Simplex
A simplex channel is unidirectional, allowing data to be transmitted in only one direction, as shown in Figure 2.10. Conventional broadcasting is an example of a simplex transmission. A radio station transmits a broadcast program, but receives nothing in return from your radio.
Rice. 2.10. Simplex transmission
This limits the use of a simplex channel for data transmission, since a constant flow of data in both directions is required to control the transmission process, confirm data, etc.
half duplex
Half-duplex transmission makes it possible to provide simplex communication in both directions along a single channel, as shown in Fig. 2.11. Here, the transmitter at station A sends data to the receiver at station B. When a transmission is required in the opposite direction, a line switching procedure takes place. After that, the transmitter of station B is able to communicate with the receiver of station A. The delay in switching the line reduces the amount of data transmitted on the communication channel.
Rice. 2.11. Half duplex transmission
full duplex
A full duplex link allows simultaneous communication in both directions, as shown in Fig. 2.12.
Fig 2.12. Full duplex transmission
2.4.2. Synchronization of digital data signals
Data transmission depends on the correct coordination of the moments of generation and receipt of signals. To determine which data element is transmitted - "1" or "0", the receiver must at the right time. The process of selecting and maintaining reference time intervals is called synchronization.
To synchronize the transmission, the transmitting and receiving devices must agree on the length of the bit (bit time) - the duration of the code element used. The receiver needs to extract the transmitted clock signal encoded in the received data stream. By synchronizing the receiver's clock bit length with the bit length encoded in the sender's data, the receiver can determine the right times to demodulate the data and correctly decode the message. Devices at both ends of a digital channel may synchronize using either asynchronous or synchronous transmission, as described below.
Simultaneously. In mode half duplex either transmit or receive information.
Half duplex mode
A mode in which transmission is carried out in both directions, but with time division is called half-duplex. At any given time, the transmission is in only one direction.
The time division is caused by the fact that the transmitting node completely occupies the transmission channel at a particular time. The phenomenon when several transmitting nodes try to transmit at the same time is called a collision and is considered normal, although undesirable, under the CSMA/CD access control method.
This mode is used when the network uses a coaxial cable or hubs are used as active equipment.
Depending on the hardware, simultaneous reception / transmission in half-duplex mode may either be physically impossible (for example, due to the use of the same circuit for receiving and transmitting in walkie-talkies) or lead to collisions.
duplex mode
A mode in which, unlike half-duplex, data transmission can be performed simultaneously with data reception.
The total speed of information exchange in this mode can reach twice the value. For example, if Fast Ethernet technology is used with a speed of 100 Mbps, then the speed can be close to 200 Mbps (100 Mbps - transmit and 100 Mbps - receive).
An illustrative example is a conversation between two people on a walkie-talkie (half-duplex mode) - when at one point in time a person either speaks or listens, and by telephone (full duplex) - when a person can both speak and listen.
Duplex communication is usually carried out using two communication channels: the first channel - outgoing communication for the first device and incoming for the second, the second channel - incoming for the first device and outgoing for the second.
In some cases, duplex communication using one communication channel is possible. In this case, the device, when receiving data, subtracts its sent signal from the signal, and the resulting difference is the sender's signal (modem communication over telephone wires, GigabitEthernet).
Wikimedia Foundation. 2010 .
See what "Full duplex" is in other dictionaries:
Double helix with Watson-creek duplex- Double helix, p. Watson Cry, duplex * double helix, p. Watsana kryka, duplex * double helix or d. h. DNA or Watson Crick h. or duplex model of Watson Crick, describing the structure of DNA as a helix, which is formed from two ... ... Genetics. encyclopedic Dictionary
full duplex mode- — [E.S. Alekseev, A.A. Myachev. English Russian explanatory dictionary of computer systems engineering. Moscow 1993] full duplex Simultaneous two-way transmission. (full) duplex… …
UTP cable with 8P8C connector (erroneously called RJ 45) used in 10BASE T, 100BASE T (x) and 1 Ethernet networks ... Wikipedia
Name: Teletype network Level (according to the OSI model): Applied Family: TCP / IP Port / ID: 23 / TCP Purpose of the protocol: virtual text terminal Specification: RFC 854 / STD 8 ... Wikipedia
Duplex and half-duplex modes of operation of transceiver devices (modems, network cards, walkie-talkies, telephones). In duplex mode, devices can transmit and receive information at the same time. In half-duplex mode, either transmit, or ... ... Wikipedia
Duplex and half-duplex modes of operation of transceiver devices (modems, network cards, walkie-talkies, telephones). In duplex mode, devices can transmit and receive information at the same time. In half-duplex mode, either transmit, or ... ... Wikipedia - network card network adapter network interface A computer component for connecting to a computer network. network adapter A peripheral device (card) that provides a connection between a computer and a LAN. ... ... Technical Translator's Handbook
The IEEE 802.3-2012 standard defines two modes of operation of the MAC sublayer:
● half duplex (half-duple x) - Uses the CSMA/CD method to access nodes to a shared environment. A node can only receive or transmit data at one time, subject to gaining access to the transmission medium;
● full duplex (full duplex) - allows a pair of nodes that have a point-to-point connection to simultaneously receive and transmit data. To do this, each node must be connected to a dedicated switch port.
