Data transmission media. Data transmission medium types of communication lines Data transmission medium performance term
Depending on the data transmission medium, communication lines are divided into the following:
- wired (aerial);
- cable (copper and fiber optic);
- radio channels of terrestrial and satellite communications.
Wired (overhead) communication lines are wires without any insulating or shielding braiding, laid between poles and hanging in the air. Such communication lines traditionally carry telephone or telegraph signals, but in the absence of other options, these lines are also used to transmit computer data. The speed and noise immunity of these lines leave much to be desired. Today, wired communication lines are quickly being replaced by cable lines.
Cable lines are quite complex structures. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical, and also, possibly, climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect various equipment to it. 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 exists in a shielded version (Shielded Twistedpair, STP), when a pair of copper wires is wrapped in an insulating shield, and unshielded (Unshielded Twisted Pair, UTP), when the insulating wrap is missing. Twisting the wires reduces the effect of external interference on the useful signals transmitted along the cable. Coaxial cable has an asymmetrical design and consists of an internal copper core and 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 consists of thin (5-60 microns) fibers through which light signals travel. This is the highest quality type of cable - it provides data transmission at very high speeds (up to 10 Gbit/s and higher) and, moreover, better than other types of transmission media, protects data from external interference.
Terrestrial and satellite radio channels are formed using a transmitter and receiver of radio waves. There are a large number of different types of radio channels, differing both in the frequency range used and in the channel range. The short, medium and long wave bands (KB, CB and LW), also called amplitude modulation (AM) bands after the type of signal modulation method used in them, provide long-distance communication, but at a low data transfer rate. The fastest channels are those operating in the ultra-short wave (VHF) range, which is characterized by frequency modulation (FM), as well as in the ultra-high frequency range (microwaves). In the microwave range (above 4 GHz), signals are no longer reflected by the Earth’s ionosphere and stable communication requires direct visibility between the transmitter and receiver. Therefore, such frequencies are used either by satellite channels or radio relay channels, where this condition is met.
In computer networks today, almost all described types of physical data transmission media are used, but the most promising are fiber optic ones. Today, both backbones of large territorial networks and high-speed communication lines of local networks are built on them. Twisted pair is also a popular medium, characterized by an excellent quality-to-cost ratio and ease of installation. Using twisted pair cables, end users of networks are usually connected 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 a channel passes 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 shielding and insulation. In some cases, the cable includes connectors that connect the cables to the equipment. In addition, to ensure fast reconnection of cables and equipment, various electromechanical devices called cross-sections, cross-boxes or cabinets are used.
Computer networks use cables that meet certain standards, which makes it possible to build a network cabling system from cables and connecting devices from different manufacturers. Today, the most commonly used standards in world practice are the following.
- 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 based on the previous version of the EIA/TIA-568 standard and 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 they impose identical requirements on cables. However, there are also differences between these standards, for example, the international standard 11801 and the European EN50173 include some types of cables that are not included in the EIA/TAI-568A standard.
Before the advent of the EIA/TIA standard, the American standard played a major role cable category systems Underwriters Labs, developed jointly with Anixter. This standard later became part of 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 when standardizing cables. This means that the standard specifies the electrical, optical and mechanical characteristics that must be satisfied by a particular type of cable or connecting product - connector, crossover box, etc. However, the standard does not specify what protocol this cable is intended for. Therefore, you cannot purchase a cable for the Ethernet or FDDI protocol, you just need to know what types of standard cables support the Ethernet and FDDI protocols.
Early versions of the standards specified only the characteristics of cables, without connectors. In the latest versions of the standards, requirements have appeared for connecting elements (documents TSB-36 and TSB-40A, which were later included in the 568A standard), as well as for lines (channels), representing a typical cabling system assembly consisting of a cord from the workstation to the outlet, the outlet itself, the main cable (up to 90 m long for twisted pair), a transition point (for example, another outlet or a hard cross-connect) and a 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.
