Basic communications and devices for. Communication network equipment

Communication, communication, radio electronics and digital devices

Data exchange is required for various purposes: transferring files, sharing peripheral devices such as printers, accessing a variety of Internet information services and private networks, receiving and transmitting fax messages, sending messages to pagers and mobile phones, establishing voice communications, IP telephony, video communications, and even playing games together over the network. COM port A serial interface for transmitting data in one direction uses one signal line along which information bits are transmitted one after another...

Lecture 13. Communication devices

Questions:

Wired communication interfaces.

Modems.

Literature: 1. Hooke. M. Hardware IBM PC. Peter, 2005, p. 6 08-660.

PC communication devices are designed to organize data exchange between computers, a computer and a remote input/output device, as well as to connect a computer to a local or global network. Data exchange is required for various purposes: transferring files, sharing peripheral devices (for example, printers), accessing a variety of information services on the Internet and private networks, receiving and transmitting fax messages, sending messages to pagers and mobile phones, establishing voice communications ( IP -telephony), video calling and even joint games over the network. Modern technologies used for these purposes are focused specifically on communications: COM port, wireless interfaces, modems, local network adapters. Communication between computers, although with a number of restrictions, can be established by other means: through LPT -ports, serial buses FireWire and USB.

1. Wired communication interfaces.

1.1. COM port

A serial interface for transmitting data in one direction uses one signal line, along which information bits are transmitted one after another sequentially. English interface and port names Serial Interface and Serial Port. Serial transmission reduces the number of signal lines and improves communication over long distances.

Since the first models, in PC there is a serial interface - COM port (Communications Port communication port). This port provides asynchronous standard exchange RS-232C. Synchronous exchange in PC support only special adapters, for example SDLC or V .35. COM ports are implemented on universal asynchronous transceiver chips(UART), family compatible 18250/16450/16550. They occupy 8 adjacent 8-bit registers in the I/O space and can be arranged according to standardbase addresses:

3F8h (COM1), 2F8h (COM2), 3E8h (COM3), 2E8h (COM4).

Ports can generatehardware interrupts IRQ 4 (usually used for COM1 and COM3) and IRQ 3 (for COM2 and COM4). On the external side, the ports have serial data lines for transmit and receive, as well as a set of control and status signals that comply with the standard RS-232C . COM ports have external male connectors DB 25 P or DB 9 P, displayed on the back panel of the computer. A characteristic feature of the interface is the use of non-TTL signals - all external signals of the port are bipolar. There is no galvanic isolation; the circuit ground of the connected device is connected to the circuit ground of the computer. The transmission speed can reach 115.2 Kbps.

The name of the port indicates its main purpose - connecting communications equipment (for example, a modem) to communicate with other computers, networks and peripheral devices. Peripheral devices with a serial interface can also be directly connected to the port: printers, plotters, terminals, etc. The COM port is widely used to connect a mouse, as well as organize direct communication between two computers. Electronic keys are also connected to the COM port.

Currentlydevices that traditionally use a COM port are recommended to be switched to serial buses USB and Fire Wire.

1.2. RS-232C interface

RS-232C protocol

Standard RS -232 C describes single-ended transmitters and receivers signal is transmitted relative to common circuit ground (balanced differential signals are used in other interfaces e.g. RS-422). Interface does not provide galvanic isolation devices. Logical unitcorresponds to the voltage on receiver input in the range -12...-3 V.Logical zerocorresponds to the range +3...+12 V. Range -3...+3 V dead zone, which determines the hysteresis of the receiver: the line state will be considered changed only after crossing the threshold. The signal levels at the transmitter outputs must be in the ranges -12...-5 V and +5...+12 V to represent one and zero, respectively.

Logical 0

Logical 1 0 U input

12V +3V +12V

Figure 13.1. Voltage levels and logic signal

The interface assumes the presenceprotective groundingfor the devices being connected, if they are both AC powered and have surge protectors.

Connecting and disconnecting interface cablesself-powered devices must be madewhen the power is off.Otherwise, the difference in uneven potentials of devices at the time of switching may be applied to the output or input (which is more dangerous) interface circuits and damage the microcircuits.

In table 13.1 shows the purpose of the contacts of the COM port connectors (and any other data transmission equipment). For modems, the names of the circuits and contacts are the same, but the roles of the signals (input-output) are reversed.

Table 13.1. Connectors and interface signals RS-232C

1 Ribbon cable for 8-bit multicards.

2 Ribbon cable for 16-bit multicards and ports on motherboards.

3 Ribbon cable option for ports on motherboards.

4 Wide ribbon cable to 25-pin connector.

Subset of signals RS-232C, related to asynchronous mode, consider from the point of view of the COM port PC . For convenience, we will use the name mnemonic adopted in the descriptions of COM ports and most devices (it differs from the faceless designations RS-232 and V .24). The purpose of the interface signals is given in table. 10.2.

Table 13.2. Interface signal assignment RS-232C

The normal sequence of control signals for the case of connecting a modem to a COM port is shown in Fig. 13.1. Let us recall that a positive level corresponds to the logical state “off”, and a negative level corresponds to the logical state “on”.