Access method CSMA/CD
The basic idea of Ethernet was to use a bus topology based on coaxial cable. The cable was used as a shared transmission medium over which workstations connected to the network performed a bi-directional (in all directions) broadcast. Terminators (plugs) were installed at both ends of the cable.
Rice. 5.21 Ethernet network
Since a common transmission medium was used, control over the access of nodes to the physical medium was required. To organize the access of nodes to a shared transmission medium, we used carrier sense multiple access method with collision detection(Carrier Sense Multiple Access With Collision Detection, CSMA/CD).
The CSMA/CD method is based on competition(contention) nodes for the right to access the network and includes the following procedures:
● carrier control;
● collision detection.
Before starting the transmission, the network device must make sure that the data transmission medium is free. This is achieved by listening to the carrier. If the medium is free, then the device starts transmitting data. While a frame is being transmitted, the device continues to listen on the transmission medium. This is done in order to ensure that no other device has started data transmission at the same time. After the end of the frame transmission, all network devices must withstand the technological pause (Inter Packet Gap) equal to 9.6 μs. This pause is called frame interval and is needed to reset the network adapters and to prevent the exclusive capture of the environment by one network device. After the end of the technological pause, the devices have the right to start transmitting their frames, since Wednesday is free.
Network devices can start transmitting data at any time when they determine that the channel is free. If a device tries to start transmitting a frame, but detects that the network is busy, it is forced to wait until the transmitting node completes the transmission.
Rice. 5.22 Frame transmission on an Ethernet network
Ethernet is a broadcast medium, so all stations receive all frames transmitted over the network. However, not all devices will process these frames. Only the device whose MAC address matches the destination MAC address specified in the frame header copies the contents of the frame to an internal buffer. Then the device checks the frame for errors, and if there are none, it passes the received data to the overlying protocol. Otherwise, the frame will be discarded. The sending device is not notified whether the frame was successfully delivered or not.
In Ethernet networks, conflicts are inevitable ( collisions), because the possibility of their occurrence is inherent in the CSMA/CD algorithm itself. This is because there is some time between the moment of transmission, when the network device checks whether the network is free, and the moment the actual transmission begins. It is possible that some other device on the network will start transmitting during this time.
If several devices on the network start transmitting at approximately the same time, bitstreams coming from different devices collide with each other and become distorted, i.e. collision occurs. In this case, each of the transmitters must be able to detect a collision before it finishes transmitting its frame. Having detected a collision, the device stops transmitting the frame and amplifies the collision by sending a special sequence of 32 bits to the network, called jam-sequence. This is done so that all devices on the network can recognize the collision. After all devices have recognized the collision, each device is switched off for some randomly selected time interval (its own for each network station). When the time expires, the device can start transferring data again. When transmission resumes, the devices involved in the collision do not have data transmission priority over the rest of the devices on the network.
If 16 attempts to transmit a frame cause a collision, then the transmitter MUST stop trying and discard the frame.
Rice. 5.23 Ethernet collision detection
Collision domain
In half-duplex Ethernet technology, regardless of the physical layer standard, there is a concept collision domain.
Collision domain(collision domain) is a part of the Ethernet network, all nodes of which recognize a collision, regardless of in which part of the network it originated.
An Ethernet network built on repeaters and hubs forms one collision domain.
Recall that the repeater was an OSI physical layer device used to connect segments of a data transmission medium in order to increase the total length of the network.
