MODEM
Introduction:
Modems a modem converts digital signals from your computer into audible
tones which can be transmitted over ordinary analog phone lines. This process is called
modulation. Modem is a acronym of modulator and demodulator. On the receiving end, a
modem demodulates the analog signals arriving over the phone line into electrical signals
which are then fed to the computer. MOdulation and DEModulation accurately describe
what modems do. The purpose of a modulator-demodulator is to convert the digital signal to
a modulated sine wave. Dialup modems can also perform phone functions such as dial,
answer and hang-up. A modem is a device or program that enables a computer to transmit
data over, for example, telephone or cable lines. Computer information is stored digitally,
whereas information transmitted over telephone lines is transmitted in the form of analog
waves There are many different types of modems in use operating at different speeds and
conforming to different standards. To function, the modems on both sides of the connection
must agree to use the same standard. When modems connect, they must negotiate:
o quality of the line
o speed
o Options such as encoding and compression.
Block Diagram:
History:
News wire services in 1920s used multiplex equipment that met the definition, but the
modem function was incidental to the multiplexing function, so they are not commonly
included in the history of modems.
TeleGuide terminal
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Modems grew out of the need to connect teletype machines over ordinary phone lines
instead of more expensive leased lines which had previously been used for current loopbased teleprompters and automated telegraphs. George Stibitz connected a New Hampshire
teletype to a computer in New York City by a subscriber telephone line in 1940In 1943,
IBM adapted this technology to their unit record equipment and were able to transmit
punched cards at 25 bits/second Mass-produced modems in the United States
began as part of the SAGE air-defense system in 1958, connecting terminals at various
airbases, radar sites, and command-and-control centers to the SAGE director centers
scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs
as conforming to their newly published Bell 101 dataset standard. While they ran on
dedicated telephone lines, the devices at each end were no different from commercial
acoustically coupled Bell 101, 110 baud modems.
In the summer of 1960, the name Data-Phone was introduced to replace the earlier term
digital subset. The 202 Data-Phone was a half-duplex asynchronous service that was
marketed extensively in late 1960.In 1962, the 201A and 201B Data-Phones were
introduced. They were synchronous modems using two-bit-per-baud phase-shift keying
(PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the
201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive
channels running on their own set of two wires each. The famous Bell 103A dataset standard
was also introduced by Bell Labs in 1962. It provided full-duplex service at 300 baud over
normal phone lines.
Features of Modems:
1. Speed
:
The speed at which the modem can send data in bps (bits per second). Typical modem
speeds are: 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K, 28.8K bps.
2. AutoDial/Redial:
Smart modems can dial the phone number and auto re dial if a busy signal is received.
3. Auto
Answer:
Most modems have Ring Detect capability and can automatically answer the telephone
when an incoming call comes in.
4. Self
Testing
Newer modems have self-testing features. They can test the digital connection to the
terminal/computer and the analog connection to a remote modem. They can also check
the modem's internal electronics.
Voice Over Data : Voice Over Data modems allow a voice conversation to take place
while data is being transmitted. This requires both the source and destination modems to
have this feature.
5. Synchronous or Asynchronous Transmission :
Newer modems allow a choice of synchronous or asynchronous transmission of
data. Normally, modem transmission is asynchronous (we send individual characters with
just start and stop bits). Synchronous transmission or packet transmission is used in specific
applications.
6. Baud :
It is the speed at which the analog data is changing on the voice channel and bps is the
speed at which the decoded digital data is being transferred.
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Types of Modem:
1) External Modem :
External modems sit next to the computer and connect to the serial port using a straightthrough serial cable. It is the second term we have to consider from different types of
computer modem. An External modem can be used to the same purpose and in the same
conditions as internal computer modem. However external modem is a small box that uses
other kind of First of all external modem can be a Dial-Up modem but more expensive
than the internal. As well as another type of external modem you may consider two
important types of modem: cable modem and DSL modem if you need high-speed internet
services. .
