Media performance is a term. Classification of networks by type of medium for data transmission

Selection and justification of the data transmission medium

1. General characteristics of the data transmission medium

Data transmission media fall into two categories. Cable transmission medium (carrier) - with a central conductor enclosed in a plastic sheath.

Cables are widely used in small local area networks. The cable usually carries signals in the lower part of the electromagnetic spectrum, which is normal electrical current and sometimes radio waves.

Wireless transmission media involves the use of higher frequencies in the electromagnetic spectrum.

These are radio waves, microwaves and infrared rays. Such an environment is required for mobile computers or networks that transmit data over long distances. It is commonly used in enterprise networks and WANs (a cell phone uses a microwave signal to transmit the signal).

Networks spanning multiple locations often use a combination of wired and wireless media.

When choosing the optimal media type, you should know the following characteristics of the data transmission medium:

- price;

- the complexity of the installation;

- throughput;

- signal attenuation;

- susceptibility to electromagnetic interference (EMI, Electro-Magnetic Interference);

- the possibility of unauthorized listening.

Price. The cost of each transmission medium should be compared with its performance and available resources.

Complexity of installation. The complexity of the installation depends on the specific situation, but it is possible to make some generalized comparison of data transmission media. Some types of media are installed using simple tools and do not require much training, while others require lengthy training of employees, and their installation is better left to professionals.

Bandwidth. The capabilities of the data transmission medium are usually evaluated in terms of bandwidth. In communications, "bandwidth" refers to the range of frequencies that are transmitted by the data transmission medium. In networks, it is measured by the number of bits that can be transmitted through a given medium per second. Signal transmission methods also affect cable bandwidth.

The number of nodes. An important characteristic of a network is the number of computers that can be easily connected to network cables. Each network cabling system has a natural number of nodes for it, the excess of which requires the use of special devices: bridges, routers, repeaters and hubs, allowing to expand the network.

Attenuation of signals. Electromagnetic signals are weakened during transmission. This phenomenon is called attenuation.

Electromagnetic interference. Electromagnetic Interference (EMI) affects the transmitted signal. They are caused by external electromagnetic waves that distort the desired signal, making it difficult for the receiving computer to decode. Some communication media are more susceptible to electromagnetic interference than others. Interference is also called noise.

As a data transmission medium in electronic communication, you can use:

· coaxial cable;

Twisted pair of wires;

· Fiber optic cable;

· infrared radiation;

· Microwave range of radio air;

· Radio range of the air.

Today, the overwhelming majority of computer networks in most cases use wires or cables for connection.

For example, Belden, a leading manufacturer of cables, publishes a catalog where it offers more than 2,200 cable types. Fortunately, most networks use only three main cable groups:

1.coaxial cable;

2.twisted pair:

unshielded (Unshielded Twisted Pair, UTP);

shielded (Shielded Twisted Pair, STP);

3. fiber optic cable.

2. Cables based on twisted pairs

Twisted pairs of wires are used in the cheapest and perhaps the most popular cables today.

Twisted-pair cable consists of several pairs of twisted insulated copper wires in a single dielectric (plastic) sheath. It is quite flexible and easy to lay.

Typically, the cable contains two or four twisted pairs. Unshielded twisted pairs are characterized by poor immunity from external electromagnetic interference, as well as poor protection from eavesdropping for the purpose of, for example, industrial espionage.

Interception of transmitted information is possible both using the contact method (by means of two needles stuck into the cable), and using the non-contact method, which reduces to radio interception of electromagnetic fields emitted by the cable. To eliminate these disadvantages, shielding is used.

In the case of STP shielded twisted pair, each of the twisted pairs is placed in a metal braid-shield to reduce cable radiation, protect against external electromagnetic interference and reduce the mutual influence of the pairs of wires on each other (crosstalk - crosstalk). Naturally, shielded twisted pair is much more expensive than unshielded, and when using it, it is necessary to use special shielded connectors, so it is much less common than unshielded twisted pair.

The main advantages of unshielded twisted pairs are the ease of mounting connectors at the ends of the cable, as well as the ease of repairing any damage compared to other types of cable. All other characteristics are worse than those of other cables.

According to the EIA / TIA 568 standard, there are five categories of unshielded twisted pair (UTP) cables.

3. Coaxial cables

A coaxial cable is an electrical cable consisting of a central wire and a metal braid, separated by a dielectric layer (inner insulation) and placed in a common outer sheath.

Until recently, coaxial cable was the most widespread, which is associated with its high noise immunity (thanks to the metal braid), as well as higher, than in the case of twisted pair, permissible data transmission rates (up to 500 Mbit / s) and long permissible transmission distances ( up to 1 km and more).

