In the modern world, Internet users receive information, explore the Internet space, without thinking about how they can do this. Almost always, users confuse a router with what is a modem. Let's look at what it is in this article.
The ancestor of the current data device appeared in 1962. His the creator is company AT and T. At that time, the speed of information exchange was only three hundred bits per second. Then in 1991 this data increased to fourteen kilobits per second.
What is a modem
A modem is a device for receiving and sending information through the telephone system. Streams of information enter it, where they are converted into the necessary signal that passes through the telephone line. It goes to the other end of the wire, where another similar device is already demodulating the signals, turning them into computer signals, and they enter the computer, and then are displayed on the screen user. The word itself comes from an abbreviation of two English words: modulator and demodulator.
What are these devices for?
Modems are in use for connection with the Internet via a telephone line. This device is a kind of bridge between the Internet and home or office equipment. Modern models can be used as a router, sharing the Internet between several devices.
It is worth noting that it will not be able to fully replace the router, since it is not possible to receive the Internet via rj45 from the provider.
Types and types of modems
All such gadgets can be conditionally divide by type and type. Let's look at them more specifically:
- By type of connection modems are made wired and wireless. Wireless well used by laptop owners. Since they connect to the laptop via a USB connector.
Wired connected to the computer using a cable.
- According to the operating principle divided into hardware and software. Hardware differ from software ones in that all signal processing functions are performed by the device itself. Software They give all the work to the computer processor.
- By type of connection devices are divided into telephone, mobile, Dial Up. Analog modems or Dial Up operate over the telephone network. Their speed reaches only 56 kilobits per second. ADSL technology has replaced analog gadgets and is now used everywhere. The transmission speed of information via ADSL reaches 100 MB/s. Mobile phones include those available in the form of a keychain. They work using EDGE, 3G, 4G protocols. Data transfer speed in 3G is up to 3.5 MB/s. While the speed of 4 G is 100 MB/s.
- Broadband. These are ADSL modems. Today, the fastest devices for data transfer.
Popular manufacturers
Modems are produced by many companies. But the most popular of them are Cisco, Zixel, TP LINK, ASUS. These models are famous for being complete. Can work like router.
They are often equipped with DLNA, file and FTP server. In addition, they have an interface to support up to 4 computers. Web interface support.
What does a modem consist of?
Almost the only external hardware components are the input and output ports. This also includes universal, signal and modem processors, read-only storage, RAM, and device status indicators.
The functions that the device can perform are determined mainly by the activities of the universal processor and the program located in the ROM. If update ROM or reprogram it, you can improve the functions of a particular device.
The signal processor converts incoming and outgoing signals into those needed by the device that is connected to it. Buffered in RAM incoming and outgoing data, compression algorithms and other functions occur. Adapters allow you to exchange data, on the one hand, between the modem and the Internet line, and on the other hand, between the computer and the modem. 
Principle of operation
This device (regardless of USB or landline) turns normal signal to digital. This device has a built-in modulator that converts these signals. The modulator converts signals from the computer, before starting to transmit information, into the signals required by the Internet line. Then the data is transported. And the device at the other end is already demodulating these signals to those that are necessary for the PC with which it is connected.
This is how the information needed by the user is supplied.
What is the difference between a router and a modem?
Many people confuse a router with a modem. This not the same devices. Routers have the following features:
- The modulator-demodulator converts the signal, and the router divides it among network users.
- The first one works with one user, the router with several.
- A router, unlike a signal converter, is a multifunctional device.
- Routers are assigned their own IP address.
Although it is worth noting that for the latest models such the differences are not relevant. Almost all functions of the router and modem are now identical, except that the router cannot transmit data through the telephone line. In modern devices this can be considered the main and only difference.
A modem is a device designed to modulate a signal, that is, to convert an analog signal to a digital one. It is from the word “modulation” that the name “modem” comes from. Using a modem, the user accesses the Internet. The first similar device appeared in 1979. During this time, of course, a lot has changed. The speed has also changed, which can vary greatly among users, so some people want to measure the Internet speed.
Types of modems
1) Fiber optic modem. The device connects the computer to the global network via a fiber optic cable.
2) Cable modem. It allows you to transmit a signal through a standard television cable. At the same time, working on the Internet does not in any way affect the quality of the television signal transmission.
3) ISDN modems. Such modems are used to work in digital networks - with their help it is possible to transmit voice, text information and graphics at the same time at a constant high speed.
4) ADSL modems. They connect to a telephone line, but work using a special technology, due to which the access speed increases significantly. Such modems are not common due to the fact that they require special, complex equipment, which does not always pay off.
Modems are classified according to their functionality as follows:
1) Analog modems work to transmit information and receive signals.
2) Fax modems are convenient because they perform the fax function.
Modems are divided into external and internal.
The external modem looks like a small box and connects to the PC via the main COM port, or in some cases via a USB port. The external modem is equipped with indicators that can be used to read the necessary information.
Modems tend to freeze, in which case you need to turn it off and on again. Connecting an external modem is easier than an internal one - you need to connect the cable at one end to the modem and the other to the computer.
An internal modem is a small board that is installed in a special PCI slot located inside the computer. Internal modems are cheaper and do not require a power supply and a separate outlet for connection.
The modem performs the functions of both input and output devices. It allows you to connect to other remote computers using telephone lines and exchange information between computers. The modem converts digital signals into sounds when transmitting, and vice versa when receiving.
Modem is a device for converting digital signal information into analog (Modulation) for transmission over analog communication lines, and converting the received analog signal back into digital (DEModulation).
Why is this needed? Since computers can only exchange digital signals, and communication channels are such that analog signals pass through them best, this is why a bridge is needed that converts the signal - a modem. But the modem also has quite a few other functions, the main ones being error correction and data compression. The first mode provides additional signals through which modems check data at both ends of the line and discard untagged information, while the second mode compresses the information for faster and clearer transmission, and then reconstructs it at the receiving modem. Both of these modes significantly increase the speed and purity of information transmission, especially in Russian telephone lines.
Main characteristics of modems
Modems differ in many characteristics: design, supported data transfer protocols, error correction protocols, voice and fax data transfer capabilities.
By execution(appearance, placement of the modem in relation to the computer) modems are: internal - inserted into the computer as an expansion card; desktop (external) have a separate case and are placed next to the computer, connecting with a cable to the computer port; the modem in the form of a card is miniature and is connected to the laptop computer through a special connector; the portable modem is similar to the desktop modem, but has a reduced size and is self-powered; rack modems are inserted into a special modem rack, which increases ease of use when the number of modems exceeds a dozen.
Modems also differ by type: an asynchronous modem can only transmit over an analog telephone network and works only with asynchronous communication ports of terminal devices (in its pure form it is not currently used);
fax modem is a classic modem with added fax capability, which allows you to exchange faxes with fax machines and other fax modems;
modem with backup of a dedicated dial-up line - these modems are used when reliable communication is required. They have two independent line inputs (One connects to a leased line and the other to a dial-up line);
synchronous modem - supports synchronous and asynchronous transmission modes;
four-wire modem - these modems operate over two dedicated lines, one is used only for transmission, the second only for reception) in full-duplex mode. This is used to reduce the influence of echo;
cellular modem - used for mobile radiotelephony, which includes cellular communications;
ISDN modem - combines a regular modem and an ISDN adapter in its case;
a radio modem uses the air as a transmission medium instead of telephone wires;
network modem - these are modems with a built-in LAN network adapter for sharing on a local network;
cable modem - these modems allow you to use cable television channels for transmission. At the same time, the speed can reach 10 Mbit/s.
