entranceAccording to which parameter is determined by the receiving level of the modem. Initial configuration of a new modem
2. Classification of modems. Comparative analysis of various classes. Characteristic evaluation.
2.1 Classification of modems
At first glance, there is nothing easier than classifying modems. It goes without saying that they are divided into external and internal. Of course, someone can offer them to divide in speeds (14,400 bits / s, 28,800 bps, 33600 bps, 56k), and, last time, will recall the possibility of transmitting data in synchronous and asynchronous modes. However, it is a swing from the height of a completely ptichy flight. Near everything looks far away.
We will try to classify the devices entrusted to us.
And so, let's start with the fact that there are differences, designed to work only on the allocated or only on switched lines, as well as on those and others. Distinguish modems for digital and analog lines.
Depending on the supported data transfer mode, modems are divided into:
supporting only asynchronous operation mode;
supporting asynchronous and synchronous modes of operation;
supports only synchronous mode of operation.
By execution (this characteristic determines the appearance, dimensions and placement of the modem relative to the computer):
internal modem - inserted into the computer as an extension board. They, in addition, are divided into controller and non-beam. The first one has a majority of existing internal modems intended for ISA interface. Second - for PCI interfaces. The further development of PCI modems are SOFT modems (otherwise Win modems).
desktop modem - has a separate case and is located next to the computer, connecting the cable with the computer port. Sometimes called an external modem that is not entirely correct, because The following two types are also external (i.e. located outside system Block Computer).
the modem in the form of a card is a miniature and connects to a portable computer through a special connector (one who saw a laptop network card will understand what is being spent).
portable modem - similar to a desktop modem, but has a reduced dimensions and autonomous food.
the rack modems are inserted into a special modem rack that increases the convenience of operation, when the number of modems turns over a dozen.
By the nature of the use of modems can be divided into ordinary and professional.
Under conventional modems, we will understand the devices usually used by the end user at home or in the office. These modems use only phone channels.
Professional modems are the most advanced and high-speed devices, mostly rack. Used to integrate local networks, in modem pools, as well as for remote access to LAN resources.
Among the usual modems, 3 types can be distinguished:
data exchange devices (simply modems);
devices for exchanging data and documents (fax modems);
document exchange devices, documents and reception voice messages (Voice fax modems).
It should be noted that usually dashing data and a telephone conversation cannot be kept simultaneously. An exception is the SVD modem and RadishVoiceView technology, designed for simultaneous voice and data transmission.
Support for facsimile regime is not excluded in professional modems, they usually do not provide for oodo sound support.
As another classification feature, select the transmitted environment. By type of transmitting medium, you can allocate:
modems for 2 wired copper lines (ordinary, professional, ADSL, SR, ER modems);
modems for 4 wired copper lines (ordinary, professional, HDSL, ISDN, SR, ER, MR modems);
modems for fiber optic lines (FOM, FOM-T1 / E1, FOM-T2 / E2, FOM-T3 / E3);
2.2 Comparison of modem characteristics for selected and switched channels
2.2.1 Modems for selected channels
The dedicated channel is a channel with a fixed bandwidth or a fixed bandwidth that constantly connects two subscribers. Subscribers can be both separate devices (computers or terminals) and entire networks.
Dedicated channels are usually rented in companies - operators of territorial networks, although large corporations can lay their own dedicated channels.
Selected channels are divided into analog and digital, depending on what type of switching equipment is applied to the constant switching of subscribers. On the analog dedicated lines for data equipment, physical and channel protocols are not rigidly defined. The absence of a physical protocol leads to the fact that the bandwidth of the analog channels depends on the bandwidth of the modems that the channel user uses. The modem itself and sets the physical level protocol you need for the channel.
On digital dedicated lines, the physical layer protocol is fixed - it is set by the G.703 standard.
Modems for work on highlighted analog channels
To transfer data to dedicated analog lines, modems are used based on the analog signal modulation methods. The protocols and modem standards are defined in the CCITT Recommendations of the V series. These standards determine the operation of the modems for both the selected and switched lines.
As mentioned in paragraph 2.1, the modems can be synchronous, asynchronous and synchronous asynchronous.
Modems operating only in asynchronous mode, usually maintain a low data transfer rate - up to 1,200 bits / s. Thus, modems operating according to standard V.23 can provide a speed of 1200 bps on a 4-wire dedicated line in duplex asynchronous mode, and according to the V.21 standard - at a speed of 300 bits / s for a 2-wire dedicated line also in Duplex asynchronous mode. Duplex mode on a 2-wire end is provided by the frequency separation of the channel. Asynchronous modems represent the cheapest mode of modems, since they do not require high-precision signal synchronization schemes on quartz generators. In addition, the asynchronous mode of operation is unpretentious to the quality of the line.
Modems operating only in synchronous mode can only be connected to a 4-end end. Synchronous modems are used to highlight the signal, high-precision synchronization schemes and therefore are usually significantly more expensive than asynchronous modems. In addition, the synchronous mode of operation places high quality quality requirements.
For the dedicated tone frequency channel with a 4-wire termination, a sufficiently many standard series of V series has been developed. All of them support duplex mode:
V.26 - 2400 bt / s transfer rate;
V.27 - 4800 BIT / C transfer rate;
V.29 - transmission speed of 9600 bps;
V.32 Ter-transmission rate of 19,200 bits / s.
For a highlighted broadband kocal 60-108 kHz, there are three standard:
V.35 - transfer rate of 48 kbps;
V.36 - 48-72 Kbps transfer rate;
V.37 - transfer rate 96-168. Kbit / s.
Error correction In synchronous operation, it is usually implemented via HDLC protocol, but the Outdated SDLC and BSC protocols of IBM are allowed. Modems of Standards V.35, V.36 and V.37 are used to communicate with DTE interface v.35.
Modems operating in asynchronous and synchronous modes are the most versatile devices. Most often, they can work both on the dedicated and switched channels, providing duplex mode of operation. On the selected channels, they mainly support 2-wire ending and much less often - 4-wire.