It is more difficult to mechanically connect to it for unauthorized eavesdropping on the network, it also gives noticeably less electromagnetic radiation outside.

However, the installation and repair of a coaxial cable is much more difficult than a twisted pair cable, and its cost is higher (it is about 1.5-3 times more expensive than a twisted pair cable). It is also more difficult to install connectors at the ends of the cable. Therefore, it is now used less often than twisted pair.

The main application of the coaxial cable is in networks with a "bus" topology.

If the braid is grounded at two or more points, not only the network equipment can fail, but also the computers connected to the network. Terminators must be matched to the cable, that is, their resistance must be equal to the characteristic impedance of the cable.

For example, if a 50-ohm cable is used, only 50-ohm terminators are suitable.

There are two main types of coaxial cable:

thin (thin) cable having a diameter of about 0.5 cm, more flexible;

a thick cable, about 1 cm in diameter, is much more rigid. It is a classic version of coaxial cable, which is almost completely replaced by more modern thin cable.

A thin cable is used for transmission over shorter distances than a thick one, since the signal is attenuated more in it. But with a thin cable it is much more convenient to work: it can be quickly laid to each computer, and a thick one requires rigid fixation on the wall of the room.

Connecting to a thin cable (using BNC bayonet type connectors) is easier and does not require additional equipment, and to connect to a thick cable, you need to use special rather expensive devices that pierce its shell and establish contact - both with the central core and with the screen.

A thick cable is about twice as expensive as a thin one. Therefore, a thin cable is used much more often.

Cost per seat. The thin coaxial cable has a lower cost per workstation - about $ 25. You can purchase these cables with the connectors already attached.

Anyone can lay such cables - they are simply connected by a chain from computer to computer.

Thick coaxial cable usually costs about $ 50 per station. In addition, transceivers are required for each station (about $ 100).

Distance restrictions. The total bus length on thin coaxial cable is limited to 185 m. Thick coaxial cable has a total limitation of 500 m (in structures without repeaters).

4. Fiber optic cables

Fiber optic (aka fiber optic) cable is a fundamentally different type of cable compared to the two types of electrical or copper cable considered.

Information on it is transmitted not by an electrical signal, but by a light signal. Its main element is transparent fiberglass, through which light travels over huge distances (up to tens of kilometers) with insignificant attenuation.

The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electrical cable, except that instead of a central copper wire, thin (about 1-10 microns in diameter) fiberglass is used, and instead of internal insulation, a glass or plastic sheath does not allow light to go outside the glass fiber.

Fiber-optic cable has exceptional characteristics in terms of noise immunity and secrecy of transmitted information.

In principle, no external electromagnetic interference is capable of distorting the light signal, and this signal itself, in principle, does not generate external electromagnetic radiation.

It is almost impossible to connect to this type of cable for unauthorized eavesdropping on the network, as it requires breaking the integrity of the cable.

The theoretically possible bandwidth of such a cable reaches 10 GHz, which is incomparably higher than that of any electrical cable. The cost of fiber optic cable is constantly decreasing.

Typical signal attenuation in fiber optic cables at frequencies used in local area networks is about 5 dB / km. The most important of them is the high complexity of installation.

Although fiber-optic cables allow signal splitting (special splitters for 2-8 channels are produced for this), as a rule, they are used to transfer data in only one direction, between one transmitter and one receiver.

It is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, i.e. signal attenuation increases. Fiber optic cables are also sensitive to mechanical stress (shock, ultrasound) - the so-called microphone effect. To reduce it, soft sound-absorbing shells are used.

Use fiber-optic cable only in networks with a star and ring topology. In this case, there are no problems of matching and grounding. The cable provides perfect galvanic isolation of network computers.

There are two different types of fiber optic cables:

multimode (or multimode) cable - cheaper, but of lower quality;

singlemode cable is more expensive but has better performance.

A single-mode cable has a center fiber diameter of about 1.3 µm and only transmits light at the same wavelength (1.3 µm).

In a multimode cable, the light paths have a noticeable spread, as a result of which the waveform at the receiving end of the cable is distorted. The central fiber has a diameter of 62.5 microns, and the diameter of the outer cladding is 125 microns (this is sometimes referred to as 62.5 / 125). The wavelength of light in a multimode cable is 0.85 µm.

The permissible cable length is 2-5 km.

Typical latency for most common cables is 4-5 ns / m.

Distance restrictions. With 10Base-FL Ethernet, the distance of multimode fiber optic cable is limited to 2000 m, while Fast Ethernet 100Base-F is limited to 400 m.

