Network - modem communication and computer networks.
Modem communication and computer network
1. INTRODUCTION
Rare serious business man, a professional programmer or
system The operator can imagine the full work without the use
of such powerful, efficient and convenient combination of both
ordinary telephone line, modem and computer network. While the
first two constitute only the technical side of the new
organization of information exchange between users, computer
network - This is a global idea of combining
disparate owners of computers and modems, organize and control
requirements and randomly demands for rapid information
services, instant processing of offers, services
konfedentsial'noy personal correspondence, etc. etc.
Now, given multiple times increasing every year, information
flows, is almost impossible to imagine a strong relationship
banking structures trading and brokerage firms, government
agencies and other organizations without modern computer
technology and computer networks. Otherwise
would contain a giant staff of handlers of paper documents and
couriers with reliability and speed of such a system would
still be significantly below provides dial-up communications
and computer networks. But every minute of delay in sending
important messages may result in very tangible financial loss
and image meltdown.
Is inconceivable without a modem system of electronic
communications. This device allows join the fun, but today,
using the latest inventions of the world of telecommunications,
and just have a vital, world information flows electronic
databases, e-mail electronic manuals, electronic bulletin
boards, and more. The possibility of obtaining and sharing
information using modems today can not be overestimated, and
that awaits us tomorrow, we can not even imagine. E letter sent
by e-mail anywhere in the world, will reach the destination in
less than two hours. We can place any advertisement or
advertising In a teleconference on your network
e-mail address and information through the
day finds the whole world (unless, of course,
This is very willing). Via modem
You can, for example, from Moscow to connect
directly to the server in New York and work
with information databases, which
it contains. Finally, we can send
fax. Even today, no solid brokerage firm can not do without the
operational and transmitting information using computer
communication channels and, consequently, modems.
2. WHAT IS A MODEM AND HOW IT WORKS
When the computer is used to exchange
information over the telephone network, it is necessary
device that can receive the signal
from the telephone network and convert it into
digital information. The output of the device information is
subjected to modulation and demodulating at the entrance, hence
the name MODEM. Appointment of a modem is to replace the signal
from the computer ( combination of zeros and ones), electric
signal with a frequency corresponding to the working range of
the telephone line. Acoustic channel of the line modem shared
on bands of low and high frequency. Band
Low frequency is used to transfer
data and high frequency - for
admission. Used many ways to encoding information, the most
famous which are the method of FSK (Frequency
Shift Keying) for speeds of up to
300 baud (baud - unit rate
information, which is equal to 1 bit / s) and method of PSK (
Phase Shift Keying) for a faster modem, transmission speed to
2400 baud.
FSK uses four selected frequencies. The transmission of
information signal with frequency 1070 Hz is interpreted as a
logical zero, and the signal frequency of 1270 Hz - as logical
unit. When receiving a signal from zero corresponds to 2025 Hz,
and the unit - 2225 Hz.
PSK uses two frequencies: data - 2400 Hz, for the reception
- 1200 Hz. Data is transmitted by two bits, while
encoding is carried out by the phase shift signal. Use the
following phase shifts for encoding: 0 degrees to
combination of bits 00, 90 degrees to 01,
180 degrees for 10 and 270 degrees for 11.
There are also other types of modulation (DPSK, QAM, TCM).
The modem is performed either in the form of an external
a device that one output is connected to a telephone line, and
the other to standard COM-port (connector
RS232 on the recommendations of CCITT V.24), or
in the form of an ordinary printed circuit board, which is
installed on a common bus computer. Internal modem options can
be adapted as a standard ISA, so and PCI buses.
The controller is a modem - it is usually
dedicated microcomputer type
SC1107 and SC1108, containing the eight-ALU, ROM 8 Kbytes, RAM
128 bytes timer, a command register, interrupt controller,
stack, I / O port. If board modem connected to the system bus
PC, then use "parallel" controller SC1107. If the board works
with the computer via RS232, is used "Serial" controller
SC1108. In Some constructions of the role of controller
performs processor 8031 with the external ROM
(I2732, 2764) and IC 74LS373.
3. INTERNATIONAL STANDARDS FOR MODEMS
The most widely used because
called HAYES-compatible modem, for
behalf of the manufacturer of one of the first
modems. These modems use AT-commands (from the English word
ATtention), Compatible with Hayes Smartmodem. Except
standard for all Hayes-compatible
modem command set, each manufacturer
alone offers the user a wide range of specific commands that are
force only in models of this brand (eg,
USRobotics, Rockwell, ZyXEL, etc.).
In addition to the compatibility of a set of commands
modem should comply with any
standard data transmission over telephone lines. These
standards are recommendations of the CCITT (International
Consultative Committee on Telegraph and Telephone, France.
