Micro #25 21 января 2000

Интернет

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.