Cable standards stipulate quite a lot of characteristics, the most important of which are listed below (the first two of them have already been discussed in some detail).
The focus of modern standards is on twisted pair cables and fiber optic cables.
Various criteria, such as data speed and cost, help determine the most suitable data transmission medium. The type of material used in the network to provide connections determines parameters such as data transfer speed and data volume. Another factor influencing the choice of the type of data transmission medium is its cost.
To achieve optimal performance, it is necessary to ensure that the signal attenuates as little as possible when moving from one device to another. Signal attenuation can be caused by several factors. As will be discussed later, many media use shielding and technical solutions to prevent signal attenuation. However, the use of shielding increases the cost and diameter of the cable, and also complicates its installation.
In addition, different types of shells can be used in network data transfer environments. The jacket, which is the outer covering of the cable, is usually made of plastic, non-stick coating or composite material. When designing a local network, it should be remembered that a cable laid between walls, in an elevator shaft, or passing through the air duct of a ventilation system can become a torch that contributes to the spread of fire from one part of the building to another. In addition, the plastic shell may cause toxic smoke if it catches fire. To avoid such situations, there are relevant building codes, fire codes and safety codes that specify the types of cable sheaths that can be used. Therefore, when determining the type of data transmission medium for use when creating a local network, these standards should also be taken into account (along with factors such as cable diameter, its cost and installation complexity).
The type of transmission medium used to create the network determines the volume and speed of data transfer.
Data Link Layer
All data on the network is sent by the source and moves towards the destination. The function of the physical layer is to transmit data. Once the data is sent, the data link layer of the OSI reference model provides access to the network media and physical transmission in a medium that allows the data to locate its destination on the network. The data link layer is also responsible for issuing error messages, accounting for network topology, and controlling data flow.
In the OSI reference model, the data link and physical layers are adjacent. The data link layer ensures reliable transit of data through the physical layer. This level uses the address media access control (MAC). As stated earlier, the data link layer deals with issues of physical addressing (as opposed to network or logical addressing), network topology, link discipline (how the end system should use the network link), error notification, frame ordering, and information flow control. In addition, the link layer uses the MAC address as a means of specifying a hardware or link address, allowing multiple stations to share the same data transmission medium and simultaneously uniquely identify each other. In order for data packets to be exchanged between physically connected devices belonging to the same local network, each sending device must have a MAC address, which it can use as the destination address.
MAC addresses
Every computer, whether connected to a network or not, has a unique physical address. No two physical addresses are the same. The physical address (or MAC address) is hardcoded on the board network adapter(Fig. 2.7).
Figure 2.7. The physical address of the computer is protected
on the network adapter board
Thus, in a network, it is the network adapter card that connects the device to the data transmission medium. Each network adapter card that operates at the data link layer of the OSI reference model has its own unique MAC address.
In a network, when one device wants to send data to another device, it can establish a communication channel with that other device using its MAC address. The data sent by the source contains the MAC address of the destination.
As the packet travels through the data medium, the network adapters of each device on the network compare the destination MAC address contained in the data packet with its own physical address. If the addresses do not match, the network adapter ignores the packet and the data continues on to the next device.
If the addresses match, then the network adapter makes a copy of the data packet and places it at the data link level of the computer. After this, the original data packet continues to move through the network, and each subsequent network adapter carries out a similar comparison procedure.
Network adapters
Network adapters convert data packets into signals for transmission over the network. During manufacturing, the manufacturer assigns each network adapter a physical address, which is entered into a special chip installed on the adapter board. In most network adapters, the MAC address is programmed into ROM. When the adapter is initialized, this address is copied into the computer's RAM. Since the MAC address is determined by the network adapter, when the adapter is replaced, the physical address of the computer will also change; it will match the MAC address of the new network adapter.