Modems are also characterized by data transfer speed. It is measured in bps (bits per second) and is set by the manufacturer at 2400, 9600, 14400, 16800, 19200, 28800, 33600, 56000 bps.
Drives for CDs. Purpose. Main characteristics.
Operating principle of a CD-ROM drive. The surface of the optical disk moves relative to the laser head at a constant linear speed, and the angular speed varies depending on the radial position of the head. The laser beam is directed onto the track and focused using a coil. The beam penetrates the protective layer of plastic and hits the reflective layer of aluminum on the surface of the disk.
When it hits the protrusion, it is reflected onto the detector and passes through a prism, which deflects it onto a light-sensitive diode. If the beam hits the hole, it is scattered and only a small part of the radiation is reflected back and reaches the photosensitive diode. On the diode, light pulses are converted into electrical ones, bright radiation is converted into zeros, and weak radiation into ones. Thus, the pits are perceived by the drive as logical zeros, and the smooth surface as logical ones.
CD-ROM capacity is 640-700 MB. The information carrier on a CD is a relief polycarbonate substrate, on which a thin layer of light-reflecting metal is applied.
CD-ROM discs are designed for reading information only, not writing.
CD-ROM drive performance. It is usually determined by its speed characteristics during continuous data transfer over a certain period of time and the average data access time, measured in KB/s and ms, respectively. There are one-, two-, three-, four-, five, six and eight-speed drives that provide data reading at speeds of 150, 300, 450, 600, 750, 900, 1200 KB/s, respectively. An important characteristic of the drive is the buffer fill level, which affects the quality of playback of animated images and videos.
Design features of CD-ROM drives
As you know, most drives are external and built-in (internal). CD drives are no exception in this sense. Most CD-ROM drives currently offered are built-in.
The front panel of each drive provides access to the CD loading mechanism. One of the most common is the CD-ROM loading mechanism using a caddy.
CD-R. A disk drive with the ability to write information once onto a special disk. Recording on CD-R discs is carried out due to the presence of a special photosensitive layer on them, which burns out under the influence of a high-temperature laser beam.
The speed of writing information to CD-R discs on modern drive models can reach up to 20 times. However, it is very important to select discs for recording, the markings of which coincide with the speed marking of your drive (4x, Sx, 10x, 12x, 14x, etc.). Most blanks sold today should support at least eight times the write speed.
CD-RW. Today, CD-R drives have virtually disappeared from the scene. They have been replaced by new standard drives that can burn not only CD-Rs, but also rewritable discs - CD-RWs. When recording these discs, a completely different technology is used, different from CD-R, and they are designed differently.
A CD-RW disc is like a layer cake, where the working, active layer rests on a metal base. It consists of a special material that changes its state under the influence of a laser beam. Being in a crystalline state, some parts of the layer scatter light, while others - amorphous - transmit it through themselves, onto the reflective metal substrate. Thanks to this technology, information can be written to the disk, and not just read.
Speed characteristics are usually indicated in the name of the drive - for example, 12x8x32, where the lower value corresponds to the CD-RW write speed, and the maximum corresponds to the read speed.
ROM. Purpose. Compound.
Read-only memory (ROM) stores information that does not change during computer operation. This information is made up of test-monitor programs (they check the functionality of the computer when it is turned on), drivers (programs that control the operation of individual computer devices, for example, a keyboard), etc. ROM is a non-volatile device, so the information in it is saved even when the power is turned off.
Persistent memory(ROM - read-only memory) - non-volatile memory, used to store data that will never need to be changed. The memory contents are specially “hardwired” into the BIOS chip during its manufacture for permanent storage. ROM can only be read.
BIOS is the basic input/output system. BIOS is a complex system consisting of a large number of utilities designed to automatically recognize the equipment installed on a computer, configure it and check its operation.
This system includes various input-output programs that provide interaction between the operating system, application programs on the one hand, and devices included in the computer (internal and external) on the other.
Originally, BIOS was intended to test the computer when it was turned on. Currently, BIOS is a complex system consisting of a large number of utilities designed to automatically recognize the equipment installed on a computer, configure it and check its operation. The most promising BIOS for system storage is flash memory(replaceable memory cards). It allows you to modify functions to support new devices connected to your computer. The BIOS system is inextricably linked with CMOS RAM.
CMOS(semi-permanent memory) - a small area of memory for storing computer configuration parameters, which is regulated using the CMOS Setup Utility. Has low power consumption. The contents of CMOS memory do not change when the computer's power is turned off because it uses a special battery to power it. It is used to store information about the configuration and composition of the computer’s equipment, stores information about floppy and hard drives, about the processor, as well as readings from the clock system.
RAM. Purpose. Compound.
Random access memory (also random access memory, RAM) - in computer science - memory, part of the computer memory system, which the processor can access for one operation (jump, move, etc.). It is designed to temporarily store data and instructions necessary for the processor to perform operations. RAM transmits data to the processor directly or through cache memory. Each RAM cell has its own individual address. RAM can be manufactured as a separate unit or included in the design of a single-chip computer or microcontroller.
Random access memory (RAM) is used for short-term storage of variable (current) information and allows its contents to change as the processor performs computational operations. This means that the processor can select a command or processed data from RAM (read mode) and, after arithmetic or logical processing of the data, place the result in RAM (write mode). New data can be placed in RAM in the same places (in the same cells) where the original data was located. It is clear that previous commands (or data) will be erased.
RAM is used to store programs compiled by the user, as well as initial, final and intermediate data resulting from the operation of the processor.
RAM uses either flip-flops (static RAM) or capacitors (dynamic RAM) as storage elements. RAM is a volatile memory, so when the power is turned off, the information stored in the RAM is lost forever.
Today, the most common types of RAM are SRAM (Static RAM). RAM collected on flip-flops is called static random access memory or simply static memory. The advantage of this type of memory is speed. Since the triggers are assembled on gates, and the gate delay time is very short, switching the trigger state occurs very quickly. This type of memory is not without its drawbacks. First, the group of transistors that make up a flip-flop is more expensive, even if they are etched in the millions on a single silicon substrate. In addition, a group of transistors takes up much more space because communication lines must be etched between the transistors that form the flip-flop.
DRAM (Dynamic RAM)
A more economical type of memory. To store a discharge (bit or trit), a circuit consisting of one capacitor and one transistor is used (in some variations there are two capacitors). This type of memory solves, firstly, the problem of high cost (one capacitor and one transistor are cheaper than several transistors) and secondly, compactness (where one trigger, that is, one bit, is placed in SRAM, eight capacitors and transistors can be accommodated). There are also some disadvantages. Firstly, capacitor-based memory works slower, because if in SRAM a change in voltage at the trigger input immediately leads to a change in its state, then in order to set one digit (one bit) of capacitor-based memory to one, this capacitor must be charged , and in order to set the discharge to zero, discharge accordingly. Memory on capacitors got its name Dynamic RAM (dynamic memory) precisely because the bits in it are not stored statically, but “drain” dynamically over time. Thus, DRAM is cheaper than SRAM and its density is higher, which allows more bits to be placed on the same space of the silicon substrate, but at the same time its speed is lower. SRAM, on the contrary, is faster memory, but also more expensive. In this regard, conventional memory is built on DRAM modules, and SRAM is used to build, for example, cache memory in microprocessors.
