Transcript How Modems Work?

Wired Physical Layer
EMC 165 Computer and
Communication Networks
Lecture 8
Feb 10, 2004
Outline of today’s lecture
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How Modems Work?
How ISDN Works?
How DSL Works?
How Cable Modems Work?
How Fiber Optics Work?
How Modems Work?
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Origin of Modems
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Modem – contraction of the words “modulatordemodulator”.
A modem Is typically used to send digital data over a
phone line
Sending modem modulates the data into a signal that
is compatible with the phone line.
Receiving modem demodulates the signal back into
digital data.
Wireless modems convert digital data into radio
signals and back.
How Modems Work? - contd
How Modems Work? – contd
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In this configuration, a dumb terminal at an off-site office
can “dial-in” to a large central computer
Modem speeds went through a series of improvements
over the years
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300 bps – 1960s through 1983
1200 bps – 1984-1985
2400 bps
9600 bps – first appeared in late 1990 and early 1991
19.2Kbps
28.8 Kbps
33.6 Kbps
56 Kbps – became standard in 1998
ADSL – theorectical max of up to 8 Mbps
300bps Modems
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A 300 bps modem is a device that uses Frequency Shift Keying (FSK) to
transmit digital information over a telephone line.
In FSK, a different tone (frequency) is used for the different bits.
The originating modem transmits 1070-Hz tone for a 0 and a 1270-Hz tone
for a 1.
The answering modem transmits a 2025-Hz tone for a 0 and a 2225-Hz
tone for a 1.
Because they use different tones, they can use the line simultaneously. This
is known as full-duplex operation.
When the letter “a” is typed, the ASCII code for this letter is 97 decimal or
01100001 binary.
A device inside the terminal called a Universal Asynchronous
Receiver/Transmitter (UART) converts the byte into its bits and sends them
out one at a time through a serial port called RS-232 port.
The terminal’s modem is connected to the RS-232 port, so it receives the
bits one at a time and its job is to send them over the phone line
Faster Modems
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To create faster modems, Phase-Shift Keying (PSK) and
Quadrature Amplitude Modulation (QAM) are used.
What is PSK? Shifting the phase of the wave.
Higher speed modems incorporate a concept of “gradual
degradation”, meaning they can test the phone line and fall back to
slower speeds if the line cannot handle the modem’s fastest speed.
What is QAM?
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Simply a combination of amplitude modulation and PSK
 Assume use 2 amplitudes and 4 phase shift
 Bit value Amplitude Phase
 000
1
None
 001
2
None
 010
1
¼
 011
2
¼
etc
ISDN
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ISDN stands for Integrated Services Digital
Network
Using the same copper phone lines that
modems use, ISDN delivers a 5-fold speed
improvement (compared to 28.8 Kbps
modem) (up to 128 Kbps).
ISDN can combine voice and data services
over the same wires.
ISDN Basics
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ISDN provides a raw data rate of 144 Kbps over a
single twisted pair.
This 144 Kbps channel is divided into 2 64-Kbps
channels (refer to as Bearer channels) and one 16
Kbps channel (refers to as Data channel).
Each B channel can carry a separate telephone call
and has its own telephone number called a Directory
Number (DN).
One can combine the 2 B-channels together to form
a single 128 Kbps data channel through a process
called bonding.
How ISDN does It?
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If ISDN can squeeze 144Kbps out of my phone line, why can’t
modem do the same thing?
 A given pair of wires connecting 2 parties for communication can
carry electrical signals in two forms: analog or digital.
 An analog signal changes gradually through an infinite number of
values, while a digital signal changes instantly (in theory)
between just two values.
 An analog signal’s infinite number of variations makes it
impossible to reproduce exactly. An analog signal will go only so
far in copper wire; to go further the signal must be regenerated
electronically with a device called a repeater. The repeater
converts the weak input signal to a stronger signal, unavoidably
distorting it in the process. Each regeneration degrades the
signal a bit more.
 Digital signals, on the other hand, are easy to regenerate
precisely because there are only 2 possible states for the signal.