For example, let's imagine a hotel. Let us further assume that room 207 has a lock that can be opened with key A, and room 410 has a lock that can be opened with key F. The decision is made to change the locks in rooms 207 and 410. After changing, key A will open room 410, and key F will open room 207. B In this example, the locks play the role of network adapters, and the keys play the role of MAC addresses. If the adapters are swapped, the MAC addresses will also change.
Summary
The function of the physical layer is to transmit data.
Several types of data transmission media can be used to connect computers.
A coaxial cable consisting of an outer cylindrical hollow conductor surrounding a single inner conductor.
Unshielded twisted pair cable, used in many networks and consisting of four pairs of wires twisted together.
Shielded twisted pair cable that combines shielding, noise suppression, and wire twisting techniques.
A fiber optic cable is a carrier that is capable of carrying a modulated light signal.
Various criteria, such as data speed and cost, can be used to determine the most suitable type of communication medium.
The data link layer of the OSI reference model provides access to the data transmission medium and the physical data transfer itself, in which the data has the ability to determine the location of the recipient on the network.
The data link layer ensures reliable transit of data through a physical communication channel.
This level uses the MAC address - a physical address, information about which is located on the network adapter board.
Network adapters convert data packets into signals, which are sent to the network.
Each adapter is assigned a physical address by the manufacturer.
Control questions
1. What are the names of all the materials that provide physical connections in a network?
A. Application Environment.
B. Learning environment.
Concept of data transmission medium
The data transmission medium should be understood as a set of equipment using
Which interaction is carried out between the participants of the connection within the framework
Communication session.
In the simplest case, the transmission medium can be implemented in the form of a cable
(single or as part of a group) and/or use any of the types
Wireless technologies.
To use a cable in a computer network, the following must be clearly described:
Type of cable system and its physical characteristics;
Forms and levels of information signal;
Methods for branching the transmission medium and connecting to it;
Requirements for network equipment.
When using wireless technologies, there are even more restrictions and requirements,
Since each of these environments has special ways of encoding, decoding and
Applications of the signal in the environment.
Typically the transmission medium operates in one of the following modes:
Simplex transmission. Unidirectional channel, signals always pass through it
Only in one direction.
Half duplex transmission. Signals can be transmitted in both directions via
A single communication channel, but at any given time signals are transmitted only
One way.
Duplex transmission. This method implements full two-way communication via
The only communication channel.
The properties of the transmission medium determine the level of protection of transmitted signals from
Interference. There are the following types of interference:
Electromagnetic interference represents intrusion by an outsider
An electromagnetic signal that disrupts the shape of the useful signal. When in useful
The signal is added by external interference, the receiving computer cannot properly
Interpret the signal.
Radio frequency interference consists of signals from radio transmitters and other
Devices that generate signals at radio frequencies. These also include
Computer processors and displays. Radio frequency is considered to be electromagnetic
Radiation at frequencies from 10 KHz to 100 GHz. Emission at frequencies from 2 to 10 GHz
Also called microwave.
The influence of radio frequency interference is eliminated using noise filters,
Used in various types of networks.
Crosstalk. This type of interference includes wire signals,
Located at a distance of several millimeters from each other. flowing through
Electric current in a wire creates an electromagnetic field that generates
Signals in another wire located nearby. Quite often, when talking
On a telephone, you can hear other people's muffled conversations. The reason for this
There is crosstalk.
Crosstalk is greatly reduced by twisting two wires together like this
Made in twisted pair. The more turns there are per unit length, the less
The influence of interference.
Signal attenuation. Passing through the cable, electrical and optical signals
They are getting weaker. The greater the distance to the source, the weaker the signal.
This weakening of the signal with distance is called signal attenuation. Attenuation
This is the reason why the specifications of various network architectures
Specifies the limit on cable length. If this restriction is met, then
The fading effect will not affect the normal operation of the communication channel.
Different cable systems have different frequency range tolerances
And the rate of signal attenuation (Figure 1).
As the frequency increases, the attenuation increases because the higher the frequency
signal, the more intense the dissipation of its electromagnetic energy into the surrounding
Space. As the frequency increases, the wire itself turns into a signal carrier
Into an antenna that dissipates its energy into space.