General provisions
Modems (the name comes from the merger of two words - modulator and demodulator)- These are devices that allow you to organize communication between computers located at a distance from each other. If the computers are nearby, then you can organize communication between them using a serial, parallel port, USB, Blutooht. However, such communication is only possible at close distances, determined by the capabilities of the port. Over long distances, the signal weakens and special devices are required that can convert the signal into a form that allows the signal to be transmitted over long distances. For this purpose, a device called a “modem” is used - from the word MOdulator-DEMOdulator. The modulator allows you to convert a digital signal to analog, and the demodulator allows you to do the reverse conversion, that is, convert from analog to digital form(in a more precise sense, modulation is a change in the characteristics of a carrier signal (usually low-frequency periodic oscillations) by a high-frequency control signal, which allows the necessary information to be transmitted). Demodulation is the separation of an information signal from a combination of carrier and information signals). A fax works on almost the same principles, which is why modems that are produced with fax transmission capabilities are called a fax modem. Modems can be internal (inserted into expansion slots), external (connected to COM, LPT, USB ports or a network cable to the RJ-45 connector of the computer’s network card, usually have an external power supply), built-in like a laptop or in the form of a connection card to PCMCIA connector for laptop computers(the latter is also called an expansion card PC Card and is practically obsolete. The standard currently used ExpressCard with bus connection USB and PCI Express ). Recently, wireless modems (called a module or gateway) using communication lines from cellular operators have become widespread (the most famous are USB modems) . The operating principles of all devices are the same.
Modems can be analog And digital. Analog modems (dial-up) were the first to be used. Due to the fact that the data transfer speed through these modems was not high (up to 56 Kbps), they began to switch to digital modes (with operating frequencies from 4 KHz to 2 MHz and, accordingly, speeds up to several megabits/sec). In addition, you cannot conduct a conversation while transmitting data via an analog modem.
Most users used the telephone network to transmit data. In order to use digital transmission, it is necessary that both the sender and the recipient have a digital telephone exchange. In addition, there should not be a paired telephone and burglar alarm on the telephone line. Some users still use analog modems.
Main characteristics of modems:
- interior or external. An internal modem is a card that plugs into a slot on the motherboard. This modem is inserted like a regular card, but you need to connect the wires as indicated below. An internal modem is usually cheaper than an external one. But it doesn't require any desk space or take up the computer's serial port.
External modems (new) are connected to a USB, PCMCIA or ExpressCard connector and do not require additional power, as they receive it from the connector.
An external modem (old ones) is connected to the serial port and is located in a separate housing. This type requires connection to the electrical network via a transformer. Its advantages include the fact that it does not occupy an expansion slot and makes it easy to transfer it from one computer to another.
Supported standard And transmission speed;
Size of RAM or flash memory.
Additional modem features: digitizing voice and converting it into an analog signal for conversation when transmitting data; Fax; automatic caller number identification; answering machine; electronic secretary and other capabilities that telephone sets have.
As a rule, a modern modem has the following phone capabilities, which we will present. These are: negotiations with several subscribers; temporarily muting the microphone; turning on external speakers; memory for subscriber numbers; calling the subscriber again; auto dialer; automatic number identification; remembering called numbers and time of call; detection of the second ring during a conversation; protection from unwanted calls; recording received messages; answering machine; remote control; the phone panel may have buttons with functions: auto-repeat, listening to left messages, turning off the phone, turning off external speakers, etc.; there may be indicators on the telephone panel that determine the operating mode, picking up the handset, etc.; there may be a display with data on incoming and outgoing calls, talk time, etc.; voice dialing, the user calls the subscriber’s last name by voice, and the modem connects with his number; speed dialing, dialing a number using one or two keys; auto attendant, answering incoming calls during a conversation with another subscriber; collecting statistics on the number of calls received, their numbers, call time during the day, etc.; other functions, for example, dialing a specific number at a certain time of the day, alarm clock, etc.
If the modem freezes, you can restore its functionality by resetting the power (remove the external one and reinsert it again), but you do not need to turn off the computer. In addition, it has an indication by which you can determine the status of the modem.
Digital modems.
Several are currently in use formats: ADSL, HDSL, IDSL, ISDN, HPNA, SHDSL, SDSL, VDSL, WiMAX and wireless modems using wireless communication (Wi-Fi). They are often called xDSL (Digital Subscriber Line).
ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) appeared in 1987 and is one of the very first and most common digital data transmission formats. Allows you to send data from the user to the network at speeds from 16 to 640 kbit/s (according to standards 0.5, 0.8, 1.2, 1.3, 3.5 Mbit/s, and receive data at speeds of 1.5, 0.8, 5, 8, 12, 25 Mbit/s sec). Since the user usually receives data rather than sends it, this separation of speeds is not felt by the user, except in cases of video communication. Therefore, over time, other types of formats began to appear using coaxial cable (cable television, speeds up to 100 Mbit/s) and an Ethernet connector (local network with speeds up to 1 Gigabit/s). In a number of European countries, the ADSL standard has become the standard by which every resident receives Internet access.
A regular telephone line uses frequencies from 0.3 to 3.4 KHz to pass through; for an ADSL modem, the lower frequency for the outgoing stream is 26 kHz, the upper frequency is 138 KHz, and for the incoming stream it is from 138 kHz to 1.1 MHz. This way, you can talk on the phone and send and receive data at the same time.
However, the first modems did not allow comfortable conversations on the phone, since the high-frequency part of the modem introduced extraneous noise into the telephone conversation (and, conversely, the conversation introduced distortions into the data transmission). To avoid this, they began to use a frequency filter (Splitter), which allowed only low frequencies to pass through to the phone.
HDSL (H igh D ata rate digital Subscriber Line (high-speed digital subscriber line) developed in the late 80s. It uses not one, but two pairs of wires and has a speed of either 1.5 Mbit/s (American standard) or 2.0 Mbit/s (European standard) and allows you to transmit a signal up to 4 kilometers, and in some cases up to 7 kilometers. Mainly used for organizations.
IDSL(ISDN Digital Subscriber Line - IDSN digital subscriber line) allows data transmission at a speed of 144 Kbps.
ISDN(Integrated Services Digital Network) appeared in 1981 and has a data transfer rate of 64 Kbps.
HPNA(Home Phoneline Networking Alliance is the name of a joint association of non-profit industrial companies) works with either standard telephone or coaxial cable. The latest standard (3.1) allows you to transfer data at speeds of up to 320 Mbit/s, according to standard 2.0 – 10 Mbit/s.
SHDSL (Symmetric High-speed DSL - symmetric high-speed DSL) allows you to transmit data over one pair of wires at speeds from 192 Kbps to 2.3 Mbps, and over two pairs twice as much over a distance of up to 6 km.
SDSL(Symmetric Digital Subscriber Line - symmetric digital subscriber line) uses one pair of cables with speeds from 128 to 2048 Kbps. Valid at a distance of 3 to 6 km.
VDSL(Very-high data rate Digital Subscriber Line - ultra-high-speed digital subscriber line) has a high data transfer rate from 13 to 56 Mbit/s from the network to the user and 11 Mbit/s in the opposite direction over a distance of up to 1.2-1.4 km.
WiMAX(Worldwide Interoperability for Microwave Access) is a wireless communication 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 to Wi-Fi, but differs in that it can transmit a signal over a long distance and, in addition, is somewhat more expensive.
Bluetooth(translation - blue tooth) was developed in 1998 and is used for wireless communication with a computer in the license-free range of 2.4 - 2.4835 GHz. It does not have a connector and is located inside the computer (device), used to transmit data using radio waves between various types of computers, cell phones, printers, cameras, keyboards, mice, joysticks, headphones, MFPs, scanners and others.The essence of the method is that in a certain range the frequency changes abruptly 1600 times per second. This change in frequency occurs simultaneously for the receiver and transmitter, which operate synchronously according to this scheme.The devices can be located at a distance of up to 200 meters from each other, depending on the obstacles between them (walls, furniture, etc.).
