra High Frequency: 109 Hz): TV, terrestrial
microwave
• SHF (Super High Frequency:1010 Hz): Satellite, microwave
• EHF (Extremely High Frequency, 1011 Hz)
• THF (Tremendously High Frequency, 1012 Hz)
• Higher frequency: IHF?, AHF?, PHF? (Incredibly,
Astonishingly, Prodigiously).
29
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
30
Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
31
Politics of the Electromagnetic Spectrum
• Allocate spectrum policies
– Beauty contest requires each carrier to explain why its
proposal serves the public interest best.
– Lottery
– Auction
• Open band: Frequencies are not allocated but
restrained in a short range.
The ISM bands in the United States. 32
Lightwave Transmission
Convection currents can interfere with laser communication systems.
A bidirectional system with two lasers is pictured here. 33
Communication Satellites
• Geostationary Satellites (GEO)
• Medium-Earth Orbit Satellites (MEO)
• Low-Earth Orbit Satellites (LEO)
• Satellites versus Fiber
34
Communication Satellites
• Geostationary Satellites (GEO)
– VSAT (Very Small Aperture Terminals): 1-meter
antennas, DirecPC
• Low-Earth Orbit Satellites (LEO)
– Iridium: 66 satellites
– Globalstar: 48 satellites
– Teledesic: 30 satellites
35
Communication Satellites
Communication satellites and some of their properties,
including altitude above the earth, round-trip delay time
and number of satellites needed for global coverage. 36
Communication Satellites
The principal satellite bands.
37
Communication Satellites
VSATs using a hub.
38
Low-Earth Orbit Satellites
Iridium
(a) The Iridium satellites from six necklaces around the earth.
(b) 1628 moving cells cover the earth. 39
Globalstar
(a) Relaying in space: Iridium
(b) Relaying on the ground: Globalstar 40
Satellites versus Fiber
• A single fiber has more bandwidth but is not available
to most users.
• Satellites are possible for mobile communication.
• Satellites are cheaper for Broadcasting.
• Satellites can be deployed in places with hostile terrain
or a poorly developed terrestrial infrastructure such as
Indonesia.
• Satellites can be deployed in areas where obtaining the
right for laying fiber is difficult.
• Satellites is possible for rapid military communication
deployment.
41
Public Switched Telephone System
• Structure of the Telephone System
• The Politics of Telephones (FYI)
• The Local Loop: Modems, ADSL and Wireless
• Trunks and Multiplexing
• Switching
42
Structure of the Telephone System
• The PSTN (Public Switched Telephone Network) is the world's
collection of interconnected voice-oriented public telephone
networks. It's also referred to as the POTS (Plain Old Telephone
Service).
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy. 43
Structure of the Telephone System
A typical circuit route for a medium-distance call.
44
Major Components of the Telephone
System
• Local loops
– Analog twisted pairs going to houses and businesses
• Trunks
– Digital fiber optics connecting the switching offices
• Switching offices
– Where calls are moved from one trunk to another
45
The Politics of Telephones
• LATA (Local Access and Transport Area) is a
geographic area covered by one or more local
telephone companies, which are legally referred to as
local exchange carriers (LECs).
• LEC (Local Exchange Carrier) is a public telephone
company in the U.S. that provides local service. Some
of the largest LECs are the Bell operating companies
(BOCs).
• IXC (IntereXchange Carrier) is a company handling
inter-LATA traffic such as AT&T, MCI, and Sprint.
• A POP (Point of Presence) is a switching office built
to handle calls from a LATA. 46
The Politics of Telephones
The relationship of LATAs, LECs, and IXCs. All the
circles are LEC switching offices. Each hexagon
belongs to the IXC whose number is on it. 47
The Local Loop: Modems, ADSL, and
Wireless
• Transmission lines suffer from three major
problems:
– Attenuation
– Delay distortion
– Noise
• The square waves used in digital signals have a
wide frequency spectrum (usually, high
frequency) and thus are subject to strong
attenuation and delay distortion.
48
Modems
The use of both analog and digital transmissions for a computer to
computer call. Conversion is done by the modems and codecs.49
Modems
• The modulation is introduced to solve this
problem.
– Amplitude: two different amplitudes are used to
represent 0 and 1.
– Frequency: different tones are used.
– Phase: the wave is systematically shifted (45, 135,
225, or 315º).
• A modem (modulator-demodulator) is a device
that modulates outgoing digital signals to
analog signals. 50
Modems
(a) A binary signal (c) Frequency modulation
(b) Amplitude modulation (d) Phase modulation 51
Modems
• The number of samples/symbols per second is
measured in baud.