Fig. 13.1. Sequence of interface control signals RS-232C

1. Setting the signal DTR the computer indicates a desire to use a modem.
2. Setting the signal DSR The modem signals that it is ready to establish a connection.
3. RTS signal the computer requests permission to transmit and declares its readiness to receive data from the modem.
4. CTS signal The modem notifies that it is ready to receive data from the computer and transmit it to the line.
5. Removing the CTS signal the modem signals that further reception is impossible (for example, the buffer is full) the computer must pause data transmission.
6. Signal restoration CTS The modem allows the computer to continue transmission (there is space in the buffer).
7. Removing the RTS signal can mean either the computer's buffer is full (the modem must pause data transmission to the computer) or there is no data to transfer to the modem. Typically, in this case, the modem stops sending data to the computer.
8. The modem confirms that the signal has been removed RTS signal reset CTS.
9. The computer resets the signal RTS to resume transmission.
10. The modem confirms its readiness for these actions.
11. The computer indicates the exchange is complete.
12. The modem responds with confirmation.
13. The computer picks up the signal DTR , which is usually a signal to disconnect (“hang up”).
14. Modem reset signal DSR reports that the connection is broken.

start bit, stop bit, guaranteeing a pause between sendings (Fig. 13.2). The start bit of the next byte is sent at any moment after the stop bit, that is, pauses of arbitrary duration are possible between transmissions. The start bit, which always has a strictly defined value (logical 0), provides a simple mechanism for synchronizing the receiver with a signal from the transmitter. It is assumed that the receiver and transmitter operate at the same baud rate.

In asynchronous transmission, each byte is preceded by start-bit, signaling to the receiver the start of a transmission, followed by data bits and possibly a parity bit (parity). Completes the parcel stop bit, guaranteeing a pause between sendings (Fig. 13.2). The start bit of the next byte is sent at any moment after the stop bit, that is, pauses of arbitrary duration are possible between transmissions.

Rice. 13.2. Asynchronous Transfer Format

The start bit, which always has a strictly defined value (logical 0), provides a simple mechanism for synchronizing the receiver with a signal from the transmitter. It is assumed that the receiver and transmitter operate at the same speed

The asynchronous sending format allows you to identify possibletransmission errors:false start bit, lost stop bit, parity error. Format control allows you to detect a line break: in this case, a logical zero is accepted, which is first interpreted as a start bit and zero data bits, then the stop bit control is triggered.

For the asynchronous mode, the following series is adopted:standard baud rates:50, 75, 110, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bps. Sometimes instead of the unit of measurement “bit/s” they use “baud” ( baud ), but when considering binary transmitted signals this is incorrect.

It is customary to measure in baud the frequency of changes in the line state signal, and with a non-binary coding method in a communication channel, bit rates (bit/s) and signal changes (baud) can differ several times.

Number of data bits may be 5, 6, 7 or 8 (5- and 6-bit formats are not very common). Quantity stop bit can be 1, 1.5 or 2 (“one and a half bits” means only the duration of the stop interval).

Asynchronous mode isbyte-oriented(character-oriented) minimum transferred unit of information byte (character). In contrast, synchronous mode (not supported by COM ports) is bit-oriented: the frame sent over it can have an arbitrary number of bits.

1.3. Application of Som-ports

The COM port is widely used for connecting various peripheral and communication devices, communication with technological equipment, control and monitoring objects, programmers, in-circuit emulators and other devices via the RS-232C protocol.

A COM port can also function as a bidirectional interface that has

3 software controlled output lines and

4 program-readable input lines with bipolar signals.

Their use is determined by the developer. There is, for example, a circuit of a one-bit pulse-width converter that allows you to record an audio signal to a PC disk using the input line of the COM port. Playing this recording through a regular PC speaker allows speech to be transmitted.

Direct connection of devices

A). Mouse manipulator.

COM ports are most often used to connect manipulators (mouse, trackball). In this case, the port is used in serial input mode. A serial mouse Serial Mouse can be connected to any working port. To match the port and mouse connectors, you can use the DB-9S-DB-25P or DB-25S-DB-9P adapter. An interrupt is required for the mouse, for the C0M1 port IRQ4, for COM2 IRQ3. The fact that the C0M1 port requires the IRQ4 interrupt to operate the mouse is a feature of its driver, but the very fact of the limitation is important for the user. Each event - moving a mouse or pressing and releasing a button - is encoded by a binary message via the RS-232C interface. Asynchronous transmission is used; Bipolar power supply is provided from the interface control lines.

b). External modem.

To connect external modems, a complete (9-wire) ADF-AKD cable is required, the diagram of which is shown in Fig. 13.3. The same cable is used to match connectors (according to the number of contacts); It is possible to use adapters 9-25 intended for mice. Communication software usually requires interrupts to operate, but there is freedom in choosing the port number (address) and interrupt line. If operation is expected at speeds of 9600 bps and higher, then the COM port must be implemented on a UART 16550A or compatible chip. The ability to work using FIFO buffers and exchange via DMA channels depends on the communication software.

V). Computer communication.

To connect two computers located a short distance from each other, they also use a direct connection of their COM ports with a null modem cable (Fig. 13.4). MS-DOS programs such as Norton Commander and Interlnk allow you to exchange files at speeds of up to 115.2 Kbps without the use of hardware interrupts.

Rice. 13.3. Modem connection cables

Fig. 13.4 Null modem cable a) minimal, b) full.