The transmit/receive device is located inside the computer and is not visible. If your computer does not have such a device, you can connect an external device via a USB connector that allows you to work with this type of data transfer.
There are standards: 1.0 (1998), 2.0 EDR (2004) with a data transfer rate of 3 Mbit/s, in practice about 2 Mbit/s, 2.1 (2007) using energy-saving technology, simplified communication between devices, has also become more protected, 2.1 EDR required even less power, connecting devices was further simplified and reliability increased, 3.0 HS (2009) with transmission speeds of up to 24 Mbit/s. 4.0 began to be used in the iPhone in 2011, allowing data transfer at a speed of 1 Mbit/s. in portions from 8 to 27 bytes.
There are profiles for this standard, which are a set of functions. In order for devices to work using a specific profile, both devices must support this profile. For example, A2DP (two-channel stereo audio), AVRCP (standard TV functions), BIP (image forwarding), BPP (text, email to printer forwarding) and so on
WiFi used to create a wireless network. Developed in 1991 by NCRCorporation and AT@T, supported by the Wi-Fi Alliance and compliant with the IEEE 802.11 standard. Used to connect computers and cell phones to a network (local and Internet).
The transmitting and receiving device is located inside the computer and is not visible. If your computer does not have such a device, you can connect an external device via a USB connector that allows you to work with this type of data transfer.
The following standards are available: 802.11a uses frequencies of 5 GHz, providing speeds (in theory) of up to 54 Mbit/s; 802.11b uses frequencies of 2.4 GHz, providing speeds (in theory) of up to 11 Mbit/s. (practically not used); 802.11g uses frequencies of 2.4 GHz, providing speeds of up to 54 Mbps. (the most common); 802.11n uses frequencies of 2.4 and 5 GHz, providing speeds from 150 to 600 Mbit/s. (newly developed, starting to gain momentum). This standard increases the data transmission range and reduces communication barriers. This standard uses MIMO (Multiple Input Multiple Output) technology, which allows the use of reflected waves from walls. If the device has one antenna, it can operate at a speed of 150 Mbit/s, two antennas - 300 Mbit/s, three - 450 Mbit/s, four (not yet available) - 600 Mbit/s. However, the declared data transfer speed differs from the actual one. So, instead of 300 Mbit/sec, it turns out about 100-130 Mbit/sec (since half of the transmitted information is service characters), which is also enough for work. And if there are walls, the speed drops further, for example, for three walls it will drop to 50 Mbit/sec.
Since some household appliances operate on the 2.4 GHz frequency (such as a microwave oven), they may cause interference. Therefore, it is advisable to have a device that operates at two frequencies: 2.4 and 5 GHz.
There are also cable modems for connecting to a cable television channel.
Typically, digital modems may contain elements that are used as Gateway between the local network and the Internet: router, firewall, etc.
Modem indicators
The following may be available indicators:
A.A.(Auto Answer - auto answer) - auto answer mode, providing an answer to the subscriber's request in automatic mode;
CD(Carrier Detect - carrier detection or DCD) - lights up during a communication session;
CTS or C.S.(Clear To Send) - the modem is ready to receive data from the computer. Goes off while receiving data;
DATA– lights up when data is being transmitted;
DC (Data Compression) - compression data ;
FAX– when the modem operates as a fax;
H.S.(High Speed) – lights up when the modem is operating at maximum speed;
E.C. (Error Control or ARQ) - error correction mode;
M.R.(Modem Ready – modem readiness or DSR) - indicates that the modem is connected to the power supply and is ready for operation;
OH(Off Hook – off hook) - lights up when the hook is hung up;
ON(PWR) - power indicator;
PWR (PoWeR) – power on;
R.D.(Receive Data - receiving data or RX or RXD) - indicates that data is being sent to the computer;
SD(Send Data – sending data or SX or TXT) - indicates that data is being received from the computer;
TEL– lights up when the handset on a parallel connected telephone is lifted;
RTS (Request To Send) - the modem is ready to receive data from the computer. Lights up when waiting for data from the computer, goes off during data transfer;
T.D. (Transmit Data or TXD) – lights up or flashes when data is being transferred from the computer to the modem. May light up when transmitting data at maximum baud rate;
TST (TeST) - flashes during testing;
TR(Terminal Ready – device readiness or DTR) - lights up when receiving a control signal;
USB– lights up when the modem is connected to the computer via the USB bus.
The modem body may also have a volume control.
On the back external modem may have connectors with icons:
A.C. IN – connecting the power adapter;
LINE – connection to a telephone line;
ON / OFF – turning on/off the modem;
PHONE – connecting a telephone;
R.S. -232 – connector for connecting to a computer serial port;
USB – connector for connecting to the USB bus.
Analog modem
Data transfer. Telephone lines are adapted to analog signals. Due to the fact that human speech has a range from 30 Hz to 10 KHz (music has a larger range), to save money, the telephone line passes a signal from 100 Hz to 3 KHz. It is this limitation that limits the ability to transmit data at high speeds. Computers can be connected not only through a telephone line, but also using radio waves and infrared radiation. In this case, no wires are needed.
Ultimately, the data sent in the parallel channel is converted into a serial transmission with start-stop bits at the serial port, transmitted to the modem, where it is simulated, that is, superimposed on the carrier frequency of the signal transmitted along the line, then sent to another modem. Next, they are converted into digital form, sent to the serial port, where they are converted into parallel form, and then sent to the processor for processing.
Digital data is sent bit by bit, and the sending can be of two types: synchronous and asynchronous. In synchronous transmission, a data packet consists of a header, which includes the destination address, the data itself, and a checksum. Asynchronous transmission transmits a start bit, 8 data bits, possibly a parity bit, and a stop bit indicating the end of the transfer. This type is used in a serial channel.
In addition, three modes can be used when transmitting data: duplex, in which data is transmitted in both directions simultaneously, half-duplex, in which data can be transmitted in both directions, but in one direction at a time, and simplex - data transmission only in one direction.
Data transfer from modem to modem and from modem to computer has different speeds, therefore, to prevent data from being lost, the modem has a buffer where the received data is stored.
Some modems compress data before sending it, and when received, another modem decrypts the data. There are files that have already been compressed, so this method may not provide any transfer benefits. To avoid data loss, the data transfer speed from the modem to the computer must be several times higher than between modems, which is actually implemented in practice.
When transmitting data, the unit is often used baud, which is sometimes confused with bits/sec. In fact, these are different quantities. 1 baud is one character sent per unit of time, and this can be not only data, but also control signals. A character can represent multiple bits. If the signal consists of two types: 0 and 1, then the symbol indicates 1 bit, if 512, then 9 bits (2 9 = 512). When transmitting data at low speeds, 1 baud is approximately equal to 1 bit/sec. At high speeds, the modem sends data at several frequencies, so at each moment in time, not one, but several bits are transmitted, that is, the speed measured in bits/sec, and not baud/sec, will be several times higher than the baud speed . Often the baud rate indicated implies the speed in bits/sec.
When transmitting via a modem, you can approximately determine how long it takes to transfer by dividing the transfer speed by 10, for example, if the transfer occurs at a speed of 28,800 bps, then approximately 2,880 bytes or characters will be transferred per second (28,800/10= 2 800).
The modem connects to the computer's serial port and processes serial data. Typically, a modem is used to work on the Internet, but it can also serve to communicate directly between two arbitrary computers. Modems are also used as fax machines to transmit fax messages. They may have a built-in adapter for creating voice messages in answering machine mode.