Simple ISDN hookup
UTP – Unshielded Twisted Pair
ISDN Basics
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The B channels carry customer voice or data signals.
The D channel carries signals between your ISDN equipment and the
phone company’s central office.
The 2 bearer plus one data channel is called the Basic Rate Interface
(BRI) in telco lingo.
One can buy 23 B channels with a single 64 Kbps D channel. This
service is called the Primary Rate Interface (PRI).
A single 4-wire cable carries the 2B+D channels into another box called
the Terminal Adapter (TA). Unlike the Network Terminator (NT1), which
provides only a single function (creating the 2B+D channels), the TA
can do many things.
The TA can connect any of the terminal equipment (TE) – computers,
fax machines, LANs or telephone sets to one or both of the B channels.
External ISDN reference points labeled R, S/T, and U. Each interface
point requires an electrically different device connection and cabling.
The U reference point is the incoming unshielded twisted pair. The S/T
reference point is a four-wire UTP cable.
ISDN Basics
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A typical TA for data-only applications might simply emulate a
pair of ordinary modems, transmitting standard modem setup
and dialing commands into ISDN call-setup commands.
One connects a computer to this kind of TA with a normal RS232
cable and uses the usual modem or fax software to set the speed
to 64 Kbps.
The TA provides automatic rate adaptation to match whatever
data rate the computer supports with ISDN’s 64 Kbps channel.
Advantage of ISDN: data is sent digitally so higher reliability
when compared to the analog modems which suffer from all
kinds of maladies ranging from intermittent line noise to speed
mismatches and protocol conflicts.
Beyond ISDN Basics
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One can set up ISDN to simulate the features of an office PBX,
using advanced TAs or direct computer-integrated ISDN
hardware.
ISDN offers flexible options for mixing voice and data
 Up to 8 devices can share access to the channels using a feature
of ISDN called passive bus. Passive bus uses a 2nd kind of
network terminator, called NT2 to let up to 8 separate TAs share
a single 2B+D circuit.
 TAs that support passive bus have a port labeled S/T to indicate
that you are making the connection at the S/T ISDN reference
points.
ISDN also allows you to construct sophisticated integrated
voice/data applications e.g. call appearances. POTS allow you to
put a call on hold while taking a second call. ISDN expands that
capability to up to 15 separate calls.
Multiple ISDN circuit appearances
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For incoming ISDN calls, the telco’s Central Office sends a call setup
message to the TA via the D channel, indicating that a call is available to be
picked up.
The TA answers the call and assign it to an available B channel. If both B
channels are used, it can free a channel by placing an active call on hold
and making the new call active. These calls can be either data or voice, in
any combination. Thus a single TA can handle as many as 15 simultaneous
calls in progress, with any 2 of those calls active.
Using analog modems in ISDN environment
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This kind of TA accepts an ordinary voice or modem audio signal
through a standard RJ11 modular jack and digitizes it for transport
across the ISDN interface. It interprets the touch-tone dialing signals put
out by the telephone set or modem and generates the required ISDN
call setup signals.
If the number you call is not an ISDN POP, the telco equipment at the
remote end automatically trnaslates the digitized audio back to analog
audio, where the destination modem hears what it’s always heard before
ISDN came along.
Cost of ISDN
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Cheapest service (PacBell) - $30/month for
local access plus msg-unit charges of 4 cts
for 1st minute and 1ct for each additional
minute.
Long-distance digital charges 2-3 times
higher than voice long-distance calls.
NT1 costs between $100-$200
ISDN TAs cost range from $300 to $1500.
Alternatives to ISDN
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Copper-wire digital services such as T1 (1.544Mbps)
Frame Relay services (56 Kbps to 1.544 Mbps)
Asynchronous Transfer Mode (ATM) (25Mbps to 100 Mbps).
Alternative is to replace copper wire with fiber optic cabling
 The last mile also called the local loop is telco talk for the twisted
wire pair between the CO and the subscriber.
 Each telephone user requires a dedicated pair of copper wires.
The length is more than 1 mile but fewer than 20 miles and
averages over 5 miles in metropolitan areas.
 Faster digital services require digital repeaters at least once per
mile.