Ethernet networks (10BASE2 and 10BASE5 specifications) based on coaxial cable used two-port repeaters linking two physical segments. The repeater worked as follows: it received signals from one network segment, amplified them, restored synchronization and transmitted to another. Repeaters did not perform complex filtering and other traffic processing, because were not smart devices. Also, the total number of repeaters and segments connected by them was limited due to time delays and other reasons.
Later, multiport repeaters appeared, to which workstations were connected with a separate cable. Such multiport repeaters are called "hubs". The reason for the emergence of multiport repeaters was as follows. Since the original Ethernet technology used coaxial cable and a bus topology as the transmission medium, it was difficult to install building cabling. Later, the international standard for structured building cabling defined the use of a star topology, in which all devices were connected to a single point of concentration using twisted-pair cables. Token Ring technology was well suited to these requirements, and therefore, in order to survive in the competition, Ethernet technology had to adapt to new requirements. This is how the 10BASE-T Ethernet specification was born, which used twisted-pair cables and a star topology as the transmission medium.
Hubs worked at the physical layer of the OSI model. They repeated the signals received from one of the ports to all other active ports, after restoring them, and did not perform any traffic filtering or other data processing. Therefore, the logical topology of networks built using hubs has always remained a bus one.
At one point in time, in networks built on repeaters and concentrators, only one node could transmit data. In the case of simultaneous receipt of signals in the common transmission medium, collision, which led to damage to the transmitted frames. Thus, all devices connected to such networks were in the same collision domain.
Rice. 5.24 Collision domain
With an increase in the number of network segments and computers in them, the number of collisions increased, and the network throughput decreased. In addition, the bandwidth of the segment was divided between all devices connected to it. For example, if ten workstations were connected to a segment with a bandwidth of 10 Mbps, each device could transmit on average at a rate of no more than 1 Mbps. The task arose network segmentation, i.e. dividing users into groups (segments) according to their physical location, in order to reduce the number of clients competing for bandwidth.
Switched Ethernet
The task of segmenting the network and improving its performance was solved using a device called bridge(bridge). The bridge was developed by Digital Equipment Corporation (DEC) engineer Radia Perlman in the early 1980s and was an OSI link-layer device designed to connect network segments. The bridge was invented a little later than routers, but since it was cheaper and transparent to network layer protocols (it worked at the link layer), it became widely used in local networks. Bridge connections ( bridging) are a fundamental part of the IEEE LAN standards.
The bridge worked according to the algorithm transparent bridge(transparent bridge), which is defined by the IEEE 802.1D standard. Before forwarding frames from one segment to another, it analyzed them and transmitted them only if such a transfer was really necessary, that is, the MAC address of the destination workstation belonged to another segment. Thus, the bridge isolated the traffic of one segment from the traffic of another and divided one large collision domain into several small ones, which increased the overall performance of the network. However, the bridge transmitted broadcast frames (for example, necessary for the operation of the ARP protocol) from one segment to another, so all devices on the network were in the same broadcast domain (broadcast domain).
The transparent bridge algorithm will be discussed in more detail in Chapter 6.
Switched Ethernet(Ethernet-switched network) is an Ethernet network, the segments of which are connected by bridges or switches
Rice. 5.25 Connecting two network segments using a bridge
Because bridges were typically two-port devices, they were only as efficient as long as the number of workstations in a segment was relatively small. As soon as it increased, congestion occurred in the networks, which led to the loss of data packets.
The increase in the number of devices connected in a network, the increase in the power of workstation processors, the emergence of multimedia applications and client-server applications demanded more bandwidth. In response to these growing demands, in 1990 Kalpana launched the first switch (switch), called EtherSwitch.
The switch is a multiport bridge and also operates at the data link layer of the OSI model. The main difference between a switch and a bridge is that it is more productive, can simultaneously establish several connections between different pairs of ports, and supports advanced functionality.
Rice. 5.26 Local area network built on switches
In 1993, Kalpana introduced Full Duplex Ethernet Switch (FDES) technology into its switches. Over time, with the development of Fast Ethernet technology, full-duplex operation became part of the IEEE 802.3 standard.
Operation in full duplex mode provides the possibility of simultaneous reception and transmission of information. only two devices are connected to the transmission medium. Reception and transmission are carried out on two different physical channels "point-to-point". For example, over different pairs of twisted-pair cable or different fibers of an optical cable.