For asynchronous synchronous modems, a number of standards of the V series have been developed:
V.22 - transfer rate up to 1200 bps;
V.22 BIS - transfer rate up to 2400 bps;
V.26 TER - transmission speed up to 2400 bps;
V.32 - transmission speed up to 9600 bps;
V.32 BIS - transfer rate of 14,400 bps;
V.34 - transfer rate up to 28.8 kbps;
V.34 + - transfer rate up to 33.6 kbps.
Standard V.34, adopted in the summer of 1994, marks a new approach to data transmission over the tonal frequency channel. This standard was developed by CCITT for quite a long time - since 1990. A great contribution to its development was made by Motorola, which is one of the recognized leaders of this industry. Standard V.34 was developed for transmitting information on the channels of almost any quality. The feature of the standard is the procedures for dynamic adaptation to changes in the channel characteristics during information sharing. Adaptation is carried out during the communication session - without termination and without breaking the established compound.
The main difference of V.34 from the previous standards is that it defines 10 procedures for which the modem after testing the line selects its basic parameters: carrying and bandwidth (the selection is carried out from 11 combinations), transmitter filters, optimal transmission level and other . The initial compound of modems is carried out according to the V.21 standard at the minimum speed of 300 bits / s, which allows you to work on the worst lines. To encode data, excess QAM quadrature amplitude modulation codes are used. The use of adaptive procedures immediately allowed to raise the data transfer rate of more than 2 times compared with the previous standard - V.32 BIS.
The principles of adaptive setup to the parameters of the line were developed in the V.34 + standard, which is an advanced version of the standard V.34. Standard V.34 + allowed to slightly increase the data transfer rate due to the improvement of the coding method. One transmitted code symbol carries in a new standard on average not 8.4 bits, as in protocol V.34, A 9.8. At a maximum transmission rate of code symbols in 3429 baud (this limitation cannot be overcome, since it is determined by the tone channel bandwidth) an improved coding method gives the data transfer rate of 33.6 kbps (3429 x 9,8 - 33604). True, experts note that even in America, only 30% of telephone lines will be able to provide such a low interference levels so that the V.34 + modems can work at maximum speed. Nevertheless, the modem of the V.34 + standard have advantages compared with V.34 modems, even on the roaring lines - they are better to "hold" the connection than the modem v.34.
Protocols V.34 and V.34 + allow you to work on a 2-wire dedicated line in duplex mode. Duplex transmission mode in standards V.32, V.34, V.34 + is provided not using the frequency separation of the channel, but by simultaneous data transmission in both directions. The received signal is determined by subtracting using the signal processors (DSP) of the transmitted signal from the common signal in the channel. Echo suppression procedures are also used for this operation, since the transmitted signal, reflecting from the near and far ends of the channel, makes distortion in the general signal (data transfer method described in the 802.3AB standard defining the operation of Gigabit Ethernet technology on twisted pair of category 5 , I took a lot from the standards V.32-V.34 +).
At high speed modem V.32-V.34 + in fact, synchronous mode is always used in the communication channel. At the same time, they can work with DTE both by asynchronous interface and synchronous. In the first case, the modem converts asynchronous data to synchronous data.
Modems for work on highlighted digital channels
Digital selected lines are formed by constant switching in the primary networks built on the basis of switching equipment operating on the principles of channel separation in time - the TDM described in Chapter 2. There are two generations of digital primary network technologies - Pleeio-Chronoi technology ("Plesia" means " Almost ", that is, almost synchronous) digital hierarchy (PLESOCHRONIC DIGITAL HIERARCHY, PDH) and later technology - synchronous digital hierarchy (Synchronous Digital Hierarchy, SDH). In America, SDH technology meets the SONET standard.
Digital multiplexing and switching equipment was developed at the end of the 60s at AT & T to solve the problem of communication of major telephone network switches. Channels with frequency seal applied before this in the PBX-PBX sites have exhausted their capabilities on the organization of high-speed multichannel communication by one cable. The FDM technology for simultaneous data transmission 12 or 60 subscriber channels was used to be a twisted pair, and to increase the speed of communication, it was necessary to launch cables with a large number of wires of wires or more expensive coaxial cables. In addition, the frequency seal method is highly sensitive to various kinds of interference, which are always present in the territorial cables, and the high-frequency carrier speech itself creates interference in the receiving equipment, being poorly filtered.
To solve this problem, the T1 equipment was developed, which allowed multiplexed, transmit and switch (on an ongoing basis) data 24 subscribers in digital form. Since subscribers still used by conventional telephones, that is, the transfer of voices was in analog form, the T1 multiplexers themselves were digitized with a frequency of 8000 Hz and encoded a voice using a pulse code modulation (Pulse Code Modulation, PCM). As a result, each subscriber channel formed a 64 kbps digital data stream. To connect the main PBX, the T1 channels were too weak multiplexing tools, so the technology was implemented by the idea of \u200b\u200bthe formation of channels with the velocities hierarchy. Four channels of type T1 are combined into the channel of the following digital hierarchy - T2, transmitting data at a rate of 6.312 Mbit / s, and seven channels T2 are given when combining the channel T3, transmitting data at a speed of 44.736 Mbps. Equipment T1, T2 and T3 can interact with each other, forming a hierarchical network with trunk and peripheral channels of three speeds.
From the mid-70s, the dedicated channels built on the T1 apparatus began to give up by telephone companies for rent on commercial terms, having ceased to be internal technology of these companies. T1 networks, as well as more speed T2 and T3 networks allow us to transmit not only the voice, but also any data presented in digital form - computer data, television image, faxes, etc.
The digital hierarchy technology was later standardized by CCITT. At the same time, some changes were made to it, which led to the incompatibility of American and international versions of digital networks. The American version is distributed today except the United States also in Canada and Japan (with some differences), and in Europe an international standard applies. The analogue of channels T in the international standard are inali type E1, E2 and eZ with other speeds - 2.048 Mbps, 8.488 Mbps and 34,368 Mbps. The American version of the technology was also standardized ANSI.
