Difference between circuit switching and packet switching. Global networks - three stages of development, circuit switching methods

All currently existing telecommunication networks can use two types of communication to provide the required functions - circuit switching and (or) packet switching. What is it and how do they differ from each other?

Let's start with how circuit-switched networks work. They appeared earlier than their dial-up counterparts, so, not surprisingly, they are easier to implement. A striking example of a network that uses circuit switching is a telephone line. Obviously, in order for two subscribers to start communicating, it is necessary to establish a connection between them. The initiating subscriber dials a number, which, in fact, is a command to the equipment located between them to properly connect two lines - from the initiator and from the responder (we take an example when subscribers are served by one station). Previously, mechanical probes were used for this, but with the advent of digital solutions, the implementation changed, although the principle remained the same. Circuit switching provides subscribers with an independent line that remains assigned to them until the end of the communication session. The advantages are obvious: high reliability, no need to transmit control packets. However, this connection method becomes too wasteful as the number of subscribers increases, since the number of channels is physically limited. Even an attempt to solve this problem by using seals is only a temporary measure, a defined intermediate solution. In addition, circuit switching has one significant drawback - the communication line is busy all the time, even if there is no exchange of information between subscribers. For example, when telephone conversation you can hang up next to the device and go about your business - the channel will remain reserved for them until a signal is received to disconnect the connection.

That is why circuit switching was subsequently replaced by packet switching. The principle of its operation involves encoding and splitting the transmitted data stream into a number of separate packets, which are transmitted over a common communication line to the recipient and there combined into the original stream. To understand the differences between these two methods, you can use an analogy with a transport line: when switching channels, the line is represented by a railway track, and the data flow is a train of many cars. It is quite clear that delays along the route are extremely rare, and the reliability is one of the highest. At the same time, several trains cannot move along this track at the same time. But lines with packet switching are a high-speed highway with multi-lane traffic. The transported cargo (transferred packages) is divided into several vehicles which, maneuvering in the flow of other modes of transport, reach their destination, where the initial structure is assembled. IN in this example the road is a communication channel, and cars are data packets. They calmly coexist on the same road, almost without interfering with each other’s movements. Exceptions are traffic jams, traffic lights and emergency situations (these are delays). Even if any machine does not arrive at the recipient, a copy of it can be resent upon request. The total volume of information transmitted per unit of time during packet switching is significantly higher than in the case of channels.

In general, switching is switching something, changing states. In network technology, it forms a route for the passage of data. The peculiarity lies in the way it is organized. Switching should not be confused with routing, the task of which is to find the optimal path.

) is connected using a terminal device (T), which sends information to the network at the same speed. This speed is equal to the channel. If situations arise when the user transmits volumes of information that are less than the channel capacity, then the Terminal device fills the void with empty data. This is shown in Fig. 2.

Figure 2

That some of the information actually exists complemented by emptiness The recipient's Terminal device also knows, which discards the supplemented information.

Establishing a connection

To exchange information, you first need to establish a connection via . During installation, a connection may arise. Let's say two objects A and B want to exchange data (see Fig. 1). First you need to send request V switching network, where the object indicates the address of object B. The task of sending a request is to make the connection between objects an information channel, the characteristics of which are similar to a continuous connection, that is, throughout the entire duration of the established connection, data is transmitted at the same speed and volume. This means that in transit switches no need to buffer information objects.

To create a connection, the request must go through a series of switches that lie on the channel from A to B, and make sure that all sections of the path are in this moment free.

Connection refused

The only one positive thing such a connection, this means that the delay level is minimal and transmit real time/(voice, video) will be very convenient.
The negative aspects are that each physical line always transmits data at the same speed, which is inefficient. And the use of resources is also not efficient as shown in Fig. 1. The solution to circuit switching problems is multiplexing.

Packet switching

The packet switching algorithm was specially made for the efficient exchange of computer traffic. When an entity transmits switched packets, the data is broken up at the originating node into small pieces called frames. Each package is given title, which contains the delivery address. Figure 3 shows the breakdown of the data stream into packets. Another additional field that is added to the end of the packet is limit switch. Placed there check sum , which allows you to check whether the information was changed during transmission or not.

Figure 3

Packets enter the network without pre-reservation of network channels and not with preset speed, as implemented in switched networks. And it is transmitted at the rate at which the source generates. It is assumed that a packet-switched network is always ready to receive a packet from an object, unlike a circuit-switched network.