• In quadrature phase-shift keying (QPSK), the
four angles, usually out of phase by 90°, are
used to transmit 2 bits/symbol. The bit rate is
twice the baud rate.
• QAM-64 (Quadratrue Amplitude Modulation-
64) allows 64 different combinations, so 6 bits
can be transmitted per symbol.
52
Modems
Constellation Diagrams:
(a) QPSK.
(b) QAM-16.
(c) QAM-64. 53
Modems
• To reduce the chance of an error, standards for higher
speeds modems do error correction by adding extra bits
to each sample. The schemes are known as TCM
(Trellis Coded Modulation).
• In V.32, 14,400 bps is achieved by transmitting 6 data
bits and 1 parity bit per sample at 2400 baud. It uses
QAM-128.
• In V.34, the modem can run at 28,800 bps at 2400 baud
with 12 data bits/symbol or 33,600 bps at 2400 baud
with 14 data bits/symbol.
54
Modems
(a) (b)
(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.
55
Modems
• Why are 56 kbps modems in use?
– The telephone channel is about 4000 Hz (300 ~
3400 Hz).
– The maximum data rate = 2 x 4000 log2 2 = 8000
sample/sec
– The number of bits per sample is 8, one for control
purpose, allowing 8000 x 7 = 56,000 bit/sec.
• V.90 provides 33.6 kbps upstream and 56 kbps
downstream.
• V.92 provides 48 kbps upstream.
56
Modems
• A connection that allows traffic in both
directions simultaneously is called full duplex.
• A connection that allows traffic either way, but
only one way at a time is called half duplex.
• A connection that allows traffic only one way is
called simplex.
57
Bandwidth, Baud Rate, Bit Rate
• The bandwidth of a medium is the range of
frequencies that pass through it with minimum
attenuation, usually, measured in Hz.
• The baud rate is the number of samples/sec
made. Each sample sends one symbol.
• The bit rate is the amount of information sent
over the channel and is equal to the number of
symbols/sec times the number of bits/symbol.
58
Digital Subscriber Lines (DSL)
• xDSL is made to work by connecting to a different
switch instead of the filter that attenuates all
frequencies below 300 Hz and above 3400 Hz.
• The xDSL services have been designed with the
following goals:
– They must work over the existing category 3 twisted pair
local loops.
– They must not affect existing telephones and fax machines.
– They must be faster than 56 kbps.
– They must be always on.
59
Digital Subscriber Lines
Bandwidth versus distanced over category 3 UTP for DSL.
60
Digital Subscriber Lines (DSL)
• DMT (Discrete MultiTone) divides the 1.1 MHz
spectrum available on the local loop into 256
independent channels of 4312.5 Hz each.
– Channel 0: POTS
– Channel 1-5: not used
– One for upstream and one for downstream control
– 32 channels for upstream and rest for downstream
• The ADSL standard (ANSI T1.413 and ITU G.992.1)
allows speeds of 8 Mbps downstream and 1 Mbps
upstream.
61
Digital Subscriber Lines
Operation of ADSL using discrete multitone modulation.
62
Digital Subscriber Lines
A typical ADSL equipment configuration.
63
Wireless Local Loops
• Business practice of a long-distance telephone
company for the local phone service:
– It must buy or lease a building for the end office.
– It must fill the end office with switches.
– It must run a fiber between the end office and the toll office.
– It must acquire customer.
• How is the new local phone company to connect
customer telephones and computers in the end office?
– Buy the right to lay the new wires. Costly
– Buy/lease from other local phone company. Costly
– Use the WWL (Wireless Local Loop).
64
Wireless Local Loops
• A fixed telephone using a wireless local loop is
different from a mobile phone in three ways:
– The wireless local loop customer often wants high-speed
Internet connectivity.
– A directional antenna is needs to be installed.
– The user does not move.
• LMDS (Local Multipoint Distribution System) is a
system for broadband microwave wireless transmission
direct from a local antenna to homes and businesses
within a line-of-sight radius, a solution to the so-called
last-mile technology problem of economically bringing
high-bandwidth services to users.
• The IEEE 802.16 can be used for wireless local loops
standard. 65
Wireless Local Loops
Architecture of an LMDS system.
66
Trunks and Multiplexing
• Two categories of multiplexing schemes are used to
multiplex many conversations over a single physical
trunk:
– In FDM (Frequency Division multiplexing), the frequency
spectrum is divided into frequency bands. For fiber optic
channels, WDM (Wavelength Division Multiplexing) is
used.
– In TDM (Time Division Multiplexing), the entire
bandwidth is used for a chunk of time period.
67
Frequency Division Multiplexing
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel. 68
Wavelength Division Multiplexing
Wavelength division multiplexing.