The physical level of PDH technology supports various types of cables: twisted pair, coaxial cable and fiber optic cable. The main option for subscriber access to Channels T1 / E1 is a cable of two twisted pairs with RJ-48 connectors. Two pairs are required to organize a duplex data transfer mode at a speed of 1.544 / 2,048 Mbps. To view signals, uses: in channels T1 bipolar potential B8ZS code, in the channels EL-bipolar potential HDB3 code. To enhance the signal on the T1 lines every 1800 m (one mile), regenerators and the line control instrument are installed.
Coaxial cable due to its wide bandwidth supports channel T2 / E2 or 4 channel T1 / E1. To work T3 / E3 channels, either a coaxial cable or a fiber optic cable, or microwave channels, is usually used.
Thus, modems designed to work in digital dedicated lines belong to the following classes:
modems for 4 wired copper lines;
modems for fiber optic lines;
modems for radio channels (radio modem, cellular modem);
cable modems (use coaxial cable).
In more detail, they will be discussed below.
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Selection of modem.
All you need to know about the modem operation: a modem is a device that allows you to connect computers through the telephone network. Opportunities available to you with such a connection are defined exclusively by software that you will use, and the quality of the modem itself determines the connection speed. All characteristics of the modem that you should know:
All other characteristics of modems are of interest only to specialists.
External modems work, as a rule, better than internal, visually - light bulbs flashes on the panel, and produce a stronger impression on your friends (the more modem and the more light bulbs on it - the stronger the impression), but the internal occupy less space in your Room (since it is located entirely inside the computer).
By purchasing a modem and connecting it with a computer (or putting it into a computer), you can call for sample and in order of curiosity to call Data Force IP (tel. 755-9363) and get the necessary data for a trial connection to the Internet.
External modems
To connect an external modem to a computer, it is necessary (and enough) so that it has a free serial port (som port) and a cable for connecting a modem with this port. Usually there are two consecutive ports in the computer, "Mouse" will be connected to one of them. Connectors of serial ports are 9-pin and 25-pin. Usually, the computer has one 9-pin connector (the "mouse" is connected to it) and one 2 5-pin (if you do not have a modem, then this jack is usually free), both - the "dad" type, that is, with pins. The modem usually has a 25-pin connector of the "Mom" type, that is, with holes. In this case, you need a Mama-Dad type cable, from both sides of which 25-pin connectors. If the computer is free only a 9-pin connector, then you need a cable that has a 9-pin "mother" and a 25-pin "dad". You can almost certainly purchase the cable in the same place where I purchased a modem.
If you purchase a high-speed modem, then for you the important characteristics of the port of your computer are becoming important. You need you to have a high-speed serial port (for example, Magic words-UART16550A). Usually, there is a number of light bulbs on an external modem, each of which two beaks are signed. Here are the most common symbols:
- HS - High speed
- AA - Willingness to answer
- CD - carrier frequency detected
- It is the initialization of the dialing
- RD - Data Receive
- SD - Sending data
- Tr - readiness for work
- MR- modem included
- RS - Data Transfer Request
- CS - readiness for sending data.
If you have purchased an internal modem, pay attention to the following: as standard, the computer usually has two consecutive ports, denoted by som1 and COM2. In fact, serial ports may be more. Internal modems have a built-in serial port and there are jumpers (Jumpers), with which you can set, which number will be from this port and after which interruption will need to work with it. As a rule, the factory setting is COM3 or COM4. However, the IBM PC architecture initially did not provide for the presence of several consecutive ports from the computer, and access to such ports is organized via the "Interrupt request" - Interrupt Request - IRQ.
Two IRQ - IRQ3 and IRQ4 are usually allocated for operation with sequential ports. Between the first four sequential ports, these IRQ are distributed as follows:
- Som1 - IRQ4
- COM2 - IRQ3.
- COM3 - IRQ4
- COM4 - IRQ3.
Modem speeds
The speed of the basic variants of modems (in order of speed increases): 2400 Bod, 9600, 14400, 19200, 21600, 28800 and 33600.
Higher speeds on Russian telephone lines are difficult to achieve. Any modem is capable of working not only at its maximum speed, but at all lower speeds. Full range of speeds: 300, 1200, 2400, 4800, 7200, 9600, 12000, 14400, 16800, 12000, 14400, 16800, 19200, 21600, 24000, 26400, 28800, 31200, 33600. That is, a modem on 33600 bodes is able to work on all speeds here .
The speed of the modem 2400 baud means that a second is sent 300 bytes (byte \u003d 8 bits, one character), per minute - 18 kilobytes, per hour - 1 megabyte. The speed of 28800 baud means that 3600 bytes are sent per second (per minute -216 kilobyte, in an hour - 13 megabytes).
Really, the modem efficiency is usually below the shipping rate - due to the low quality of the telephone line there are two or three (and even more) to repeat the transfer of information portions.
Modems protocols
To combat poor quality telephone lines, various correction protocols and data seals have come up with.
Main protocols:
- Bell 209A 9600.
- V.29 9600.
- V.32 9600.
- V.32BIS 14400.
- V.33 14400 V.32TERBO 19200
- V.34 28800 and above
- V.FC Simplified Option
- V.34 HST 16800 and above
- ZYX 16800 and above
- others.
In case the speed of the steady connection does not suit you (all modem programs are always reported by the user this information), try calling back again - the connection through the telephone network occurs every time different wires, and it is likely that another connection will be better quality .
On Russian telephone lines, the best results are given by the HST and ZYX protocols. ATTENTION: Modems that have only V.34 are connected to modems, which also have only V.34 protocol, at speeds not higher than 14400.
Fax modemes
A fax modem is such a modem that is capable of taking (and stored on a hard disk) faxes and send faxes specially prepared on the computer.