Serial Interface Conversions

At the physical level, the serial interface has various implementations that differ in the way they transmit electrical signals. There are a number of international standards related RS-232C . In Fig. 13.5 shows the connection diagrams for their receivers and transmitters, and also shows the restrictions on the line length (L) and the maximum data transfer rate (V). Unbalanced lines of the RS-232C and RS-423A interfaces have the lowest immunity to common mode interference. The best parameters are provided by the point-to-point interface RS-422A and its trunk (bus) analogue RS-485 , operating on symmetrical communication lines. They use differential signals to transmit each signal, with a separate (twisted) pair of wires for each signal chain. Since these interfaces are logically related, it is possible to use simple signal converters that provide the transition from one interface to another.

Rice. 13.5. Serial Interface Standards

In the above standards, the signal is represented by a potential. There are serial interfaces where the current flowing through the common transmitter-receiver circuit “current loop” and MIDI is informative

"Current loop" is a common serial interface option. In it, the electrical signal is not the voltage level relative to the common wire, but the current in the two-wire line connecting the receiver and transmitter. A logical one (the “on” state) corresponds to a current of 20 mA, and a logical zero to the absence of current. This representation of signals for the above-described asynchronous sending format allows the detection of a line break - the receiver will notice the absence of a stop bit (the line break acts as a permanent logic zero).

A current loop usually involves galvanic isolation of the receiver input circuits from the device circuitry. In this case, the source of current in the loop is the transmitter (this option is called an active transmitter). It is also possible to supply power from the receiver (active receiver), while the output switch of the transmitter can also be galvanically isolated from the rest of the transmitter circuit. There are simplified options without galvanic isolation, but this is already a degenerate case of the interface.

A galvanically isolated current loop allows signals to be transmitted over distances of up to several kilometers. The distance is determined by the resistance of the pair of wires and the level of interference. Since the interface requires a pair of wires for each signal, it is common to use only two interface signals. In the case of bidirectional exchange, only transmitted and received data signals are used, and flow control is implemented by software. If bidirectional communication is not required, one data line is occupied, and for flow control the return line is used for the CTS signal (hardware protocol) or the counter data line (software protocol). With proper software, a single current loop can provide bidirectional half-duplex communication between two devices. In this case, each receiver “hears” both the signals of the transmitter on the opposite side of the channel and the signals of its own transmitter. They are regarded by communication packets simply as an echo signal. Therefore, for error-free reception, the transmitters must operate alternately.

2. Modems

To transmit data over long distances (within the whole world), public telephone networks (PSTN) have long been used. However, conventional analogue telephone networks are not suitable for direct transmission of digital data; modems are required on both sides of the subscriber.

A modem (modulator-demodulator) is used to transmit information over long distances that are inaccessible to local networks, using dedicated and dial-up telephone lines. The modulator converts binary information coming from the computer into analog signals with frequency and (or) phase modulation, the spectrum of which corresponds to the bandwidth of ordinary voice telephone lines. The demodulator extracts the encoded binary information from this signal and transmits it to the receiving computer.

Fax modem (fax-modem) allows you to send and receive fax images, compatible with conventional fax machines. Faxing also involves the transmission of digital data, although the “digit” is not visible to end users: the fax machine scans the image, digitizes it (1 bit per dot), compresses the data and transmits it via a modem to the telephone line. On the receiving side, inverse transformations are performed. A fax modem works similarly, but instead of scanning, its software support accepts graphic or text data from other programs. Received faxes are formatted as files in graphic formats that are available to applications for further processing or printing.

Modern modems have a number of additional features that expand their scope of application. A voice modem is capable of converting an audio signal into digital form, in which it is transmitted over a communication line. On the receiving side, inverse transformations are performed. The audio signal is compressed, for example, using the ADPCM (Adaptive Differential Pulse Code Modulation) method.

Modems can operate in simplex, full-duplex or half-duplex mode during a communication session. To increase the effective speed, various information compression methods are used, implemented both by the modems themselves and by communication software.

2.1. Modem designs

ITL DS SOS SP

USG K

KU

USPC

Rice. 10.7 Modem block diagram

In Fig. 10.7 shows a typical block diagram of an external modem, which includes:

ITL telephone line interface;

DS Differential system for separating input and output signals, transition from 2-wire to 4-wire line;

SOS signal processing system with DAC and ADC.

SP signal processor for signal encoding;

K SP control controller, providing: error correction, information compression, memory work;

USP device for interfacing with a loudspeaker;

KU control keys;

USPK device for connecting to a personal computer.

1 . Interface with telephone line - ITL

(Direct Access Arrangement - DAA)

In the GOSTs of the ex-USSR, “Joint 1 PM” is regulated. In the USA, modems are tested for compliance with FCC Part 65, Part 15, in the UK the corresponding standard is BS6305... Telephone companies around the world strictly regulate the requirements for equipment connected to channels.

Providing physical connection, protection against overvoltage and radio interference, dialing and recording telephone calls, galvanic isolation and impedance matching - this is not a complete list of functions supported by the DAA circuit. The listed functions are provided as follows.

1) RJ11 connectors provide a physical connection to a dial-up telephone line and telephone set. In cheap products, the telephone is connected parallel to the modem input; in high-quality products, telephone/modem switching is supported, implemented using a relay.

