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Contents
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TABLE OF CONTENTS
CHAPTER TITLE PAGE NO
I INTRODUCTION 4
1.1 Introduction-Modems 5
1.2 Characteristics of Modems
7
1.3 Functions of the Modems 8
II MODEM STANDARDS 9
2.1 Bell Modem Standards 10
2.2 ITU-T Modem Standards 14
2.3 Intelligent Modems 19
III CONCLUSION 22
IV BIBILIOGRAPHY 24
V APPENDIX-A 26
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Chapter – I
Introduction
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1.1 INTRODUCTION-MODEMS
A modern stands for modulation/demodulation. A modulator converts a digital
signal to an analog signal. A demodulator converts an analog to a digital signal.
The most familiar type of DCE is a modem. Both modulators and
demodulators, however, do use the same techniques as digital-to-analog encoders: modulators to
further encode a signal, and demodulators to decode it. A modulator treats a digital signal as a
series of 1s and 0 s, and so can transform it into a completely analog signal by using the digital–
to–analog mechanisms of ASK, FSK, PSK, and QAM.
Each DCE must be compatible with both its own DTE and with other DCEs.
A modem must use the same type of encoding (such as NRZ- L), the same voltage levels to
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mean the same things, and the same timing conventions as its DTE. A modem must also be able
to communicate with other modems.
Theoretical Bit Rates for Modems
Encoding Half- duplex Full- Duplex
ASK, FSK, 2- 2400 1200
PSK
4-PSK, 4- QAM 4800 2400
8-PSK, 8- QAM 7200 3600
16- QAM 9600 4800
32- QAM 12000 6000
64- QAM 14400 7200
128- QAM 16800 8400
256- QAM 19200 9600
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. A
modem converts between these two forms.
Fortunately, there is one standard interface for connecting external modems to
computers called RS-232. Consequently, any external modem can be attached to any computer
that has an RS-232 port, which almost all personal computers have. There are also modems that
come as an expansion board that you can insert into a vacant expansion slot. These are
sometimes called n board or internal modems.
While the modem interfaces are standardized, a number of different protocols
for formatting data to be transmitted over telephone lines exist. Some, like CCITT V.34, are
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official standards, while others have been developed by private companies. Most modems have
built-in support for the more common protocols -- at slow data transmission speeds at least, most
modems can communicate with each other. At high transmission speeds, however, the protocols
are less standardized.
1.2 CHARACTERISTICS OF MODEMS
Aside from the transmission protocols that they support, the following characteristics
distinguish one modem from another:
bps : How fast the modem can transmit and receive data. At slow rates, modems are
measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher
speeds, modems are measured in terms of bits per second (bps). The fastest
modems run at 57,600 bps, although they can achieve even higher data transfer rates by
compressing the data. Obviously, the faster the transmission rate, the faster you can send
and receive data. Note, however, that you cannot receive data any faster than it is being
sent. If, for example, the device sending data to your computer is sending it at 2,400 bps,
you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast
modem. In addition, some telephone lines are unable to transmit data reliably at very high
rates.
voice/data: Many modems support a switch to change between voice and
data modes. In data mode, the modem acts like a regular modem. In voice mode, the
modem acts like a regular telephone. Modems that support a voice/data switch have a
built-in loudspeaker and microphone for voice communication.
auto-answer : An auto-answer modem enables your computer to receive calls in your
absence. This is only necessary if you are offering some type of computer service that
people can call in to use.
data compression : Some modems perform data compression, which enables them to
send data at faster rates. However, the modem at the receiving end must be able to
decompress the data using the same compression technique.
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flash memory : Some modems come with flash memory rather than
conventional ROM, which means that the communications protocolscan be easily
updated if necessary.
Fax capability: Most modern modems are fax modems, which means that they can
send and receive faxes.
To get the most out of a modem, you should have a communications software
package, a program that simplifies the task of transferring data.
Modems are classified on the basis of two criteria,
•Data sent per unit time
•Change in the state of the signal per unit time
1.3 FUNCTIONS OF THE MODEMS
1. Error Correction
In this process the modem checks if the information they receive is undamaged.