All standards related to the data transmission medium are described on the physical
OSI model level.
The data transmission medium is understood as a physical substance through which electrical signals are transmitted, used to transfer certain information presented in digital form.
The data transmission medium can be natural or artificial. The natural environment is the environment existing in nature; Most often, the natural environment for transmitting signals is the Earth's atmosphere, but it is also possible to use other environments - airless space, water, soil, ship hull, etc. Accordingly, artificial means media that were specially manufactured for use as a data transmission medium. Representatives of the artificial environment are, for example, electrical and fiber optic (optical) cables.
Artificial environments. Classification and application
The typical and most common representatives of the artificial data transmission medium are cables. When creating a data transmission network, the choice is made from the following main types of cables: fiber-optic (fiber), coaxial (coaxial) and twisted pair (twisted pair). In this case, both coax (coaxial cable) and twisted pair use a metal conductor to transmit signals, and a fiber-optic cable uses a light guide made of glass or plastic.
When choosing a cable, especially an electrical cable, there is a tension between achieving high transmission speeds and covering long distances. The fact is that you can increase the data transfer rate, but this reduces the distance over which the data can travel without recovery (regeneration
Coaxial cable
Broadband networks and cable television use an important advantage of coaxial cable - its ability to transmit many 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.
Coaxial cable has a wide bandwidth; this means that it can transmit traffic at high speeds. It is also resistant to electromagnetic interference (compared to twisted pair) and is capable of transmitting signals over long distances.
There are several sizes of coaxial cable. There are thick (0.5 inch diameter) and thin (0.25 inch diameter) coaxial cables. Thick coaxial cable is stronger, more resistant to damage and can transmit data over longer distances, but the disadvantage of such cable is that it is difficult to connect.
twisted pair
Twisted pair (TP - twisted pair) is a cable in which an insulated pair of conductors is twisted with a small number of turns per unit length. In the last few years, twisted pair manufacturers have learned to transmit data over their cables at high speeds and over long distances. Modern advances have made it possible to transmit data over a twisted pair cable at a speed of 1 Gbit/s (250 Mbit/s in each of 4 pairs).
Compared to fiber optic and coaxial cables, the use of twisted pair cables has a number of significant advantages. This cable is thinner, more flexible and easier to install. It's also inexpensive. And as a result, twisted pair cable is an ideal means of data transmission for offices or workgroups where there is no electromagnetic interference.
However, twisted pair has the following disadvantages: strong exposure to external electromagnetic interference, the possibility of information leakage and strong signal attenuation. In addition, twisted pair conductors are subject to skin effect - at high current frequencies, electric current is displaced from the center of the conductor, which leads to a decrease in the useful area of the conductor and additional attenuation of the signal.
Fiber Optic Cable
Fiber-optic cable was touted as the solution to all the problems caused by copper cable. This cable has enormous bandwidth and can transport voice, video and data signals over very long distances. Because fiber optic cable uses pulses of light rather than electricity to transmit data, it is immune to electromagnetic interference. A distinctive feature of fiber optic cable is that it provides higher information security than copper cable. This is due to the fact that the intruder cannot eavesdrop on signals, but must physically connect to the communication line. In order to access the information transmitted over such a cable, an appropriate device must be connected, and this, in turn, will lead to a decrease in the intensity of the light radiation. The disadvantages of fiber optic cables include high cost and fewer possible reconnections compared to electrical cables, since during reconnections microcracks appear at the commutation site, which leads to deterioration in the quality of the optical fiber.
Fiber optic cable can be single-mode or multi-mode. A single-mode cable has a smaller fiber diameter (5-10 microns) and allows only rectilinear propagation of light radiation (along the central mode). In the core of a multimode cable, light can propagate not only in a straight line (along several modes). The more modes, the narrower the cable capacity. For a single-mode cable the light source is a laser, for a multi-mode cable it is an LED.