When connected, the modem sends signals that are also output to the speakers and can be heard as a continuously changing sound for several seconds. The receiving modem determines the standard by which it can operate, and also makes adjustments to the clock frequency, that is, it performs phase modeling. After this, the speaker turns off, but the signals continue to arrive, in particular, they can be listened to through a parallel telephone.
Modems come in two types: internal and external. The internal ones are made in the form of expansion cards and are inserted into the motherboard connector, the external ones have their own housing and are connected to the serial port using a cable. The latest types of modems can be connected via USB (and sometimes receive power from the computer), so they can be used while the computer is running, free up a connector, and have other advantages. When connecting a modem to a serial port, high-speed models require that the port also be fast. So, for modems with a speed of 56 Kbps, a speed on the serial port of 115 Kbps is required. The higher port speed is needed because it also sends control signals between the computer and the modem that are not transmitted over the telephone line. If the port does not support high speeds, data may be lost. External devices can be turned off by turning off the power supply, and internal devices can only be turned off when the computer is turned off, which is inconvenient when the modem freezes.
Modems can be divided into two categories: the first type (Class2) has an internal processor that processes the data, in the second the data is processed by the central processor (Class1), they are also called Windows modems, somewhat cheaper than the first type. Such a modem, if the processor is old, can greatly slow down the computer, but if the user rarely accesses the Internet and sends only a small number of e-mails from time to time, then this is acceptable. It is quite advisable to use it even if the computer has a powerful processor.
Often the modem is characterized protocol with whom he works. Exist signal modulation protocols, error correction protocols, data compression And work with fax communication (fax). The modem has several protocols for each of these types. Error correction protocols include V.42, MNP2-4, MNP10, data compression protocols – V42bis, MNP5.
One of the main characteristics of the modem is the data transfer speed, and the indicated maximum speed can be 33.6 or 56 Kbps for modern devices. If a speed of 33.6 Kbps is specified, then the entire bandwidth is used and data is transmitted in both directions at a speed of 33.6 Kbps. if the line allows it. If the line does not allow this, then a transition to a lower speed occurs. Speed 56 Kbps. ensures that data is received at a higher speed than when sending it, since there are more frequencies for reception than for transmission, but transmission from the modem is carried out at a lower speed.
In addition, it is necessary that both modems have the same characteristics, otherwise data transfer will not reach the maximum speed. To do this, before purchasing a modem from your provider, you need to clarify the type of modem with which it works best. Below is a table of correspondence between some protocols and their transmission speed.
The prefix bis indicates that the standard has been revised. Starting from speed 14,400, all protocols are duplex, that is, they transmit messages in both directions simultaneously. The names of not only standards that define a data transfer protocol, but also other types of protocols can begin with the symbol V, for example, V.24 contains a list of specific signals between two modems, V.25bis is a command language for controlling a modem, etc. there are other names, for example, MNP, some begin with the symbol V, but then there are not numbers, but symbols, for example, V.FC.
The following MNP protocols are in effect: MNP1 And MNP2- outdated and not currently used; MNP3– provides synchronous transmission; MNP4- transmits data in synchronous mode in packets from 32 to 256 bytes of data, while the size of the packet depends on the quality of the telephone line. For a lower quality line, a smaller package is used, for a higher quality line, a larger one is used; MNP5- provides synchronous mode, while data compression is used, has two algorithms for compressing repeated messages; MNP6- provides synchronous mode, also uses data compression; MNP7, MNP8, MNP9- provides synchronous mode, while using more advanced compression methods; MNP10- used when the data transmission line is of poor quality. At the moment of starting work, it sets the lowest speed, and if the line is capable of operating with a higher gear, then the speed increases.
The following protocols also exist:
Xmodem- protocol issued in 1977. The transmitting modem sends a special NAK signal, then, upon reception, the receiving modem issues a NAK signal until it receives a data packet, which consists of the start of data character (SOH), block number, 128 byte data and a checksum (CS) . When data is received and checked for correctness using a checksum, a signal is sent that the data has been received (ACK), and if it is received incorrectly, a signal (NAK) is sent. If there are multiple failed data transfers, the communication session is terminated. At the end of the transmission, an EOT character is sent indicating the end of the session.
There are modifications to this protocol, for example in Xmodem CRC the checksum has been increased to 16 bytes, which increases transmission reliability, Xmodem 1k– data block size increased to 1 kilobyte, Xmodem G- transmits data, and the checksum is located at the end not of the data block, but of the file.
Ymodem- based on the Xmodem protocol, with a transmitted data size of 1 kilobyte, transmits the file name and its attributes. In addition, the first block contains information about whether there are further files to transfer.
Kermit- uses data packets up to 94 bytes, mainly used in Unix systems.
Zmodem- transmits data ranging in size from 64 to 1024 bytes with compression. If there is a failure, it sends data from the moment when the failure occurred.
Bimodem– further development of the Zmodem protocol with the ability to send data in two directions simultaneously.
Sometimes may be required modem commands, for example, to test it. Below is a list of some modem commands (note that modifications of modems may have a different set of commands):
ATA- the modem is ready for operation;
ATADP number– pulse dialing of a telephone number;
ATADT number– tone dialing of a telephone number;
ATW– carrier waiting;
ATMx– loudspeaker operation, where 0 is off, 1 is on;
ATLx– loudspeaker volume from 0 to 7;
ATQx– modem messages about command execution: 0-enabled, 1-disabled;
ATHx– 0—disconnect the modem from the line, 1—connect;
ATZ– restoration of the original operating mode;
AT&W– recording current modem parameters into memory;
ATSx=value– determination of modem characteristics;
+++ - switching the modem to command mode;
A\– repeating the last command.
When transmitting data via modem, special protocols are used to compress data, for faster transmission, and error correction methods. Such standards are designated MNP (Microcom Networking Protocol), as well as some of the standards starting with the letter V (V.41, V42 and V42bis).
To transmit data, a special protocol is used, that is, a rule according to which data is transmitted and received. For normal operation, both modems (sending and receiving) must be able to work with these protocols. With data correction methods, in addition to them, a special CRC combination is sent, which is used to identify errors. Upon reception, the data is checked, that is, calculations and comparisons of CRC blocks (calculated and verification) are performed and, in the case of normal operation, a signal is sent that the data was received correctly.
Notes. The country code on your computer matches the international phone prefix. The telephone number consists of the following digits: Country code (10 for Russia), + region code (495 or 499 for Moscow) + PBX number (3 digits) + telephone number within the PBX (4 digits)
If you have experimented with the modem and it does not work, then to reset the parameter values, you can restart the computer, while turning the modem off and on, or enter the AT&F command, and enter AT&V to determine the modem parameters.
The transmission of text information over telephone channels is called dayphone communication.
Modems contain contains: I/O port adapter for working with a telephone line; I/O port adapter for working with a computer; a processor that modulates/demodulates the signal and provides a communication protocol; memory where the chip control program is stored, modem parameters are maintained, and RAM; a controller that manages communications with the computer and modem components.
The modem may have some of these components, and the missing part will be modeled by a central processor, for example, a controller. Such modems are called software modems.
The most important characteristic is the data transfer speed. More recently, the standard speed was 14.4 Kbps (of course, there were lower speeds), then devices appeared that allowed information to be transmitted at speeds of 28.8 and 33.6 Kbps. Now the maximum transmission speed has reached 128 Kb/sec and has provided maximum transmission capability over the telephone network.
