the main / Navigators / Switching (computer networks). The terminal device consisting of a monitor and keyboard used to exchange data with a computer The device commuting several link channels is called a concentrator.

Switching (computer networks). The terminal device consisting of a monitor and keyboard used to exchange data with a computer The device commuting several link channels is called a concentrator.

Circuit Switching and Packet - IT "S Methods for Solving The Generalized Problem of Switching Data in Any Network Technology. Complex Technical Solutions of the Generalized Switching Tasks in Its Entirety Consists of the Particular Problems of Data Transmission Networks.

By The Special Problems of Data Networks Include:

dEFINE FLOWS AND APPROPRIATE ROUTES;
fixation Route Configuration Parameters, and Tables of Network Devices;
rECOGNITION FLOWS AND DATA TRANSFER BETWEEN ONE DEVICE INTERFACE;
multiplexing / Demultiplexing Streams;
separation medium.

Among The Many Possible Approaches to the Solution of the Generalized Problem of Subscribers Switching Networks Allocate Two Basic, Which Include Channel Switching and Packet Switching. Thus There Are Traditional Applications of Each Switching Techniques, For example, Telephone Networks Continue to Be Built and Constructed using Circuit Switched Technology, Computer Networks and The Vast Majority Are Based on Packet Switching Technique.

Theraefore, AS Information Flow in Circuit-Switched Networks Are The Data Exchanged Between A Pair of Subscribers. Accordingly, The Global Flow Feature Is A Pair of Addresses (Telephone Numbers) Subscribers Communicate with Each Other. One Feature of Circuit-Switched Networks Is The Concept of An Elementary Channel.

Elementary Channel.

Elemental Channel (Or Channel) - Is a Basic Technical Characteristics of the Circuit Switched Network, Which Is Fixed Within A Given Type of Network ThroughPut Value. Every Link In The Circuit Switched Network Has A Capacity Of A MultiPle Channel Elementary Adopted for This Type of Network.

In Traditional Telephone Systems The Value of the Elementary Channel Speed \u200b\u200bIS Equal to 64 Kbit / S, Which Is Sufficient for High-Quality Digital Voice.

For High-Quality Voice Use the Frequency of Sound Vibrations Amplitude Quantization 8000 HZ (Sampling Time 125 MS Intervals). To Represent A Measure of the Amplitude is Most Often Used 8-bit Code, Which Makes 256 Tone Gradation (by sampling values).

In This Case, The Transmission of One Voice Channel Is Needed Bandwidth 64 Kbit / S:

8000 x 8 \u003d 64000 BITS / S or 64 kbit / s.

SUCH A VOICE CHANNEL IS CALLED AN ELEMENTARY CHANNEL DIGITAL TELEPHONE NETWORKS. A Feature of the Circuit Switched Network is That The Bandwidth of Each Link Must Be Equal To An Integer Number of Elementary Channels.

THE COMPOSITE CHANNEL

Communication Constructed by Switching (Connection) of Elementary Channels, Called A Composite Channel.

Composite Channel

THE PROPERTIES OF THE COMPOSIE CHANNEL:

composite Channel Throughout ITS Length Is Made Up of The Same Number of Elementary Channels;
composite Channel Has a Constant and Fixed Bandwidth Throughout Its Length;
composite Channel Is Created Temporarily for The PERIOD OF THE SESSION TWO SUBSCRIBERS;
at the Session, All The Basic Channels That Are Included in The Composite Channel, ENTER THE EXCLUSIVE USE OF SUBSCRIRS, FOR WHICH THE CONPOSE CHANNEL HAS BEEN CREATED;
during The Communication Session In The Subscribers CAN Send Network Data Rate Not Exceeding A CHANNEL CAPACITY OF THE COMPOSITE;
data Received in A Composite Channel, The Called Subscriber Is Guaranteed to Be Delivered Without Delay, Losses, and at the Same Rate (Source Rate) Regardless of Whether Here Is At This Time in the Other Network Connection or Not;
after the End of the Session The Basic Cronesponding Composite Channel, Declared Free and Returned to the Pool of Resources Allocated for Use by Other Users.