The received faxes through a special program to work with a fax modem can be printed on the printer.
In the preparation of the sent fax nothing complicated, on the contrary, you do not need to print beautiful font On the printer, what you are going to shove into a fax apparatus - in many test editors there is an opportunity to turn the document with which you work to fax (or even immediately send it to a fax modem).
But if you work with the Internet, your modem does not have to be a fax.
Introduction
The development of computing networks demanded transmission during intermasket exchange
large digital information with high speed and loyalty.
That is why the problem of designing channels of channels arose
data transmission effectively using existing bandwidth
continuous telecommunication channels and based on modern techniques and
technologies of digital integrated circuits.
Basic functions in coordinating sources and data receivers with continuous
frequency-limited channels entrusted to signal conversion devices
(UPS), which largely define such characteristics of digital
channels like speed and loyalty. Therefore, the development of UPS providing
required information characteristics of data transmission systems between
geographically remote terminal points, is one of the relevant
tasks included in the problem of problems technical support intermadal exchange
information B. computing networks.
Signal conversion devices
The main task of creating UPS was to make such a "translator", which allowed
would convert a digital signal, a more understandable computer or terminal, in
used in telegraph, telephone and some other communication channels
When the ODO devices (terminal data of the data - they can be a computer,
terminal, etc.) exchange data with each other using, for example,
telephone line, the signal must adapt to speech-oriented
analog world. However, the ADD devices interact through digital (
discrete) signals. The form of a digital signal is significantly different from the form
analog signal. The similarity is that the signal is continuous, repeats
itself and periodic, but it is very different in that the discrete is changed
states (level electric voltage) Very sharp. EUM and terminals
use digital, binary forms, since semiconductor transistors in
basic - discrete devices with two states. Digital Transmission
implemented at present in many systems, for example - in local networks,
where cars are not removed on a large distance, and there is an opportunity to associate them
tire. She is also wide
cO is used when directly connected between computers through
asynchronous ports (so-called zermodems). Digital transmission
there are a number of explicit advantages compared to analog communication systems. but
analog channels are still dominated by local devices connecting systems.
Oyody to telephone service channels.
There are several types of UPS:
Telegraph type signals conversion devices;
Low-level signals conversion devices;
Auto-call devices (AVU),
and also, perhaps some other, specific, devices.
The abstract in more detail the most famous and frequently used from
they are modems, as well as auto-calling devices, as a possible (and very valuable)
supplement (and for the most modern modems - an integral part)
Recently, modems become an integral part of the computer. Installing
modem on your computer, you actually open up for yourself a new world. Your
the computer turns out of a separate computer in the global network link.
The modem will allow you without leaving the house, access databases that
can be removed from you for many thousands of kilometers, place a message on
BBS (electronic billboard), accessible to other users, copy
with the same BBS files you are interested in, integrate home computer network
your office, while not counting the low data exchange rate) is created
full sense of work in the office network. In addition, taking advantage of global
networks (Relcom, Fidonet) can be accepted and sent emails not
only within the city, but in fact, any end of the globe. Global networks
make it possible not only to exchange mail, but also to participate in all sorts of
conferences, receive news in almost any subject of interest.
Modem (modulator demodulator) is a device transforming
serial digital signals into analog signals and vice versa.
In other words, the modem provides a digital / analog interface that allows two
devices to communicate with each other through the telephone network. It changes either
amplitude or frequency or phase to submit digital data in the form
analog signals.
To be accurate, modulation determination is: it is a frequency modification for
representations of data. This frequency is called the carrier frequency. Data that
modulate carrier (i.e., the data transmitted by the terminal or computer) is called
by modulating signal. The term "modulating" is usually applied to
non-modulated signal.
The modem modifies the carrier signal (amplitude, frequency, or phase) in order to
he could carry a modulating signal.
Modem with amplitude modulation (AM-modem) changes the amplitude of his carrier in
accordance with the sequence of bits to be transmitted. Usually
a higher amplitude is zero, and a lower one - one. More
common modem is a hormower (modem with frequency modulation).
the amplitude is preserved constant, and the frequency changes. Binary unit
represented by one frequency, and binary zero is another frequency. Another type
modems are FMMMOD (modem with phase modulation). This modem, in order to
present a change with on or with on, dramatically changes the signal phase.
Standardization organizations use generally accepted ADF abbreviations (DCE) for
modem and OOD (DTE) for computer, terminal or any other display device,
connected to the modem.
2. In the designations of organizations on standards, each conductor in a multi-wire
the digital interface is called a "exchange chain". "Circuit" is used for
data transmission, management and synchronization.
Modem work can be easier to submit if we consider the modulator and
the demodulator constituting in a modem one integer in the form of separate devices. We will
consider the well-known and simple two-wire connection (also
there is a 4 conductive connection. This type of connection is used, for example, on
When the modem is connected to the two-wire line, you must connect two wires.
immediately and to the linear output of the modem (modulator), and to the linear entrance
(demodulator). They are connected in parallel, but through a hybrid
transformer. In the perfect hybrid transform
tore Analog signals from the modulator pass through the transformer in
two-wire line, and the analog signals from the line pass through the transformer
in the demodulator. However, in a real hybrid transformer there is a reverse
communication in the form of weak analog signals from the modulator to the demodulator. Hybrid
the transformer is part of the modem. Two wires are outward in the form
two-contact shoe or twin-tie cord and can be connected
directly to the phone outlet.
4. Additional information about equipment
4.1. Channels
The simplest network in which modems are used is the two-point channel, in
which two modem is connected using one link. In the example "Channel"
connects oodevm with an oodterminal, while the "line" connects the Ademode with
another ADD modem. Therefore, the "channel" consists of a "line" and two modems.
When the modem is selected, the type of communication provided by a combination is important.
modem with line. Duplex channel allows you to transmit simultaneously
consistent data in both directions, while half-duplex - in
each moment of time is only one of two.