This eliminates media collisions (no longer requires CSMA/CD as there is no media contention), increases the time available for data transmission, and doubles the usable channel bandwidth. Each channel provides transmission at full speed. For example, for the 10BASE-T specification, each channel transmits data at 10 Mbps. For the 100BASE-TX specification, at 100 Mbps. At the ends of a duplex connection, the connection speed is doubled because data can be transmitted and received at the same time. For example, in the 1000BASE-T specification, in which data is transmitted over channels at a rate of 1000 Mbps, the total throughput will be 2000 Mbps.
Rice. 5.27 Data transmission in full duplex mode
Also, thanks to the full duplex mode, the restriction on the total length of the network and the number of devices in it has disappeared. The only thing left is the limitation on the length of cables connecting neighboring devices.
Full duplex operation is possible only when connecting network devices whose ports support it. If a shared media segment is connected to a device port, the port will operate in half-duplex mode and recognize collisions. Ports of modern network devices support the function of auto-detection of half-duplex or full-duplex operation.
When the port is operating in full duplex mode, the sending interval between successive frames must not be less than the technological pause equal to 9.6 µs. In order to avoid overflowing the receive buffers of devices when operating in full duplex mode, it is required to use a frame flow control mechanism.
It should be noted that the 10, 40 and 100 Gigabit Ethernet specifications only support full duplex operation. This is due to the fact that modern networks have become fully switched, and switches almost always use full duplex mode when interacting with other switches or high-speed network adapters.
Lecture 4. Methods of network communication.
Network communication methods
Signals
As mentioned earlier, there are many ways to physically create and transmit a signal. Electrical pulses can travel through a copper wire, light pulses through a glass or plastic fiber, radio signals are transmitted through the air, and so are laser pulses in the infrared or visible range. Converting ones and zeros representing data in a computer into pulses of energy is called coding (modulation).
Similar to the classification of computer networks, signals can be classified based on their various characteristics. The signals are as follows:
analog and digital,
modulated and modulated,
synchronous and asynchronous,
simplex, half duplex, duplex and multiplex
Analog and digital signals
Depending on the form of electrical voltage (which can be seen on the oscilloscope screen), signals are divided into analog and digital. You are most likely already familiar with these terms, since they are quite often found in the documentation of various electronic equipment, such as tape recorders, televisions, telephones, etc. etc.
In a sense, analog equipment represents the outgoing era of electronic technology, and digital equipment represents the latest, coming to replace it. However, it should be remembered that one type of signal cannot be better than another. Each of them has its own advantages and disadvantages, as well as its own areas of application. Although digital signals are being used more and more, they will never replace analog signals.
Parameters of analog signals
Analog signals change smoothly and continuously over time, so they can be graphically represented as a smooth curve (Figure 4.1).
In nature, the vast majority of processes are fundamentally analog. For example, sound is a change in air pressure, which can be converted into electrical voltage using a microphone. Applying this voltage to the input of the oscilloscope, you can see a graph similar to that shown in Fig. 4.1, i.e. You can see how air pressure changes over time.
To better visualize analog information, think of a traditional speedometer in a car. As the speed of the vehicle increases, the needle moves smoothly along the scale from one number to the next. Another example is tuning to a station in a radio receiver: when you turn the knob, the received frequency changes smoothly.
Most analog signals are cyclic or periodic in nature, such as radio waves, which are high frequency oscillations of an electromagnetic field. Such cyclic analog signals are usually characterized by three parameters.
Amplitude. The maximum or minimum signal value, i.e. wave height.
Frequency. The number of cyclic signal changes per second. Frequency is measured in hertz (Hz); 1 Hz is one cycle per second.
Phase. The position of a wave relative to another wave or relative to some point in time that serves as a reference point. The phase is usually measured in degrees, and it is considered that the full cycle is equal to 360 degrees.
Parameters of digital signals
Another name for digital signals is discrete The term discrete states is quite common. Digital signals change from one discrete state to another almost instantly, without stopping in intermediate states (Fig. 4.2).
An example of a digital signal would be the latest digital speedometer in a car (compare with the analog speedometer example in the previous section). As the vehicle speed increases, the digits showing the speed value in kilometers per hour switch jumps, and the signal value is fundamentally discrete: for example, there are no intermediate values between the discrete states "125 km/h" and "126 km/h". Another example of digital information is the latest radio receiver, in which the user enters an exact number equal to the frequency of the radio station to tune in to a specific station.