69
Time Division Multiplexing
• The analog signals are digitalized by a device called a codec
(coder-decoder) producing a 7 or 8 bit number.
• PCM (Pulse Code Modulation) is a technique to digitalize
analog data.
– T1 carriers can handle 24 channels multiplexed together. 24 x 8 = 192
bits + 1 bit for framing = 193 bits/frame
– Since each analog signal must be sampled 8000 times per second, we
must repeat this process every 1/8000 sec = 125 microseconds.
– So, the transfer rate on the T1 carrier is: 192 bits / 0.000125 seconds =
1.544 Mbps.
• DPCM (Differential Plus Code Modulation) is a method,
which consists of outputting the difference between the current
value and the previous one, to reduce the number of digitalized
bits, 70
Time Division Multiplexing
The T1 carrier (1.544 Mbps).
71
Time Division Multiplexing
Delta modulation.
72
Time Division Multiplexing
Multiplexing T1 streams into higher carriers.
73
SONET/SDH
• SONET (Synchronous Optical NETwork) is the
American National Standards Institute standard for
synchronous data transmission on optical media.
• SDH (Synchronous digital hierarchy) is the
international standard for synchronous data
transmission on optical media.
• The goal of SONET:
– Possible for different carriers
– Unify the U.S., European, and Japanese digital systems
– Provide a way to multiplex multiple digital channels
– Provide support for operations, administration, and
maintenance (OAM)
74
SONET/SDH
• Synchronous Optical Network (SONET)
– The full specification is larger than this book.
– It addresses both the framing and encoding
problems.
– It multiplexes several low-speed links onto one
high-speed link.
75
SONET/SDH
• SONET Frame Structure: (Synchronous Transport
Signal-1)
– 9 x 90 = 810 bytes
– The first three columns are reserved for system management
information.
– The first 9 rows contain the overhead. Overhead has multiple
functions: across different links, specify voice channel,
concatenation frames.
– The remaining 87 columns hold the user data, called the SPE
(Synchronous Payload Envelope). The first column is the
overhead for the sublayer.
– STS-N frame can be thought of as consisting of N STS-1
frames. 76
Time Division Multiplexing
Two back-to-back SONET frames.
77
Time Division Multiplexing
SONET and SDH multiplex rates.
• STS (Synchronous Transport Signal)
• OC (Optical Carrier): OC-256 – 13.271 Gbps, OC-768 – 40 Gbps
• Synchronous Transport Modules (STM) 78
Switching
• Circuit switching – seek out a physical path from
sender to receiver. An end-to-end path must be
(conceptually) established before data is sent.
• Message switching – no path is established in advance.
The message is stored in the first switching office and
forwarded later one hop at a time.
– Example: store-and-forward network
– Problem: No restriction of block size
• Packet switching – place a restriction on block size, to
allow packets to be buffered in main memory at the
switching office.
– Advantages:
Well-suited for interactive traffic
• Improved response time and throughput 79
Circuit Switching
(a) Circuit switching.
(b) Packet switching. 80
Message Switching
81
(a) Circuit switching (b) Message switching (c) Packet switching
Packet Switching
A comparison of circuit switched and packet-switched networks.
82
The Mobile Telephone System
• First-Generation Mobile Phones:
Analog Voice
• Second-Generation Mobile Phones:
Digital Voice
• Third-Generation Mobile Phones:
Digital Voice and Data
83
Politics and Issues of Mobile Telephones
• At first, the U.S. had a single mobile phone system. In Europe
every country devises its own system.
• Then Europe learned from mistake and standardized on a single
system (GSM). By then, the U.S. deregulated the standard. As a
consequence, the U.S. has two major and one minor
incompatible system.
• Mobile phone ownership and usage in Europe is far greater than
in the U.S.
– A single system for all of Europe
– In the U.S. the telephone companies charge the mobile phone owners for
incoming call to keep callers from getting nervous about using the
telephone.
• The widespread use of prepaid mobile phones in Europe (up to
75% in some areas) and Asia.
• http://www.gsmworld.com/news/statistics/index.shtml
84
The Mobile Telephone System
• Every cellular system digital or analog is
comprised of four parts.
1. Cells and cell sites (base stations)
2. Switching station ( mobile telephone switching
office, MTSO )
3. System operator and its local office
4. Cellular telephones
85
The Mobile Telephone System
• The heart of the system is made up of individual radio
coverage areas called cells. Each cell is a self-contained
calling area.
• Within the cell, a cell site is strategically positioned as
a base station for receiving, sending and routing the
radio signals of cellular phone calls.