CCITT (Comite Consultatie International Telegraphique et
Telephonique). In the U.S. and Canada, there are a similar
standard Bell, the only difference from CCITT is lexical.
Below, in Table 1 are the most common standards for modems.
Modems that meet the standards for
speed to 2400 baud, can freely exchange information. It should
be noted that the recommendation CCITT V.32 is not
standard in the full sense of the word,
Because virtually every major manufacturer of modems speeds
above 2400 baud has a habit of additional applications
one or more specific data transfer protocols. Their use
possible only with regard similar modems, and thus achieved, as
Typically, a higher transmission speed,
noise immunity and speed of the connection.
The most common and cheapest (
why, and enjoying success in the user's preference) is a
protocol HST (High Speed Transfer), developed
by USRobotics in the late 80's.
There are varieties of this Protocol:
H96, H14, H16, H19, H21, H28, a difference which is only in the
speed of information transmission, which is respectively 9600,
14400, 16800, 19200, 21600 and 28800 baud. Due to low cost, wide
upgradeability, and high noise immunity and high-speed data
protocol HST users prefer to buy well-known model, USRobotics,
such as the Sportster, Worldport, Courier.
Widespread and
Modems firm ZyXEL, having a specific protocol ZYX, enabling the
data transmission speed of 19200 baud full duplex. Increasingly
popular modems ZyXEL purchased in the early 90's solely because
of unavailability for domestic buyers of other brands of
modems. Their main drawback - high price deters a wide range of
consumers. But Despite this, banking institutions and
government agencies on the basis of tradition, prefer modems
this firm.
Less common, very expensive,
but has a strong and stable signals, ability to ignore even the
protective filters that put the PBX in order to avoid
free use of cable modems. This modem company Telebit
TrailBlazer and brand famous protocol PEP (Packet Ensemble
Protocol).
Virtually all high-speed modems
consistent with less rapid standards.
4. MODEM AS A MEANS OF COMMUNICATION BETWEEN COMPUTER
TERAMI
If one computer at work
two people, they already have a desire to use this computer to
share information with each other. On larger machines, which
use both dozens, even hundreds of people for this
There are special programs that allow users to send messages to
each other, and an administrator - to notify users about the
news in the system.
Needless to say that as soon as
You can combine several
machines in the network, users have seized upon
This opportunity not only to
use the resources of remote machines, but also
to broaden their communication. In
within the enterprise, small town or
just a limited area may
creation of an ordinary local area network based on
standard Ethernet or Arcnet and combining them by standard
cables. But when it comes to connecting computers at a distance
of many thousands of kilometers from each other, then
immediately gets up quite a reasonable question: why not use
such an old and proven means of communication such as telephone
lines? After all, it is no secret that the entire globe in the
literal sense the words "wrapped" strings of telephone cables.
Appointment of one of these cables - to transmit
signals of certain frequencies. That's when
It took a mysterious device
Modulation / demodulation, which could
translate information into signals of a certain frequency. The
first such device was submitted by developers and CCITT
standardized in 1964.
In parallel with the hardware inventions
began to go out and software, providing a convenient, affordable
and the simple dialogue in a chain of modem-computer-man.
Created programs designed messaging users located on different
machines. Because of the variety of computers, operating
systems, ways to connect machines to the network and objectives
persecuted while people of these programs turned out to be
quite a lot and they do not always compatible with each other.
Almost each programmer is able to create such a "mailer" on the
basis of which could to create a computer network.
So we came very close to the time when you should determine,
or at At least, identify the key services
which provides us with the acquisition of a modem.
4.1. SERVICES
4.1.1. TRANSFERRING FILES
If we want to quickly pass a file to our friend or a member
of Vladivostok, then we can proceed in two
ways. Someone chooses to write this
file to the floppy and buy a plane ticket
hire a courier and request him to deliver
this floppy on such an address. And someone
reasonably decide to buy a modem USRobotics brand COURIER
company and within minutes, using the existents in every
respectable establishment of a telephone line to transmit the
necessary information in Vladivostok. For this purpose, in
addition to the telephone line, you must have only as existents
in every decent institution 6kompyuter and simple communication
program.
4.1.2. Electronic bulletin boards
Second, no less important service is
to use with a modem so-called electronic bulletin board
(Bulletin Board System, later BBS). Physically, they represent
a sufficiently powerful PC is usually an IBM-compatible, the
based 486 or Pentium processor with special software (Maximus,
Remote Access, PCBoard, etc.), which
via a modem connected to a conventional telephone
network. BBS node contains a large number
useful (and not) software very different trends are logically
broken down by category. Operating system mode on-line, we can
find a list of suggested files and "Pumped" on your computer,
all interested us. In addition to the BBS are accessible area
of personal and public correspondence between the users of
this station. Thus it is possible to place a commercial ad, ads
for the search software foolish to anonymous letters, and
declarations of love and devotion. Quite different question
that the unusual behavior can be at the behest of the System
Operator (hereinafter sysop) to lose future access to the BBS.