Both limitations are related to the timing of the Ethernet, not the properties of the cable itself.

The bandwidth limit for modern fiber optic cables is 622 Mbps at 1000 meters. Each time the cable is cut in half, its bandwidth doubles.

The radio channel uses the transmission of information using radio waves, so it can provide communication over many tens, hundreds and even thousands of kilometers.

The transmission speed can reach tens of megabits per second (here a lot depends on the selected wavelength and the encoding method). However, in local networks, the radio channel has not become widespread due to the rather high cost of transmitting and receiving devices, low noise immunity, complete lack of secrecy of transmitted information and low reliability of communication.

But for global networks, the radio channel is often the only possible solution, since it makes it relatively easy to provide communication with the whole world with the help of repeater satellites. A radio channel is also used to connect two or more local networks located far from each other into a single network.

Table 1

900 MHz from 1 ch! broad spectrum signal editing	These solutions typically provide 2 Mbps of bandwidth over 5,000 meters. These radio networks function much like cell phones and do not require a transmitter and receiver to be in line-of-sight. Their cost is, as a rule, about $ 5,000 per station.
with transmission in wide	The use of the 2.4 GHz band is licensed by the FCC, and devices are currently planned to operate in this band.
with transmission in wide	Solutions in the 5.8 GHz band provide data transfer rates of about 6 Mbps over distances of up to 244 meters. These devices consume little power and provide more bandwidth than the 900 MHz options, but are not suitable for communication over long distances. Cost is about $ 1,000 per station
Microwave transmission at 23 GHz	Microwave transmission at 23 GHz has the best performance and distance among wireless solutions. Such solutions are implemented on a point-to-point basis, and the receiver and transmitter must be in the line of sight. They allow transferring data at a speed of 6 Mbps for a distance of up to 50 km, but they are very susceptible to the weather and are quite expensive. Cost per station is usually $ 15,000

The infrared channel also does not require connecting wires, as it uses infrared radiation for communication (like a home TV remote control).

Its main advantage in comparison with a radio channel is its insensitivity to electromagnetic interference, which makes it possible to use it, for example, in industrial conditions.

True, in this case, a rather high transmission power is required so that no other sources of thermal (infrared) radiation are affected. Infrared communication does not work well even in dusty environments.

The maximum data transfer rates via the infrared channel do not exceed 5-10 Mbit / s.

Infrared channels are divided into two groups.

Line-of-sight channels, in which communication is carried out on beams coming directly from the transmitter to the receiver. In this case, communication is possible only if there are no obstacles between the computers on the network. The length of the line-of-sight channel can be up to several kilometers.

Scattered radiation channels that operate on signals reflected from walls, ceilings, floors, and other obstacles. Obstacles in this case are not terrible, but communication can be carried out only within the same room.

Question Evolution of computing systems

1) Batch processing systems:

1950s - The first computers appear.

Batch processing systems were built on the basis of the mainframe - a powerful and reliable universal computer. Users had punched cards containing data and program commands, operators entered these cards into a computer, and the printed results were received the next day.

Maximizing the Efficiency of Computing Power

Disregard for the interests of users

2)Multi-terminal system

Distributed data input-output.

Centralized processing.

1960s the emergence of multi-terminal time sharing systems.

LAN prototype.

The computer was placed at the disposal of several users at once, each with a terminal, the response time of the aircraft is quite short.

Computing networks

BC is a collection of computers connected by communication lines (cables, network adapters, telecommunications equipment).

Classification of networks on a territorial basis

LAN - MAN - WAN

Wide Area Networks (WAN).

Data transmission over hundreds and thousands of kilometers

Chronologically appeared the first (50s-60s)

Evolved from telephone networks

Initially slow and unreliable

Today WAN:

Are rings or backbone

Main speed 2.5 Gbit / s

10-Gbit / s, 40-Gbit / s solutions are widespread

Complicated data control and recovery procedures are applied

Local area networks - Local Area Networks (LAN).

Concentrated on the territory of 1-2 km.

Speed ​​up to 10 Gbps

Wide range of services

The most important stage of development is the formation of standard LAN technologies: Ethernet, Token Ring, FDDI.

Metropolitan Area Networks (MAN)

Distances of several tens of kilometers

Cheaper than WAN

Connection speeds 1-40 Gbit / s

Used to connect existing LANs and connect to the WAN

Modern tendencies

Global networks are closely matched in quality to local ones

2) The LAN began to use switches, routers, gateways => the ability to build complex networks

Question. Seven-level OSI model.