The modems involved in error correction divide the information into packets called frames.
Before sending this information, the modems tag each of the frames with check sums. Check
sum is a method of checking redundancy in the data present on the computer. The modems that
receive the information, verify if the information matches with checksums, sent by the error-
correcting modem. If it fails to match with the checksum, the information is sent back.
2. Compressing the Data
For compressing the data, it is sent together in many bits. The bits are grouped
together by the modem, in order to compress them.
3. Flow Control
Different modems vary in their speed of sending signals. Thus, it creates
problems in receiving the signals if either one of the modems is slow. In the flow control
mechanism, the slower modem signals the faster one to pause, by sending a 'character'. When it
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is ready to catch up with the faster modem, a different character is sent, which in turn resumes
the flow of signals.
Chapter-II
Modem Standards
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2. MODEM STANDARDS
- Bell moderns
- ITU-T modems
2.1 BELL MODEMS
The first commercial modems were produced by the Bell telephone company in the early 1970s.
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.
Modems grew out of the need to connect tele printers over ordinary phone lines
instead of more expensive leased lines which had previously been used for current loop–
based tele printers and automated telegraphs. In 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 AT&T in 1962. It
provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was
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used with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem
transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the
use of remote low-speed terminals such as the KSR33, the ASR33, and the IBM 2741. AT&T
reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C
modems.
-
For many years, the Bell System ( AT&T) maintained a monopoly on the use of its
phone lines, allowing only Bell-supplied devices to be attached to its network. Before 1968,
AT&T maintained a monopoly on what devices could be electrically connected to its phone
lines. This led to a market for 103A-compatible modems that were mechanically connected to
the phone, through the handset, known as acoustically coupled modems. Particularly common
models from the 1970s were the Novation CAT and the Anderson-Jacobson, spun off from an in-
house project at Stanford Research Institute (now SRI International). Hush-a-Phone v. FCC was
a seminal ruling in United States telecommunications law decided by the DC Circuit Court of
Appealson November 8, 1956. The District Court found that it was within the FCC's authority to
regulate the terms of use of AT&T's equipment. Subsequently, the FCC examiner found that as
long as the device was not physically attached it would not threaten to degenerate the system.
Later, in the Carterfonedecision of 1968, the FCC passed a rule setting stringent AT&T-designed
tests for electronically coupling a device to the phone lines. AT&T's tests were complex, making
electronically coupled modems expensive, so acoustically coupled modems remained common
into the early 1980s.
In December 1972, Vadic introduced the VA3400. This device was remarkable
because it provided full duplex operation at 1,200 bit/s over the dial network, using methods
similar to those of the 103A in that it used different frequency bands for transmit and receive. In
November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in
design to Vadic's model, but used the lower frequency set for transmission. It was also possible
to use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency
assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out
many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple
modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-
bit/s mode, AT&T's 212A mode, and 103A operation.
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Bell Modem Standards:
– 103/113 series
– 202 series
– 212 series
– 201 series
– 208 series
– 209 series
103/113 Series The Bell 103/113 series modems operate in FDX mode over two-wire switched
telephone line. Transmission is asynchronous. Using FSK encoding, session originator
frequencies are 1,070 HZ=0 and 1,270 HZ=1. Answerer frequencies are 2,025 HZ=0 and 2,225
HZ=1. Data rate is 300 bps. The 113 series is a variation of the 103 series with additional testing
features.
202 Series The bell 202 series modems operate in HDX mode over two-wire switched telephone
lines. Transmission is asynchronous, using FSK encoding, because the 202 series is HDX, only
one pair of transmission frequency is used: 1,200 HZ=0, and 2,400 HZ=1. 202 series includes a
secondary transmission frequency operating in either direction at 387 Hz, using ASK encoding,
with a data rate of only 5 bps, used for flow control or error control.