Connection refused

Connection Requests Are Not Always Successful.

If The Path Between The Calling and Called Subscribers Are No Free Calnels or Called Basic Node Is Busy, The Malfunction Occurs in the Connection Setup.

THE ADVANTAGE OF CIRCUIT SWITCHING

Circuit Switching Technology Is Aimed At Minimizing The Accidental Events in The Network, Ie a Technology. In Order to Avoid Any Possible UNCERTAINTY MUCH OF THE WORK ON INFORMATION EXCHAND IS CARRIED OUT IN ADVANCE, EVEN BEFORE THE START THE DATA TRANSFER. FIRST, FOR A GIVEN ADDRESS, THE AVAILABILITY OF THE REQUIRED BASIC CHANNELS ALL THE WAY FROM THE SENDER TO THE RECIPIENT. But in the Case of Bursty, This Approach Is Inefficient, Since 80% of The Time Channel May Be IDle.

Packet Switching.

The most Important Principle of The Networks with a Packet Switched Data Submission IS Transmitted Over The Network in The Form of Structurally Separated from the Each Other Pieces of Data Called Packets. Each Packet Has a Header, Which CONTAINS THE DESTINATION ADDRESS, AND OTHER SUPPORTING INFORMATION (LENGTH OF THE DATA FIELD, A CHECKSUM, AND OTHERS.), USED FOR THE DELIVERY TO THE ADDRESSEE OF THE PACKAGE.

Having Address in Each Packet Is One of the Most Important Features of Packet Switching Technology, Since Each Packet Can Be Processed Independently of The Other Switch Packets Constituting The Network Traffic. INDITION TO TITLE IN THE PACKAGE MAY HAVE ONE ADDITIONAL FIELD TO BE PACKED AT THE END OF THE PACKAGE AND SO-CALLED TRAILER. In The Trailer Is Usally Placed Checksum, Which Allows You to Check Whether The Information Has Been Corrupted During Transmission Over The Network or Not.

Partitioning the Data Into Packets

Partitioning the Data Into Packets Takes Place in Several Stages. Chain Sender Node Generates Transmission Data, Which Is Divided Into Equal Parts. Aftert That Occurs The Formation of A Package by Adding the Header Overhead. And The Last Stage Is Assembled Packets Into The Original Message To the Destination Node.

Partitioning the Data Into Packets

TRANSFERRING DATA OVER A NETWORK AS A PACKET

Packet Transmission Network.

AS IN THE CIRCUIT SWITCHED NETWORKS, PACKET SWITCHED NETWORKS, FOR EACH OF THE STREAMS IS DETERMINED MANUALLY OR AUTOMATICALLY ROUTE FIXED IN THE STORED TABLES FOR COMMUTION SWITCHES. Packets Entering The Switch Are Processed and Sent On a Particular Route

UNCERTAINTY AND ASYNCHRONOUS MOVEMENT OF DATA IN PACKET-SWITCHED NETWORKS MAKES SPECIAL DEMANDS ON THE SWITCHES IN SUCH NETWORKS.

The Main Difference Between A Paccuit Switch of The Switches In The Circuit-Switched Networks Is That They An Internal Buffer Memory for Temporarily String Packets. Switch Buffers Need to Harmonize Data Rates In Communication Links Connected to Its Interfaces, As Well As to Harmonize The Rate of Arrival Packets with ther Switching Speed.

Methods of TRANSFER PACKAGES

A Switch Can Operate On The Basis of One Of Three Methods Promote Packages:

datagram Transmission;
TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION;
TRANSFER TO THE ESTABLISHMENT OF A VIRTUAL CHANNEL.