Comparison of analog and digital signals
Computers are digital machines. The information they process is represented by zeros and ones. A binary digit is either 0 or 1, with nothing between or outside of them. Because of this fine definition, digital signals are very convenient for representing and transmitting computer data, which is why they are used in the vast majority of networks.
Due to the simplicity of the technology, digital signals have a number of advantages:
Digital equipment is generally cheaper than analog equipment.
Digital signals are less susceptible to interference.
However, analog signals also have some advantages:
They are easy to multiplex, i.e. transmit a large number of signals over a single channel.
They are less susceptible to attenuation (signal weakening with increasing distance), so with the same power of the transmitting device, they can be transmitted over a greater distance.
In general, both analog and digital signals are useful. However, in computer networks, digital signals can achieve greater levels of security, bandwidth, and reliability. In addition, digital lines are significantly less error prone than analog lines.
Local area networks are almost always based on the transmission of digital signals over cable. Analog signals are used in some wide area networks.
Modulated and unmodulated signals
An important characteristic of the transmission method is the channel capacity, which is directly related to the signal modulation. A digital signal is called unmodulated if the transitions from one discrete state to another are voltage surges in a cable or other medium. At the same time, in a modulated signal, the transition between discrete states is a change in the amplitude of the so-called carrier signal, which is a high-frequency voltage fluctuation.
The unmodulated signal occupies the entire communication channel. In addition to it, nothing more can be transmitted over the communication channel. An example of unmodulated signals are signals in an Ethernet cable.
If modulation is used, then one channel can transmit several digital signals at different carrier frequencies. In addition, not only digital, but also analog signals can be transmitted at different carrier frequencies. An example is a cable television system, where a single cable serves dozens of television channels, each with a different program.
Unmodulated signals
Unmodulated signals are quite simple: only one signal is transmitted through the cable at a time. Unmodulated is most often a digital signal, although it can be analog.
In computer and communication technology, mainly unmodulated digital signals are used. For example, a computer exchanges modulated digital signals with monitors, printers, keyboards, and so on. An example of the application of modulated digital signals is the ISDN (Integrated Services Digital Network) system, in which many signals are transmitted on separate channels over a single cable. Unmodulated signals can be transmitted in two directions, i.e. both transmitter and receiver can be installed on each end of the cable, working simultaneously.
Modulated signals
Using modulated signals, you can organize several communication channels over one cable, while each communication channel can operate at its own carrier frequency without interfering with other channels.
Modulated signals are unidirectional. This means that the signal is transmitted only in one direction: a transmitter is installed on one end of the cable, and a receiver is installed on the other. However, several channels in different directions can work simultaneously on one cable.
In addition to cable television, modulated signals are used in a DSL (Digital Subscriber Line) system in which data and voice are transmitted simultaneously over the same line, possibly via satellite or via radio waves.
To accommodate several communication channels on one line, multiplexing methods are used.
Multiplexing
Multiplexing refers to the simultaneous transmission of many signals over a single line. On the receiving side, the multiplexed signals are restored, i.e. are separated from each other. Let's go back to the cable TV example. The TV has a built-in signal decoder that singles out one channel and discards the rest. Thanks to this, the viewer can choose the desired program.
Many literature sources mention multiplexing methods only in relation to analog signals, but digital signals can also be multiplexed. The following basic multiplexing methods are used:
frequency division of channels (Frequency Division Method - FDM);
time division of channels (Time Division Method - TDM);
high density wavelength (Dense Wavelength Division Multiplexing - DWDM).
Frequency division channels
With frequency separation of channels occupying the same line, each channel operates at its own frequency (Fig. 4.3). Usually analog signals are multiplexed by this method. In order for two-way communication to be possible with frequency division, both a multiplexer and a demultiplexer must be installed on each side.
Temporary channel separation
Typically, this technique is used to multiplex digital signals. With time division, each channel is allocated its own time intervals. At the receiving end, the signals of different channels are separated by a demultiplexer (Fig. 4.4).
High Density Wavelength Multiplexing
This multiplexing method is used when transmitting signals over fiber optic cables. The signals of each channel are transmitted by a light beam with its own wavelength. Physically, this method coincides with frequency division of channels, since the wavelength of a light beam is uniquely related to its frequency. However, the differences in the hardware implementations of these methods are so great that they are still considered as separate methods. As shown in Fig. 4.5, different data can be transmitted simultaneously over one optical fiber, and by different methods (for example, SONET and ATM).
