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 acquire a high-speed modem, then for you become important characteristics Serious port of your computer. 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 programs running with modems always report to the user this information), try to rebound again - the connection through the telephone network occurs every time on different wires, and it is likely that another connection will be more good 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.
Modems (The name comes from the confluence of two words - modulator and demodulator)- These are devices that allow you to organize a link between computers located on distance from each other. If computers are nearby, you can organize a link between them using a serial, parallel port, USB, Blutooht. However, this connection is possible only at close distances defined by the port features. At high distances, the signal is weakening and special devices are required that can convert the signal in the view that allows you to transmit a signal over long distances. To do this, serves a device called "modem" - from the word modulator demodulator. The modulator allows you to convert a digital signal to analog, and the demodulator is to make the reverse transformation, that is, translate from analog to digital form(In a more accurate sense, modulation is a change in the characteristics of the carrier signal (as a rule, low-frequency periodic oscillations) by a high-frequency signal control signal that allows you to transmit the necessary information). Demodulation is the allocation of an information signal from the set of carrier information signals). Practically, the same principles work fax, therefore modems that are produced with facsimile transmission capabilities are called a fax modem. Modems can be internal (inserted into expansion slots), external (connected to the COM, LPT, USB ports or network cable To the RJ-45 connector of the computer's network card, usually have an external power supply unit) embedded in a laptop or in the form of a card to connect to the PCMCIA connector for portable computers.(the latter is also called the expansion cardPC Card And practically outdated. Currently used standardExpressCard with connections to tiresUSB and PCI Express ). Recently, wireless modems (called the module or gateway) have been widely distributed, using cellular operator communication lines (the greatest fame receivedUSB - modems) . The principles of all devices are the same.
Modems can be analogand digital. The first began to be used analog modems (Dial -Up). Due to the fact that the data transfer rate on these modems was not large (up to 56 kbps), began to move to digital species (with a frequency of work from 4 kHz to 2 MHz and respectively speed up to several megabits / s). In addition, when transmitting data on an analog modem cannot be conversing.
Most users for data transfer used telephone network. In order to be able to use a digital type of transmission, it is necessary that the sending and receiving would have a digital PBX. In addition, the telephone line should not be a paired telephone and security alarm. Until now, some users use analog modems.
Main characteristics of modems:
- interior or external. Internal modem is a map that is inserted into the socket on the motherboard. This modem is inserted as normal mapHowever, you need to connect the wires as indicated below. Internal modem is usually cheaper than external. But it does not require space on the table, does not occupy a serial port of the computer.
External modems (new) are connected to the USB, PCMCIA or ExpressCard connector and do not require additional power, as they receive it from the connector.
The external modem (old) is connected to the sequential port and is located in a separate case. This species Requires an electrical network connection through a transformer. Its advantages should be attributed to the fact that it does not occupy expansion slot makes it easy to transfer it from one computer to another.
Supported standard and transmission speed;
The size of RAM or flash memory.
Additional modem features: voting digitization and transfer it to analog signal for conversation during data transfer; Fax; Automatic determination of the number of the caller; answering machine; Electronic secretary and other features that have telephone sets.
As a rule, the modern modem has the following phone featuresthat we give. This is: negotiations with several subscribers; Time shutdown of the microphone; inclusion of external loudspeakers; Memory for subscriber numbers; repeated call subscriber; autodial; automatic determination of the number; Memorizing the call numbers and call time; determining the second call during the conversation; protection against unwanted calls; Recording received messages; answering machine; remote control; On the phone panel there may be buttons with functions: auto drive, listening to messages left, disconnect the phone, turning off external speakers, etc.; On the phone panel there may be indicators that determine the mode of operation, withdrawal of the tube, etc.; may be a display with data on incoming and outgoing calls, talk time, etc.; Voice dialing, the user calls the name of the subscriber, and the modem connects with its number; Quick set, dialing with one or two keys; auto attendant, response to received calls when talking with another subscriber; Collection of statistics on the number of calls received, their numbers, talk time during the day, etc.; Other features, such as dialing at a specific number at a certain time, alarm clock, etc.
When the modem hangs, you can restore its performance of the power reset (external removing and insert again), while not required to turn off the computer. In addition, it has an indication at which you can determine the state of the modem.