Datagram Transmission

Datagram Transfer. Method Based on the Promotion of Packet Independent from Each Other. Packet Processing Procedure Is Only Determined by The Values \u200b\u200bof Parameters That It Carries and The Current State of the Network. AND EVERY SINGLE PACKET NETWORK IS CONSIDERED AS A COMPLEYLY INDEPENDENT UNIT TRANSFER - DATAGRAM.

Illustration Datagram Packet Principle

TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION

Procedure for Harmonization of The Two End Nodes of A Network of Some Paramers of Packet Exchange Process IS Called The Establishment of a Logical Connection. Options NEGOTIATED by The Two Interacting Nodes, Called A Logical Connection Parameters.

Virtual Channel

The Only Pre-Padded Fixed Route Connecting End Nodes to the Packet-Switched Network, Referred to As A Virtual Channel. (Virtual Circuit or Virtual Channel) Virtual Channels Are Laid for Sustainable Information Flow. In Order to Isolate The Data Flow of the Total Traffic Flow of Each Packet Is Marked With A Special Kind of Sign - Label. As With the Establishment of A Logical Network Connections, The Virtual Channel Begins with a Special Package - The Connection Request.

Table Switching Networks using Virtual Channels Is Different from the Switching Table in Datagram Networks. IT Contains Entries Only Passing Through The Switch Virtual Channels, And Not All The Possible Destination Address, As Is the Case in Networks with Datagram Algorithm Transfer.

COMPARISON CIRCUIT-SWITCHED AND PACKET

Switching Channels.	Packet Switching.
You Must First Establish A Connection	No Stage of Establishing A Connection (Datagram Method)
Location Is Only Required WHEN ESTABLISHING A Connection	ADDRESS AND OTHER SERVICE INFORMATION ARE TRANSMITTED WITH EACH PACKET
The Network May Refuse a Connection to the Subscriber	The Network Is Always Ready To Receive Data From Subscriber
Guaranteed Bandwidth (Bandwidth) for Interacting Subscribers	Network Bandwidth for Users Is Unknown, Transmission DELAYS ARE RANDOM
Real-Time Traffic Is Transferred Wort Delay	Network Resources Are Used Effectively WHEN TRANSMITTING BURSTY TRAFFIC
High Transmission Reliability	Possible Data Loss Due to Buffer Overflow
IRRATIONAL USE OF CHANNEL CAPACITY, REDUCING THE OVERALL EFFICIENCY OF THE NETWORK	Automatic Dynamic Bandwidth Allocation Of A Physical Channel Between Subscribers

Lecture number 8.

Characteristics of information channels

Information channel can also be characterized by three relevant parameters: channel use time T K. , bandwidth bands transmitted by the canalF K., I. dynamic range canalD K. characterizing his ability to transmit various levels Signal.

Validates capacity Channel.

Unlisted signaling is possible only if the signal is "fitted" in its volume into the channel capacity.

Consequently, the overall condition for negotiating the signal with information channel is determined by the ratio

However, the ratio expresses the necessary, but insufficient condition for matching the signal with the channel. A sufficient condition is the coordination in all respects:

For information canal Enjoy the concepts: information entry speed, information transfer rate and channel bandwidth.

Under the speed of information entry (information flow) I. ( X. ) Understand the average amount of information entered from the source of messages to the information channel per unit of time. This feature of the message source is determined only by the statistical properties of messages.

Information transfer rate I. ( Z. , Y. ) - The average amount of information transmitted via channel per unit time. It depends on the statistical properties of the transmitted signal and on the properties of the channel.

Bottomy S. - The largest theoretically achievable information transfer rate for this channel. This is the characteristic of the channel and does not depend on the statistics of the signal.

The bandwidth of the information channel is determined by two parameters: bit and frequency. It is proportional to their work.

Bottomy Call the maximum amount of information that can be simultaneously placed in the channel.

Frequency Shows how many times the information can be placed in the channel for a unit of time.