All ISPs usually supply a specialized modem named digital modem in the broadband
package. It’s important to notice that cable modem has to connect to an Ethernet card,
placed on the computer PCI slot to provide a broadband internet connection to the user. This
is true if you opt to use Ethernet connection. However you won't need it if your option goes
to use an USB connection.
 Types Of External Modem
 Cable modem :
The cable modem uses a coaxial cable television lines to provide a greater bandwidth than
the dial-up computer modem. An extremely fast access to the Web is providing by the cable
modem with downstream transmission up to 38 Ambits/s and an upstream transmission up
to 1 Ambits/s.
 DSL modem :
DSL (Digital Subscriber Line) modem is exclusively used for connections
from a telephone switching office to the user. This technology, available and frequently
usable, split up into two main categories:
 ADSL or Asymmetric Digital Subscriber Line :
It is used in North America and supports from 1.5 Ambits/s up to 9 Ambits/s of
downstream transmission rate and up to 3 Ambits/s of upstream transmission rate.
 SDSL. SDSL or Symmetric Digital Subscriber Line
It is used in Europe and has the same data rates for downstream and upstream
transmission which is 128 Kbits/s.
Diagram Of External Modem:
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2) Internal modems :
An internal modem is basically an external modem and serial port mounted upon a PC bus
card. These are cheaper than external modems as they do not require a power supply or a
chassis. Internal modems work fine for remote serial console applications.
Internal
modems are a plug-in circuit board that sits inside the computer. It incorporates the serial
port on-board. They are less expensive than external modems because they do not require a
case, power supply and serial cable. Be careful not to confuse an internal modem with a Win
Modem. An internal modem does not need a special device driver, but appears to Linux as a
standard serial port
 Diagram Of Internal Modem:
3) Asynchronous :
The common modem used today. Each byte is placed between a stop and a start bit.
Each modem must operate with the same start and stop bit sequence, operate at the same
baud rate and have the same parity settings for the data checking in order to
communicate correctly. Define parity checking.
4) Synchronous:
Synchronous modems can be faster than asynchronous. They depend on timing to
communicate. Data is transmitted in frames with synchronization bits which are used to
be sure the timing of the transmission and reception of data is accurate. Synchronous
modems are normally used on dedicated leased lines. Synchronous modems are one of
binary synchronous communications protocol (bisync), high level data link control
(HDLC), or synchronous data link control (SLDC). Three methods can be used to
control synchronization:
5) Analog modem:
A device that converts the computer's digital pulses to tones that can be carried Over
analog telephone lines, and vice versa. Prior to the turn of the century, analog modems
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used to be standard equipment on most desktop and all laptop computers. Contrast with
digital modem. See modem.
6) Digital modem :
A device that adapts digital signals from one transmission system to another. For
example, the term may refer to an ISDN, DSL or cable modem. Contrast with analog
modem. See cable modem, DSL, ISDN and modem. These are used with ISDN services
and are not actually modems, although they are called modems. They can provide
connection speeds of 128Kbps.
7) Voice/Data/Fax/ Modem :
Most modems can be classified with these characteristics:
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Data/fax
Data/fax/voice
Data/fax/voice/speakerphone
Voice generally means that the modem is capable, with appropriate software, of supporting
telephone answering machine functions: the modem can 'record' and 'play' to the Windows
wave device. The answering machine software will also use the sound card on the machine
to play and record. The software may also include functions to dial or answer calls using
your sound card's microphone and speakers or headphones.
Windows comes with a Phone Dialer. This utility will dial a number with any type of
modem; after Phone Dialer dials a number, you use any normal phone (connected to the
'Phone' jack of the modem) to complete a voice call.Third-party software can be used with a
voice modem to enhance functionality. One such package: ModemSpy can record phone
calls to wav files.
8) PCMCIA MODEM:
In computing, PC Card (originally PCMCIA Card) is the form factor of a peripheral
interface designed for laptop computers. The PC Card standard (as well as its successor
ExpressCard) was defined and developed by a group of companies called the Personal
Computer Memory Card International Association (PCMCIA). The United States computer
industry created the Personal Computer Memory Card International Association to
challenge the Japanese
JEIDA memory card devices by offering a competing standard for memory-expansion cards.