Of course, devices that operate at 33.6 KB/sec can also operate at slower speeds, namely 28.8 and 14.4 KB/sec, but not vice versa. So, if there is a modem at one end that provides a transfer speed of 28.8 Kbps, and at the other - 14.4, then the transfer will occur at a speed of 14.4 Kbps.
Modem installation
Installing a modem. Installing a modem, as a rule, is not a big problem, since after installation the operating system itself finds it and installs the standard driver. If a driver is supplied with the modem, it is advisable to install it, since compared to the standard driver, it provides additional capabilities.
To install, you need to perform the following sequence of actions:
Turn off the computer (if you are connecting an internal modem or an external one to the serial port);
If it is an internal modem, install it as an expansion card. At the same time, hold the board by the edges, without touching the conductors and microcircuits on the boards. If it is an external modem, then connect to a serial port or USB port. If the number of pins on the serial port connector does not match, you will need an adapter, since one of the ports may already be occupied;
If the modem has one output for a telephone, then you need to connect the wire at one end to the modem, and the other end to the telephone socket. In this case, you can use a special type of socket that 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 has two types of connectors.
One coincides with the standard in force in our country, and the second with that adopted in the West; it is found in many modems sold.
You can use a special splitter, which has one connector at one end and two at the other. One connector is installed in the telephone, the other two connect the wire to the telephone socket and the wire to the modem.
If there are two telephone connectors on the modem, then you need to connect the wire from the telephone socket to one (inscription near the line connector), the other - to the telephone set (inscription phone). If there is no inscription, then look at the back wall of the modem, where there may be a contact diagram, or refer to the documentation. If the connection is made incorrectly, the modem will not work. In this case, change the contacts. The external modem must also be connected to the network via a power supply. To install an internal modem, use the description of installing boards in the system unit;
After installation, turn on your computer and install the software that came with your modem.
Laptops have one output for connecting to a telephone line. When working with a modem, it is better not to use a parallel telephone or connect it through the corresponding socket on the modem, otherwise interference from the telephone line may occur and noise may appear.
In Windows, after installing the modem, a message will appear on the screen stating that the system has detected a new device, after which the system itself will try to determine its characteristics. Follow the instructions that came with your modem. It is necessary to make the correct 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 turn on the computer, a “installation wizard” will appear on the screen, which will help you install the modem with the help of questions and answers. If the modem does not support the Plug & Play standard (for very old models), then you need to use the mode: Start → Settings → Control Panel → Modems (2) → Properties (modems) → add → (do not define modem type) Next. If you have a disk for the modem, then you need to use the “Install from Disk” mode or, if it is not available, select the manufacturer (if unknown, then “Standard modem types”) and Model → Next → having selected the appropriate model, click Next → (select required port) Next.
One of the most important parameters that needs to be set is the type of dialing, which should be pulse, since in our country no other type is used. To install it, in the Properties window: Modems: General, click “Communication settings”, where select pulse dialing.
To check, whether the installation was completed correctly, use the mode: Start → Settings → Control Panel → System (2) → Devices, where there is a list of devices. If there is a plus sign next to the name “Modem”, then you need to click on this icon to expand the list of modems. Then you should make sure that there are no question marks or exclamation marks near the installed device.
Modem parameters can be look And change via: Start →Settings →Control Panel →Modems →Properties →General, where you change the port, speaker volume, and indicate the maximum speed. In this case, the maximum speed is meant between the modem and the computer, and not between modems. Usually the maximum speed is set, and in case of poor communication it is reduced.
Other questions
In general, communication channels are divided into:
Analogue (for example, telephone), through which information is transmitted in the form of a continuous signal;
Digital, transmission of digital (discrete or pulse) signals
or
Simplex,
Half duplex,
Duplex
or
Switched networks created for the duration of information transfer are then disconnected;
Non-switched (dedicated), dedicated for a long time
or
Low-speed (telegraph) with a speed of 50-200 bytes/sec.;
Medium-speed (telephone) with a speed of 300-56,000 bytes/sec.;
High-speed, over 56,000 bps.
To transmit data at high speed, twisted pair wire (twisted together), coaxial cable (as in a television antenna), fiber optic (made of glass fibers) and radio channel (via radio waves) are used.
Radio waves can be ultra-long (3-30 kHz), long (30-300 kHz), medium (300-3000 kHz), short (3-30 MHz), ultra-short (30 MHz-3 GHz), submillimeter (300-6000 GHz).
When transmitting data, several types of modulation are used: frequency (V21), phase (V22), amplitude and quadrature amplitude modulation, in which the phase and amplitude change, more noise-resistant than the previous ones, therefore it is used in the V22.bis standard and higher.
The protocol also contains the ability to split messages into blocks, restore communication, correct errors, 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 act as an intermediary between the computer and the network for data transfer. The network card has its own processor and memory. The main characteristics of a network card are the bus to which it is connected, memory size, card capacity (8, 16, 32 bits), types of connectors for thin and thick cables. Network cards require setting an interrupt line (often 3 or 5), a DMA channel, and a memory address (C800).
Network cable can be of several types:
twisted pair. Consists of several copper conductors twisted together in one cable, which can be unshielded (UTP) or shielded (STR).
Coaxial cable consists of a central and shielding wires, between which there is insulation. There are two varieties of this cable: thin (0.2 inches thick) and thick (0.4 inches thick).
Fiber optic cable consists of two wires consisting of light fibers. It has a large capacity, but is very expensive, so it is rarely used.
When using a cable, pay attention to the characteristic impedance, often 50 ohms. When laying, you need to have cables of the same brand, preferably from the same manufacturer. After laying a thin cable, connectors are installed, for example, Russian-made (CP50) or crimp BNC connectors. A plug is installed at the ends and one of them must be grounded.
Thick cables are laid through transceivers, using one transceiver per computer, and the ends of the cables leading to the computer must have 15-pin DIX connectors (or AUI). At the end of the cables are installed: N-terminators, one of which is grounded. To increase the length of the local network (for a thin cable it cannot be more than 185 meters), repeaters are used.
A twisted pair cable is used together with a hub or hub, from which a cable no more than 100 meters long is laid to each computer. At the ends is an RJ-45 connector, which looks similar to a telephone connector, but has 8 pins (rather than 4). Hubs can have a different number of ports, for example, 8, 12, 16, corresponding to the maximum number of connected computers.
When the modem is operating as fax, he works according to his own standards. When sending faxes at 14.4 Kbps, the standard is V.17 (14,400), V27 ter (4,800), V29 (9,600) and T.30 for the protocol itself. When transmitting an image of a sheet, the following resolution modes for fax transmission can be used: Standard – 100x200 dpi; high-quality (Fine) – 200x200 dpi; high quality (Superhigh) – 400x200dpi; photo mode (Photo) transmits 64 shades of gray.
A modern modem supports most standards, at least those that operate at less than the maximum speed of the modem.
In addition to ordinary modems, there may be very specific modems, for example, cable modems, when the signal is transmitted through tv cable. In this case, the cable is connected to a special socket, which has a connector for the TV and for the serial channel of the computer. Working over cable networks allows you to transfer data at high speed. However, over time, as the number of users increases, the throughput per user may become low. And now, while there are few users, they give a small number of users great advantages of working on the Internet.
Can be used satellite devices, in which users send a message to the provider via phone about which pages he wants to receive, and receives them via satellite.
Nowadays, more and more information is used to transmit mobile connection. In this case, the modem is connected to the mobile phone via a special cable.
In our country, the most widespread data transmission is 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 how busy the line is, one interval or another may be used. But this method of mobile communication is intended primarily for the transmission of voice messages, which take priority over digital data. Ultimately, the data transfer speed does not exceed 9.6 Kbps.