 But normal copper pairs don’t have such repeaters.
 The copper wires have been there for 50 years. The cost of
replacing existing copper with fiber would be $250 billion.
How DSL Works?
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DSL is a high-speed connection that uses the same
wires as a regular telephone line.
The wires themselves have the potential to handle
frequencies up to several million Hertz but for voice
communications, we limit them to 3.4 KHz
By limiting the frequencies carried over the lines, the
telephone system can pack lots of wires into a very
small space without worrying about interference
between lines.
Modern equipment that sends digital rather than
analog data can safely use much more of the
telephone line’s capacity. DSL does just that
ADSL
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ADSL stands for asymmetric digital subscriber line.
Asymmetric – data sent in one direction is faster than in the other
direction.
An ADSL modem has a dedicated copper wire running between it
and phone company’s nearest multiplexer (MUX) or central office.
This dedicated copper wire can carry far more data than the 3Kbz
signal needed for our phone’s voice channel.
With a dedicated copper wire between the phone company and the
home, the capacity is something like 1Mbps between the home and
the phone company (referred to as upstream) and 8 Mbps between
the phone company and the home (referred to as downstream)
under ideal conditions.
ADSL
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Most homes and small business users are connected to an
ADSL.
ADSL divides up the available frequencies in a line on the
assumption that most internet users look at or download much
more information than they send, or upload.
Under this assumption, if the connection speed from the Internet
to the user is 3-4 times faster than the connection from the user
back to the Internet, then the user will see the most benefit.
Other types of DSL include
 Very high bit-rate DSL (VDSL)
 Symmetric DSL
 Rate-adaptive DSL – a variation of ADSL where the modem
adjusts the speed of connection depending on the length and
quality of the line.
ADSL - contd
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ADSL is distance-sensitive.
As the connection length increases, the signal
quality decreases and the connection speed goes
down.
The limit for ADSL services is 5460 m (18,000 ft).
ADSL technology can provide a max of 8 Mbps
downstream at a distance of 6000 ft and upstream
speeds of up to 640 Kbps.
In practice, the best speeds widely offered today are
1.5 Mbps downstream and 64-640 Kbps upstreams.
ADSL – contd
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The approach an ADSL modem takes is as
follows
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The phone line’s bandwidth between 24KHz and
1,100KHz is divided into 4KHz bands and a virtual
modem is assigned to each band. Each of these 249
virtual modems tests its band and does the best it can
with the slice of bandwidth it is allocated. The
aggregate of the 249 virtual modems is the total
speed of the pipe.
ADSL - contd
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There are 2 competing standards for ADSL, namely “discrete
multitone (DMT) and carrierless ampltitude/phase (CAP) system.
CAP operates by dividing the signals on the telephone line into
distinct bands: voice conversations are carried in the 0-4KHz
band. Upstream channel is carried in a band between 25-160
KHz and downstream begins at 240KHz and goes up to a point
that varies depending on a number of conditions (line length, line
noise, no of users in a particular phone switch) but has a max of
about 1.5MHz.
DMT divides the data into 247 separate channels, each 4KHz
wide. You get an equivalent of 247 modems connected to your
computer at once. Each channel is monitored and if the quality is
too impaired, the signal is shifted to another channel. This
system constantly shifts signals between different channels.
DMT is more complex to implement than CAP but gives it more
flexibility on lines of differing quality.
DSL Equipment
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ADSL uses 2 pieces of equipment, one on the customer end and
one at the Internet service provider, telephone company or other
DSL service provider.
At the customer end, we have the DSL transceiver. At the service
provider end, we have the DSL access multiplexer (DSLAM) to
receive customer connections.
DSL Equipment
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DSL Transceiver – also called ATU-R. It is the point where data from
the computer is connected to the DSL line
DSL DSLAM – takes connections from many customers and
aggregates them onto a single, high-capacity connection to the
Internet.
DSLAMs are generally flexible to support multiple types of DSL.
DSLAM may provide additional functions including dynamic IP
address assignment.
ADSL provides a dedicated connection from each user back to the
DSLAM but cable modem (which we discuss next) users generally
share a network look that runs through a neighborhood. Thus, ADSL
users won’t see performance decrease as new users are added.