The discharge of the postal channel is enormous. So, shipping by mail, for example, a laser disk, can be placed simultaneously in the channel of more than 600 MB of information. At the same time, the mail channel frequency is very low - the removal of mail from the boxes occurs more often than five times a day.

Telephone channel information SingleBody: Simultaneously on the telephone wire you can send or unit (current, impulse), or zero. The frequency of this channel can reach tens and hundreds of thousands of cycles per second. This property of the telephone network allows you to use it for communication between computers.

In order to make the most efficient use of the information channel, it is necessary to take measures to ensure that the information transfer rate is as close as possible to bandwidth Channel. At the same time, the speed of entering information should not exceed the channel bandwidth, otherwise not all information will be transferred via the channel.

This is the basic condition for the dynamic coordination of the source of messages and the information channel.

One of the main issues in the theory of information transfer is to determine the dependence of the speed of transmitting information and bandwidth from the channel parameters and characteristics of signals and interference. These questions were for the first time deeply studied by K. Shannon.

1. Methods of increasing noise immunity

In the basics of all methods of increasing the noise immunity of information systems, the use of certain differences between the useful signal and an interference. Therefore, a priori information about interference and signal properties is necessary to combat interference.

Currently, a large number of ways to increase the noise immunity of systems are known. These ways are convenient to break into two groups.

I. The Group is based on choosing a messaging method.

II. The Group is associated with the construction of noise-resistant receivers.

Simple and applied method of increasing noise immunity is increase signal / interference ratio By increasing the power of the transmitter. But this method may be economically unable, as it is associated with a significant increase in the complexity and cost of equipment. In addition, an increase in the transmission capacity is accompanied by the enhancement of the interfering effect of this channel to others.

An important way to increase the noise immunity of transmission of continuous signals is rational choice of modulation type signals. Applying modulation types that provide a significant extension of the signal frequency band, one can achieve a significant increase in the noise immunity of the transmission.

Radical way to increase the noise immunity discrete signals Is use special noise-resistant codes . At the same time there are two ways to increase the noise immunity of the codes:

1. The choice of transmission methods that provide a smaller probability of code distortion;

2. Increase the correction properties of code combinations. This path is associated with the use of codes that allow you to detect and eliminate distortions in code combinations. This encoding method is associated with the introduction of additional, redundant symbols into the code, which is accompanied by an increase in the transmission time or frequency of transmission of code characters.

An increase in the noise immunity of transmission can also be achieved by re-transmitting the same message. On the receiving side, the received messages are compared and those that have the greatest number of coincidences are taken as true. To eliminate uncertainty when processing adopted information and ensure the selection by the criterion of the majority, the message should be repeated at least three times. This method of increasing noise immunity is associated with increasing transmission time.

Systems with repetition of discrete information are divided into systems with group summation, in which the comparison is made according to code combinations, and on the system with seductive summation, in which the comparison is carried out by code combinations in symbols. Suitable check is more efficient than groupage.

A variety of systems that have an increase in noise immunity due to an increase in transmission time are systems with feedback. If there are distortion in the transmitted messages, the information coming over the reverse channel provides a repetition of the transmission. The presence of a reverse channel leads to a complication of the system. However, in contrast to the transmission repetition systems in feedback systems, the transmission repetition will take place only in case of distortion in the transmitted signal, i.e. Redundancy generally turns out to be less.

Noise resistant reception It consists in using redundancy, as well as a priori information about signals and interference to solve the optimal method of receiving task: signal detection, signal differences or message recovery. Currently, the apparatus of the theory of statistical solutions is widely used for the synthesis of optimal receivers.

The receiver errors decrease with an increase in the signal / interference ratio at the receiver input. In this regard, it often produces preliminary processing of the received signal in order to increase the relationship of the useful component to interference. Such methods of pre-processing signals include the show method (combination of broadband amplifier, limiter and narrowband amplifier), the selection of signal signals, the measurement method, the filtering method, the correlation method, the accumulation method, etc.