It is good practice to implement multi-line telephone system modes (Key Telephone System) - RJ12, RJ13 and support for operation on four-wire leased lines RJ45, JM8. Table 1 shows the pin assignments of these connectors.

Table 1

2) The input lines are protected from overvoltage by a varistor, which sharply reduces its resistance at a voltage of 400...500 V. The second cascade of high-speed protection is installed in the secondary winding of the transformer and is implemented on back-to-back zener diodes.

3) Protection of the line from radio interference emitted by the modem is carried out using conventional LC filters (1000 pF plus three turns on ferrite).

4) For dial-up lines, the functions of pulse dialing, “hang up” (DC current less than 0.5 mA) and “line hold” (DC current more than 8 mA) are supported.

The most universal implementation is in which the dialing is performed by a relay, and direct current flows through a transformer.

New designs often use an Electronic Holding Coll Circuit (EHCC). It has a low DC impedance, sufficient to carry the line, but maintains a high impedance for the AC wanted signal. In this case, the number is dialed by a relay or the EHCC unit itself with optocoupler control isolation.

The EHCC scheme has limited use on some types of telephone exchanges (for example, Kvant).

5) The most conservative unit for recording telephone calls. It hasn't changed much over the past ten years. A high-voltage capacitance, a resistor, a zener diode, an optocoupler isolation LED (with a little play on values ​​and types) - that’s probably all.

6) An important requirement for the interface with the line is to ensure symmetry of the input and its galvanic isolation. Transformers are used for this. Optocoupler junctions today are interesting, rather, as exotic.

The transformers themselves, continuously improving, have undergone two waves of fashion. At first, conventional ones were used - with capacitive decoupling for alternating current. Then models were developed that did not degrade the parameters at significant constant bias currents. When moving to high speeds everything returned to the beginning...

7) Impedance matching. The input and output resistance of the modem to alternating current (300...3400 Hz) should be 600 Ohm + -15%.

A high-quality transformer and an accurate load resistor are the key to this. To reduce the dependence of impedance on frequency, additional capacitance is installed parallel to the secondary winding of the transformer.

2. Differential system (HYBRID) - DS

The purpose of the differential system is to transition from a two-wire line to a four-wire analog end of the modem. The node compensates for the penetration of the output signal into the input signal (near echo), which increases the actual sensitivity.

There are several types of "passive" implementations:

• transformer, in which the secondary winding of the transformer has a midpoint connected through a ballast resistor to the ground;
• electronic, for circuits with unipolar and bipolar power supply; in this case, the output signal is subtracted from the input signal at the op-amp, and the frequency dependence is minimized by using a boost stage.

The sore point of these schemes is the dependence on the resistance of a particular telephone line. Several types of modems have hardware adjustment, but it is not possible to completely cope with the dependence of resistance on frequency in passive systems.

An active differential system is used in expensive models. The signal required for compensation is constantly calculated by the signal processor. Generated by an additional DAC and smoothed by a filter, it is subtracted from the input signal, providing high quality compensation.

SOS signal processing system.

Galvanically isolated from the outside world by a transformer and divided into input and output by a differential system, the signal reaches the “analog front”, where the struggle for millivolts and decibels unfolds.

The output signal is generated by a DAC. For medium transmission speeds it is usually 10-bit, and for high-speed modems it is 14...16-bit. Data sampling frequency from 7.2 to 9.6 kHz. The anti-aliasing filter is usually based on integrated switching capacitor technology. It provides over 32 dB attenuation at frequencies above 4.6 kHz.

The input signal is fed to a bandpass filter. For modems corresponding to V.22bis, this is 900...1500 Hz or 2100...2700 Hz. For high-speed, the band can reach 300...4000 Hz (V.34). The “ennobled” signal is amplified by a software-controlled AGC circuit and measured by an ADC. The sampling rate and bit depth of the ADC are approximately the same as the DAC.

4. Signal processor (Digital Signal Processor - DSP) JV

The times when “ones” and “zeroes” were separated from noise using hardware comparators are long over. The transmission speed and its quality are now determined by the computing resources involved in signal processing. Their average values ​​are given in Table 2.

DSP ROM is executed either using mask technology on the processor chip, or in the form of RAM chips into which the program is loaded from the controller ROM. Data RAM is implemented on the processor or combined with instruction RAM.

Table 2

5 . Controller (Modem controller - MC) - K

Support for an interface with a computer, DSP control, implementation of error correction and information compression protocols, user interface management and interaction with non-volatile memory - this is an incomplete list of controller functions.

The average values ​​of the required resources are given in Table 3.

Support for the "upgrade" ideology has led to the gradual concentration of storage of "firmware" DSP and controller in one chip with the possibility of replacing it.

Table 3

6. Computer interface device (Data Interface - DI) USPD

External modems interact with the computer via RS-232C / V.24 interface circuits. A complete set of circuits allows operation in both asynchronous and synchronous modes. Level converter chips 1488, 1489 provide interface bipolar logic with internal TTL levels.

Internal products can only operate in asynchronous mode, because they include an asynchronous COM port microcircuit - UART (16C450 or 16C550, which has a built-in receive buffer). There are implementations in which the port is emulated by the controller. A buffer and a decoder are enough to connect the UART to the general computer bus. Jumpers allow you to configure the COM port number (COM1...COM4) with a standard or extended interrupt number.