Single-mode cable has the best performance, but is also the most expensive. Multimode cable made of plastic is the cheapest, but has the worst characteristics.
Natural environments
Considering natural data transmission media, we will make the following assumptions: 1) since the most used natural medium is the atmosphere (mainly the lower layer - the troposphere), and different signals propagate in the atmosphere in different ways, then when considering this environment, we will consider different types of signals separately ; 2) since in satellite communications the airless environment does not impose any restrictions on the signal passing through it, and the satellite communications signal experiences the main difficulties when passing through the atmosphere, we will not consider the airless environment separately.
Atmosphere
Electromagnetic waves are the most widely used data carriers in the atmosphere. It should be noted here that the nature of the propagation of electromagnetic waves in the atmosphere depends on the wavelength. The spectrum of electromagnetic radiation is divided into radio radiation, infrared radiation, visible light, ultraviolet radiation, X-ray radiation, and gamma radiation.
Radio waves
In data transmission networks, VHF radio waves have been used, which propagate linearly and are not reflected by the ionosphere (like HF) and do not go around obstacles encountered (like LW or SW). Therefore, communication in data networks built on VHF radios is limited in distance (up to 40 km). To overcome this limitation, repeaters are usually used.
Data transmission networks can be narrowband (usually single-frequency) and broadband (broadband, as a rule, organized on unlicensed frequencies). Broadband networks can use either Direct Sequence Code Division Multiple Access (DS-CDMA, DFM) or Frequency Hopping Code Division Multiple Access (FH-CDMA, FHM).
It is worth adding that when using radio waves with millimeter wavelengths or less, you will have to face the fact that the quality of radio communication will depend on the state of the atmosphere (fog, smoke, etc.).
Data transmission media can be:
1. Copper-electric cables:
-, consists of a central core, its insulation, and all this is placed in a braid of thin copper wire or aluminum or copper foil. The braid is designed to protect the central core from interference and reduce the emission of a useful signal by it. At the ends of the coaxial cable segments, connectors called CP-50 are installed; BNC. In computer networks, coaxial cables with a basic connection of 50 ohms are used. Coaxial cable can be in 2 versions: Thin with an outer diameter of 5-6 mm. and thick with a diameter of 12-14; mm.
It consists of 8 wires twisted in pairs and laminated with insulation. Twisted pairs are marked by color: orange - white-orange; blue - white-blue; brown - white-brown; green - white-green. A connector is installed at the ends of segments made on twisted pair RJ45. In the operation of local networks, 2 pairs take part, connected respectively to the first, second and third, sixth contacts of the connector RJ45(if you look at the connector so that the cable goes down and the latch is behind the connector). Wire curling is done to improve the noise immunity and consistency of network components. The development of twisted pair wires during installation is allowed to a length of 1 cm. Twisted pair wire is available in 2 versions: UTP - unshielded twisted pair; STP - shielded.
2.
- has two types of transmission:
- at multimode transmission uses a light source in the visible spectrum. The rays are chaotically reflected from the walls of the light guide.
- at single-mode transmission uses a laser beam that moves along the axis of a thinner light guide. The quality of transmission and its range with single-mode transmission are much higher. Light guides are connected to devices that convert an electrical signal into a light signal and vice versa (modem)
3. Infrared rays:
The main feature of transmission using IR rays is the need to ensure line of sight and a short distance between 2 IR modems.
4. Radio waves:
Features of the passage of radio waves are as follows:
The ionosphere surrounding the earth reflects long, medium and short range radio waves. VHF (ultra-high frequency) radio waves penetrate the ionosphere and go into space, so the microwave range is used to communicate with the satellite. Also, the microwave range is used to build computer networks using technology WI-FI. It is necessary to take into account that microwave radio waves do not have the ability to bend around obstacles and any obstacle encountered in their path absorbs them. Therefore, the main requirement for WIFI networks direct visibility between