Digital modems.
Currently used several formats: ADSL, HDSL, IDSL, ISDN, HPNA, SHDSL, SDSL, VDSL, WiMAX and wireless modems using wireless communication (Wi -fi). They are often referred to as HDSL (Digital Subscriber Line - digital subscriber line).
ADSL ASYMMETRIC DIGITAL Subscriber Line is an asymmetric digital subscriber line) appeared in 1987 and is one of the very first and most common digital data transfer format. Allows you to send data from the user to the network at a speed of 16 to 640 kbit / s (according to standards 0.5, 0.8, 1.2, 1.3, 3.5 Mbps, and receive data at a rate of 1.5, 0.8, 5, 8, 12, 25 Mbps s). Since the user usually receives data, and does not send, then this speed separation is not felt by the user except the cases of video link. Therefore, other types of formats using a coaxial cable appear with the use of coaxial cable ( cable TV, speed up to 100 Mbps) and Ethernet connector (local network at a speed of up to 1 gigabit / s). In a number of European countries, the ADSL standard has become the standard for which each resident gained access to the Internet.
The usual telephone line uses the frequency to pass from 0.3 to 3.4 kHz, the ADSL modem of the lower frequency for the outgoing flow is 26 kHz, and the upper 138 kHz, and for an incoming flow from 138 kHz to 1.1 MHz. Thus, you can talk on the phone and transmit and receive data at the same time.
However, the first modems did not allow to talk quite comfortably by phone, since the high-frequency part of the modem made extraneous noises in telephone call (And on the contrary, the conversation brought distortion into data transmission). To avoid it began to apply a frequency filter (Splitter-Partly separator), which passed only low frequencies to the phone.
HDSL. (H IGH D ATA RATE DIGITAL S UBSCRIBER L INE The high-speed digital subscriber line) was developed at the end of the 80s. It uses not one, but two pairs of wires and has a speed or 1.5 Mbps (American standard) or 2.0 Mbit / s (European standard) and allows you to transmit a signal to 4 kilometers, and in some cases up to 7 kilometers. Used mainly for organizations.
IDSL. (ISDN Digital Subscriber Line - Digital Subscriber Line IDSN) Allows you to transmit data at a speed of 144 kbps.
ISDN.Integrated Services Digital Network - a digital network with maintenance integration) appeared in 1981 and has a data transfer rate of 64 kbps.
HPNA (Home PhoneLine Networking Alliance - The name of the United Association of Non-Profit Industry Companies) works either with standard telephone or coaxial cable. Last Standard (3.1) allows you to transmit data at a speed of up to 320 Mbps., According to the standard 2.0 - 10 Mbps.
SHDSL (Symmetric High-Speed \u200b\u200bDSL - Symmetric High Speed \u200b\u200bDSL) Allows you to transmit data on one pair of wires at a speed of 192 kbps to 2.3 Mbps, and two pairs are twice as many times to 6 km.
SDSL (Symmetric Digital Subscriber Line - a symmetric digital subscriber line) uses one pair of cables with speeds from 128 to 2048 kbps. Acts a distance from 3 to 6 km.
VDSL (Very-High Data Rate Digital Subscriber Line is an ultrahigh-speed digital subscriber line) has a high data transfer rate from 13 to 56 Mbps from the network to the user and 11 Mbps in the opposite direction to the Rschuchnya to 1.2-1.4 km.
Wimax (WorldWide Interoperability for Microwave Access) is a wireless connection in the wave range from 3.5 to 5 GHz according to the 802.16-2004 standard (or fixed WiMAX) and 2.3-2.5, 2.5-2.7, 3.4-3.8 GHz according to the 802.16-2005 standard (or mobile WiMAX). It has many similar parameters with Wi -fi, but it differs in that it can transmit a long distance signal and, moreover, somewhat more expensive.