7. User Interfaces

1) Sound (SPEAKER) Interface device with loudspeaker - USG.

The speaker built into the modem sounds the processes occurring in the telephone channel. Good models use magnetoelectric speakers with a linear band playback, while cheaper ones use piezoelectric ones. For user convenience, the sound volume can be adjusted (volume node).

Most often, a sound node is built according to the following scheme:

• the signal is taken after the filter, but before the AGC;
• the volume is controlled by the controller using a 4052 voltage switch chip;
• the filter introduces frequency response prediction to linearize the characteristics of a specific type of speaker;
• the LM386 microcircuit, powered from +5 V, amplifies the signal;
• For four-wire devices, both the input and output signals are played simultaneously.

2) Indication panel (INDICATOR). Internal modems do not have display panels. In external ones, light emitting diodes (LEDs) are most often used. Relatively expensive devices use symbolic two-line liquid crystal displays (LCD). Using the control panel, you can display the modem status, characteristics of the physical line, and display a menu for programming modes. The use of standard (HD44780A00-compatible) indicators does not greatly increase the cost, but allows the manufacturer to significantly increase the price.

3) Control panel (CONTROL KEY).

In most modems, the panel is reduced to a set of jumpers and switches (SW), both inaccessible without disassembling the product, and having special “windows” and “lids” that provide “foolproofing”.

In products with LCD, the keypad (KEY) concentrates all functions for controlling operating modes.

8. Power (POWER)

Built-in modems are powered from the computer with voltages of +-5

In and only in some cases use +-12 V.

External mass-produced modems use external adapters that convert the primary supply voltage of 220 V into a secondary voltage of 9..12 V. The built-in stabilizer forms:

• main power +5 V; voltage suppression from +12 V on a linear stabilizer was usually used, now switching stabilizers are being introduced;
• 5 V for analog circuits;
• +-12 V for RS-232C interface.

Older designs used half-wave rectification circuits to produce positive and negative voltages. The new ones use full-wave, and the negative voltage is formed by separating capacitors.

9. Manufacturers

A review of modem architecture would not be complete without touching on the issue of their manufacturers. All companies can be divided into three groups.

1) Developers of the “modem heart” - a set of specialized LSIs (chip set).

For medium transfer speeds, relatively many companies entered the race for the prize (although not all received it): Intel, Rockwell, ATI, EXAR, Sierra Semiconductor, Silicon Sistems, Hayes, Sharp, Cermetek, Texas Instrument, etc.

For high-speed modems, the leaders became more clearly defined. This is the communications and telecommunications giant on the American continent AT&T and the “product of American conversion” Rockwell International. The presence of leaders does not in any way diminish the results achieved by other companies.

2) Manufacturers using universal processors and, as a result, developing their own signal processing algorithms: Motorola Codex, Telebit Corp., U.S. Robotics Inc., ZyXEL, etc. To implement compression and error correction protocols, they usually purchase a license from the R. Scott Association. All of these firms additionally support their own physical layer protocols.

Standing a little aside are the so-called soft modems, the software of which is downloaded from a computer - beautiful in concept, they have not yet become widespread.

3) Assemblers of modems based on chip set. The term “pickers” should not be understood in a disparaging tone. The quality of work is largely determined by how well the capabilities inherent in the chip set are supported, how “silently” the analog transmission path is implemented, and a thousand other reasons. Many companies make their own adjustments and implement additional functions in the software of basic chipsets.

Here are just a few large manufacturing companies : AMT International Industries Inc., Archtek America Corp., ATI Tecnologies, AT&T Paradyne, Boca Research Inc., Calpak Corp., Cardinal Technologies Inc., GVC Technologies Inc., Hayes Microcomputer Products Inc., Microcom Inc., MultiTech Systems, Practical Peripherals Inc., Racal-Datacom Inc., Zoom Telephonics Inc.

PAGE 11

Communication equipment is divided into passive and active equipment. Passive equipment includes a structured hierarchical cabling system. Active communications equipment includes many types of equipment, including:

Equipment for local computer networks;

Global computer network equipment – ​​internetworking equipment.

The main components of local networks are (Figure 7.6):

1. Network server, which plays an important role in LAN management. It must manage hard drives and support shared peripherals. Server performance is of great importance when managing large volumes of information and with a large number of workstations.

2. Workstations – autonomous computer systems connected to a network and called workstations, automated workstations and network stations. In a LAN, a personal computer is used as a workstation, which has its own processor with its own internal storage and I/O device.

3. Network amplifiers, or switches, as well as concentrators (hubs) are used to connect more workstations.

4. The modem can connect remote users.

5. Peripheral equipment (laser printers, plotters, fax devices, modems) connected to the file server (or other server device) can be used from any workstation.

6. Network adapters. The central processor is connected to peripheral equipment by a special device. To connect one PC to another, a pairing device is required, which is called a network adapter or network interface, module, card. It is inserted into an empty slot on the motherboard.

Figure 7.6 – LAN components

Internetworking equipment may include the following devices:

1. Repeater - a device for coordinating the physical parameters of parts (called segments) of the same type of network, increasing the length of the information transmission path. It provides restoration of intersegment packets, connection of different segments, amplification of a weakened signal to the required level and filtering it from noise and interference.