The bandwidth reservation scheme can also be used in packet networks. But the basic idea of ​​such a reservation is fundamentally different from the idea of ​​reserving bandwidth in circuit-switched networks. The difference is that the channel capacity of a packet-switched network can dynamically change between information communication lines depending on the current tasks of each channel, which circuit switching technology cannot implement.

Circuit switching

When switching channels, such a network implements a permanent, integral physical channel between end nodes from successive connected intermediate sections using a switch. The main condition for such a channel is the same data transfer rate in each section. Equality determines that the switches of such a network should not buffer transported data. Figure 4 shows a network operating using circuit switching technology. In order for node 1 to transfer data to node 7, a special request must first be received to implement the connection to switch A, indicating the destination address 7. Switch A must designate the route of the composite channel, and then transmit the request to the next switch, in Fig. this is switch E. Then switch E sends a request to switch F, which then passes it on to node 7. Node 7 accepts the request to establish a connection, and then it responds to the original node along the assigned route.

Figure - 4

Advantages of circuit switching:

• Known and constant speed of information transmission over an established channel
• Constant and low latency in transporting information across the network

Disadvantages of circuit switching:

• Irrational implementation of the capacity of physical channels. The transmission of information may be uneven, and the dedicated channel may be idle
• Mandatory delay before transporting information due to connection establishment

The pros and cons of any network technology are relative, since in different situations pros can act as cons and vice versa.

Figure - 5

Comparison of switching methods:

Dynamic and permanent switching

Dynamic switching networks:

• it is allowed to implement a connection at the initiative of the user of this network
• switching is implemented only for the duration of the communication session, and then is terminated at the user’s initiative
• The user can implement a connection with any network user
• The time required to establish a connection between a pair of users can be from a couple of seconds to several hours and ends after completion of work - file transfer, etc.

Examples of such networks are local area networks or TCP/IP.

Constantly switched networks:

• Allows a pair of users to order a connection for a long period of time
• The connection is created by special personnel who maintain the network, and not by users
• The permanent switching mode in circuit-switched networks is called dedicated or leased circuit service.

The most popular networks in permanent switching are SDH.

Topic 3.3: Applications for creating websites

Topic 3.4: Application of the Internet in the economy and information protection

Global networks

3.2. Network technologies. Global networks and global network technologies

3.2.1. WAN networks with circuit and packet switching

Wide Area Networks (WAN), which refer to territorial computer networks, are designed, like local networks, to provide services, but significantly more users located over a large area.

Switching methods

There are three fundamentally different switching schemes in global networks:

• circuit switching;
• message switching
• packet switching;

Circuit switching in global networks– a process that, on demand, connects two or more data stations and maintains exclusive use of the data channel until disconnection occurs. Channel switching implies the formation of a continuous composite physical channel of individual channel sections connected in series for direct data transfer between nodes. Individual channels are connected to each other by special equipment - switches, which can establish connections between any end nodes of the network.

Message switching in global networks– the process of sending data, including reception, storage, selection of the original direction and further transmission of messages without violating their integrity. Used in cases where an immediate response to a message is not expected. Messages are transmitted between transit computers on the network with temporary buffering on the disks of each computer. Messages are data that are united by semantic content, have a specific structure and are suitable for processing, sending or using.

Message sources can be voice, images, text, data. To transmit sound, the telephone is traditionally used, images are transmitted by television, text is transmitted by the telegraph (teletype), and data is transmitted by computer networks. Establishing a connection between the sender and the recipient with the ability to exchange messages without noticeable time delays characterizes the online operating mode. With significant delays in remembering information in intermediate nodes We have offline mode.

Packet switching in wide area networks– this is the switching of messages presented in the form of addressed packets, when the data transmission channel is occupied only during the transmission of the packet and, upon its completion, is freed for the transmission of other packets. Network switches, which act as gateways and routers, receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to destination stations.

In global networks, they are used to transmit information. the following types switching:

• channel switching (used when transmitting audio information over regular telephone lines;
• message switching (used mainly for transmitting email, teleconferencing, electronic news);
• packet switching (for data transmission, currently also used for transmission of audio and video information).

The advantage of circuit switching networks is the ease of implementation (the formation of a continuous composite physical channel), and the disadvantage is the low utilization of channels, the high cost of data transmission, and increased waiting time for other users.

When switching messages, data (message) is transmitted after the channel is released until it reaches the recipient.