In 1991 the two standards merged as JEIDA 4.1 or PCMCIA 2.0 (PC Card).
PC Card was originally designed for computer storage expansion, but the existence of a
usable general standard for notebook peripherals led to many kinds of devices being made
available in this form. Typical devices included network cards, modems, and hard disks.
The cards were also used in early digital SLR cameras, such as the Kodak DCS 300 series.
The original use, as storage expansion, is no longer common.
Application:
Versatile & Effective
Data Remote offers wireless communication solutions that span numerous applications.
Our real-time technologies provide the solutions to move data wirelessly, in a cost-effective
and reliable way, enabling greater efficiency in the management of mission-critical data.
Modem
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Remote Meter Reading
Our cellular modems are uniquely suited for meter reading applications--DataRemote has
assisted its diverse customer base with thousands of installations in both urban and remote
areas where landlines are either unavailable or cost prohibitive.
Power Station Monitoring & Control
Our rugged and reliable wireless solutions are a perfect fit for a wide range of environmental
monitoring applications that require low power consumption and dependable remote
monitoring options.
Variable Message Signs
Control, monitor and change information on signs in remote locations digitally and from the
ease of a desktop. Our VMS cellular modem solutions also provide innovative two-way
communication options for optical character recognition (OCR).
Gas, Water & Oil Flow Monitoring
Gas and water metering solutions are currently being explored. Data Remote's experience
with defining, designing, developing and manufacturing communications solutions for a
broad variety of industrial applications provide us with valuable insights into the resulting
product.
Fire, Law & Rescue Operations
Today's communities rely on their local municipal groups to have the most cutting-edge
communications available to serve their complex needs. Therefore, it is essential that these
teams have the exact locations and vital data they require available to them in real-time.
Cellular ATM Transactions
The benefit of leveraging wireless technology in POS applications is rapidly becoming the
watchword across numerous ATM markets. CDMA technology offers advanced security
measures coupled with the speedand convenience of the digital landscape.
Cellular Lottery Terminal Transactions
A wireless system can allow lottery terminals to communicate remotely, while receiving a
wide variety of data, from accounting and ticket information to the ability to closely monitor
all aspects of each machine's security features-including being able to sense when a machine
door has been tampered with.
Fleet Management Solutions
Our innovative solution allows users to keep abreast of real-time locations and other
important information regarding vehicles and assets over the cellular infrastructure with
Trakit Now Fleet Management Solution.
Advantages of a Modem:
modems have several advantages .
Internal modems must be installed inside of a computer. Make sure to check that the
internal modem is compatible with the system it is to be used in. The modem is connected
directly to the motherboard, normally through a PCI (peripheral component interconnect).
Internal modems have several advantages over their external counterparts.
Price:
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Internal modems are usually considerably cheaper than external modems. This is mainly because of
the lower cost of production as no materials are used building a case around it. Internal modems do
not have LED lights to reflect connectivity, which also lowers the cost of production.
Power Consumption
modems use the power of the running computer to function. No extra plug is necessary.
Internal modems are very efficient and use less power than externals. All of the power
needed is provided by the computer's power supply.
Desk Space
modems are installed inside the computer, thus taking up no space on the desk. This
frees up desk space for other peripherals or working room.
Cable Clutter
Modems require only the telephone line to be connected. The power and computer
connection are all taken care of by the PCI slot the modem is plugged into. This means
less cables snaking around and a clutter-free work environment.
Cleanliness
External modems are magnets for dust. Internal modems will not be covered with
unsightly dust as it is inside the computer. This means less dusting to keep your desk
clean.
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Fast data transfers, up to 30 Mbps if using a 100BaseT NIC
Competitive pricing against competing technologies
Easy to install - home prewired.
Disadvantages of a Modem:

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The available bandwidth depends on the number of users on the local cable TV line
segment.
There is an asymmetrical transfer rate. Upstream is slower than downstream.
There can be a bottleneck at the communication server at the head end.
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