• All cell sites are connected to the Mobile Telephone
Switching Office (MTSO).
– It provides connection into the Public Switched Telephone
network ( PSTN ) - the local telephone company.
– It provides other central functions, including call processing,
traffic management, and transferring calls as a phone moves
between cell sites.
86
The Mobile Telephone System
87
The Mobile Telephone System
• Making a call
– When a cellular user makes a call from a cellular
phone, radio signals are transmitted to the cell site.
– The cell site alerts the Mobile Telephone Switching
Office (MTSO) switching station. The MTSO, in
turn, provides an open channel ( frequency ) and
connects the call to the Public Switched Telephone
Network ( PSTN ).
– The PSTN put the call through to the number to be
reached. This process takes the same amount of time
that it takes to make a call from a land line phone.
88
The Mobile Telephone System
• Receiving a call
– Once the MTSO receives a call, it searches for the correct
cellular phone by sending out data over the radio waves.
– Cellular phones in standby mode continuously scan the radio
waves being transmitted by the MSTO. If a phone hears its
telephone number, it sends back a signal that informs the
closest cell site of its Electronic Serial Number (ESN) and its
telephone number (Mobile Identification Number or MIN).
– The cell site passes this information to the MTSO, where the
ESN and MIN are verified and a channel (frequency) is
assigned for the call.
– The cellular phone receives the message directing it to tune to
the correct voice channel. The cell site makes the voice
channel available, and the call is completed.
89
The Mobile Telephone System
• Hand-off is the transfer of a call from one cell site to
another as the cellular phone moves through the service
coverage area.
– The cell site warns the MSTO that the mobile's signal
strength is falling below a predetermined level.
– The MTSO then alerts all cell sites bordering on the first one.
They measure the mobile's transmitting signal and report
back to the MTSO.
– The MTSO, which is programmed to select the site receiving
the strongest signal, then switches the call from the weak cell
to the strongest cell without interrupting the call.
90
The Mobile Telephone System
• Roaming is a service offered by most cellular service
providers that allows subscribers to use cellular service
while traveling outside their home service area.
– When they are outside their home service area and come
within range of another cellular system, the ROAM indicator
on the cellular phone will light to show that they are in range.
– When they roam (operate outside their home system), their
cellular phone will seek service from the same type of
cellular system as the one they subscribe to at home. But if
that type is not available where they are roaming, the phone
will try to obtain service from the non-home-type system. A
blinking light indicates a non-home-type system. There is an
extra charge for calls placed while roaming. 91
Advanced Mobile Phone System
(a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used. 92
Advanced Mobile Phone System
• AMPS (Advanced Mobile Phone System) is the analog
system (1G) first developed and used in the U.S.
• The AMPS system uses FDM to separate 832 full-
duplex channels.
– 832 simplex transmission channels from 824 to 849 MHz
– 832 simplex receive channels from 869 to 894 MHz
– Each simplex channel is 30 kHz wide.
• These channels are divided into four categories:
– Control (base to mobile) to manage the system (21 channels)
– Paging (base to mobile) to alert users to calls for them
– Access (bidirectional) for call setup and channel assignment
– Data (bidirectional) for voice, fax, or data (45 channels)
93
D-AMPS
• D-AMPS (Digital-AMPS) is the first digital version
(2G) of AMPS.
– It uses the 800 or 1900 MHz spectrum.
– Each simplex channel is 30 kHz wide.
– It is described in IS-54 and IS-136.
• It is also known as TDMA (Time Division Multiple
Access).
– Several physical channels are located by dividing one
frequency channel into several time slots.
– The advantage of TDMA is that several channels are co-
located on one carrier frequency, so there are less transmitters
required.
94
D-AMPS
Digital Advanced Mobile Phone System
(a) A D-AMPS channel with three users.
95
(b) A D-AMPS channel with six users.
GSM
• GSM (Global System for Mobile communications)
is a digital voice or data cellular network used
throughout the world.
– The European version of GSM operates at the 900 MHz and
1800 MHz frequencies.
– The North American version of GSM, called GSM 1900,
operates at the 1900 MHz frequency.
– Each simplex channel is 200 kHz wide.
– Connection rate is up to 9.6K bps
– American Personal Communications (APC), a subsidiary of
Sprint, is using GSM as the technology for a broadband
personal communications service (PCS).
96
GSM
Global System for Mobile Communications
GSM uses 124 frequency channels, each of which
uses an eight-slot TDM system 97
GSM
A portion of the GSM framing structure.
98
CDMA
• CDMA (Code Division Multiple Access) is a
standard using spread spectrum transmission
(2G).