212 Series The Bell 212 series modems have two speeds. The option of a second speed allow for
compatibility with a wider number of system. Both speeds operate in full-duplex mode over
switched telephone lines. The lower speed 300 bps uses FSK encoding for asynchronous
transmission. Just like the 103/113 series. The higher speed, 1,200 bps, can operate in either
asynchronous a synchronous mode, and uses 4- PSK encoding. While the 1,200 bps in the same
data rate as that achieved by the 202 series, the 212 series achieves that rate in full-duplex rather
than half-duplex mode. By moving from FSK to PSK encoding, the designers have dramatically
increased the efficiency of transmission. In 202 series, two frequencies are used to send different
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bits in one direction. In series 212, two frequencies represent two different direction of
transmission.
208 series The 208 series modems operate in full-duplex mode over four-wire teased lines.
Transmission is synchronous, using 8-PSK encoding. Like the 201 series, the 208 series modems
achieve full-duplex status by doubling the number of wires used and dedicating the equivalent of
an entire line of each direction of transmission. The difference here is that the encoding/decoding
technology is now able to distinguish btw. Eight different phase shifts. This modem has a baud
rate of 1,600. At three bits per baud (8-PSK creates tribit), that rate translates to a bit rate of
4,800 bps.
201 Series The 201 series modems operate in either half- duplex mode over two-wire switched
lines or full- duplex mode over four- wire leased lines. Transmission is synchronous, using 4-
PSK encoding, which mean that only one frequency is needed for transmission over each pair of
wire. Splitting the two directions of transmission into two physically separate lines allows each
direction to use the entire bandwidth of the line. This means, that with essentially the same
technology, the date rate is double to 2,400 bps (or 1,200 baud) in both half – duplex modes
(2,400 bps is still half the theoretical max data rate for 4-PSK encoding over two- wire phone
lines. 1,200 baud).
209 series The 209 series modems operate in full-duplex mode over four-wire leased lines.
Transmission is synchronous, using 16-QAM encoding. These modems achieve full-duplex
status by doubling the number of wires so that each direction of transmission has a channel to
itself. This series however, allows for use of the entire bandwidth of each channel. Each shift
represents a quadbit, with 16-QAM, the data rate 9,600 bps.
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2.2 ITU-T MODEM STANDARDS
The ITU Telecommunication Standardization Sector (ITU-T) is one of the
three sectors (divisions or units) of the International Telecommunication Union (ITU); it
coordinates standards for telecommunications.
The standardization work of ITU dates back to 1865, with the birth of the
International Telegraph Union. It became a United Nations specialized agency in 1947, and the
International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité
Consultatif International Téléphonique et Télégraphique) was created in 1956. It was renamed
ITU-T in 1993.[1]
ITU has been an intergovernmental public-private partnership organization since
its inception and now has a membership of 191 countries (Member States) and over 700 public
and private sector companies as well as international and regional telecommunication entities,
known as Sector Members and Associates, which undertake most of the work of the Sector.[2]
ITU-T has a permanent secretariat, the Telecommunication Standardization Bureau (TSB), based
at the ITU HQ in Geneva, Switzerland. The elected Director of the Bureau is Malcolm
Johnson of the UK. Johnson was elected by the ITU Membership to the directorship for a 4-year
term in November 2006 and was reelected for a second term starting January 2011.
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ITU-T Modem Standards
• V.21
• V.22bis (2nd generation of V.22 series) ~ two-speed modem (1200 bps or 2400 bps)
• V.32 ~ enhanced version of V.29 (9600 bps)
• V.32bis ~ (14.4 Kbps / 64-QAM)
• V.32terbo ~ (19.2 Kbps / 256 QAM)
• V.33 ~ enhanced version of V.32
• V.34 ~ (28.8 Kbps / 33.6 Kbps)
• V.42
• V.42bis
ITU-T/Bell Compatibility
Bell ITU-T Baud Rate Bit Rate Modulation
103 V.21 300 300 FSK
212 V.22 600 1200 4-PSK
202 V.23 1200 1200 FSK
201 V.26 1200 2400 4-PSK
208 V.27 1600 4800 8-PSK
209 V.29 2400 9600 16-QAM
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ITU-T modems that do not have equivalent in Bell series are given below:
V.22 bis The term bis means the second generation. The V.22 bis is a two-speed modem, i.e. it
can operate at either 1,200 or 2,400 bps. The used speed depends on the speed of the DCE at the
other end. In 1,200 bps mode, the V.22 bis uses 4-DPSK (dibit) encoding at a transmission rate
of 600 baud.