There is also a simplex channel where data is always transmitted only in one
direction. Separate characters, data blocks can be transmitted or
sequences of bits / characters used in data channel protocols.
When transfer rates up to 20 kbps, most modems use an interface
V.24 / V.28 MKTT (or, similar, RS232C) carried out using
25Contact socket connector on the rear wall of the modem. At speeds of transmission
from 48 to 68 kbps Wanted broadband modems that use the interface
V.35 MKTT, carried out using a 34-pin connector on the rear wall
4.2. About synchronization
When transfer rates up to 20 kbps, three main types of modems are used:
Asynchronous modem (asynchronous transmission only).
These modems are low-speed and operate in asynchronous mode.
start stop-stop gender transmission. They do not generate synchronization signals.
By the way, it is the modem that we used to see near our PC, because all
COM ports personal computerscorresponding to the RS232C standard asynchronous.
Synchronous modems (for synchronous transmission).
These modems work in synchronous block mode and generate signals
sync. More often used on large machines.
Asynchronous synchronous modems (for asynchronous and synchronous transmission).
These synchronous modems when using special formats
signs can operate in asynchronous startstop data. General
the number of bits in the startstop sign should be from 8 to 1.Mide removes startstop
bits before transfer and restores them after receiving. Modems of this type
synchronization signals generate and have built-in asynchronous synchronous
converter.
Asynchronous modems can work with any transmission speed within
velocities installed for them. Synchronous and asynchronous synchronous modems can
work only with fixed transmission rates.
4.3. Modems with error correction.
To avoid mistakes arising from noise in line, used:
asynchronous modems for two-point communication that provide a separate
asynchronousCanal with error correction. They use the ARQ type protocol and
stored in buffer memory transmitted data until you get
confirmation or request for re-transmission from the receiving modem.
synchronous modems operating with speeds from 9600 to 9200 BIT / s,
using "Cross Modulation" for direct correction of synchronous errors
data. This modulation is based on the use of the protective system of alternating
(cross) redundant codes in the stream of transmitted information. Excessive
codes allow the receiving device to select those data that most accurately
correspond to the transmitted originals.
4.4. Data compression devices
Available data compression devices are made in the form of separate blocks or
embed into synchronous modems. They use adaptive compression algorithms
data before transferring and recovery after reception. They can work with
byte-oriented or with bit-oriented synchronous protocols or with
bIT / C can be sent (or accepted) modem operating at a speed of 9600
4.5. Auto-call devices
Manual Method for setting the connection when transferring data via telephone network
general use is that the first subscriber manually dials the number
phone of the second person. He, in turn, answers the challenge, shooting
handset, after which, the connection between these subscribers is considered
installed. After the verbal certificate that the connection is established correctly,
both people press the "data" buttons on their telephones (or
modems) to enable modems in the TFP line.
Instead of set telephone number manually when installing a connection for transmission
data can be used by computer automatically gaining need number. it
called the operation of Avtovyzov, which until recently required a special
software and equipment.
The equipment consisted of a special computer interface (Avtovyzov Interface
V.25) and a separate device of the Avtovyzzov connected, as shown on.
The situation with AVU has changed after the appearance of modems with the possibility of Avtovyzov.
EUM connected to one of these modems uses the only interface
V.24 / V.28 (RS232C) and for operational avtovyzovis, and for data transmission. First
modems with avtovyzovis were asynchronous and used procedures for Avtovyzov,
suggested modem suppliers. New recommendation V.25 BIS standardizes
the Avtovyzov procedure for asynchronous synchronous modems with the ability
avtovyzova.
Some synchronous modems contain a built-in automatic call scheme,
which establishes an additional connection through the TFP with the purpose of
reservations. The procedure is included when the modem damage is detected in
lines. This operation is called the automation operation.
For final establishment of communication between machines, equipment in place
appointments usually forwards automatic response to avtovyzov from
causing equipment.
In conclusion, we can say that now modern
multifunctional modems that combine almost all achievements
in the field of computer communication. A characteristic example of such fundamentally new
the approach can serve fairly powerful and perfect modem of the American firm
Zyxel is one of the world leaders in the production of communications. Typical
zyXEL modem - intellectual (i.e., almost completely controlled and
computer-driven, and at the same time able to determine the most optimal
data exchange rate Before a communication session to avoid errors that can
occur at a long transmission rate due to random interference
lines), a large range of permissible exchange rates, as well as the use of technology
At the same time, the presence of certain internal devices and different
serving programs Provides the ability to use zyxel modem and B.
as a fax, and as an answering machine (there is a built-in speaker on the board),
and even as a number determinant. In short, modems gradually
turn from ordinary UPS into small but powerful workstations on telephone
Any data transfer system (SPD) can be described through three main components. Such components are the transmitter (or the so-called "information source"), data channel and receiver (also called the "Recipient" of information).
With double-sided (duplex transmission), the source and recipient can be combined so that their equipment can transmit and receive data at the same time.
In the simplest case of SPD between points A and B consists of the following main seven parts:
- Terminal data equipment at point A;
- Interface (or joint) between the terminal equipment of the data and the data channel instrument;
- Data channel equipment at point A;
- Transmission channel between points A and B;
- Data channel hardware at point in;
- Interface (or joint) data channel hardware;
- Terminal data equipment at point V.
Term Data Equipment (ODO) the generalized concept used to describe the terminal device of the user or part of it. The OOD may be a source of information, its recipient or the other at the same time.
The ODO transmits and (or) receives data by using the data channel (ACD) equipment and the transmission channel. The corresponding International Term - DTE (Data Terminal Equipment). Often, a personal computer, a large computer (Mainframe Computer), a terminal or any other equipment that can transmit or receive data can act as DTE.