Other standard GPRS(General Packet Radio Service) allows you to increase this speed to 50 Kbit/s, and theoretically can reach 100 Kbit/s. Unlike GSM, here, to send information, it is possible to use other time intervals in the frame, up to all eight, and this circumstance increases the speed of data sending. In addition, this mobile communication option reduces user costs, since the volume of transmitted information is paid for, unlike GSM.
GPRS devices are divided into three classes according to their capabilities:
Class A. Such devices are capable of simultaneously transmitting both types of information – voice and digital – in each unit of time.
Class B. These models allow you to work alternately with either digital data or voice.
Class C. Only digital data is sent here.
So, modems and modulation-demodulation...
The term "modem" is short for the well-known computer term modulator-demodulator. A modem is a device that converts digital data coming from a computer into analog signals that can be sent over a telephone line. This whole thing is called modulation. The analog signals are then converted back into digital data. This thing is called demodulation.
The scheme is very simple. The modem receives digital information in the form of zeros and ones from the computer's central processor. The modem analyzes this information and converts it into analog signals, which are transmitted through the telephone line. Another modem receives these signals, converts them back into digital data, and sends this data back to the remote computer's central processing unit.
Modulation type which allows you to select frequency or pulse modulation. Pulse modulation is used throughout Russia.
Analog and digital signals
Telephone communication is carried out through so-called analog (sound) signals. An analog signal identifies information that is transmitted continuously, while a digital signal identifies only that data that is defined at a specific stage of transmission. The advantage of analog information over digital information is the ability to fully represent a continuous flow of information.
On the other hand, digital data is less affected by various types of noise and grinding noises. In computers, data is stored in individual bits, the essence of which is 1 (start) or O (end).
If we represent this whole thing graphically, then analog signals are sine waves, while digital signals are represented as square waves. For example, sound is an analog signal because sound is always changing. Thus, in the process of sending information over the telephone line, the modem receives digital data from the computer and converts it into an analog signal. A second modem at the other end of the line converts these analog signals into raw digital data.
Interfaces
You can use a modem in your computer using one of two interfaces. They are:
MNP-5 Serial interface RS-232.
MNP-5 Four-pin RJ-11 telephone cable.
For example, an external modem is connected to a computer using an RS-232 cable, and to a telephone line using an RJ11 cable.
Data compression
In the process of data transmission, a speed greater than 600 bits per second (bps or bits per second) is required. This is due to the fact that modems must collect bits of information and transmit them further through a more complex analog signal (a very sophisticated circuit). The process of such transmission itself allows for the transmission of many bits of data at the same time. It is clear that computers are more sensitive to transmitted information and therefore perceive it much faster than a modem. This circumstance generates additional modem time, corresponding to those data bits that need to be grouped somehow and certain compression algorithms applied to them. This is how two so-called compression protocols emerged:
MNP-5 (transmission protocol with a compression ratio of 2:1).
V.42bis (transmission protocol with a compression ratio of 4:1).
The MNP-5 protocol is usually used when transferring certain already compressed files, while the V.42bis protocol is applied even to uncompressed files, since it can speed up the transfer of just such data.
It must be said that when transferring files, if the V.42bis protocol is not available at all, then it is best to disable the MNP-5 protocol.
Error Correction
Error correction is a method by which modems test the transmitted information to determine whether it contains any damage that occurred during transmission. The modem breaks this information into small packets called frames. The sending modem attaches a so-called checksum to each of these frames. The receiving modem checks whether the checksum matches the information sent. If not, then the frame is sent again.
Frame is one of the key terms for data transmission. A frame is a basic block of data with a header, information and data attached to this header that complete the frame itself. The added information includes frame number, transmission block size data, synchronizing symbols, station address, error correction code, variable size data and so-called indicators Start of transmission (start bit)/End of transmission (stop bit). This means that a frame is a packet of information that is transmitted as one unit.
For example, in Windows 98 in the modem settings there is an option Stop bits which allows you to set the number of stop bits. Stop data bits are one of the varieties of the so-called boundary service bits. The table bit determines the end of the cycle during asynchronous transmission (the time interval between transmitted characters varies) of data in a short-term cycle.
MNP2-4 and V.42 protocols
Although error correction can slow down data transmission on noisy lines, this method provides reliable communication. The MNP2-4 and V.42 protocols are error correction protocols. These protocols determine how modems verify data.
Like data compression protocols, error correction protocols must be supported by both the sending and receiving modems.
Flow Control
During transmission, one modem can send data much faster than another modem can receive the data. The so-called flow control method allows you to inform the receiving modem that the modem will stop receiving data at some point in time. Flow control can be implemented both in software (XON/XOFF - Start signal/Stop signal) and in hardware (RTS/CTS) levels. Flow control at the software level is carried out through the transfer of a specific sign. After the signal is received, another character is transmitted.
For example, in Windows 98 in the modem settings there is an option Data bits which allows you to set the information data bits used by the system for the selected serial port. The standard computer character set consists of 256 elements (8 bits). Therefore, the default option is 8. If your modem does not support pseudographics (works only with 128 characters), please indicate this by selecting option 7.
In Windows 98, in the modem settings, there is also an option Use flow control
which allows you to determine how to implement data exchange. Here you can correct possible errors that occur when transferring data from the computer to the modem. Default setting XON/XOFF means that the data flow is controlled by software using standard ASCII control characters, which send the command to the modem pause/resume transfer.
Software flow control is only possible if a serial cable is used. Since flow control at the software level regulates the transmission process by sending certain characters, a failure or even termination of the communication session may occur. This is explained by the fact that this or that noise in the line can generate a completely similar signal.
For example, with software flow control, binary files cannot be transferred because such files may contain control characters.
Through hardware flow control, RTS/CTS transfers information much faster and more securely than through software flow control.
FIFO buffer and UART universal asynchronous interface chips
The FIFO buffer is somewhat similar to a transshipment base: while data arrives at the modem, part of it is sent to the buffer capacity, which gives some gain when switching from one task to another.
For example, the Windows 98 operating system only supports the 16550 series Universal Asynchronous Receiver Transmitter (UART) chips and allows you to manage the FIFO buffer itself. Using a checkbox Use FIFO buffers requres 16550 compatible UART (Use FIFO buffers) you can lock (prevent the system from accumulating data into the buffer capacity) or unlock (allow the system to accumulate data into the buffer capacity) the FIFO buffer. Pressing the button Advanced, you turn to dialogue Advanced Connection Settings whose options allow you to configure the connection of your modem.
S-registers
S-registers are located somewhere inside the modem itself. It is in these very registers that settings are stored that in one way or another can affect the behavior of the modem. There are a lot of registers in the modem, but only the first 12 of them are considered standard registers. S-registers are set in such a way that they send a command to the modem ATSN=xx, where N corresponds to the number of the register being set, and xx defines the register itself. For example, through the SO register you can set the number of rings to answer.
Interrupts IRQ
Peripheral devices communicate with the computer processor through so-called IRQ interrupts. Interrupts are signals that force the processor to suspend a particular operation and transfer its execution to the so-called interrupt handler. When the CPU receives an interrupt, it simply suspends the process and delegates the interrupted task to an intermediary program called Interrupt Handler. This whole thing works regardless of whether an error was detected in the operation of a particular process or not.
Information communication port or simply COM port
The serial port is very easy to find out. You can do this by simply looking at the connector. The COM port uses a 25-pin connector with two rows of pins, one of which is longer than the others. At the same time, almost all serial cables have 25-pin connectors on both sides (in other cases, a special adapter is required).