The principle of "you give me - I tell you," is widely used
in the systems of electronic bulletin boards. Each registered
the user is strictly limited Sysop circadian period of time to
implement its intentions and desires. This sometimes not even
enough, List available to take on this
BBS Files (Filelist). The user must deliberately seek to please
Sysop, send him the most interesting
and seldom, which may interest him, warning of the latest news,
gossip, and as often as possible to treat
beer. It was the last action the most
significant impact on raising the level of access (Access
Level) to the user of the BBS.
Based on the existence of electronic bulletin boards, our
old friend from Vladivostok could do much smarter: knowing at
what his friend from the BBS Moscow is a registered user, and
using his password to login the system, it leaves the files
in the territory of the load of users. In
turn, Muscovite, at his convenience
time, similar to calling this BBS, and takes it passed
Vladivostok Assistant (employee) files.
There are many classifications of nodes BBS. They are
amateur or professional, commercial and free, 24 hours and with
the only restriction of time work (usually at night, the day is
it regular voice phone), various
compatible terminal emulation (TTY, ANSI-BBS, VT102, VT52,
AVATAR, ANSI) and hardware support.
For professional BBS include large network servers such as
Elvis, Izhma, Kiae, Simte, Chci and others in the network
Relcom, and well as small commercial and advertising
station. Their main differences - the provision of access for
the monthly fee, a 24-hour work schedule, a large selection of
the proposed software, professionally paid Sysop, etc.
The same characters are sent to the
computer can, according to the standard adopted by the BBS
terminal emulation perceived totally differently. In some of
the above standards the most widely used ANSI and ANSI-BBS,
which in principle are quite subtle differences in the general
compatible.
4.1.3. E-MAIL
Third, the most interesting and convenient
service - e-mail. This is the most
universal means of computer communication. It allows you to
send messages (Files, information) with virtually any
machine on any, since most of the known PCs running on
different systems, support it. E-mail - it's
A global network of communication, where
can combine a variety of computer configurations, and
compatibility. The main distinguishing features
e-mail from the BBS are:
- A wider range of users, hence more opportunities for
communication, advertising and search. Of course, because the
BBS brings only a limited range of their
users (average 300), and the network can reach hundreds of
thousands of computers around the world! - Working in the
off-line, where no permanent existents on the mail node sysop.
Suffice it to indicate a special program-mailer (Mailer) during
system events, and address where you want to collect mail,
everything else will happen automatically!
- Access to newsgroups (Echo Conference)
- Access to newsgroups file (File Echo Conference)
- A wide range of other services
Network usually consists of several components that provide
the most rapid and secure exchange of information
within the network. The highest level are the primary Hub-nodes
(born Hub - the focus of interest) (typically three or four
powerful server). Followed by secondary, tertiary, etc.
Hub-nodes. Number levels and the number of Hub-nodes,
determined by the size and adopted the structure of the
network. This network "backbone", usually consisting of
modern computers working around the clock. Each Hub-node
attached Non Hub-nodes (Node), taking the set time (ZMH - Zone
Mail Hour) e-mail. Node, in turn, may also
have a Point-station, is the lowest stage of the network
hierarchy. Node from which the station receives mail nazyvetsya
uplink (Uplink), and who pays - the downlink (Downlink). We
illustrate the scheme in figure 1.
Fig.1 Scheme of the network hierarchy with two
levels of Hub-nodes
Points Points Points Points
Node1 Node2 Node3 Node4
2-Level Hub Jan. 2-Level Hub 2
Node9 1-Level Hub 1 (1)
Points Points (2)
(3)
2-Level Hub 5
Node11 Node12
Points Points
-----------------------------------------
Points Points Points Points
(1)
Node5 Node6 Node7 Node8
(2)
2-Level Hub on March 2-Level Hub 4
(3)
1-Level Hub 2 Node10
1-Level Hub 3 Points
2-Level Hub June 2-Level Hub 7 Points
Node13 Node14 Node15 Node16
Points Points Points Points
As we see, Hub-nodes are different from
Node only in that they have to not only as the downlink station
Points, and Node. The higher level hub, so, usually less than
He launlinkov Status Point and Node, since the main burden of
work site rests with the Hub-nodes of the lower level.
The above structure is usually maintained within fairly
closed and compact areas (eg, city). A
exchange of mail between such localities is through one of the
Hub-nodes (which has the desire, capacity and authority
to communicate with other zones, networks, regions).