Data channel equipment is also called data transfer equipment (ADF). Internationalterm DCE (DATA Communications Equipment).The DCE function is to ensure the possibility of transmitting information between two or large number DTE on the channel of a certain type, for example, by phone. For this, the DCE must provide a connection to DTE on the one hand, and with the transmission channel - on the other. DCE can be an analog modem if an analog channel is used, or, for example, a service device. Channel / Data (CSU / DSU - CHANNEL SEMIS UNIT / DATA Service Unit) if a digital channel is used.
Analog and digital communication channels.
Link -a set of distribution environment and technical means Transmissions between two channel interfaces.
Depending on the type of transmitted signals, two large-class communication channels are digitally and analog.
The digital channel is a bit path with a digital (pulse) signal at the input and channel output.
A continuous signal comes to input to the analog channel, and a continuous signal is also removed from its output.
Signal parameters can be continuous or received only discrete values. Signals may contain information either at each time (continuous in time, analog signals), or only to certain, discrete moments of time (digital, discrete, pulse signals).
The newly created SPDs are trying to build on the basis of digital channels with a number of benefits before analog.
Information regardless of its specific content and form is always transmitted from the source to the consumer. The information presented in a certain form is called message. To send a message from the source to the consumer, remote from each other requires a communication system.
Communication system (System exchange) Call a combination of technical means and mathematical methods intended for organizing messaging between points. The scheme of such a communication system between two points includes a transmitter P, Channel TO and receiver Etc.
Transmitter - This is a complex of technical devices designed to convert a message of a source into a signal that can be transmitted through this channel.
Link - A combination of technical means and a physical environment intended for signal transmission.
The physical environment on which the signal is distributed (for example, electromagnetic oscillations) is called linie .
Receiver - a complex of technical devices that transform a signal appearing at the channel output to the message.
Converting a message to the transmission signal is reduced to encoding and modulation operations, for the implementation of which there is a coding device and modulator in the transmitter. Accordingly, the receiver includes a demodulator and a decoding device.
Channels are classified on various features.
Depending on the destination Systems that include channels, they are divided into telephone, television, telegraph, telemetry, telemetry, telecommunications, transmission of digital information, etc.; According to the communication lines used - on cable, radio relay, etc.; By the strip of the frequencies occupied - on tonal, suponal, high-frequency, short-wave, light, etc.
Depending on the structures Signals channels are divided into continuous, discrete and combined (continuous-discrete or discrete-continuous). In continuous communication channels, continuous signals are used to transmit messages, in discrete - discrete and, finally, in the combined - signals of the other type.
This unit of communication channels and the previously introduced signaling of signals for continuous and discrete leads to the four possible varieties of the organization of messaging from the source to the consumer:
- The source of information generates a continuous signal delivered to the consumer in the form continuous function- Channel communication is continuous.
- The source of information generates a continuous signal delivered to the consumer in discrete form, the communication channel is continuous-discrete.
- The source of information produces discrete signal, Differed by the consumer in the form of a continuous function, is a discrete-continuous communication channel.
- The source of information generates a discrete signal delivered to the consumer in discrete form - the communication channel is discrete.
The classification of discrete and continuous channels is conditional, as often the discrete channel contains a continuous channel, at the input and output of which there are continuous signals.
Theoretically, the discrete channel is determined by setting the alphabet of code symbols at the inlet, alphabet of code symbols at the output, the amount of information transmitted by the channel per unit of time, and the value of probabilistic characteristics.
Depending on the number of code symbols in the alphabet (used number system used), the channel is called binary if am. \u003d 2, Tropic - t.\u003d 3, etc.
Sources and consumers of information can be combined with each other by direct (non-commutable) channels and on transit paths made up of several channels by switching them (QC - Channel Switching) or phased transmission of messages through switching centers as the channels are released (COP - Switching messages).
Channels combining terminal devices (sources, consumers) and switching centers, called subscriber (AK).
Analog channels are the most common due to the long history of their development and simplicity of implementation. When data transfer at the input of an analog channel there must be a device that would convert digital data coming from DTE into analog signals sent to the channel. The receiver must contain a device that would convert back-received continuous signals into digital data. These devices are modems.
Similarly, when transmitted by digital channels, DTE data has to be given to the species adopted for this particular channel. Digital modems are engaged in this transformation.
Basic model communication Systems
Theoretical basis of modern information networks Specifies the basic reference model of interaction of open systems (OSI - Open Systems InterConnection) of the International Standards Organization (ISO - International Standards Organization). It is described by ISO 7498 standard. The model is an international standard for data transmission.
According to the reference model of OSI interaction, seven levels are distinguished by the area of \u200b\u200binteraction of open systems.
The main idea of \u200b\u200bthis model is that a specific role is assigned to each level. Due to this, the overall data transfer task is split into from-del specific tasks. Level functions, depending on its number, can be fulfilled by software, hardware or software and hardware. As a rule, the implementation of higher level functions is programmed, channel and network layer functions can be executed by both software and hardware. The physical layer is usually performed in hardware form.
Each level is determined by a group of standards that include two specifications: protocol and provided to a higher level service.
Under protocol It is understood as a set of rules and formats that determine the interaction of objects of one model level.
Modems .
The history of modems began in the 1930s. It was then that the equipment appeared, which makes it possible to transmit human speech over long distances, officially referred to as "tone telegraph equipment" and only highly advanced specialists called "modem". Generally speaking, human speech is transmitted by telephone wires in the form of electrical voltage oscillations. In order for the quality to be impeccable, it is necessary to transmit oscillations with frequencies from 50 to 10,000 Hz. But to ensure the transfer of such a wide range of frequencies is too expensive, therefore, limited to the frequency range providing satisfactory intelligibility of speech - from 300 to 3,400 Hz.
The signal at the output of the telegraph apparatus has frequencies from 0 Hz (i.e. direct current) up to 200 Hz. It is clear that such a frequency range did not fall into the bandwidth boundaries and therefore could not be transmitted via telephone equipment intended for long-distance communications, and it was unprofitable to create special lines for the telegraph.