2. Modern technical means Data exchange and channel-forming equipment

The receiver can be a computer, terminal or any digitally device.

To ensure the transfer of information from the computer to communication

This may be a database file, a table, response to a request, text or image.

For messaging in computing networks are used different types Communication channels. The most common selected telephone channels and special channels for transmitting digital information are most common. Radio channels and satellite channels are also used.

A mansion in this regard is a LAN, where the transmission medium uses twisted pair of wires, coaxial cable and fiber optic cable.

To ensure the transmission of information from the computer to the communication environment, it is necessary to coordinate the signals of the internal interface of the computer with the parameters of the signals transmitted through the communication channels. At the same time, both physical coordination (shape, amplitude and duration of the signal) and code can be performed.

Technical devices performing computer pairing functions with communication channels are called adaptersor network adapters. One adapter to ensure compilation with a computer of one communication channel. In addition to single-channel adapters, multichannel devices are used - data Multiplexersor simply Multiplexers.

Data transfer multiplexer - A compilation device with multiple communication channels.

Data Multiplexers were used in data telework systems - the first step on the way to creating computing networks. In the future, in the appearance of networks with a complex configuration and with a large number of subscriber systems, special connection processors began to be applied to implement the interface functions.

As mentioned earlier, it is necessary to transform the bits to the analog channels to transmit digital information on the communication channel, and when receiving information from the communication channel to the computer, perform the opposite action - convert analog signals into the bits that can process computers. Such transforms performs a special device - modem.

Modem- device performing modulation and demodulation of information signals when transmitting them from computer to communication channel and when receiving a computer from a communication channel.

The most expensive component of the computing network is the communication channel. Therefore, when building a number of computing networks, they are trying to save on communication channels by commuting several internal communication channels to one external. Special devices are used to perform switching functions - hubs.

Concentrator - A device that switches several communication channels to one by frequency separation.

In LAN, where the physical transmission medium is a limited length cable, special devices are used to increase the length of the network - repeators.

Repeater - A device that ensures the preservation of the shape and amplitude of the signal when transmitting it to more than is provided by this type of physical transmitting medium, distance.

There are local and remote repeaters. Local Repears allow you to connect network fragments located at a distance of up to 50 m., and distance - up to 2000 m.

The most common types of network topologies:

Linear network. It contains only two terminal nodes, any number of intermediate nodes and has only one path between any two nodes.

Ring network. The network in which two and only two branches are attached to each node.

Tree network. The network that contains more than two terminal nodes and at least two intermediate nodes, and in which there is only one path between two nodes.

Star network. The network in which there is only one intermediate node.

Cheap network. A network that contains at least two nodes having two or more paths between them.

Faln beloved network. The network in which there is a branch between any two nodes. The most important characteristic of the computer network is its architecture.

Network architecture - This is a implemented data network structure defining it. topology, device composition and rules of their interaction on the network. As part of the network architecture, issues of information coding, addressing and transferring, messaging management, error control and network analysis in emergency situations and deterioration of the characteristics are considered.

The most common architectures:

Ethernet (eng. ether - Ether) - broadcast network. This means that all network stations can receive all messages. Topology - linear or star-shaped. Data rate of 10 or 100 Mbps.
Arcnet (ATTACHED RESOURCE COMPUTER NETWORK - Computer network of connected resources) - broadcast network. Physical topology - wood. Data transfer rate of 2.5 Mbps.
Token Ring. (relay ring network, network with a marker transmission) - a ring network, in which the data transmission principle is based on the fact that each knot of the ring awaits the arrival of some short unique bits sequence - marker - From the adjacent previous node. Marker's arrival indicates that you can send a message from this node further along the course of the flow. Data transfer rate 4 or 16 Mbps.
FDDI (Fiber Distributed Data Interface) - Network architecture of high-speed data transmission over fiber-optic lines. Transmission speed - 100 Mbps. Topology is a double ring or mixed (with the inclusion of star-like or tree subnets). The maximum number of stations in the network is 1000. Very high cost of equipment.
Atm (ASYNCHRONOUS TRANSFER MODE) - promising, while still very expensive architecture, ensures the transfer of digital data, video information and voices according to the same lines. Transmission rate up to 2.5 Gb / s. Optical communication lines.