00 0° phase change
01 90° phase change
10 180° phase change
11 270° phase change
In 2,400 bps mode, the V.22 bis uses 16- QAM (quad bit)
V.32 The V.32 is an enhanced version of the V.29 that uses a combined modulation and
encoding technique called trellis-coded modulation. Trellis is essentially QAM plus redundant
bit. The data stream is divided into four bit sections. Instead of a quadbit, however, a quintbit
(five-bit pattern) is transmitted. The value of the extra bit is calculated from the value of the data
bits. V.32 calls for 32-QAM with a baud rate of 2,400. Because only four bits of each quintbit
represent data, the resulting speed is 4 * 2,400 = 9,600 bps.
V.32 modems and used with two-wire switched Line in what is called pseudo-duplex.
Mode pseudo-duplex is based on a technique called echo cancellation.
V.32 bit The V.32 bis modem was the first of the ITU-T standards to supported 14,400 bps
transmission. The V.32 bis uses 64-QAM transmission (six bits per baud) at a rate of 2,400 baud
(2,400*6 = 14,400 bps).
An addition enhancement provided by the V.32 bis is the inclusion of an automatic
fall-back and fall-forward feature that enables the modem to adjust its speed upward and
downward depending on the quality of the line or signal.
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V.32 turbo The V.32 turbo is an enhanced version of V.32 bis. It uses 256-QAM to provide a bit
rate of 19,200 bps.
V.33 The V.33 is also based on V.32 this modem, however, uses trellis-coded modulation based
on 128-QAM at 2,400 baud. Each signal change represents a pattern of seven bits: six data bits
and one redundant bit. Six bits of the data per change (baud) give it a speed of 6*2,400=14,400
bps.
V.34 This modem provides a bit rate of 28,800 bps, earning the nickname V.fast. It achieves this
rate by representing 12 bits with each signal change. In addition, the V.34 is designed to provide
data compression. With data compression, the V.34 can achieve data rates as fast as two to three
times its normal speed.
Telephone Line Requirements Vs Modem Speed
300 bps (Bell 103 & ITU-T V.21 protocol)
If this works, it means your line is at least as good as the top two strands of
barbed wire on a farmer's fence. A 300bps FSK modem will literally work over miles of barbed
wire. If 300 baud doesn't work, maybe the string between the tin cans is too dry? :-)
1200 bps (Bell 212A & ITU-T V.22 protocol)
212A is, much more sensitive than 103, but is not bothered by phase shifts that
higher speed modems cannot tolerate. If 1200 works and 2400 and up won't, it is likely that a
digital carrier system is experiencing "uncontrolled clock slips," which generate phase jumps that
212A protocol can just barely handle, while higher speed protocols cannot.
Uncontrolled clock slips can be detected by making a connection at 2400 bps with all error
correction turned off, and then looking for garble on received data that repeats the exact same
character sequences frequently.
A typical (and real) example looks like this:
Local> {{{r{{{{m{xD{{rw3{{r{{{m{t({{xD{{{{v{{{t(t^O5rw3{{v:{{
Xyplex -701- Command syntax error
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Note the repeated '{{{' sequences. There are others too, such as '{r', 'rw', 'xD' in that example.
Some instances might be just a single character repeated every few seconds or minutes. The key
is that it repeats the same sequence.
2400 to 12,000 bps (ITU-T V.32, V.32bis and V.34 protocols)
Connections limited to this range indicates severe channel impairment.
Usually noise and bandwidth restrictions cause connect speeds this low. Long cable runs of over
5 or 6 miles can also cause it, as can very old analog carrier systems.
14.4 kbps (ITU-T V.32bis and V.34 protocols)
This is the minimum speed a V.34 modem should be able to connect at if
telephone line is just able to meet specs. If this speed cannot be obtained either the modem is
defective or line is out of specification.