2. Switches (hubs) expand the topological, functional and speed capabilities of computer networks. A hub with a set of different types of ports allows you to combine network segments with different cable systems. You can connect either a separate network node or another hub or cable segment to the hub port.

3. Bridge is a means (hardware or software) of logical connection at the data link level of two networks. Within the permissible length, a network segment is built - a network segment. To connect network segments, bridges are used - devices that connect two networks that use the same data transfer methods. For a network of personal computers, a bridge is a separate computer with special software and additional equipment. A bridge can connect networks of different topologies, but running the same type of network operating systems.

4. A router connects networks with a common protocol more efficiently than a bridge. It allows, for example, large messages to be split into smaller pieces, thereby ensuring the interaction of local networks with different packet sizes. A router can forward packets to a specific address (bridges only filter out unnecessary packets), choose the best path for the packet to take, and much more. The more complex and larger the network, the greater the benefits of using routers.

5. A bridge router is a hybrid of a bridge and a router that first attempts routing where possible and then switches to bridge mode if it fails.

6. A gateway, unlike a bridge, is used in cases where the connected networks have different network protocols. A message from one network arriving at the gateway is converted into another message that meets the requirements of the next network. Thus, gateways do not simply connect networks, but allow them to operate as a single network. With the help of gateways, local networks are also connected to mainframes - universal, powerful computers.

In addition to communication equipment, any computer network requires software integrated into an operating system. The world-famous operating system for computer networks is NetWare from Novell. Operating system functions are provided to systems based on the Windows platform.

3. Local network communication devices

It was previously shown that working as part of a local network requires the correct and effective implementation of certain link layer procedures, ranging from frame formation taking into account data control algorithms and restoration of data distorted or lost during transmission (LLC sublayer) and ending with access control to the shared transmission medium data (MAC sublayer). All these link level tasks must be solved by each node of the local network, therefore all nodes necessarily contain communication devices (in computer networks - network adapters, in CAN networks - CAN controllers) that implement these link level procedures. Often, the tasks solved by these devices go beyond the data link layer and, for example, include quite complex procedures for monitoring and diagnosing the operation of network elements.

In local networks, even without division into logical segments, the use of additional communication devices may be required. For example, the transformation of a physical “star” topology into a logical “common bus” or “ring” topology is carried out by concentrators (hubs). A hub is the simplest communication device; its functions are to relay messages arriving at one of the input ports to other output ports. By performing such operations, the hub changes the logical topology of the network. Typically, hub ports are bidirectional (input/output) and there are several such ports. Naturally, the hub must operate strictly in accordance with MAC procedures. As a rule, hubs perform additional control functions and can disable ports with incorrectly functioning network nodes. The hub is not a full-fledged local network node, i.e. it does not have its own MAC address and can neither be the sender nor the recipient of messages. Thus, the hub combines individual physical network segments into a single shared data transmission medium in accordance with the network technology used.

To divide a single data transmission medium into logical segments in local networks, more complex communication devices are used: bridges and switches. Bridges and switches also implement only MAC procedures and, therefore, are devices for solving link layer problems. As a rule, they also do not have independent MAC addresses and cannot be either senders or recipients of messages. Unlike hubs, bridges and switches provide selective relaying of incoming messages, dividing the local network into relatively independent logical segments. If the message recipient is in the same logical segment as the sender, the incoming message is not forwarded to other output ports. If an incoming message is addressed to another segment, it is either repeated on all output ports, or only on the one that corresponds to the addressed logical segment. For such selective relaying, bridges and switches must analyze the MAC addresses of all incoming messages. Obviously, this need significantly complicates the operation of these communication devices. Typically, a bridge contains one common message processor for all ports and therefore performs message processing on only one logical segment (port). The remaining ports must at this time receive incoming messages into buffer memories and wait in the queue for processing. This can significantly reduce network performance.

Switches contain message processors on each port and therefore provide independent and simultaneous processing of messages on each logical segment (port). Switches can process messages fully buffered or on the fly. Due to the relatively low productivity, bridges are currently practically not used.

Quite often there is a need to use different network technologies in different logical segments of the network. In these cases, communication devices must implement different MAC procedures on different output ports. Devices that allow segments with different network technologies to be combined into a single network are usually called gateways. Gateways, in addition to the functions of switches, must perform message format conversion and implement the necessary MAC procedures for each segment. Such tasks are especially relevant in complex composite networks.

4. Technical implementation of switches

To operate effectively in switches, it is necessary to ensure simultaneous transmission of messages between different ports, i.e. the throughput must correspond to the total throughput of the ports. Each port must contain a buffer memory to store messages in cases where the output port (or its communication channel) is busy transmitting another message. The most stringent performance requirements are placed on switches when processing messages on the fly.

Currently, three main schemes for implementing switches are used: switching matrices, shared memory, and a common bus. Quite often these circuits can be combined in one switch. But in any implementation, all switch ports must form a fully connected configuration, i.e. messages from each port must be sent to any other port as needed.

The switch fabric (Figure 2) provides the fastest way for ports to communicate and is a combinational logic circuit that allows signals to be passed from each port to every other port. Very often it is built like a multi-stage matrix decoder, signals controlling the direction of transmission are formed based on the analysis of the MAC address and added to the original message (the so-called tag), the speed of the matrix elements is commensurate with the data transfer speed. However, the complexity of the switch fabric increases very significantly as the number of switch ports increases.