We should add that the scheme presented
rather simplified and represents
rough picture of routing mail
FTN-compatible networks, which will be
described below.
Teleconferences are a powerful means of communication
between network members. Their meaning clearly visible in their
English title (Echo Conference), which in a primitive sense,
means: one node auknulos - the others echoed. Assume 50% of the
network is strongly concerned about the quality of beer
produced in Russia. For discussion of the topic created
so-called conference, where each signer it may publicly express
their opinions, advise the questioner to refute the misguided,
and his thoughts see all who read this conference. By
essence of the idea of teleconferencing is a lot like
the idea of newspapers or privately financed toll free
classified ads, only with the difference teleconference that
much operative in delivery of information and for it not
necessary to go to the mailbox. Computer conference can be
useful for those who want learn about new products, books or
movies through it very convenient to distribute information
about the bugs in the programs and how to fix them, it just
indispensable for fans to chat on his favorite subject with
their peers in all corners of the earth, and, of course, for
scientific discussions. With the help of the conference You can
discuss the topic that interests in such companies, which
collect in one place for a personal interview would be worth
big money and unpredictable amount of time and effort.
List of existing groups, only takes a few pages. It is possible
to find groups for specialists in ancient Greek culture and for
fans of rock music, to discuss sex and exchange cookies
recipes, a discussion about women's rights and groups devoted
to different computer Games.
Everything is simple enough. In
Each network existents list of allowed
and affordable teleconferencing (Backbone),
which necessarily existents at all
Hub-nodes of the higher levels. Hub-nodes of the lower
Levels and Node-nodes, depending on the interested in this
newsgroup can subscribe to receive it. Thus, it becomes
possible to access Conference and downlink of the site. Not
true is very similar to subscribe to the print edition of the
nearest branch communications? Only in the electronic mailboxes
boys will not be able to pour the wet snow, the attackers -
break it, and treacherous neighbors - empty.
File teleconference differ from
Usually only the fact that as a message
in them, there are no letters, and files. For example, create a
file teleconference, dedicated to music, where everyone can put
the file (song, converter, text) and each signed it certainly
will.
In addition to the above, there are a number of other
opportunities offered by network members. You can send an order
for sending or receiving faxes. Drawn
regular e-mail that has been formulated
properly, and sent to the address
computer node in charge of fax operations. The text of the
letter a fax will be delivered to the fax
destination device.
More recently, in some networks
practice service for access to electronic databases, which was
first embodied in a number of departmental networks. Although
it is one of the growing industries in the information
industry, but There are high-quality professional
domestic policy framework that allows developers to quickly and
easily take advantage of DB. According to IAS (Institute of
Automated Systems, Moscow) are most often used a database on
natural sciences, with the ability to search by chemical
formulas of various substances, the search of physical
parameters, the search for scientific journals. Able to obtain
information on any foreign firm, enterprise. There are various
bases Data on medicine, agriculture,
periodical publications (refereed and
polnotekstnye), coursework, degree projects, space, politics,
sports and etc.
The newest and most promising
form of dial-up and power
Computer network is an electronic
Exchange. This is a very reliable and fast way
find a business partner, supplier of goods / raw material,
purchaser of the product, etc. The main challenge is in
attracting of the digital exchange as much as possible
number of potential users and develop the most suitable and
convenient software.
Also note that with the recent
Time began periodic dissemination of publications through
computer networks. More and more, can now be encounter such
seemingly incomprehensible notes at the end of our legible
favorite newspaper or magazine: "The last numbers ... You can
get on the server ... Network ... . Moreover, the scope of
concepts with each "..." month, becoming wider and wider.
Motley variety of computer networks
generates the urgent problem of Integrating
or, at least at the level of connection
messaging. Different networks are different ways of connecting
machines to each other, the speed with which messages are
transmitted, a system by which machines are given names, and
agreements on how and in what form should be a message (for
example, the maximum size of the letter, which takes e-mail, or
write an address is large or small letters). To
to send a message from a machine connected to a network, a
machine to another network, we must find an intermediate machine
connected to both, through which the message and go. Such a
machine is called Gateway (Gate) between these networks.
Clearly, between the two networks may be multiple gateways
(though there may be no good, and if the messaging is not, or
He goes through an intermediate network, which is a gateway for
both). Gateway Takes message from the network A, rewrites it in
form adopted in the network B, and sends it via the B on that
machine, for which it intended. Unfortunately, in this case
markedly slowed down while the addressee messages, but usually
all the same it takes a shorter period, rather than using the
services of the usual "paper" mail.
Other articles:
Network - modem communication and computer networks.
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