Bluetooth (translation - blue tooth) was developed in 1998 and is used for wireless communication with a licensing free range of 2.4 - 2.4835 GHz. It does not have a connector and is located inside the computer (devices), is used to transfer data using radio waves between various types of computers, cell phones, printers, cameras, keyboard, mouse, joysticks, headphones, MFP, scanners and other.The essence of the method is that in a certain range pseudo-rapidly changing the frequency is jumps for 1600 times per second. Such a change in frequency occurs simultaneously for the receiver and transmitter, which work synchronously by such a scheme.Devices can be apart from each other up to 200 meters depending on the obstacles between them (walls, furniture, etc.).
The receiving and transmission device is inside the computer and is not visible. If there is no such device in the computer, you can connect an external device through a USB connector that allows you to work with this type of data transmission.
There are standards: 1.0 (1998), 2.0 EDR (2004) with a data transfer rate of 3 Mbps, in practice about 2 Mbps, 2.1 (2007) with the use of energy-saving technology, simplified communication between devices has also become more Protected, 2.1 EDR was required even less electricity, the connection of devices was further simplified and reliability increased, 3.0 HS (2009) with a transmission rate of up to 24 Mbps. 4.0 began to be used in the iPhone in 2011, allows you to transmit data at 1 Mbps speed. portions from 8 to 27 bytes.
There are profiles for this standard, which are a set of functions. In order for the devices to work on a specific profile, you need both devices to support this profile. For example, A2DP (dual-channel stereo transmission of audio data), AVRCP (transmitting standard TV functions), BIP (forwarding), BPP (text shipment, email letters per printer) and so on
Wi-Fi used to create a wireless network. Developed in 1991, NCRCorporation and [Email Protected], accompanied by the Wi-Fi Aliance company alliance and meets the IEEE 802.11 standard. Used to connect to the network (local and Internet) computers and cell phones.
The receiving and transmission device is inside the computer and are not visible. If there is no such device in the computer, you can connect an external device through a USB connector that allows you to work with this type of data transmission.
The following standards are available: 802.11A uses frequencies in 5 GHz, providing speed (in theory) to 54 Mbps; 802.11b uses frequencies in 2.4 GHz, providing speed (in theory) to 11 Mbps. (practically not used); 802.11g uses frequencies in 2.4 GHz, providing speeds up to 54 Mbps. (most common); 802.11n uses frequencies in 2.4 and B5 GHz, providing a speed from 150 to 600 Mb / s. (Recently designed, begins to gain strength). In this standard, the data transmission range is increased, the bonds are less. This standard uses MIMO technology (multiple Input Multiple Output - multiple input, multiple output), which allows the use of reflected waves from the walls. If the device has one antenna, it can work at a speed of 150 Mbps, two antennas - 300 Mbps, three - 450 - Mbit / s, four (not yet available) - 600 Mbps. However, the declared data transfer rate differs from the real one. So, instead of 300 Mbps, it turns out about 100 -130 mbits / s (as half of the information transmitted - service characters), which is also sufficient to work. And in the presence of walls, the speed drops, for example, for three walls will drop to 50 Mbps.
Since some household appliances operate at a frequency of 2.4 GHz (for example, a microwave), they can create interference. Therefore, it is desirable to have a device that works in two frequencies: at 2.4 and 5 GHz.
There are also cable modem to connect to the cable television channel.
Typically, digital modems may contain elements that are used as gateway Between the local network and the Internet: the router, the firewall and so on.
Modem indicators
Perhaps the following indicators:
AA. (AUTO ANSWER - auto answer) - Automotive mode, providing an answer to the subscriber's request in automatic mode;
CD (Carrier Detect - the definition of carrier or DCD) - on the communication session;
CTS. or CS. (Clear to Send) - the readiness of the modem to receive data from the computer. Goes out during data receipt;
Data. - on data transmission;
DC (Data Compression) - Compression Data ;
Fax - when working as a modem as a fax;
HS. (High Speed \u200b\u200b- high speed) - lights up when the modem is operated with the maximum speed;
EC (Error Control or ARQ) - error correction mode;
Mr. (Modem Ready - the readiness of the modem or DSR.) - shows that the modem is connected to the power supply and ready to work;
Oh. (Off Hook - removed tube) - glows with a hanging tube;
ON.(PWR) - power indicator;
PWR. (POWER) - power on;
Rd. (Recieve Data - Getting Data or RX. or RXD. ) - shows that the data is received to the computer;
SD. (Send Data - Sending data or SX.or TXT.) - shows that data is received from the computer;
TEL. - Lights when the tube is removed on the parallel connected telephone;
Rt.S. (Request to Send) - the readiness of the modem to receive data from the computer. Lights while waiting for data from the computer, goes out during data transfer;
TD. (Transmit. Data. or TXD. ) - burns or flashes when sending data from the computer to the modem. May burn when transmitting data at the maximum transmission rate;
TST (Test) - flashes when testing;
Tr(Terminal Ready - device readiness or Dtr.) - lights up when the control signal is obtained;
USB - Lights when the modem is connected to a computer through uSB bus.