16.8 kbps (ITU-T V.34 protocol)
This is the minimum speed most V.34 modems will actually be able to
connect at on a minimally specified line. That means if modem can get this speed, the line must
be within specification.
This is also the upper limit for connections through various types of digital
carrier systems that use ADPCM or 32k bits per channel instead of standard 64 kbps PCM
encoding. Due to lower sampling rate and fewer bits per sample more quantization noise is
generated by the analog-to-digital conversions in these systems, thus reducing the Signal to
Noise Ratio (SNR) limiting speed.
19.2 kbps to 26.4 kbps (ITU-T V.34 protocol)
Speed in this range generally indicates a bandwidth limited channel. Some
older carrier systems and some digital systems that packetize and compress data are limited to
19.2 or 21.6 kbps. Long cable runs are also reason for connection speed of 26.4 kbps or below.
26.4 kbps, perhaps with occasional 24 and 28.8 (ITU v.34 protocol)
Usually indicates Subscriber Line Carrier (SLC) that has a "universal"
interface to the Telco switch. Such an interface adds an extra DA/AD conversion, which prevents
V.90/92 mode, and also adds at least 3 dB of quantization noise and a small amount of
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bandwidth restriction, all of which combine to usually disallow 28.8 kbps connection. Note that
for modems, which measure Signal-to-Noise Ratios, 37 dB is the best that can be obtained on a
connection through any form of 64 kbps channel PCM digital carrier. The specification for a
voice grade telephone line is only 24 dB.
28.8 kbps to 33.6 kbps (ITU-T V.34 protocol)
Indicates a line is so close to perfect that it would be difficult for Telco to
measure any change that would improve speed. This means SNR is better than about 32 dB,
channel bandwidth is about 3,400 Hz, and circuit must not be very long. 33.6 kbps simply means
the line is about as good as it can get.
2.3 INTELLIGENT MODEMS
Intelligent modems contain software to support a number in addition to modulation
and demodulation of functions, such as automatic answering and dialling.
Intelligent modems were first introduced by Hayes Microcomputers products, Inc.
more recently; other manufactures have come out with what are referred to as Hayes-compatible
modems.
Instructions in the Hayes and- compatible modems are called AT commands format is:
AT command [parameter] command [parameter]…
Each command starts with the letters AT followed by one or more commands, each of
which can take one or more parameters. E.g., to have the modem dial (408) 486- 8902, the
command is TD 4088648902
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Some AT commands
Command Meaning Parameters
A Put modem in answer The number to dial
B mode 0 or 1
D Use V.22 bis at 1200 0 or 1
E bps n
H Dial the number
L Enable/disable echo
P printing
T Put modem on/off hook
Adjust speaker volume
Use pulse dialing
Use tone dialing
AT Attenuation
DT Dial using DTMF tones
DP Dial using pulse dialing
FO HDX
F1 FDX
H Hang up
O Switch from command to on-line mode
Z Reset modem
+++ Switch from on-line to command mod
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Chapter-III
Conclusion
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CONCLUSION
Basically, modem is used to connect the analog world to the digital world. Modems
grew out of the need to connect tele printers over ordinary phone lines instead of more expensive leased
lines which had previously been used for current loop–based tele printers and automated telegraphs. AT
& T laboratory introduced many modems. The Bell standard, IBM, ITU-T standard for modems
paved a more attention towards the users. It marked a prominent start and now, this has evolved
a lot. Nowadays Many companies have come up with their one unique and different product all
of which have different functionalities, but some standard protocols is only liked by a user.
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Chapter-IV
Bibiliography
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BIBILIOGRAPHY
http://en.wikipedia.org/wiki/Modem
www.doc.ic.ac.uk/~costa/cn_slides/v34.pdf
en.wikipedia.org/wiki/List_of_modem_standards
www.scribd.com/.../Data-Communications-and-Networking-by-Behr...
www.4shared.com/rar/.../Data_Communication_and_network.html
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Chapter-V
Appendix-A
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Four modem usb 3G Key
Cable Modem and Router
Connection to Modem
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Bell-Modem
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