Each server receives, verifies, assembles, routes, and transmits messages. The advantages include a reduction in the cost of data transmission. Disadvantage this method is the low speed of information transfer, the impossibility of conducting a dialogue between users.

Packet switching involves the exchange of small packets (part of a message) of a fixed structure, which do not allow the formation of queues at switching nodes. Advantages: fast connection, reliability, efficient use of the network.

In networks with circuit switching, subscribers are connected by a composite channel formed by network switches at the request of one of the subscribers; with this switching method, before transmitting data, it is always necessary to perform a connection establishment procedure, during which a composite channel is created.

Circuit-switched networks are good at switching data streams of constant intensity, for example, data streams created by interlocutors talking on the phone, but cannot dynamically redistribute the capacity of trunk channels between streams of subscriber channels.

To jointly share channels between network switches with multiple subscriber channels, two technologies are used: frequency division technology (FDM) and time division technology (TDM).

Frequency division is typical for analog modulation of signals, and time division is characteristic for digital coding. Frequency Division Division (FDM) technology was developed for telephone networks, but is also used for other types of networks, such as cable television networks and computer networks. During the transition to a digital form of voice representation, a new technology was developed that focuses on the discrete nature of the transmitted data - this is the time division technology (TDM).

Packet switching.

Packet switching is a subscriber switching technique that was specifically designed for the efficient transmission of computer traffic.

With the packet switching method, all messages sent by users are broken up at the source node into relatively small parts called packets. Each packet is equipped with a header, which necessarily indicates the address information necessary to deliver the packet to the destination node and other service information. WAN switches receive packets and, based on address information, transmit them to each other, and ultimately to the destination node.

Packet network switches differ from circuit switches in that they have internal buffer memory for temporary storage of packets if the output port of the switch is busy transmitting another packet at the time the packet is received. In this case, the packet remains for some time in the packet queue in the buffer memory of the output port; when its turn reaches it, it is transferred to the next switch.

Circuit-switched networks operate efficiently in the sense that the amount of data transmitted from all network subscribers per unit time is greater than when using a circuit-switched network. However, for each pair of subscribers, the network throughput may be lower than that of a circuit-switched network due to packet queues in the switches.

Packet sizes have a significant impact on network performance. Typically, packets on networks are 1–4 KB in size.

Packet switched networks can operate in one of two modes: datagram mode or mode virtual channels.

At datagram mode packet transmission assumes independent routing of each packet. In this case, the switch can change the route of any packet depending on the state of the network. The datagram method does not require prior connection establishment and therefore operates without delay before data transmission.

Virtual channel mode involves transmitting packets along a predetermined path - over a virtual channel. In this case, before data can be transmitted between two end nodes, a virtual circuit must be established, which is the only route connecting these nodes. The time spent on establishing a virtual channel is compensated by the subsequent rapid transmission of the entire stream of packets. The virtual channel can be dynamic And permanent.

A dynamic virtual channel is established for one communication session; for this purpose, a special service packet is sent to the network - a request to establish a connection. This packet, passing through network devices, “lays” a virtual channel through which these packets will be transmitted.

Permanent virtual circuits are created by the network administrator by manually configuring switches.

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;

Among the many possible approaches to the solution of the generalized problem of subscribers switching networks allocate two basic ones, which include channel switching and packet switching. 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.

Therefore, 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 is equal to 64 kbit/s, which is sufficient for high-quality digital voice.

For high-quality voice uses 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 = 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 composite 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 subscribers, for which the composite 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 there is at this time in the other network connection or not;
• after the end of the session the basic channels that were included with the corresponding composite channel, declared free and returned to the pool of resources allocated for use by other users.

Connection refused

Connection refused

Connection requests are not always successful.

If the path between the calling and called subscribers are no free channels 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 exchange is carried out in advance, even before the start of 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 ineffective, 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 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. In addition to the title in the package may have one additional field to be placed at the end of the package and so-called trailer. In the trailer is usually 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. After 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 commutation 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 packet switch of the switches in the circuit-switched networks is that they have an internal buffer memory for temporarily storing 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 their 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 ​​of parameters that it carries, and the current state of the network. And every single packet network is considered as a completely independent unit transfer - datagram.

Illustration datagram packet principle

Transfer to the establishment of a logical connection

Transfer to the establishment of a logical connection

Procedure for harmonization of the two end nodes of a network of some parameters 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

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 gasket from the source node 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