A COM port (serial port) is a port through which computers communicate with devices such as a modem and mouse. Standard personal computers have four serial ports.
COM 1 and COM 2 ports are usually used by a computer as external ports. By default, all four serial ports have two IRQs:
COM 1 is bound to IRQ 4 (3F8-3FF).
COM 2 is tied to IRQ 3 (2F8-2FF).
COM 3 is tied to IRQ 4 (3E8-3FF).
COM 4 is tied to IRQ 3 (2E8-2EF).
This is where conflicts can arise, since external ports of other I/O devices 1/0 or controllers can use the same IRQs.
Therefore, having assigned a COM port or IRQ to the modem, you must check other devices to see if they have
the same serial ports and interrupts.
It must be said that devices connected to the telephone line in parallel to the modem (especially Caller ID) can very significantly degrade* the quality of operation of your modem. Therefore, it is recommended to connect phones through the dedicated socket in the modem. Only in this case will he disconnect them from the line during operation.
Your modem's flash memory
Flash memory is a read-only memory or PROM (read-only reprogrammable memory) that can be erased and reprogrammed.
All modems whose names contain the line “V. Everything” are subject to reprogramming. In addition, "Courier V.34 dual standart" modems are subject to software upgrade if the line Options the response to the ATI7 command contains the V.FC protocol. If the modem does not have this protocol, then the upgrade to "Courier V. Everything" is carried out by replacing the daughter board.
There are two modifications of Courier V. Everything modems - with the so-called supervisor frequency of 20.16 MHz and 25 MHz. Each of them has its own firmware versions, and they are not interchangeable, i.e. The firmware from the 20.16 MHz model will not work for the 25 MHz model, and vice versa.
Field programmable NVRAM
All modem settings come down to correctly setting the values of the NVRAM registers. NVRAM is a user-programmable memory that retains data when the power is turned off. NVRAM is used in modems to store the default configuration that is loaded into RAM when turned on. NVRAM programming is done in any terminal program using AT commands. A complete list of commands can be obtained from the documentation for the modem, or obtained in a terminal program using commands AT$ AT&$ ATS$ AT%$. Write factory settings with hardware data control to NVRAM - AT&F1 command, then make adjustments to the modem settings in conjunction with a specific telephone line and write them to NVRAM by command AT&W. Further initialization of the modem must be done using the command ATZ.4.
Application software for data transfer
Data transfer programs allow you to connect to other computers, BBS, Internet, Intranet and other information services. You may have a very extensive range of such programs at your disposal. For example, in Windows 98 you have at your disposal a very good terminal client, Hyper Terminal.
If you have problems establishing communication with other modems
First you need to assess the nature of the communication line. To do this, after a successful session, before re-initializing the modem, enter the commands ATI6- communication diagnostics, ATI11- connection statistics, ATY16- amplitude-frequency characteristic. The received data must be written to a file. After analyzing the received data, it is necessary to make changes to the current configuration and then write them to NVRAM using the command AT&W5.
Russian telephone lines and imported modems
The choice of modems today is quite large, and the difference in their cost is quite significant. Transmission speeds of more than 28,800 bps are usually unattainable on Russian telephone lines. Above 16,900 bps can only be obtained if the Internet service provider has lines on the PBX to which your phone is connected. In other cases, working on the Internet is too tedious, because at a typical (and not always achievable) speed of 9,600 bps, it becomes a complete wait. Therefore, for stable data transmission in the event of interference in the telephone line, you need a high-quality modem that costs at least $400.
Which modem is better - internal or external?
The internal modem is installed in a free expansion slot on the computer's motherboard and connected to the built-in power supply, while the external modem is a stand-alone device connected to the computer via a standard serial port.
Each of the designs has its own advantages and disadvantages. The internal modem occupies a system bus slot (and, as a rule, there are not enough of them), it is difficult to monitor its operation due to the lack of indicators, and besides, the described models are fundamentally not suitable for notebook-type portable computers that have a narrow-profile case and, in most cases, are not with expansion connectors. At the same time, the internal modem is several tens of dollars cheaper than external analogues, does not take up space on the table and does not create a tangle of wires. Using an external modem means that the computer to which it is connected has the most modern serial port control chips (UART). UART chips appeared in the first PCs, since even then it became clear that data exchange via a serial port was too slow and complex an operation and it was better to entrust it to a special controller. Since then, several UART models have been released. Computers such as IBM PC and XT, as well as those fully compatible with them, used the 8250 chip; in AT it was replaced by UART 16450. Until recently, most computers based on i386 and i486 processors were equipped with a 16550 controller, which included internal hardware buffers of the " queue", and today the UART 16550A is becoming the standard - a chip similar to the previous one, but with the defects eliminated. The lack of buffers in all chips except the last one causes data transfer through the serial port at speeds above 9600 bps to become unstable (using MS Windows reduces this threshold to 2400 bps).
If you need to connect a high-speed external modem to a computer that uses an older UART chip, you must either change the multicard or add a special expansion card (which will take up one bus slot and deprive the external modem of a critical advantage). Internal modems do not have this problem - they do not use a COM port (more precisely, they contain one). Now internal modems have another advantage, also related to speed. According to the V.42bis specification, data can be compressed by approximately four times during transmission, therefore a modem operating at 28800 bps must receive data from or send data to the computer at a speed of 115600 bps, which is the limit for serial PC port. However, 28,800 bps is not the limit for a telephone line, where the maximum lies somewhere in the region of 35,000 bps, and on digital lines (ISDN) the throughput exceeds 60,000 bps. Consequently, in this situation, the serial port will become a bottleneck of the entire system, and the potential capabilities of the external modem will not be realized. Modem manufacturers are currently developing models that can connect to a faster parallel port, but it is obvious that devices sold now will not be able to accommodate this.
At the same time, many modems can be upgraded to operate at high speeds, even being able to operate on ISDN. But everything depends on the restrictive barrier on the computer side, which for the internal modem is significantly higher than 4 MB/s (ISA bus bandwidth). By the way, all ISDN modems are internal. True, all this will happen tomorrow (or maybe the day after tomorrow), but today we can say one thing: choose a device of the type that you like - there are no functional differences between internal modems and their external analogues.
Which modem to choose and how to choose it
The modem cannot be unique. Your modem must be understood by other modems. This means that the modem must support the maximum number of standards, that is, error correction, data exchange methods and data compression. The most common standard is V.32bis for modems with an exchange rate of 14000 bps. For modems with a speed of 28800 bps, the standardized protocol is V.34.
In addition, it must be emphasized that modems with a data exchange rate of 16800, 19200, 21600 or 33600 are not standard.
No error correction should be done in software. Everything must be built into the modem by its manufacturer.
About the outside and the inside. An external modem is connected to your serial port via a special cord. Such a modem, as a rule, has a volume control, information indicators, a power supply and other, sometimes useful accessories. If you are a professional, then you should not care which modem you choose - internal or external. Usually, a good internal modem, through special software, does a good job of emulating all the clarity of an external modem.
Do not buy purely imported modems. These pieces of iron do not get along on our ancient lines. Buy only certified modems, that is, hardware specially tailored for our dirty telephone exchanges.
In Russia, such a choice is very small. This market is dominated by two companies: ZyXEL from sunny Taiwan and U.S. Robotics from the USA. Modems from the latter company are chosen by professionals (Courier), while the former is chosen by everyone else, that is, all those users who choose the so-called ultra-reliable ZyCell protocol.
So, choose Courier. And, believe me, this is not advertising.