A sound volume control can also be on the modem housing.
On the back panel the external modem can be connectors with icons:
AC IN. – connecting the power adapter;
Line – connecting to the telephone line;
ON. / Off – enable / disable modem;
Phone – connecting a telephone;
RS. -232 – connector for connecting to a serial port of the computer;
USB – connector for connecting to the USB bus.
Analog modem
Data transfer. Telephone lines are adapted to analog signals. Due to the fact that the human speech has a range of 30 Hz to 10 kHz (music has a larger range), then to save the telephone line transmits a signal from 100 Hz to 3 kHz. It is this restriction that binds the ability to transfer data at high speeds. Computers can be connected not only through the telephone line, but also using radio waves and infrared radiation. In this case, the wires are not needed.
Ultimately, the data sent in the parallel channel, in the serial port to be converted to serial transmission with start-stop bits, are transmitted to the modem, where they are modeled, that is, it is superimposed on the carrier frequency of the signal transmitted over the line, then sent to another modem. Next, they are transformed into a digital form, are sent to the serial port, where they are converted into a parallel view, after which the processor is sent to the processor.
Digital data is premissible, and the parcel can be two types: synchronous and asynchronous. When synchronous transmission, the data packet consists of a header, which includes the address of the destination, the data itself and the checksum. When asynchronous transmission passes the start bit, 8 data bits, possibly a parity check bits, and a stop bit indicating the end of the transmission. This species is used in the serial channel.
In addition, three modes can be used during data: duplex, in which the data is transmitted in both directions at the same time, half-duplex, in which data can be transmitted in both directions, but at each time in one direction, and the simplex - data transmission only in one direction.
Transmission of data from the modem to the modem and from the modem to the computer has a different speed, so that the data is not lost, the modem has a buffer where the data obtained is stored.
Some modems compress the data before shipping, when receiving another modem decrypts this data. There are files that have already been compressed, so this method may not benefit when transmitting. To avoid data loss, the data transfer rate from the modem to the computer must be several times higher than between modems, which is in practice and implemented.
When data transfer is often used unit baudwhich is sometimes confused with bits / sec. In fact, these are different quantities. 1 Bud is one character sent per unit of time, and this may be not only data, but also control signals. The symbol may be several bits. If the signal consists of two types: 0 and 1, the character denotes 1 bits if 512, then 9 bits (2 9 \u003d 512). When data transfer with a small speed of 1, the bodes is approximately 1 bits / s. With a high speed, the modem sends the data on several frequencies, so at each moment of time is transmitted not one, but a few bits, that is, the speed measured in the bit / s, and not bode / s, will be several times higher than the speed in bodies . Often the specified speed in the bodes implies speed in bits / sec.
When transmitted via modem, it is possible to approximately determine how much time is required for transmission, making the transmission rate by 10, for example, if the transmission occurs at a speed of 28,800 bits / s, approximately 2,880 bytes or characters (28 800/10 \u003d 2 800).
The modem connects to the serial port of the computer and works with sequential data. Typically, the modem is used to work on the Internet, but it can also be for communication between two arbitrary computers directly. Modems are also used as fax for transmission of fax messages. They can have a built-in adapter to create voice messages in the answering machine mode.
When connecting the modem, signals are sent, which are also displayed on the speakers and can be heard in the form of an ongoing changing sound within a few seconds. The receiving modem defines the standard on which it can work, and also makes the clock frequency settings, that is, performing phase modeling. After that, the speaker is turned off, but the signals continue to do, in particular, they can be heard through a parallel phone.