Physical layer

The physical layer defines the electrical, mechanical, procedural and

functional characteristics of activation, maintenance and deactivation of a physical channel between end systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical information transfer rates, maximum communication distances, physical connectors, and other similar characteristics. Data unit: Bit (bit)

Link layer

The data link layer provides reliable data transit over the physical channel. In accomplishing this task, the link layer solves the issues of physical addressing, network topology, linear discipline (how the end system uses the network link), notification of faults, orderly delivery of data blocks, and information flow control. Data unit: Frame

Network layer

The network layer is a complex layer that provides connectivity and route selection between two end systems connected to different "subnets" that may be located in different geographic locations.

In this case, a "subnet" is essentially an independent network cable (sometimes called a segment).

Because two end systems wishing to communicate can be separated by a significant geographic distance and many subnets, the network layer is the routing domain. Routing protocols select optimal routes through a series of interconnected subnets. Traditional network layer protocols transport information along these

Routes. Data unit: Packet

Transport layer

The transport layer is concerned with issues such as performing reliable transport of data across the internetwork. By providing reliable services, the transport layer provides mechanisms for establishing, maintaining, and orderly termination of virtual circuits, transport troubleshooting systems, and traffic management (in order to prevent the system from flooding with data from another system). Data unit: Datagram / data block (datagramm)

Session level

As its name indicates, the session layer establishes, manages, and terminates communication sessions between applications. Sessions consist of a conversation between two or more presentation objects. The session level synchronizes the dialogue between the objects of the representative level and manages the exchange of information between them. The session layer provides a means to send information, class of service, and exception notification of session, proxy, and application layer problems. Data unit: Message

Representative level

The presentation layer is responsible for ensuring that information sent from the application layer of one system is readable to the application layer of another system. If necessary, the representative layer translates between a plurality of information presentation formats using a common information presentation format.

Data unit: Message

Application level

The application layer is the OSI layer closest to the user. It differs from the other layers in that it does not provide services to any of the other OSI layers; however, it provides them for applications that are outside the scope of the OSI model. Examples of such application processes are programs for processing large-scale tables, programs for processing words, programs for bank terminals, etc.

Data unit: Message

As a data packet moves from top to bottom, each new level adds its own service information to the packet in the form of a header and, possibly, a trailer (information placed at the end of the message). This operation is called encapsulation top-level data in a lower-level package

question. Classification of data transmission media.

Under data transmission medium they understand the physical substance through which the transmission of electrical signals used to transfer one or another information presented in digital form takes place.

The natural environment is the environment that exists in nature - Not natural. - specially designed (cables, etc.)

Natural environments

- Atmosphere Electromagnetic waves are most widely used as data carriers in the atmosphere.

- Radio waves - electromagnetic waves with a frequency of less than 6000 GHz (with a wavelength of more than 100 microns).

- Infrared and visible light (laser)

Artificial environments The main types of cables are fiber-optic (fiber), coaxial (coaxial) and twisted pair (twisted pair). At the same time, both coaxial and twisted pair use a metal conductor to transmit signals, and a fiber-optic cable uses a light guide made of glass or plastic.

Coaxial cable

An important advantage is its ability to transmit multiple signals at the same time. Each such signal is called a channel. All channels are organized at different frequencies, so they do not interfere with each other. It has a wide bandwidth; this means that it can organize the transmission of traffic at high speeds. It is also immune to electromagnetic interference and is capable of transmitting signals over long distances.

Twisted pair

A cable in which an insulated pair of conductors is twisted with a small number of turns per unit length. Twisting is carried out to reduce external interference.

Advantages: Thinner, more flexible, easier to install, inexpensive.

Disadvantages: strong influence of external electromagnetic interference, the possibility of information leakage,

strong signal attenuation.

Unshielded Twisted Pair (UTP)

CAT5 (100 MHz frequency band) - 4 pairs, up to 100 Mbps when using 2 pairs and up to 1000 Mbps when using 4 pairs, is the most common network media used in computer networks so far.

Shielded Twisted Pair (STP)

Foil Twisted Pair (FTP)

Foil Shielded Twisted Pair (SFTP)

Similar information.

Communication lines also differ in the physical medium used to transfer information. The physical transmission medium can be a set of conductors that carry signals. On the basis of such conductors, wire (air) or cable communication lines are built (Fig. 1). The earth's atmosphere or outer space is also used as a medium, through which information signals propagate. In the first case, they talk about wired environment, and in the second - about wireless.

In modern telecommunication systems, information is transmitted using electric current or voltage, radio signals or light signals - all these physical processes are oscillations of the electromagnetic field of various frequencies.