Rice. 2. Implementation of a switching matrix using binary switches

In shared bus switches (Figure 3), the ports are connected by a high-speed bus that provides performance greater than the total performance of the ports. Messages on the internal bus must be transmitted in small portions - cells, this is necessary to prevent delays in the transmission of other messages. The common bus does not provide cell buffering. Each port receives all cells, using tags, accumulates in a buffer those cells that are addressed to it and transmits them to the output channel.

Rice. 3. Common bus architecture

The interaction of switch ports can be organized using two-input shared memory (Fig. 4). Writing to shared memory from input ports is also carried out by cells using a queue manager, and data is read in a similar way for transmission to output ports.

Rice. 4. Shared memory architecture

The last two methods place very high demands on the performance of switch elements.

CAN controllers. These controllers are available either as separate integrated circuits or as built-in elements of more complex devices. The CAN controller, complete with the CAN transceiver IC, ensures the operation of the local network, implementing all the necessary functions: from controlling access to a shared data transfer medium (MAC procedures) to transmitting signals over a communication line. For HLP protocols...

Three types of addresses: local, IP addresses and symbolic domain names. A local address is an address used by an autonomous system (a subnet of a composite network). It is assumed that each autonomous system can be built using its own network technology, can have an independent addressing system and use its own internal addresses. If the autonomous system is also an IP network, the local (internal) ...

As the goals of education, the content of education, the teacher, students, the technological subsystem, which includes means, methods and forms of teaching. 2. Improving the quality of knowledge with the help of the telecommunication environment in computer science lessons Recently, the means of “virtual worlds” on the Internet, three-dimensional objects, which are an improved...

Such a program will be included in the FPKP plan, then remuneration for specialists conducting advanced training will be made from FPKP funds. Chapter 2. Educational capabilities of a computer network 2.1 E-mail The most common communication technology and corresponding service in computer networks has become the technology of a computer method of sending and processing...

Communication stations use a large amount of equipment. Basic devices and components communication station:

1. network adapters (network cards);
2. network cables;
3. transceivers;
4. repeaters;
5. concentrators;
6. switches;
7. bridges;
8. routers;
9. gateways

Now let's figure out what these devices are and how they work.
First, let's look at what it is network adapter. A network adapter is a device that provides information transfer between a computer and a computer network. The network adapter also plays the role of temporary storage of information. In other words, it serves to buffer information. But the main function of a network adapter is to provide a connection between the computer and the network. The network adapter runs physical layer processes.

The network adapter is like a mini-computer, equipped with its own processor and memory. Most often in stores, network adapters are called network cards. In most cases, network cards are classified into different types. The most common classification is by type port, through which the connection to the computer occurs. The following types are distinguished:
1. ISA
2.PCI
3.USB
The most popular are network cards with PCI exit. The PCI connector is located on the computer's mother card. Depending on the model and brand of your computer, PCI slots may vary. After the network card is connected to the motherboard, a cable with a connector is connected to the network card RJ-45 or BNC.

Depending on the place of use, network cards can be of two types:
1. for personal computers;
2. for servers
Thanks to modern technologies, network cards provide speeds from 10 to 1000 Mbit/s.

The next item is network cables. To connect a personal computer and a communication network, three main types of cable are used:
1. Twisted pair
2. Coaxial cable
3. Optical cable
The optical cable has the best characteristics, but the price of the optical cable is significantly lower than that of its analogues.

The following seven items can be combined into one category, “communications intermediate equipment.” Intermediate equipment includes:

1. Transceivers
2. Repeaters
3. Hubs
4. Switches
5. Bridges
6. Routers
7. Gateways

Additional equipment is used as amplifiers and signal converters. The equipment is used to increase the speed and efficiency of the cable network, and also connects all parts of the communication network into a single whole.
The physical structuring of communication systems is carried out, thereby connecting all personal computers to the network. Additional equipment allows a personal computer to quickly connect to the network and maintain high data transfer rates.
Parts of the system such as transceivers And repeaters serve to amplify and convert the signal. A hubs And switches unite a certain number of computers into a local communication network configuration.

Hub plays the role of a device for structuring a computer network. And then, switch receives the signal and divides it into logical segments. This is done in order to avoid system errors and crashes.

For the process of interaction of all parts of the communication network and as an internetwork interface, switches, bridges, routers and gateways are used.
Repeaters are a hardware device for maintaining a stable signal. The functions of a repeater are to restore and amplify the signal. This equipment is used to increase the length of the cable network.
Transceivers And heat transferrs are devices for bidirectional transmission between an adapter and a network cable or two cable segments. The main function of the device is to amplify the signal. The transceiver can also be used as a signal converter. Such a device can convert an electrical impulse into a radio signal or an optical impulse. This allows you to transfer information to other information transfer devices.
Hubs are hardware multiple access devices that combine separate physical sections of cable at one point. They form a common data transmission medium or physical network segments.

An important element of a communication network is the switch. Switch is a complex device that divides the entire network into logical elements. Each logic element interacts with the network card and the communication network.