Modems are two types: internal and external. Internal performed in the form of extension cards and inserted into the motherboard connector, external have their own body and with the cable are connected to a sequential port. Recent modem types can be connected via the USB bus (and sometimes the power supply from the computer), which is used during the computer, release the connector and have other advantages. When the modem is connected to the sequential port for high-speed models, it is necessary that the port is also high-speed. So, for modems with a speed of 56 kbps, the speed in the sequential port of 115 kbps is required. The highest port speed is needed because control signals are also sent between the computer and the modem that are not transmitted by the telephone line. In the case when the port does not support high speeds, the data may be lost. External devices can be turned off by turning off the power supply, and internal - only when the computer is turned off, which is inconvenient when the modem hangs.
Modems can be divided into two categories: the first view (Class2) has an internal processor that processes data, in the second data is processed by a central processor (Class1), they are also called Windows modemami, somewhat cheaper than the first type. Such a modem if the processor is old, can slowly slow down the computer, but if the user rarely goes into the Internet and sends a small number of email messages from time to time, it is permissible. It is quite advisable to use it in the event that a powerful processor is on the computer.
Often the modem is characterized protocolwith which he works. Exist signal modulation protocols, error correction protocols, data compression and facsimile (fax). The modem has several protocols for each of these species. Error correction protocols include V.42, MNP2-4, MNP10, data compression - V42BIS, MNP5.
One of the main characteristics of the modem is the data transfer rate, with the specified maximum speed may be for modern devices 33.6 or 56 kbps. If the speed of 33.6 kbps is specified, then the entire band is used and the data is transmitted in both directions at a speed of 33.6 kbps. In case the line it allows. If the line does not allow this, then the transition to more low speed. Speed \u200b\u200b56 kbps. It provides data from a greater speed than when sent, since frequencies to receive here more than for transmission, however, the transmission from the modem is carried out at a lower rate.
In addition, it is necessary that both modem have the same characteristics, otherwise the data transmission will not reach the maximum speed. To do this, before buying a modem, the provider needs to clarify the type of modem with which it works better. Below is a correspondence table between some protocols and the transfer rate over it.
BIS prefix means that the standard has been revised. Starting from a speed of 14,400, all protocols are duplex, that is, transmit messages in both directions at the same time. With the S symbol, the names of not only standards defining the data transmission protocol can begin, but also other types of protocols, for example, V.24 contains a list of specific signals between two modems, V.25bis - command language for control of the modem, etc., There are other names, for example, MNP, come with a symbol V, but further are not numbers, but characters, for example, V.FC.
The following MNP protocols operate: MNP1 and MnP2. - outdated and are not currently not used; MnP3. - Provides synchronous transmission; MNP4. - transmits data in synchronous register packages from 32 to 256 bytes of data, while the size of the package depends on the quality of the telephone line. For a less high-quality line, a smaller package is used for more - more; MnP5 - Provides synchronous mode, the data compression is used, has two compression algorithm for repetitive messages; MNP6 - Provides synchronous mode, also uses data compression; MNP7, MNP8, MNP9 - Provides synchronous mode, while uses more advanced compression methods; MNP10 - Used with a poor-quality data line. At the time of starting the work, sets the lowest speed, and if the line is capable of working with increased transmission, then the speed increases.
There are also the following protocols:
Xmodem. - The protocol was released in 1977. The transmitting modem sends a special NAK signal, then after receiving the receiving modem displays the NAK signal until it receives a data packet that consists of a data start symbol (SOH), block numbers 128 bytes and checksum (CS) . Upon receipt of data and checking on the correctness of the checksum, the signal is sent to the fact that the data is taken (ACK), and if taken incorrectly, the signal (NAK) is sent. If there are several unsuccessful data transmission, the communication session is stopped. At the end of the transmission, the EOT character is sent, reporting on the end of the session.
There are modifications of this protocol, for example, in XModem CRC. check sum increased to 16 bytes, which increases the reliability of the transfer, XModem 1K. - the size of the data block is increased to 1 kilobyte, XModem G. - transmits data, and the checksum is at the end of the data block, but the file.
Ymodem. - Based on the XModem protocol, with the value of the transmitted data of 1 kilobyte, transmits the file name and its attributes. In addition, the first block contains information about whether the following files are available for transmission.