Then the device was invented for connecting the telegraph apparatus to the telephone channel, which required adaptation to the bandwidth band. At the output of the telegraph apparatus, the voltage can take two fixed values \u200b\u200bcorresponding to zero and one. If you first encode, and then on the same algorithm to decline the signal, the prototype of modern modems is obtained.
Creating a device that was transmitted to the telephone channel of a negative polarity to the telephone channel signal of an arbitrary frequency, and for the voltage of the positive polarity, the signal of another frequency, allowed to enter a signal in the telephone range. At the other end there was a device that determines the frequency of the received signal and converting signals of various frequencies into signals of different polarity. The first of the processes is called modulation, and the second, inverse to it, demodulation. Since the telephone channel is possible simultaneous communication in two directions, then at each of the channels, devices that carried out both modulation and demodulation were installed. From the reduction of the words "modulation" and "demodulation" and was formed by the word "modem".
The most first modem for PC was the production of Hayes Microcomputer Products, which in 1979 released Micromodem II for popular personal computers apple II. The modem cost $ 380 and worked at a speed of 110/300 BPS. Prior to this, there were only specialized devices that combined mainframes.
By the way, Hayes released in 1981 and the first modem of SmartModem 300 BPS, whose team system has become a sectoral standard and remains it to this day. The first modems with the "commercial" transmission rate of 2400 BPS were represented by several companies in December 1981 at the COMDEX exhibition at a price of $ 800-900. And then it is time U.S. Robotics. In 1985, this company began issuing his famous Courier series, significantly reduced the cost bar of the cost of 2400 bits / s. At the beginning of next year, the first Courier HST modem with a transmission rate of 9600 bps, and in 1988, the Courier Dual Standard modem, which supported HST and V.32 communication protocols ($ 1600), and Courier v.32 ($ 1500). After another two years, Courier V.32bis modem was released, in 1994 - Sportster v.34 with a transfer rate of 28.8 kbps ($ 349), and in 1995 - Courier V.EveryThing 33.6 kbps .
Digital signals generated by a computer cannot be directly transmitted on the telephone network, because it is designed to transmit human speech - continuous signals Sound frequency.
The modem provides conversion digital signals Computer B. alternating current Sound Range Frequencies - This Process is called modulation as well as the reverse transformation called demodulation . Hence the device name: modem - mO double / dem. tool.
Modulationthe process of changing one or several output parameters by the law of the input signal.
In this case, the input signal is typically digital and is called modulating. The output signal is usually analog and often be called the modulated signal.
Currently, modems are most widely used to transfer data between computers through a commutable public telephone network (CTSOP, GTSN - General Switched Telefone Network).
To communicate, one modem calls another by phone number, and he answers the call. The modems are then sent to each other signals, harmonizing them with both communication mode. After that, the transmitting modem begins to send modulated data with a consistent rate (number of bits per second) and the format. The modem on the other end converts the received information in digital view And transfers it to its computer. After completing the communication session, the modem is disconnected from the line.
Model communication scheme
Modems
you can also classify in accordance with the protocols implemented in them.Protocol- This is a set of rules that manage the information exchange of interacting devices.
All protocols regulating those or other aspects of the functioning of modems can be attributed to two large groups: international and branded.
The international-level protocols are developed under the auspices of the standardization sector of the International Telecommunication Union (ITU-T - International Telecommunications Union - Telecommunications) and are accepted as recommendations. All ITU-T Recommendations relative to modems belong to the V series. The branded protocols are developed by individual companies - manufacturers of modems, in order to succeed in competitive struggle. Often, branded protocols become standard de facto protocols and are partially either fully accepted as ITU-T Recommendations, as happened to a number of Microcom protocols. The most actively developing new protocols and standards are engaged in such well-known firms as AT & T, Motorolla, U. S. Robotics, ZyXEL and others.
Types of modems
Currently, a huge number of all kinds of modems are produced, ranging from the simplest, providing the transmission speed of about 300 bits / s, to complex fax modemics, allowing you to send a fax or sound letter from your computer to any point in the world.
Consider only the so-called hayes-compatible modems. These modems support the AT commands of the modem management team developed by Hayes. Currently, such modems are widely used all over the world for communicating personal computers through telephone lines.
The hardware modems are made either as a separate board inserted into the slot on the computer's motherboard, or as a separate housing with a power supply that connects to the serial asynchronous port of the computer.
The first one is called internal modem, and the second - external .
Internal modems is usually stronger than the effects of interference and less resistant to work. In addition, they have a rather unpleasant property "hob" and withdraw them from this state, you can only with the computer RESET button. They have both a big plus: they do not interfere with you, without taking place on your desktop and, moreover, they receive food over the computer bus. In addition, they have the ability to store any data when the computer is turned off (similar to the CMOS of the computer).
External modems More for the fact that you can always on the display of the modem status display: what is busy in this moment. In addition, they are less susceptible to interference.
Modems can work in synchronous and asynchronous mode. In addition, there are duplex and half-duplex modes. Their difference is that in half duplex mode, the transmission at one point is only in one direction, while in duplex mode, the transmission is carried out in both directions at the same time.
Fax standards
According to the recommendations of the Standardization Sector of the International Telecommunications Union (ITU-T - International Telecommunications Union - Telecommunications), depending on the modulation type used, the faxes of four groups are distinguished. The first facsimile standards belonging to group 1 were based on an analogue method of transmitting information. The text page faxes group 1 passed in 6 minutes. Standards of Group 2 improved this technology in the direction of increasing the transmission rate, with the result that the transfer time of one page decreased to 3 minutes.
The radical difference of groupxes of group 3 from earlier consists in a fully digital transmission method with speeds up to 14,400 bits. As a result, applying data compression, the Fax group 3 transmits a page for 30-60 s. With a deterioration in the quality of communication, the faxes of group 3 go into emergency mode, slowing down the transfer rate. According to the Group 3, two degrees of resolution are possible: standard, providing 1728 points horizontally and 100 points vertically; And a high, doubling number of points vertically, which gives the resolution of 200x200 points / inches and twice the speed.