How Cable Modems Work?
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Cable Modem Basics
Inside the Cable Modem
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Tuner
Demodulator
Modulator
MAC
Microprocessor
Cable Modem Termination System
Cable Modem Basics
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Each television signal is given a 6 MHz channel on the cable. The
coaxial cable used to carry cable TV can carry hundreds of megahertz
of signals.
When a cable company offers internet access over the cable, the cable
modem system puts downstream data into a 6MHz channel.
On the cable, the data looks just like a TV channel. So, internet
downstream data takes up the same amount of cable space as any
single channel of programming.
Upstream data requires even less of the cable’s bandwidth, just 2MHz,
since the assumption is that most people download far more
information than they upload.
Putting both upstream and downstream data on the cable TV system
requires two types of equipment, a cable modem on the customer end
and a cable modem termination system (CMTS) at the cable provider’s
end.
Inside the Cable Modem.
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All cable modems contain certain key components:
 A tuner
 A demodulator
 A modulator
 A media access control (MAC) device
 A microprocessor
Inside the Cable Modem
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Tuner
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connects to the cable outlet.
Sometimes, with the addition of a splitter to separate the Internet data
channel from normal CATV programming.
In some cases, tuner will contain a diplexer which allows the tuner to
make use of one set of frequencies for downstream traffic and another
set for upstream data.
In some cases, the cable modem tuner is used for downstream data and
a dial-up telephone modem is used for upstream traffic.
The tuner passes the received signal to the demodulator.
Inside the Cable Modem
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Demodulator
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A QAM demodulator takes a radio-frequency signal that has had
information encoded in it by varying both the amplitude and phase of the
wave, and turns it into a simple signal that can be processed by the
Analog/Digital (A/D) converter.
The A/D converter takes the signal and turns it into a series of digital 1s
and 0s.
An error correction module then checks the received information against
a known standard so that problems in transmission can be found and
fixed.
Network frames may be in MPEG format, so an MPEG synchronizer
may be used to make sure the data groups stay in line and in order.
Inside the Cable Modem
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Modulator
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If cable system is used for upstream, a modulator is used to convert the digital
computer network data into radio-frequency signals for transmission. This component
is called a burst modulator, because of the irregular nature of most traffic between a
user and the Internet.
Consists of 3 parts
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MAC
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A section to insert information used for error correction on the receiving end.
A QAM modulator.
A digital to analog (D/A) converter.
Sits between the upstream and downstream portions of the cable modem, and acts as
the interface between the hardware and software portions of the various network
protocols.
Some of the MAC functions will be assigned to a central processing unit (CPU).
Microprocessor
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Depends on whether the cable modem is designed to be part of a larger computer
system or to provide internet access with no additional computer support. In systems
where the cable modem is the sole unit required for internet access, the
microprocessor picks up MAC slack and much more. Motorola’s PowerPC processor is
one of the common choices for system designers.
CMTS
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CMTS takes the traffic coming in from a group of customers on a single
channel and routes it to an internet service provider (ISP) for connection to
the Internet
At the head-end, the cable providers will have servers for accounting and
logging, Dynamic Host Configuration Protocol (DHCP) for assigning and
administering IP addresses of all the cable system’s users, and control
servers for a protocol called CableLabs Certified Cable Modems (or
DOCSIS), the major standard used by US cable systems in providing
internet access to users.
CMTS – contd
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The downstream data is sent to all users just like in an Ethernet network.
Individual network connection decides whether a particular block of data is
intended for it or not.
On the upstream side, information is sent from the user to the CMTS. Other
users don’t see that data at all.
The narrower upstream bandwidth is divided into slices of time, measured in
milliseconds, in which users can transmit one “burst” at a time to the
Internet.
A CMTS will enable as many as 1,000 users to connect to the Internet
through a single 6MHz channel. Each channel is capable of 30-40 Mbps of
total throughput.
How does Fiber Optics Work?
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What are Fiber Optics?
How does an Optical Fiber Transmit Light?