Corded (overhead) communication lines are wires without any insulating or screening braids, laid between poles and hanging in the air. Even in the recent past, such communication lines were the main ones for the transmission of telephone or telegraph signals. Today, wired communication lines are rapidly being replaced by cable ones. But in some places they are still preserved and, in the absence of other possibilities, continue to be used, in particular, for the transfer of computer data. The speed and noise immunity of these lines leave much to be desired.

Cable lines have a rather complex design. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical, and possibly climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect to various equipment.
giving. In computer (and telecommunication) networks, three main types of cable are used: cables based on twisted pairs of copper wires - unshielded twisted pair (UTP) and shielded twisted pair (STP), coaxial cables with a copper core, fiber -optical cables. The first two types of cables are also referred to as copper cables.

Radio channels terrestrial and satellite communications are formed using a transmitter and receiver of radio waves. There is a wide variety of types of radio channels, differing in both the frequency range used and the channel range. Broadcast radio bands (long, medium and short wave), also called AM bands, or Amplitude Modulation (AM) bands, provide long-distance communications, but at low data rates. The faster channels are those that use the Very High Frequency (VHF) bands, for which Frequency Modulation (FM) is applied. For data transmission, the Ultra High Frequency (UHF) bands, also called microwave bands (over 300 MHz), are also used. Above 30 MHz, signals are no longer reflected by the Earth's ionosphere, and a line of sight between the transmitter and receiver is required for stable communication. Therefore, these frequencies are used in satellite or radio relay channels or in such local or mobile networks in which this condition is fulfilled.

In computer networks, almost all types of physical data transmission media described are used today. Fiber optic cables with high bandwidth and low susceptibility to interference provide good opportunities. They are used today as the backbone of large territorial and urban networks, as well as high-speed local networks. Twisted pair is also a popular medium, which is characterized by excellent value for money and ease of installation. Wireless channels are used most often in cases where cable communication lines cannot be used, for example, when the channel passes through a sparsely populated area or for communication with mobile network users. The provision of mobility has affected primarily telephone networks, computer networks are still lagging behind in this regard. Nevertheless, the construction of computer networks based on wireless technologies, for example Radio Ethernet, is considered today one of the most promising areas of telecommunications.

The medium of information transmission are those communication lines (or communication channels) through which information is exchanged between computers. The overwhelming majority of computer networks (especially local ones) use wired or cable communication channels, although there are also wireless networks, which are now increasingly used, especially in laptop computers.

There are 4 types of data transmission media:

Twisted pair cables

Coaxial cables

Fiber optic cables

· Wireless communication channels

Twisted pairs of wires are used in cheap and today, perhaps, the most popular cables. Twisted-pair cable consists of several pairs of twisted pair-wise insulated copper wires in a single dielectric (plastic) sheath. It is quite flexible and easy to lay. Twisting the wires minimizes inductive crosstalk between the cables and reduces the effect of transients.

Typically, the cable includes two (Fig. 4.1) or four twisted pairs.

Rice. 4 ,1.

Unshielded twisted pairs are characterized by poor immunity from external electromagnetic interference, as well as from eavesdropping, which can be carried out for the purpose of, for example, industrial espionage. Moreover, the interception of information transmitted over the network is possible both using the contact method (for example, by means of two needles stuck into the cable), and using the non-contact method, which reduces to radio interception of electromagnetic fields emitted by the cable. Moreover, the effect of interference and the amount of radiation outside increases with increasing cable length. To eliminate these disadvantages, cable shielding is used.

In the case of STP shielded twisted pair, each of the twisted pairs is placed in a metal braid-shield to reduce cable radiation, protect against external electromagnetic interference and reduce the mutual influence of the pairs of wires on each other (crosstalk - crosstalk). In order for the shield to protect against interference, it must be grounded. Naturally, a shielded twisted pair is much more expensive than an unshielded one. Its use requires special shielded connectors. Therefore, it is found much less frequently than unshielded twisted pair.

The main advantages of unshielded twisted pairs are the ease of installing connectors at the ends of the cable, as well as repairing any damage compared to other types of cable. All other characteristics are worse than those of other cables. For example, at a given transmission rate, the signal attenuation (a decrease in its level as it passes through the cable) is greater for them than for coaxial cables. Taking into account the still low noise immunity, it is understandable why communication lines based on twisted pairs are usually rather short (usually within 100 meters). Currently, twisted pair is used to transmit information at speeds up to 1000 Mbps, although the technical problems that arise at such speeds are extremely complex.

A coaxial cable is an electrical cable consisting of a central copper wire and a metal braid (screen), separated by a dielectric layer (internal insulation) and placed in a common outer sheath (Fig. 4.2).