Computer network hardware.

1.computers;

2. Compound interface devices with communication channel;

3. Communication channels

4. Devices connecting (commuting) communication channels

5. Devices connecting local networks.

EUMM pairing devices with communication channel

Technical device performing computer pairing functions with communication channel is called adapter , or a network adapter. One adapter provides compilation with a computer of one communication channel.
In addition to single-channel adapters, multi-channel pairing devices are used - multiplexers. Multiplexers - This is an EV pairing device with multiple communication channels.
To transmit digital information, you need a bits flow to convert to analog signal. And when receiving, perform the reverse transformation. Such conversions perform a modem. Modem - A device that performs modulation and demodulation of information signals when transmitting them from computer to a communication channel and when receiving a computer from a communication channel.

Network cables

(coaxial consisting of two isolated concentric conductors from which external has a view of the tube;
fiber optic ;
cables on twisted pairs Educated by two overwhelmed wires, etc.).

Devices connecting (commuting) communication channels

The most expensive component of the Sun is a communication channel. Therefore, when building computing networks, they try to save on communication channels by commuting several internal communication channels to one external. Special devices are used to perform the switching function - hubs.

Huba (hubs) I. commuting hubs (switters) Expand topological, functional and high-speed capabilities of computer networks. Hub with a set of differential ports allows combine network segments with different cable systems . You can connect both a separate network node and another hub or cable segment.
In LAN where the transmission medium is a limited length cable, special devices are used to increase the length of the network - repeaters. Repeater - A device that ensures the preservation of the shape and amplitude of the signal when transmitting it to more than is provided by this type of physical transmitting medium, distance. The local repeater connects the network fragments up to 50 m. And remote - up to 2000 m.

Connections local networks

To connect local networks, the following devices are used, which differ from each other by destination and capabilities:

· Bridge (eng. Bridge) - connects two local networks. Transmits data between networks in packet form without making any changes. Bridges can filter packages, Protecting the entire network from local data streams and passing out only those data that are intended for other network segments.

· Router (eng. Router.) Combines networks with a common protocol more efficiently than the bridge. It allows, for example, split large messages into smaller pieces, thereby ensuring the interaction of local networks with different package size.

The router can send packets to a specific address (bridges only filter out unnecessary packets), choose the best way to pass the package and much more. Than more difficult I. more networkThe greater the benefit from the use of routers.

· Bridge router (eng. Brouter.) - This is a hybrid of a bridge and a router, which first tries to perform routing, where it is only possible, and then, in case of failure, goes into the bridge mode.

· Gateway (eng. Gateway.), unlike the bridge, applies in cases where the network connected has various network protocols . The message received from a single network in the gateway is converted to another message corresponding to the requirements of the next network. Thus, the gateways do not just connect networks, but allow them to work as a single network.

Network Protocols

Separate internet sections are networks of various architecture that bind to each other using routers. Transmitted data are divided into small portions, called packages. Each package moves over the network regardless of other packages.

For overcoming interface incompatibility separate computers We produce special standards called communication protocols.

Protocol Communication - This is a consistent set of specific information exchange rules between different devices data transmission. There are protocols for transmission rates, data formats, error control, etc.

The Internet in the Internet is unboundedly switched (ie, they are associated) with each other, because all computers participating in data transmission use a single TCP / IP communication protocol (T-C-PI / AI-PI ").

In fact, the TCP / IP protocol is two different protocols that determine various aspects of data transfer on the network:

tCP Protocol (Transmission Control Protocol) - Data Transmission Management Protocol using automatic re-transmission of packages containing errors; This protocol is responsible for dividing the transmitted information to packages and properly restore information from the recipient packages;
iP Protocol (Internet Protocol) - the internet interaction protocol responsible for addressing and allowing the package on the path to the destination end item to several networks.