– The original CDMA standard, also known as
cdmaOne and still common in cellular telephones in
the U.S., offers a transmission speed of up to 14.4
Kbps in its single channel form and up to 115 Kbps
in an eight-channel form.
– It operates in the 800 and 1900 MHz bands.
– Each simplex channel is 1.25 MHz wide.
– It can carry data at rates up to 115 kbps.
99
CDMA
• Operation of CDMA:
– In CDMA, the input signals are digitized and transmitted in
coded, spread-spectrum mode over a broad range of
frequencies.
– In CDMA, each bit time is subdivided into m short intervals
called chips. Typically, there are 64 or 128 chips per bit.
– Each station is assigned a unique m-bit code called a chip
sequence.
– To transmit a 1 bit, a station sends its chip sequence. To
transmit a 0 bit, the station sends the one’s complement of its
chip sequence.
100
CDMA – Code Division Multiple Access
(a) Binary chip sequences for four stations
(b) Bipolar chip sequences
(c) Six examples of transmissions 101
(d) Recovery of station C’s signal
Third-Generation Mobile Phones:
Digital Voice and Data
• Factors which drives the telephony industry:
1. Data traffic exceeds voice traffic.
2. Design a lightweight portable device with versatile functions
(telephone, music player, gaming device, digital camera,
Web interface, and more).
• IMT-2000 (International Mobile Telecommunication-
2000) network should provide
– High-quality voice transmission
– Messaging (replace e-mail, fax, SMS, chat, etc.)
– Multimedia (music, videos, films, TV, etc.)
– Internet access (web surfing, w/multimedia.)
102
Third-Generation Mobile Phones:
Digital Voice and Data
• Two main IMT-2000 proposals (differences in
coding methods):
– W-CDMA (wideband code-division multiple access) by
Ericsson.
• W-CDMA can support communications at up from 384
Kbps to 2 Mbps
• A 5 MHz-wide channel is used.
• UMTS (Universal Mobile Telecommunication System) is
the system pushed by the EU.
– CDMA2000 by Qualcomm.
• CDMA2000 can support mobile data communications at
speeds ranging from 144 Kbps to 2 Mbps.
• A 5 MHz-wide channel is used. 103
2.5-Generation Mobile Phones:
Digital Voice and Data
• EDGE (Enhanced Data rates for GMS
Evolution) is GSM with more bits per baud.
• GPRS (General Packet Radio Service) is a data
service that can be layered onto D-AMPS or
GSM.
– It allows mobile stations to send and receive IP
packets.
– Each channel is 200 kHz wide.
– Data rates of up to 115 kbps
104
Cable Television
• Community Antenna Television
• Internet over Cable
• Spectrum Allocation
• Cable Modems
• ADSL versus Cable
105
Community Antenna Television
• The head end is an amplifier to strengthen the signals.
• Cable television was initially called community
antenna television.
An early cable television system.
106
Internet over Cable
• HFC (Hybrid Fiber Coax) system is a system with fiber
for the long-haul and coaxial cable to the houses.
107
Cable television
Internet over Cable
The fixed telephone system.
108
Internet over Cable
• Spectrum Allocation
– 5 – 42 Mhz: upstream channels
– 54 Mhz ↑: downstream channels
– A 6 Mhz or 8 Mhz downstream channel is modulated with
QAM-64 or QAM-256 for the high quality cable.
– With a 6 MHz channel and QAM-64, the net payload is 27
Mbps.
– For upstream, QPSK is used because QAM-64 does not
work well when there is too much noise.
• The head end and amplifier are upgraded to CMTS
(Cable Modem Termination System).
109
Spectrum Allocation
Frequency allocation in a typical cable TV system
used for Internet access 110
Cable Modem
• A cable modem is a device
• How cable modems work?
– Ranging: get the distance from the headend to get correct
timing to fit in one or more minislots.
– Acquiring upstream channel, downstream channel, and
minislot assignments: request minislots and wait for the
acknowledge from the headend. Otherwise, retry.
– Sending packets to request an IP address.
– Establishing a secret key between the head-end and modem.
– Log in and provide its unique identifier over the secure
channel.
111
Cable Modems
Typical details of the upstream and downstream
channels in North America. 112
ADSL versus Cable
• Which is better, ADSL or cable?
– Theoretically, coax is hundreds of times more than
twisted pair. But, the full capacity is not available
for data users (See comment on Page 175).
– In practice, ADSL providers achieve about 80% of
the bandwidth. Cable depends on how many people
are sharing the cable.
– Being a point-to-point medium, ADSL is more
secure than cable.
– The telephone system is more reliable than cable.
– Most ADSL providers offer a choice of ISPs
(sometimes, required by law). 113