A Fiber-Optic Relay System
Advantages of Fiber Optics
What are Fiber Optics?
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Fiber Optics are long, thin strands of very pure glass about the diameter
of a human hair. They are arranged in bundles called optical cables and
used to transmit light signals over long distances.
A single optical fiber has the following parts:
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Core – the glass center of the fiber where the light travels
Cladding – the outer optical material surrounding the core that reflects the
light back into the core
Buffer coating – the plastic coating that protects the fiber from damage and
moisture
What are Fiber Optics? - contd
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There are two types of fibers: single-mode and multimode fibers
Single mode fibers have small cores (about 3.5x10-4
inches or 9 microns in diameter) and transmit infrared
laser light (wavelength = 1300 to 1550 nanometers)
Multi-mode fibers have larger cores (about 2.5x10-3
inches or 62.5 microns in diameter) and transmit infrared
light (wavelength=850 and 1300 nm) from light-emitting
diodes (LEDs).
How Does an Optical Fiber Transmit Light?
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The light in a fiber-optic cable travels through the core by constantly
bouncing from the cladding, a principle called total internal reflection.
Because the cladding does not absorb light, the light wave can travel great
distances.
Some of the light signal degrades within the fiber due to impurities in the
glass. The extent of degradation depends on the purity of the glass and the
wavelength of the transmitted light e.g. 850 nm light degrades 60-70
percent/Km while 1550 nm light degrades 50 percent/Km. But some
premium optical fibers show much less signal degradation – less than 10%
at 1550 nm.
What is Total Internal Reflection?
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When light passes from one medium with one index of refraction (m1 ) to
another medium with a lower medium of refraction (m2), it bends or refracts
away from an imaginary line perpendicular to the surface (normal line). As
the angle of the beam through m1 becomes greater with respect to the
normal, the refracted light through m2 bends further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2,
but instead will travel along the surface between two media. If the beam
through m1 is greater than the critical angle, then the refracted beam will be
reflected entirely back into m1 (total internal reflection, even though m2 may
be transparent.
A Fiber-Optic Relay System
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Fiber-optic relay systems consist of the following
 Transmitter – produces and encodes the light signals
 Optical fiber – conducts the light signals over a distance
 Optical regenerator – may be necessary to boost the light signal
 Optical receiver – receives and decodes the light signals.
Transmitter
 Directs the optical fiber device to turn the light “on” or “off” in the
right sequence
 Has a lens to focus the light into the fiber. Lasers have more
power than LEDs but vary more with changes in temperature and
are more expensive.
 The most common wavelengths of light signals are 850nm, 1300
nm, and 1550 nm.
Fiber-Optic Relay System - contd
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Optical Regenerator
 Some signal loss occurs especially over long distances (more
than 0.5 mile) such as undersea cables.
 Optical regenerator consists of optical fibers with a special
coating (doping). The doped portion is pumped with a laser.
When the degraded signal comes into the doped coating, the
energy from the laser allows the doped molecules to become
lasers themselves. The doped molecules then emit a new,
stronger light signal with the same characteristics as the
incoming weak light signal. It is like a laser amplifier.
Optical Receiver
 Takes incoming digital light signals, decodes them and sends the
electrical signal to the other user’s computer. The receiver uses a
photocell or photodiode to detect the light.
Advantages of Fiber Optics
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Less expensive – several miles of optical cable can be made cheaper
than equivalent lengths of copper wire.
Thinner – Optical fibers can be drawn to smaller diameters than copper
wire
Less signal degradation – the loss of signal in optical fiber is less than
in copper wire
Low power – because signals in optical fibers degrade less, lowerpower transmitters can be used.
Digital signals – optical fibers are ideally suited for carrying digital
information, which is especially useful in computer networks.
Non-flammable – no electricity is passed through optical fibers, there is
no fire hazard.
Lightweight – an optical cable weights less than a comparable copper
wire cable.
Flexible – because they can transmit and receive light, fiber optics are
used in many flexible digital cameras.
References
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www.HowStuffWorks.com
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How Modems Work
How ISDN Works?
How Cable Modems Work?
How Fiber Optics Work?