Figure 4.2

Until recently, coaxial cable was very popular due to its high noise immunity (thanks to the metal braid), wider bandwidths (over 1 GHz) than in the case of twisted pair cables, and also large allowable transmission distances (up to a kilometer). It is more difficult to mechanically connect to it for unauthorized eavesdropping on the network, it also gives noticeably less electromagnetic radiation outside. However, the installation and repair of a coaxial cable is much more difficult than a twisted pair cable, and its cost is higher (it is about 1.5 - 3 times more expensive). It is also more difficult to install connectors at the ends of the cable. Now it is used less often than twisted pair. The EIA / TIA-568 standard includes only one type of coaxial cable used in an Ethernet network.

The main application of the coaxial cable is in networks with a bus topology. In this case, terminators must be installed at the ends of the cable to prevent internal signal reflections, and one (and only one!) Of the terminators must be grounded. Without grounding, the metal braid does not protect the network from external electromagnetic interference and does not reduce the radiation of information transmitted through the network to the external environment. But when the braid is grounded at two or more points, not only the network equipment can fail, but also the computers connected to the network. Terminators must be matched with the cable, it is necessary that their resistance is equal to the characteristic impedance of the cable. For example, if a 50-ohm cable is used, only 50-ohm terminators are suitable.

Less commonly, coaxial cables are used in star networks (for example, passive star in an Arcnet network). In this case, the matching problem is greatly simplified, since no external terminators are required at the free ends.

There are two main types of coaxial cable:

· Thin (thin) cable having a diameter of about 0.5 cm, more flexible;

· Thick (thick) cable, with a diameter of about 1 cm, much more rigid. It is a classic version of coaxial cable, which is almost completely replaced by modern thin cable.

A thin cable is used for transmission over shorter distances than a thick one, since the signal is attenuated more in it. But with a thin cable it is much more convenient to work: it can be quickly laid to each computer, and a thick one requires rigid fixation on the wall of the room. Connecting to a thin cable (using BNC BNC connectors) is easier and requires no additional hardware. And to connect to a thick cable, you need to use special rather expensive devices that pierce its shells and establish contact with both the central core and the screen. Thick cable is about twice as expensive as thin cable, so thin cable is used much more often.

As with twisted pairs, the type of outer sheath is an important parameter of the coaxial cable. Likewise, both non-plenum (PVC) and plenum cables are used in this case. Naturally, Teflon cable is more expensive than PVC cable. Typically, the sheath type can be distinguished by color (for example, Belden uses yellow for PVC and orange for Teflon).

Typical signal propagation delays in a coaxial cable are about 5 ns / m for a thin cable and about 4.5 ns / m for a thick one.

There are variants of the coaxial cable with a double shield (one shield is located inside the other and separated from it by an additional layer of insulation). These cables have better noise immunity and eavesdropping protection, but they are slightly more expensive than conventional cables.

Nowadays it is considered that coaxial cable is outdated, in most cases it can easily be replaced by twisted pair or fiber optic cable. And the new standards for cable systems no longer include it in the list of cable types.

Fiber optic (aka fiber optic) cable is a fundamentally different type of cable compared to the two types of electrical or copper cable considered. Information on it is transmitted not by an electrical signal, but by a light signal. Its main element is transparent fiberglass, through which light travels over huge distances (up to tens of kilometers) with insignificant attenuation.

Drawing. 4.3.

The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electrical cable (Figure 4.3). Only instead of a central copper wire, thin (about 1-10 microns in diameter) fiberglass is used, and instead of internal insulation, a glass or plastic sheath is used, which does not allow light to go outside the fiberglass. In this case, we are talking about the regime of the so-called total internal reflection of light from the boundary of two substances with different refractive indices (the refractive index of the glass shell is much lower than that of the central fiber). The metal sheath of the cable is usually absent, since shielding from external electromagnetic interference is not required here. However, sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called armored; it can combine several fiber-optic cables under one sheath).

Fiber-optic cable has exceptional characteristics in terms of noise immunity and secrecy of transmitted information. In principle, no external electromagnetic interference is capable of distorting the light signal, and the signal itself does not generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized eavesdropping on the network, as this violates the integrity of the cable. The theoretically possible bandwidth of such a cable reaches 1012 Hz, that is, 1000 GHz, which is incomparably higher than that of electrical cables. The cost of fiber optic cable has been steadily decreasing and is now approximately equal to the cost of thin coaxial cable.

However, fiber optic cable also has some disadvantages.

The most important of them is the high complexity of installation (when installing connectors, micron accuracy is required, the attenuation in the connector strongly depends on the accuracy of the cleavage of the fiberglass and the degree of its polishing). To install the connectors, welding or gluing is used using a special gel that has the same refractive index of light as fiberglass. In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, on both ends of which the connectors of the required type are already installed. It should be remembered that a poorly installed connector dramatically reduces the allowable cable length, which is determined by attenuation.