Asynchronous and synchronous transmission
The data embedded in a digital signal is actually represented by changes in the discrete states of the signal. We can restore our original zeros and ones by measuring the voltage with a voltmeter at certain points in time. However, you need to know exactly at what time points you should take measurements. Synchronization, i.e. timing is as important in communication technology as it is in all other areas of our lives.
In network technology, this timing is called bit synchronization. Electronic devices synchronize individual bits using asynchronous or synchronous methods.
Asynchronous transfer
This method uses the start bit located at the beginning of each message for synchronization. When the start bit hits the receiving device, it at that moment synchronizes its internal clock with the clock of the transmitting device.
Synchronous transmission
In synchronous transmission, the internal clocks of the transmitting and receiving devices are coordinated by built-in mechanisms. For example, time information may be embedded in the data signals. This method is called state-guaranteed synchronization. Among the synchronous methods, this is the most common.
Another synchronous method is synchronization using a separate time signal, in which time information is transmitted between the transmitter and receiver on a separate channel. Another synchronous method is gating. In this case, synchronization is performed using special strobe pulses.
Simplex, half duplex and full duplex transmission methods
Channels carrying data signals can operate in one of three modes: simplex, half-duplex, and full-duplex. These methods differ in the directions in which signals are transmitted.
Simplex transmission
As the name suggests, this is the simplest transfer method. It is sometimes called unidirectional because the signals only travel in one direction, like cars on a one-way street (Figure 4.6).
Television is an example of simplex communication. The data (TV programs) are transmitted to the TV. No signals are sent from the TV back to the studio or cable company. Therefore, the TV set includes only a signal receiver, but not a transmitter.
Currently, interactive television systems are becoming more widespread, allowing you to transmit signals not only from the studio to the TV, but also in the opposite direction. However, most companies' cable equipment still only supports simplex transmission. This created a serious problem with the advent of the Internet. The existing cable system was only able to transmit data in one direction, towards the user.
This shortcoming makes it impossible, for example, for a user to access Web pages, because the user's browser must send its request to the Web site. Cable companies offer two ways to solve this problem:
transmit user requests (which are always much shorter than Web pages) over telephone lines and Web pages over television cables;
install new cable equipment with two-way transmission.
Most companies used the first method as a temporary alternative to the second, more advanced one. If the cable transmission system is left as simplex, then the user will have to bear the costs only for the purchase of cable and telephone modems (with a capacity of the latter no more than 56 Kbps.) In this case, the resources of a high-speed cable channel will be fully used.
Many cable companies are immediately upgrading their equipment to support two-way communication, while others still only provide one-way Internet data over TV cable. In these areas, customers are forced to use both cable and analog modems connected to the telephone line.
Half duplex transmission
Compared to simplex, the advantages of half-duplex transmission are obvious: signals can be transmitted in both directions. Unfortunately, however, this road is not wide enough for signals to pass in both directions at the same time. In the half-duplex method, signals are transmitted in only one direction at a time (Fig. 4.7).
The half-duplex method is used in many radio communication systems, such as communication devices in police vehicles. In these systems, when the microphone button is pressed, you can speak, but you cannot hear anything. If users press the microphone buttons at both ends at the same time, neither of them will hear anything.
duplex transmission
The operation of a duplex communication system is like a two-way street: cars can move in both directions at the same time (Figure 4.8).
An example of duplex communication is a normal telephone conversation. Both subscribers can speak at the same time, while each of them hears what the other on the other end of the line is saying (although it is not always possible to make out what was said).
Problems in Signaling
The signals that computers communicate with are subject to various interferences and limitations. Different types of cables and transmission methods have different susceptibility to interference.
Electromagnetic interference
Electromagnetic interference is the intrusion of an extraneous electromagnetic signal that disrupts the shape of a useful signal. When external interference is added to the useful signal, the receiving computer cannot correctly interpret the signal.
Imagine that you are driving in a car next to a powerful industrial installation and listening to the radio at this time. A clean and legible signal is suddenly covered with noise and crackling. This is because strong signals are added to the radio signal from a setup that is closer than the radio. Therefore, electromagnetic interference is sometimes called noise.
Quite often, interference comes from an unknown source. There are many Devices in which electrical signals do not perform informational functions, but are a by-product of various production processes. The interference they create can extend over distances of up to several kilometers.