Bridges- These are signal conductors that connect all local networks into a single network.
Bridges function:
1. Network structuring
2. Connecting similar networks with slight functional differences

A bridge controls traffic on one part of the network and traffic on another part, improving overall data transfer performance. Provides a smooth and continuous flow of data.

Gateways are a device for combining heterogeneous communication networks with different information exchange protocols. Gateways adapt all incoming information for their network.
The final element of the system is the router. Router is a device for communicating a device with multiple networks.
Routers are designed to connect homogeneous networks and branched networks with several parallel routes. This allows for fast synchronization between networks.

We have looked at the main components of a standard communications network. Many other types of devices and equipment are used in modern communications stations.

PC communication devices are designed to exchange data between computers, a computer and a remote I/O device, as well as to connect computers into a local (Local Area Network, LAN) or global (Wide Area Network, WAN) network (including the Internet). Data exchange is required for various purposes: transferring files, sharing peripheral devices (for example, printers), accessing a variety of information services on the Internet and private networks, receiving and transmitting fax messages, sending messages to pagers and mobile phones, establishing voice communications (IP- telephony), video calling and even joint games over the network. Modern technologies used for these purposes are discussed in, and this chapter describes communication devices: modems and wired and wireless LAN adapters. Communication between computers, however, with a number of restrictions, can be established by other means: through LPT ports, FireWire and USB serial buses. Of course, practical (applied) benefit from connecting a computer to a network can only be obtained if you have network software, but its consideration is not the topic of this book.

Connection options

To connect an individual (for example, home) computer to the Internet, it must be connected to an Internet service provider (service provider). There are several options for such communication, differing in availability, bandwidth, and cost:

Connecting via a modem via a regular (switched) telephone line is the most widespread and accessible method. To do this, you need to install a modem (internal or external) and configure the Internet browser (application program) for this type of connection (the Windows setup wizard will ask all the necessary questions). You also need to enter into an agreement with the provider and get from him the phone number by which the modem should call the provider, a user name and password. The provider, in principle, can be located anywhere, but you should not choose a provider that is too far away (in terms of the telephone network), especially a non-resident or foreign one (it will be too expensive). Then, when you start the browser, the modem will automatically (or with a confirmation request) at the browser’s command call the provider and establish a connection (this may take minutes, if, of course, it is easy to reach the provider). Once the connection is established, you can enjoy all the benefits of the Internet. Payment terms may be different: fixed payment for a limited amount of information sent (traffic); time-based connection payment; payment based on traffic and their various combinations. When paying by the hour, you should not forget to disconnect the connection when you finish active work on the Internet (this saves money). Connecting via a modem has a number of disadvantages: the data reception speed from the network cannot exceed 56 Kbps, and transmission speeds are even lower. If the user's PBX is bad or the communication lines are bad, as well as with a bad provider, the connection will be unstable, connections will be broken, and copying large files under such conditions can be not only time-consuming, but also impossible. Naturally, it is impossible to use this telephone line for conversations while working on the Internet. It is also problematic to reach an enthusiastic Internet subscriber from the outside (although you can send him a letter by e-mail). Subscribers of interlocked phones may experience technical connection difficulties, as well as difficulties in sharing telephone time with neighbors. The likely transition to time-based payment for telephone calls may significantly increase the cost of this currently most affordable connection method.

Connection via xDSL modems via regular telephone lines. To do this, the user and the provider must have the appropriate modems installed, but the provider, in addition, must be physically located on the territory of the telephone exchange serving this user. Such modems are more expensive than regular modems, but they provide higher transfer speeds. In addition, the modem works independently of the phone connected to the same line. A small local network of computers can also be connected to an xDSL modem (a number of xDSL modems are connected directly via Ethernet).

Connection via a dedicated two- or four-wire telephone line using a special modem. This connection is no longer connected to the telephone; only telephone cables are used here. Connection can be organizationally complex, since telephone cables do not always have free pairs. The quality and speed of communication are usually higher than those of conventional modems, but the price of the equipment is also higher.

Connection via cable television network and cable modem. This is not a very common method yet, since it is beneficial to the provider (the owner of the cable television network) only if there is a significant number of subscribers wishing to connect to the Network.

Connection via ISDN digital network. This requires an ISDN connection adapter (often called an ISDN modem) and the actual ISDN line routed to the user. Transmission speed is 64 or 128 Kbps (for subscribers with the BRI interface), but even for this entry-level ISDN network is an expensive proposition.

Satellite connection. The provider provides high-speed transmission of downstream traffic (from the Network to the user) via satellite; reception requires a satellite dish and a special receiver connected to a computer. The return channel is organized using one of the traditional wire methods (usually through dial-up telephone lines).

Connection via a fiber optic communication line is the most expensive, but also high-quality connection. This requires running a fiber optic cable from the provider to the user, with one user only requiring a pair of fibers. The terminal equipment is expensive, but the transmission speed is limited only by the physical capabilities of the provider (and the financial capabilities of the user).

Connection to a local network, which is an IP subnet of the Internet. Technically, this is the simplest connection - you need a network adapter connected to the local network. The IP network protocol is connected to the adapter, an IP address is assigned, and the computer becomes a full member of the Network. Issues of communication with the provider fall on the network administrator, who must take care of separating the local network from the global network with a router. The router can communicate with the provider using one of the methods described above.