It should also be remembered that the use of a fiber-optic cable requires special optical receivers and transmitters that convert light signals into electrical signals and vice versa, which sometimes significantly increases the cost of the network as a whole.

Fiber optic cables allow signal splitting (for this purpose, special passive couplers are produced for 2-8 channels), but, as a rule, they are used to transfer data only in one direction between one transmitter and one receiver. After all, any branching inevitably greatly weakens the light signal, and if there are many branches, then the light may simply not reach the end of the network. In addition, there is an internal loss in the splitter, so the total signal power at the output is less than the input power.

Fiber optic cable is less durable and flexible than electrical cable. Typical bending radii are around 10 - 20 cm, with smaller bending radii the central fiber may break. Poorly tolerates cable and mechanical stretching, as well as crushing effects.

The fiber-optic cable is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, that is, the signal attenuation increases. Sudden changes in temperature also negatively affect it, fiberglass can crack.

Fiber optic cable is used only in networks with a star and ring topology. In this case, there are no problems of matching and grounding. The cable provides perfect galvanic isolation of network computers. In the future, this type of cable is likely to supplant electrical cables, or at least strongly suppress them. The reserves of copper on the planet are depleting, and there are more than enough raw materials for the production of glass.

In addition to cable channels, wireless channels are sometimes also used in computer networks. Their main advantage is that no wiring is required (no need to make holes in the walls, fix the cable in pipes and gutters, lay it under raised floors, above false ceilings or in ventilation shafts, look for and repair damage). In addition, computers on the network can be easily moved within a room or building, since they are not tied to anything.

The radio channel uses the transmission of information over radio waves, so theoretically it can provide communication for many tens, hundreds and even thousands of kilometers. The transmission speed reaches tens of megabits per second (here a lot depends on the selected wavelength and the encoding method).

The peculiarity of the radio channel is that the signal is freely broadcast on the air, it is not enclosed in a cable, so there are problems of compatibility with other sources of radio waves (radio and TV broadcasting stations, radars, radio amateur and professional transmitters, etc.). The radio channel uses transmission in a narrow frequency range and modulation with an information signal of the carrier frequency signal.

The main disadvantage of the radio channel is its poor protection against eavesdropping, since radio waves propagate uncontrollably. Another big disadvantage of the radio channel is its weak noise immunity.

For local wireless networks (WLAN - Wireless LAN), radio-channel connections are currently used at short distances (usually up to 100 meters) and within line-of-sight. The two most commonly used frequency bands are 2.4 GHz and 5 GHz. The transmission speed is up to 54 Mbps. A widespread version with a speed of 11 Mbit / s.

WLANs allow wireless network connections to be established in a limited area (usually inside an office or university building, or in public places such as airports). They can be used in temporary offices or other locations where cabling is not feasible, or as an add-on to an existing wired LAN to enable users to work while moving around the building.

The popular Wi-Fi (Wireless Fidelity) technology allows communication between 2 to 15 computers using a hub (called an Access Point, AP), or multiple hubs if there are 10 to 50 computers. the ability to link two local networks at a distance of up to 25 kilometers using powerful wireless bridges. For example, in Fig. 4.4 shows the combination of computers using one access point. It is important that many mobile computers (laptops) already have a built-in Wi-Fi controller, which greatly simplifies their connection to a wireless network.

Figure 4.4

The radio channel is widely used in global networks for both terrestrial and satellite communications. In this application, the radio channel has no competitors, since radio waves can reach anywhere in the world.

If we talk about possible topologies, then most naturally all wireless communication channels are suitable for a bus topology, in which information is transmitted simultaneously to all subscribers. But when using narrow-beam transmission and / or frequency division by channels, any topologies (ring, star, combined topologies) can be implemented both on the radio channel and on the infrared channel.

Depending on the data transmission medium, communication lines are divided into the following:

• wire (air);
• cable (copper and fiber optic);
• radio channels for terrestrial and satellite communications.

Wire (overhead) communication lines are wires without any insulating or screening braids, laid between the poles and hanging in the air. Such communication lines traditionally carry telephone or telegraph signals, but in the absence of other possibilities, these lines are also used to transfer computer data. The speed and noise immunity of these lines leave much to be desired. Today, wired communication lines are rapidly being replaced by cable ones.

Cable lines represent a rather complex structure. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical, and possibly climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect to various equipment. There are three main types of cable used in computer networks: twisted-pair copper cables, copper coaxial cables, and fiber-optic cables.