Electromagnetic interference causes problems not only in computer communication technologies. In cities, there are many devices that transmit and receive electromagnetic signals: mobile phones, radio communications, television transmitters and receivers. Electromagnetic interference can cause many problems, such as poor television picture, aircraft crash due to communication failure with the controller, death of a patient due to malfunction of medical equipment, etc. There are also long-term side effects of electromagnetic radiation, such as cancer or leukemia, which can be caused by prolonged exposure of a person to a powerful source of electromagnetic fields.
In communications technology, unshielded copper wires are particularly susceptible to electromagnetic interference. The metal outer jacket of coaxial cables largely protects them from interference. The same function is performed by the metal sheath of the shielded twisted pair cable. Unshielded twisted pair is quite susceptible to interference. Fiber-optic cables are completely insensitive to electromagnetic interference, because the signals in them are not electrical impulses, but a beam of light. Therefore, in conditions of strong electromagnetic interference, fiber-optic communication channels work best.
RF interference
Radio frequency interference is the signals from radio transmitters and other devices that generate signals at radio frequencies. They also include processors and computer displays. Radio frequency is electromagnetic radiation at frequencies from 10 kHz to 100 GHz. Radiation at frequencies from 2 to 10 GHz is also called microwave.
The influence of radio frequency interference is eliminated with the help of anti-interference filters used in various types of networks.
Crosstalk
This type of interference includes signals from wires located at a distance of several millimeters from each other. An electric current flowing through a wire creates an electromagnetic field that generates signals in another wire located nearby. Quite often, while talking on the phone, you can hear the muffled conversations of other people. The reason for this is crosstalk.
Crosstalk is greatly reduced if two wires are twisted together, as is done in twisted pair. The more turns per unit length, the less the effect of interference. The use of fiber optic cable completely eliminates this problem. Any number of optical fibers can be placed inside one shell, and they will not interfere with each other, because the signals in them are not electrical impulses, but light rays.
Signal attenuation
Passing through the cable, electrical signals become weaker and weaker. The greater the distance to the source, the weaker the signal. This is easy to imagine, imagining that you are trying to say something to a person who is at some distance from you. If he is 5 meters away, he will hear your voice (signal) clearly and loudly, but if he is 50 meters away, he will hardly understand what you are shouting to him about. This signal attenuation with distance is called signal attenuation.
Attenuation is the reason why the specifications of various network architectures specify a limit on the length of the cable. If this limitation is respected, then the fading effect will not affect the normal operation of the communication link.
As the frequency increases, the attenuation increases, because the higher the frequency of the signal, the more intense the dispersion of its electromagnetic energy into the surrounding space. As the frequency increases, the wire itself turns from a signal carrier into an antenna, dissipating its energy into space.
Signals in a fiber optic cable are also subject to attenuation. The two main reasons are the absorption of the light beam by impurities in the glass and the scattering of the beam due to small changes in the optical density of the glass formed during its production. However, fiber optic cables can carry a signal over much longer distances than copper cables without reducing signal strength to an unacceptable level.
Bandwidth
The bandwidth of a communication channel is usually measured in megabits per second (Mbps). Bandwidth is affected by the type of signal, the type of medium, and the distance over which the signal is transmitted.
The concepts of high and low bandwidth are very relative. For example, a lOBaseT Ethernet throughput of 10 Mbps seems very high compared to the throughput of a telephone modem (50 Kbps), while at the same time it seems frustratingly low compared to Gigabit Ethernet (1 Gbps) or high-speed WAN connections such as SONET and ATM.
An important criterion when choosing the type of cable and network architecture is the required (both now and in the future) throughput.
Network Growth Planning
At the network planning stage, it must be remembered that bandwidth is a resource that is always not enough. Purchasing equipment with higher bandwidth than currently needed is a good investment: the extra cost will definitely pay off.
Computer and communication technologies are developing rapidly. In the 1980s, typical WAN links had a bandwidth of 10 Kbps, and local networks had a bandwidth of 2.5 Mbps. At that time, no one even imagined that someday it would be necessary to transmit anything at a speed greater than 100 Mbps. After all, technologies such as videoconferencing, voice transmission, or the transfer of large files that are now widespread did not yet exist.
It is much easier and cheaper to lay a cable with increased bandwidth than to replace the cable with a new one later. Let's say you are installing a 10BaseT network, for which Category 3 cable with a bandwidth of 10 Mbps is sufficient. Buying category 3 cable instead of category 5 will save you a few dollars. However, in a few years, when you need to upgrade your network to 100 Mbps (which will almost certainly happen), you will have to replace all the cables. This will cost significantly more than if you bought and installed Category 5 cable right away.