Fax devices of the first three groups are focused on using the analog telephone channels of the KTSOP.
The standard 4 standard provides permission to 400x400 points / inches and raising speed at a lower resolution. Group 4 faxes give permission very high Quality. However, they need high-speed communication channels that can provide ISDN networks, and cannot work through the channels of the PTSOP.
The modem (modulator-demodulator) is a conversion device for conversion digital signals into analog and vice versa. Standardization Organizations use generally accepted ADF abbreviations (DCE) to designate a modem and an ADD (DTE) to designate a computer, a terminal or any other device connected to the modem. The modem has two interfaces (Fig. 2.31): Interface between DCE and analog line; Multi-wire digital interface between DCE and DTE.
Two-point channel. The simplest network using modems is a two-point channel, in which two modem are connected ("point-to-point") one line of communication (Fig. 2.32). Discrete canal Connects DTE with DTE. The line connects the DCE with the DCE. The discrete channel consists of a line and two modems (DCE). With a transmission rate of up to 20 kbps, use the V.24 / V.28 interface (RS-232C), carried out using a 25- or 9-pin socket connector. When transfer rates from 48 to 168 kbps, broadband modems operating with the V.35 interface are needed. At speeds up to 20 kbps, any of the following analog telephone communication lines can be used:
4-wire 2-point dedicated line; 4-wire multipoint dedicated line; 2-wire 2-point dedicated line; 2-wire 2-point switched line (communication by dialing through the number of KTSOP); 4-wire 2-point switched line, organized by switching two separate two-wire connections via the PTSOP. The standards of telephone channels as derivatives from the standard channel of the tonal frequency (PC) channel are presented in Table. 2.10.
Modem modes. Asynchronous. This mode is implemented asynchronous modems, such modems are low-speed and operate in the mode of asynchronous startstop of the kind. Asynchronous modems do not generate synchronization signals and can operate with any transmission rate within the speeds installed for them. Synchronous. In this mode, the data is transmitted by blocks, and the modem generates synchronization signals. Modems implementing only synchronous mode are called synchronous modems. Asynchronous synchronous. This mode is implemented asynchronous synchronous modems, which can be carried out both synchronous and asynchronous transmission. The modem removes startstop bits before transmission and restores them after receiving. The modems of this type generate synchronization signals and have a built-in asynchronous synchronous converter. Asynchronous synchronous and synchronous modems work only with fixed transmission rates. When the modem is selected, the type of communication provided by a modem combination with a line is important.
Any modem that works with a 4-wire 2-point line uses one pair to transmit, and the second for reception and, therefore, can operate in duplex mode. Modems operating with a 4-wire multipoint line operate only in half duplex mode. The modems that have only synchronous mode are operated on a 4-wire 2-point noncommutable line, or through a pushpiece, while one switching connection provides half-duplex mode, and the dual switching connection is a duplex mode. Asynchronous synchronous modems operate on 2-wire lines (or selected or switched), and they can all work in duplex mode. Modems acceptance. Transfer of data on telephone networks describe the recommendations of the V International Telecommunications Union Series (Sector technical standards) - ITU-T. Compatibility check is to check the V series number specified by the manufacturer in the modem specifications. The classification of the recommendations of the V series is shown in Fig. 2.33.
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The modem can work in two modes: command and data transfer. The modem command mode is typically set: when power is turned on; at the initial initialization of the modem; After an unsuccessful attempt to connect with a remote modem; when interrupted from the keyboard by pressing the "Put the tube" keys (most often); When you exit the data transfer mode via Escape-sequence. In the command mode, the entire data stream entering the modem via the V.24 / V.28 interface is perceived as a command. Data transfer mode (on-line) is set after sending the Connect message modem in cases: with a well-known attempt to establish communication with a remote modem; When performing a self-test modem. In the data transmission mode, the data flow entered into the DTE modem is translated with the conversion into the line, and the data flow from the line is transmitted with the reverse conversion to the DTE interface. Functional modem modem. The modem is always in one of the two functional modes (with the exception of periods when it moves from one mode to another): command (local) and in asynchronous connection mode (on line). The modem transition circuit is presented in Fig. 2.34. When power is turned on, the modem initializes its parameters according to the configuration recorded in non-volatile memory, and goes into an asynchronous command mode. Only in this mode, the modem perceives the AT team. According to Z-command, the modem restores its working configuration
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from non-volatile memory and returns to the command mode, "^-command restores the configuration of the manufacturer's profile (default setting) and returns to the command mode. The modem "raises the tube" in the auto-output mode: a) upon receipt of the A-team; b) automatically when S1 \u003d SO, when the counter of calls received (calls) becomes equal to the number set to response; c) When the dialing command arrives when the call string ends R. The functions of the exchange chains 103, 104, 109 v.24. Consider the functions of the exchange circuits associated with the transmission and reception of data: 103 (2) TXD (transmitted data) to the DCE; 104 (3) RXD (received data) to DTE; 109 (8) CD (detector received linear signal) to DTE. The input stream of serial data entering the modem via a circuit 103 is converted by the modulator to the modulated analog signal to output it in the line (Fig. 2.35). At the other end of the line, the remote modem demodulator receives a modulated linear signal and converts it to the stream of serial data to output through the data reception circuit 104.
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When the modulated carrier frequency is detected by the demodulator, the circuit 109 transfers from the state to the ON state. At the same time, between the moment of detecting the carrier and moment of changes in the status of the exchange circuit 109, a delay known as the delay in the inclusion of the carrier detection is made. There is also a "shutdown" delay of the carrier detection, which occurs when the line carrier is turned off at the other end. The circuit 109 in the internal modem scheme is necessary for fixing the data reception chain 104 (data is accepted only when the state of the circuit 109 is turned on). The activation of the CD signal and the fixation of the data reception circuit provides protection against short-term linear noise emissions that simulate false signals in the data reception circuit 104.