A twisted pair of wires is called twisted pair. Twisted pair available in shielded version (Shielded Twistedpair, STP), when a pair of copper wires is wrapped in an insulating shield, and unshielded (Unshielded TwistedPair, UTP), when the insulating wrap is missing. Twisting the wires reduces the effect of external noise on the wanted signals transmitted over the cable. Coaxial cable has an asymmetrical structure and consists of an inner copper core and a braid separated from the core by a layer of insulation. There are several types of coaxial cable, differing in characteristics and areas of application - for local networks, for wide area networks, for cable television, etc. Optical fiber cable (opticalfiber) consists of thin (5-60 microns) fibers through which light signals propagate. This is the highest quality type of cable - it provides data transfer at a very high speed (up to 10 Gbps and higher) and, moreover, better than other types of transmission medium, it provides data protection from external interference.

Radio channels for terrestrial and satellite communications generated by a transmitter and receiver of radio waves. There are a large number of different types of radio channels, differing in both the frequency range used and the channel range. The short, medium, and long wavelength bands (KB, CB, and LW), also called Amplitude Modulation (AM) based on the type of signal modulation they use, provide long-distance communication, but at a low data rate. The higher-speed channels are those operating in the ultrashort wave (VHF) bands, which are characterized by frequency modulation (Frequency Modulation, FM), as well as in the microwave bands (microwaves). In the microwave range (above 4 GHz), signals are no longer reflected by the Earth's ionosphere and for stable communication a line of sight is required between the transmitter and the receiver. Therefore, such frequencies use either satellite channels or radio relay channels, where this condition is met.

Almost all the described types of physical data transmission media are used today in computer networks, but the most promising are fiber-optic ones. Today they are used as the basis for the construction of highways of large territorial networks, as well as high-speed communication lines of local networks. Twisted pair is also a popular medium, which is characterized by an excellent quality-to-cost ratio and ease of installation. Twisted pair cables are usually used to connect end users of networks at distances of up to 100 meters from the hub. Satellite channels and radio communications are most often used in cases where cable communications cannot be used - for example, when passing a channel through a sparsely populated area or to communicate with a mobile network user, such as a truck driver, a doctor making a round, etc.

A cable is a rather complex product, “consisting of conductors, layers of shield and insulation. In some cases, the cable includes connectors that connect the cables to the equipment. In addition, various electromechanical devices called cross-sections, cross-boxes, or cabinets are used to ensure fast re-switching of cables and equipment.

In computer networks, cables are used that meet certain standards, which allows you to build a cabling network from cables and connecting devices from different manufacturers. Today, the most commonly used standards in world practice are as follows.

• American standard EIA / TIA-568A, which was developed jointly by several organizations: ANSI, EIA / TIA and Underwriters Labs (UL). The EIA / TIA-568 standard is developed on the basis of the previous version of the EIA / TIA-568 standard and the additions to this standard TSB-36 and TSB-40A).
• International standard ISO / IEC 11801.
• European standard EN50173.

These standards are close to each other and in many respects impose identical requirements on cables. However, there are differences between these standards, for example, the international standard 11801 and the European EN50173 include some types of cables that are absent in the EIA / TAI-568A standard.

Before the advent of the EIA / TIA standard, the American standard played an important role cable category systems Underwriters Labs, co-developed with Anixter. Later this standard was included in the EIA / TIA-568 standard.

In addition to these open standards, many companies at one time developed their own proprietary standards, of which only one is still of practical importance - the IBM standard.

A protocol-independent approach has been adopted for cable standardization. This means that the standard stipulates the electrical, optical and mechanical characteristics that must be met by a particular type of cable or connecting product - connector, junction box, etc. However, for which protocol this cable is intended, the standard does not specify. Therefore, you cannot purchase an Ethernet or FDDI cable, you just need to know what types of standard cables support the Ethernet and FDDI protocols.

Earlier versions of the standards defined only the characteristics of the cables, without connectors. In the latest versions of the standards, requirements for connecting elements appeared (documents TSB-36 and TSB-40A, which were then included in the 568A standard), as well as for lines (channels), representing a typical assembly of elements of the cable system, consisting of a cord from the workstation to the outlet, the outlet itself, the main cable (up to 90 m long for twisted pair), the transition point (for example, another outlet or hard crossover connection) and the cord to the active equipment, such as a hub or switch.

We will focus only on the basic requirements for the cables themselves, without considering the characteristics of the connecting elements and assembled lines.

The cable standards stipulate a lot of characteristics, of which the most important are listed below (the first two of them have already been considered in sufficient detail).

The focus of current standards is on twisted pair and fiber optic cables.