Kermit. - uses data packets up to 94 bytes, mainly used in UNIX systems.
Zmodem. - transmits data from 64 to 1024 bytes with their compression. If you fail, sends the data from the moment when it was a failure.
Bimodem. - Further development of the ZMODEM protocol with the ability to send data in two directions at the same time.
Sometimes they may be required team modem, for example, for testing it. Below is a list of some modem commands (we note that modifications modifications can have a different set of commands):
ATA - the readiness of the modem to work;
ATADPNER - pulse dialing phone number;
ATADTNETER - Tone set of phone number;
ATW. - waiting for carrier;
ATMX - the work of the loudspeaker, where 0 is turned off, 1-on;
ATLX. - the loudspeaker volume from 0 to 7;
ATQX - Modem messages about command execution: 0-on, 1-turned off;
Athx - 0-disable modem from line, 1-connection;
ATZ.- restoration of the initial mode of operation;
AT & W. - recording the current parameters of the modem in memory;
ATSX \u003d value - determination of the characteristics of the modem;
+++ - switching the modem into command mode;
A \\- Repeat the last team.
When transferring data to the modem, special data compression protocols are used, for their faster transmission and error correction methods. Such standards are indicated by MNP (Microcom Networking Protocol - MicroCom Network Protocol), as well as some of the standards starting with the letter V (V.41, V42 and V42BIS).
For data transfer, a special protocol is used, that is, the rule by which the data is transmitted and accepted. For normal operation, it is necessary that both modem (sending and receiving) can work with these protocols. With data correction methods, in addition to them, a special CRC combination is sent, which serves to determine errors. When receiving, the data is checked, that is, calculations and comparison of CRC blocks (calculated and verifying) are made and, if normal, the signal is sent to the fact that the data is received correctly.
Comments.The country code in the computer coincides with the international telephone prefix. The phone number consists of the following numbers: country code (10 for Russia), + region code (495 or 499 for Moscow) + PBX number (3 digits) + phone number inside PBX (4 digits)
If the modem is experimented and it does not work, then you can reboot the computer, while turning off and turning on the modem, or enter the AT & F command, and to determine the parameters of the modem, enter AT & V.
Transfer of text information on telephone channels is called detecton communications.
Modems contain in yourself: an input-output port adapter for working with a telephone line; I / O port adapter for working with a computer; processor generating modulation / demodulation of the signal and providing communication protocol; Memory where the microcircuit management program is stored, modem parameters are supported, and RAM; Controller messaging messages with computer and modem components.
The modem may have a part of these components, and the missing part will simulate the central processor, for example, the controller. Such modems are called software.
The most important characteristic is the data transfer rate. More recently, the standard was the speed of 14.4 kbit / s (of course there were smaller speeds), then devices appeared to transmit information at a speed of 28.8 and 33.6 kb / s. Now the maximum transfer rate has reached 128 kb / s and provided the maximum possibility of transferring the telephone network.
Of course, devices that work at a speed of 33.6 kb can also work at slower speeds, namely 28.8 and 14.4 kb / sec., But not vice versa. So, if at one end there will be a modem that provides a transmission rate of 28.8 kb / s, and on the other - 14.4, then the transmission will occur at a speed of 14.4 kb / s.
Installing modem
Installing the modem. Installing the modem, as a rule, does not constitute big problems, because after installation operating system Itself finds it and installs a standard driver. If the driver is given to the modem, it is desirable to install it, as compared with the standard driver, it gives additional features.
To install, you need to perform the following sequence of actions:
Turn off the computer (if an internal modem is connected or an external to the sequential port);
If it is an internal modem, set it as an extension fee. At the same time, hold the edge fee, without touching the conductors and microcircuits on the boards. If this is an external modem, connect to the serial port or USB port. If the number of pins in the connector of the serial port does not coincide, the adapter will be required, since one of the ports can already be busy;
If there is one output in the modem, you need to connect the wire with one end to the modem, another end - to the telephone socket. In this case, you can use a special view of the socket, which has two outputs: one for the phone, the other for the modem. The appearance of such a socket is shown in the figure on the right, it contains two such types of the connector.