Network access methods
There are several different access methods to suit different network architectures and topologies. The following methods are most widely used:
passing the token (relay access);
request priorities.
CSMA/CD method
Currently, the most common LAN access control method is CSMA/CD (Carrier Sense Multiple Access with Collision Detection). The prevalence of the CSMA/CD method is largely due to the fact that it is used in the most common Ethernet architecture today.
This is a very fast and efficient method of providing access to an Ethernet cable. To understand how it works, let's look at fragments of its name separately.
Media control. When a computer is about to send data to the network using the CSMA/CD method, it must first check if another computer is transmitting its data on the same cable at the same time. In other words, check the status of the carrier: is it busy transferring other data.
Multiple access. This means that several computers can start sending data to the network at the same time.
Conflict detection. This is the main task of the CSMA/CD method. When the computer is ready to transfer, it checks the status of the media. If the cable is busy, the computer does not send signals. If the computer does not hear other people's signals in the cable, it starts transmitting. However, it may happen that two computers listen to the cable and, not detecting signals, both start transmitting at the same time. This phenomenon is called signal collision.
When signals collide in a network cable, the data packets are destroyed. However, not all is lost. In the CSMA/CD method, computers wait for a random period of time and resend the same signals. Why should the time interval be random? If both computers wait for some fixed number of milliseconds, then their waiting times can coincide and everything will be repeated from the beginning. The computer that first repeats the transmission of the packet (whose random time period turned out to be shorter) kind of "wins" access to the network in roulette.
The probability of conflicts is low, since they occur only if the beginnings of the packets match, i.e. very short periods of time. Since the signals are transmitted at high speed (in Ethernet - 10 or 100 Mbps), the performance remains high.
The implementation of the CSMA/CD method is defined by the IEEE 802.3 specifications.
CSMA/CA method
The name of the method stands for Carrier Sense Multiple Access with Collision Avoidance (multiple access with media control and conflict avoidance).
CSMA/CA is a more "distrustful" method. If the computer does not find other signals in the cable, it does not conclude that the path is clear and you can send your precious data. Instead, the computer first sends a Request to Send (RTS) signal. By this, he announces to other computers that he intends to start transferring data. If another computer does the same at the same time, then there will be a conflict of signals, not data packets. Thus, data packets can never collide. This is called conflict prevention.
At first glance, the method with conflict prevention is much more advanced than with detection. However, its performance is lower due to the fact that, in addition to the data, it is necessary to send KTS signals, the vast majority of which are not needed. In fact, the number of signals arriving on the cable almost doubles.
The CSMA/CA method is used in AppleTalk networks.
Passing the token
Is there an access method that works without signal conflicts at all? There is such a method: it is a token-passing method.
The token passing method is non-competitive. In this method, two computers cannot start transmitting a signal at the same time. The method works like a seminar, where the participant cannot begin to speak until he is given the floor. Similarly, a computer on a token-passing network does not signal until the token passes to it.
Telecommunication systems by types of communication, as well as modes of data transmission and reception are divided into the following types of communication:
Simplex communication
Simplex communication- this is a one-way communication between two subscribers, in which the direction is carried out in one direction and via the same communication channel. Those. at simplex communication the second subscriber, to whom the message or message is sent, can neither answer nor confirm anything, but only listen.
Half duplex
Half duplex- this is a two-way communication between two subscribers, in which, over the same communication channel, data is received and transmitted alternately. The first subscriber sends a message and must release his channel. The second, having received a message, sends (sends) a response message via the same channel. And this can go on for as long as you like. Movies often feature dialogue like this:
- First, this is an iceberg - RECEPTION
— Iceberg, I heard your message, OVER
— End of communication.
In this example half duplex the word "RECEPTION" just means that the message has been sent and you can switch to the response mode.
duplex communication
duplex communication- This is a two-way communication that can be carried out simultaneously. Those. two subscribers can both receive and send a message via one communication channel. Various telephone conversations are a great example duplex communication. In practice, there is basically a separate communication channel for receiving and transmitting.
In most cases, the communication channel provides the means for one-way data transfer. With the help of just one communication line, it is possible to ensure the implementation of several communication channels at once. Such a connection is called multichannel.