Information transmission circuit for TCP / IP protocol is such: the TCP protocol splits information on packages and numbers all packages; Next, using the IP protocol, all packets are transmitted to the recipient, where using the TCR protocol is checked, all packages are obtained; After receiving all packages, the TCR protocol has them in the desired order and collects in a single whole.

Above, we looked at you that the Internet consists of a large set of computers, one of them can be connected only for a while, while others have a permanent network IP address (host). Difference network OT world Wide Web The fact that only the one on which is installed on the reference point special program To support the WWW server. Most often, such a computer is called the "server".

How does the package finds his recipient?

Each computer connected to the Internet has two equivalent unique addresses: a digital IP address and a symbolic domain address. Address assignments occurs according to the following scheme: International Organization Network Information Center issues groups of addresses to local networks, and the latter distribute specific addresses at their discretion.

The IP address of the computer has a length of 4 bytes. Typically, the first and second bytes define the network address, the third byte determines the subnet address, and the fourth is the address of the computer in the subnet. For convenience, the IP address is written in the form of four numbers with values \u200b\u200bfrom 0 to 255, separated by points, for example: 145.37.5.150. Network address - 145.37; Subnet address - 5; The address of the computer in the subnet - 150.

internet

Interlocutors. As a rule, in general access networks it is impossible to provide each couple of subscribers their own physical communication line, which they could monopolize "own" and use at any time. Therefore, a network always applies any way to switching subscribers, which ensures the separation of existing physical channels between several communication sessions and between network subscribers.

Switching in urban telephone networks

The city telephone network is a combination of linear and station structures. A network having one PBX is called necroshed. Linear structures of such a network consist only of subscriber lines. Typical value of the capacity of such a network of 8-10 thousand subscribers. With large capacities, due to a sharp increase in the length of Al, it is advisable to switch to the zoned construction of the network. In this case, the territory of the city is divided into areas, in each of which one district PBX (RATS) is constructed to which subscribers of this area are connected. The connections of the subscribers of the same area are carried out through one RATS, subscribers of different RATC - in two. RATS are associated with interconnecting lines in general, according to the principle "each with each". The total number of beams between the RATS is equal to the amount of RATS / 2. As an increase in the capacity of the network, the number of beams clutching the RATC among themselves according to the "each with each" principle begins to grow sharply, which leads to an excessive increase in the cable consumption and costs for the organization of communication and therefore, with the capacles of the network, more than 80,000 subscribers use additional switching node. On such a network, the connection between the PBX of different areas is carried out through the nodes of the incoming message (UVS), and the connection within its nodal area (ur is carried out according to the principle "Each with each" or through its UVS.

Methods for transferring digital information

Digital data on the conductor are transmitted by changing the current voltage: no voltage - "O", there is a voltage - "1". There are two ways to transfer information on a physical transmitting environment: digital and analog.

Notes: 1. If all computer network subscribers lead data on the channel at one frequency, such a channel is called narrowband (skips one frequency).

2. If each subscriber operates at its own frequency on one channel, then such a channel is called broadband (skips many frequencies). The use of broadband channels saves on their quantity, but complicates the process of controlling the exchange of data.

For digital or narrowband transmission method (Fig. 6.10) The data is transmitted in their natural form at a single frequency. A narrow method allows you to transmit only digital information, provides each this moment The time the possibility of using the transmitting media only by two users and allows for normal operation only at a limited distance (the length of the communication line is not more than 1000 m). At the same time, the narrowband transmission method ensures high data exchange rate - up to 10 Mbps and allows you to create easily configurable computing networks. The overwhelming number of local computing networks uses narrowband transmission.

Fig. 6.10. Digital transmission method

Analog The method of transmitting digital data (Fig. 6.11) provides broadband transmission due to use in one channel signals of various carrier frequencies.