One coincides with the standard operating in our country, and the second one with the west adopted in the West, it is available in many modem sold.
You can use a special splitter, which has one connector at one end, on the other - two. One connector is installed in the telephone, the wire to the telephone socket and the wire to the modem connects to the other other.
If there are two telephone connector on the modem, then in one you need to connect the wire from the phone socket (the inscription near the LINE connector), the other to the telephone set (Phone's inscription). If there is no inscriptions, then look at the rear wall of the modem, where there can be a diagram of contacts, or refer to the documentation. If the connection is made incorrectly, the modem will not work. In this case, change the contacts. External modem You also need to connect to the network through the power supply. To set the internal modem, use the description of the installation of boards into the system unit;
After installation, turn on the computer and install softwaresupplied with the modem.
In laptops there is one output to connect to the telephone line. During working with the modem, it is better not to use a parallel telephone or connect it through the corresponding socket on the modem, otherwise there may be filled from the telephone line, noise appear.
In the Windows system, after installing the modem, a message appears on the screen that the system has detected a new device, after which the system itself will try to determine its characteristics. Follow the instructions attached to your modem. It is necessary to make the right installation so that there are no conflicts due to the use of system resources.
Installation The modem is made in the same way as other devices. If the modem supports the Plug & Play standard, then when you enable the computer, the "Master" of the settings will appear on the screen, which will help to install the modem using questions and responses. If the modem does not support the Plug & Play standard (for very old models), you need to use the mode: Start → Settings → Control Panel → Modems (2) → Properties (Modems) → Add → (Do not define modem type) Next. If there is a disk to the modem, then you need to use the "Install from the disk" mode or, when it is missing, select the manufacturer (if unknown, then "Standard Modem Types") and model → Next → selecting the appropriate model, click Next → (Select The required port) Next.
One of the most important parameters to be installed is the type of dialing that should be pulsed, since in our country, another type is not used. To install it, you need in the Properties window: Modems: General Press "Communication Settings", where to select a pulse set.
To checkwhether the installation is properly performed, use the mode: Start → Setup → Control Panel → System (2) → Devices where there is a list of devices. If about the name "modem" is a plus sign, then you need to click on this icon to reveal the list of modems. After that, you should make sure that there is no question and exclamation mark about the installed device.
Modems parameters can be look and change By: Start → Setup → Control Panel → Modems → Properties → General, where the port changes, the volume of the speaker, the maximum speed is indicated. At the same time, the maximum speed is meant between the modem and the computer, and not between modems. Usually set the maximum speed, and in the case of bad connection It is reduced.
Other questions
In general, communication channels are divided into:
Analog (for example, the phone), for which the information is transmitted as a continuous signal;
Digital, transmission of digital (discrete or pulsed) signals
or
Simplex,
Half duplex,
Duplex
or
Switched, created at the time of information transfer, are then disconnected;
Non-commutable (dedicated) allocated to long term
or
Low-speed (telegraph) at a speed of 50-200 bytes / s.;
Medium-speed (telephone) at a speed of 300-56,000 bytes / s.;
High-speed, over 56,000 bits / s.
To transfer data from high speed The wire is a twisted pair (retained between themselves), a coax cable (as in the television antenna), fiber optic (from glass fibers) and radio channel (through radio wave).
Radio waves can be super long (3-30 kHz), long (30-300 kHz), medium (300-3000kHz), short (3-30 MHz), ultrashort (30 MHz-3GHz), submillimeter (300-6000GHz).
During data transmission, several modulation types are used: frequency (V21), phase (V22), amplitude and quadrature amplitude modulation at which the phase and amplitude changes are changed, more interferent than the previous ones, so it is used in the standard v22.bis and higher.
The protocol also contains the ability to split messages to blocks, recovery of communication, error correction, etc. These include Xmodem, Ymodem, Zmodem, Kermit, etc. The most common is zmodem.
Network cards Serve to connect a computer to a network of computers and are an intermediary between a computer and data network. Network card has its own processor and memory. The main characteristics of the network card is the bus to which it connects, the size of the memory, the discharge of the card (8, 16, 32, discharge), the types of the connector for a thin and thick cable. Network cards require setting the interrupt line (often 3 or 5), DMA channel, memory addresses (C800).
Cable for network Maybe several species:
Twisted para. It consists of several twisted copper conductors in one cable, which can be unshielded (UTP) or shielded (STR).
Coax cable It consists of central and shielding wires, between which isolation is insulated. There are two varieties of this cable: thin (thick 0.2 inches) and thick (0.4 inches thick).
Fiber optic cable Consists of two wires consisting of fiber fibers. It has a greater bandwidth, but very expensive, therefore it is rarely used.
When using cable pay attention to wave resistance, often 50 ohms. When laying, you need to have cables of one brand, preferably one manufacturer. After laying a thin cable, connectors are installed, for example, Russian production (CP50) or crimping BNC connectors. At the ends, the plug is installed and one of them must be grounded.
The thick-cable laying occurs through transceivers, and one transceiver is used to one computer, and at the ends of the cables following the computer must be 15-pin DIX connectors (or AUI). At the end of the cables are installed: N-terminators, one of which ground. To increase the length of the local network (for a thin cable, it cannot be more than 185 meters), use repeaters (Repeater - repeater).
The twisted pair cable is used together with a hub or hub (HUB), from which a cable is laid no more than 100 meters long. At the ends there is a RJ-45 connector, which looks like a telephone connector, but has 8 contacts (and not 4). Hubs may have a different number of ports, for example, 8, 12, 16 corresponding to the maximum number of connected computers.
When working modem as faxHe works on its standards. When sending faxes with a speed of 14.4 kbps, the standard V.17 (14 400), V27 Ter (4 800), V29 (9 600) and T.30 for the protocol itself is used. When transferring an image of a sheet, the following modes with resolution for fax transmission can be used: Standard (Standard) - 100x200 DPI; Quality (Fine) - 200x200 DPI; High quality (Superhigh) - 400x200dpi; Photoregray (Photo) transmits 64 grades of gray.
A modern modem supports most standards, in any case, those that work with less than the maximum speed of this modem.
In addition to conventional modems, there may be very specific modems, for example, cable when the signal is transmitted through television cable. In this case, the cable connects to a special outlet on which there is a TV connector and for a serial computer channel. Work on cable networks allows you to transmit data at high speed. However, with time when the number of users increases, the bandwidth for each user can be low. And now, while users are a bit, they give a small number of users great advantages of working on the Internet.
Can be used satellite devicesHowever, users through the phone pass the message to the provider, which pages he wants to get, and gets them through the satellite.
Currently, more and more information is used mobile connection. In this case, the modem is connected to a mobile phone through a special cable.
We have the greatest distribution when transferring data - voice and digital, there is a standard GSM.- Global System for Mobile Communication, which can be translated as "Global System for mobile communications" The essence of this standard is that all transmitted information is divided into so-called frames divided into eight intervals. Depending on the employment of the line, one interval can be involved or the other. But this method mobile communications Designed primarily for transfer voice messagesthat have priority to digital data. Ultimately, the rate of data pumping does not exceed 9.6 kbps.
Another standard GPRS. (General Packet Radio Service - Shared Batch Radio Radio) allows you to increase such speed up to 50 kbps, and theoretically can reach 100 kbps. In contrast to GSM, there are also other time intervals in the frame, up to all eight, and this circumstance increases the speed of data. In addition, this mobile communication option provides a reduction in user spending, as the amount of information transmitted unlike GSM is paid.
GPRS devices are divided into three classes in their capabilities:
Class A. Such devices in each unit of time are able to transmit both types of information simultaneously - voice and digital.
Class B. These models allow you to work alternately or with digital data, or with voice.
Class S. Here is the shipment of only digital data.
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 has seen network card For a laptop will understand what we are talking about).
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);
data exchange devices, documents and receiving 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.
Information about the work "Modems: Use on networks, differences in architecture, comparative characteristics, features of operation. Non-standard situations and their permission. Diagnostics and Testing »
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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). The discrete channel 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. Telephone channel standards as derivatives from standard Canal The tonal frequency (TC) is 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. Data transmission by telephone networks Describe the recommendations of the V Series of the International Telecommunications Union (Sector of 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.