With analog transmission method, control of the parameters of the carrier frequency signal for transmission over the digital data channel.

The carrier frequency signal is a harmonic oscillation described by the equation:

X \u003d x max sin (ωt + φ 0),

where x Max is the amplitude of oscillations;

ω - frequency of oscillations;

φ - the initial phase of oscillations.

You can pass the digital data on an analog channel by controlling one of the parameters of the carrier frequency signal: amplitude, frequency or phase. Since it is necessary to transmit data in binary form (sequence of units and zeros), then the following management methods can be offered ( modulations): amplitude, frequency, phase.

The easiest way to understand the principle amplitude Modulations: "0" - no signal, i.e. lack of carrier frequency oscillations; "1" - the presence of a signal, i.e. The presence of oscillations of the carrier frequency. There are oscillations - unit, no oscillations - zero (Fig. 6.11a).

Frequency Modulation provides for the transmission of signals 0 and 1 at different frequency. When moving from 0 to 1 and from 1 to 0, a carrier frequency signal changes (Fig. 6.116).

The most difficult to understand is phase modulation. Its essence is that when moving from 0 to 1 and from 1 to 0, the oscillation phase is changing, i.e. Their direction (Fig. 6.11b).

In the high-level networks of the hierarchy - global and regional are also used also broadband transferwhich provides for the work for each subscriber at its frequency within one channel. This ensures the interaction of a large number of subscribers at high data transfer rates.

Broadband allows you to combine digital data, images and sound in one channel, which is a necessary requirement modern Systems multimedia.

Example 6.5. A typical analog channel is a telephone channel. When the subscriber removes the tube, then hears the uniform beep - this is the carrier frequency signal. Since it lies in the range of sound frequencies, it is called a tonal signal. To transmit on a telephone channel, you need to control the carrier frequency signal - modulate it. The sounds perceived by microphone are converted into electrical signals, and those, in turn, and modulate the carrier frequency signal. When transmitting digital information, the Office produces information bytes - the sequence of units and zeros.

Hardware

Technical devices that perform computer pairing functions with communication channels are called adapters or network adapters. One adapter provides compilation with a computer of one communication channel.

Fig. 6.11. Methods for transferring digital information on analog signal:

a - amplitude modulation; b - frequency; V - Phase

In addition to single-channel adapters, multichannel devices are used - data Multiplexers or simply multiplexers.

Data transfer multiplexer - A compilation device with multiple communication channels.

As mentioned earlier, the bits flow to transform into analog signals to transmit digital information on the communication channel, and when receiving information from the communication channel to the computer, perform a reverse action - convert analog signals to the bits that can process computers. Such transforms performs a special device - modem.

Modem - A device that performs modulation and demodulation of information signals when transmitting them from computer to a communication channel and when receiving a computer from a communication channel.

Concentrator - A device that switches several communication channels to one by frequency separation.

In LAN, where the physical transmission medium is a limited length cable, special devices are used to increase the length of the network - repeators.

Repeater - A device that ensures the preservation of the shape and amplitude of the signal when transmitting it to more than is provided by this type of physical transmitting medium, distance.

There are local and remote repeaters. Local Repeators allow you to connect network fragments located at a distance of up to 50m, and distance - up to 2000 m.

Switching (computer networks). The terminal device consisting of a monitor and keyboard used to exchange data with a computer The device commuting several link channels is called a concentrator.

Elementary Channel.

THE COMPOSITE CHANNEL

Connection refused

THE ADVANTAGE OF CIRCUIT SWITCHING

Packet Switching.

Partitioning the Data Into Packets

TRANSFERRING DATA OVER A NETWORK AS A PACKET

Methods of TRANSFER PACKAGES

Datagram Transmission

TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION

Virtual Channel

COMPARISON CIRCUIT-SWITCHED AND PACKET

Switching in urban telephone networks

Password recovery