Tashkent university of information technology named after muhammad al-khwarizmi

System interconnection

Wireless LANs

Wireless WANs

System Interconnection

System interconnection is all about interconnecting the components of a computer using short-range radio. Some companies got together to design a short-range wireless network called Bluetooth to connect various components such as monitor, keyboard, mouse and printer, to the main unit, without wires. Bluetooth also allows digital cameras, headsets, scanners and other devices to connect to a computer by merely being brought within range.
In simplest form, system interconnection networks use the master-slave concept. The system unit is normally the master, talking to the mouse, keyboard, etc. as slaves.

Wireless LANs

These are the systems in which every computer has a radio modem and antenna with which it can communicate with other systems. Wireless LANs are becoming increasingly common in small offices and homes, where installing Ethernet is considered too much trouble. There is a standard for wireless LANs called IEEE 802.11, which most systems implement and which is becoming very widespread.

Wireless WANs

The radio network used for cellular telephones is an example of a low-bandwidth wireless WAN. This system has already gone through three generations.

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  The first generation was analog and for voice only.
  
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  The second generation was digital and for voice only.
  
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  The third generation is digital and is for both voice and data.
  

Inter Network

Inter Network or Internet is a combination of two or more networks. Inter network can be formed by joining two or more individual networks by means of various devices such as routers, gateways and bridges.

1.2. Switching methods

In any communication network, some kind of switching method is always used, which ensures, with the help of switches, the availability of available physical channels simultaneously for several communication sessions between network subscribers, each of which is connected to the nearest switch by an individual line connections. At any time, this line is used by only one subscriber, and between switches, the communication lines are shared by many subscribers.

Three fundamentally different methods of switching subscribers in networks are used: circuit switching, message switching and packet switching.
Message-switched and packet-switched networks are a type of networks with intermediate storage of transmitted information. Networks with circuit switching and packet switching are divided, in addition, into two classes - into networks with dynamic switching and networks with constant switching.
In networks with dynamic switching, the connection of a subscriber with any other is established by the network at the initiative of the subscriber, continues for a certain time (from several seconds to several hours) and is also completed at the initiative of the subscriber at the end of the exchange of information. This mode of operation is supported by public telephone networks, local networks, TCP / IP networks.
In networks with constant switching, the connection between interacting users is established by the network personnel for a long time (several months or more). The constant switching mode is popular in SDH technology networks, where dedicated communication channels with a bandwidth of several Gbit / s are created.
Some networks support both modes of operation, such as X 25 and ATM networks.
Channel switching. When circuits are switched between connected endpoints throughout the entire time interval of the connection, real-time exchange is provided, and bits are transmitted at a constant rate over a channel with a constant bandwidth. An end-to-end composite communication channel is established between subscribers before the start of information transfer. This channel is formed from separate sections with the same bandwidth. The passage of a separate call signal is provided by the serial connection of several switching devices located in circuit switching centers (CSC). Each device reserves the physical connection between one incoming and one outgoing channel. If, when establishing an end-to-end communication channel, the called party or at least one of the switching devices in the call signaling chain is busy, the latter will be blocked, and the subscriber who initiated the call must repeat it after some time.
The time to establish an end-to-end communication channel is usually large due to the need to organize the interaction of a significant number of switching devices. Once such a channel is established, the DCCs perform a minimal number of functions, although a large amount of information can be transmitted. Therefore, when using the channel switching method, information transmission is provided by two main components in the expenditure side of resources: resources for organizing a call and resources for maintaining switching devices in the CCC or for organizing the distribution of time channels . The first component does not depend on the amount of transmitted information, and the second is directly proportional to the time interval during which the connection occurs.
Switches and the links connecting them must provide simultaneous data transmission of several subscriber channels, therefore they must be high-speed and support one of two subscriber circuit multiplexing techniques:
• frequency division multiplexing technique (FDM - Frequency Division Multiplexing - frequency multiplexing of channels), when to separate subscriber channels, modulation of a high-frequency carrier sinusoidal signal with a low-frequency signal generated by sound vibrations is used (frequency division is typical for analog signal modulation);
• time division multiplexing technique (TDM - Time Division Multiplexing - time division multiplexing), when the equipment of TDM networks (multiplexers, switches, demultiplexers) operates in the time division mode, serving in turn during the cycle all subscriber channels of their work. Temporal division is characteristic of digital coding. TDM networks require synchronous operation of all equipment, so this multiplexing technique has another name - synchronous transfer mode technique. At present, almost all data (voice, image, computer data) is transmitted in digital form, so the dedicated channels of TDM technology, which provide the lower level for digital data transmission, are universal channels for building networks of any type: those phone, computer, television.
Virtual circuits in packet-switched networks. There is a mode of network operation - transmission of packets over a virtual channel (virtual circuit or virtual channel), where a virtual channel is established between two end nodes, which is the only route connecting these end nodes. A virtual channel can be dynamic or permanent.
A dynamic virtual circuit is established by requesting a connection using a special packet that passes through the switches and "lays" a virtual circuit, i.e. Switches remember the route for a given connection and, when subsequent packets arrive on a given connection, they always send them along the laid route.
Permanent virtual circuits are created by network administrators by manually configuring switches.
Each packet transfer mode has its advantages and disadvantages. When transmitting data using the datagram method, i.e. based on a packet transmitted through the network independently of other packets without establishing a logical connection and acknowledgment, and therefore operates without delay before transmitting data. This is especially advantageous for small data transfers where the connection setup time can be comparable to the data transfer time. In addition, the datagram method adapts faster to changes in the network.
When using the virtual circuit method, the time spent on establishing a virtual circuit is compensated by the subsequent rapid transmission of the entire packet stream. Switches recognize that a packet belongs to a virtual channel by a special label - the virtual channel number, and do not analyze the addresses of end nodes, as is done with the datagram method.
As disadvantages of the channel switching method, the following can be indicated:
• a long time to establish an end-to-end communication channel due to the possible waiting for the release of its individual sections;
• the need to retransmit the call signal due to the busyness of the called party or any switching device in the chain of passage of this signal (in this regard, the system in which the channel switching method is implemented belongs to the class of systems with loss of requests for service);
• lack of choice of information transfer rates;
• escalation of functions and capabilities of the network is limited;
• Uniform loading of communication channels is not ensured (opportunities for smoothing the load are very limited).
Advantages of the circuit switching method:
• development of channel switching technology (the first switching device appeared at the end of the 19th century);
• the ability to work in interactive mode and in real time;
• providing both bit transparency and transparency in time, regardless of the number of switching between subscribers;
• guaranteed network bandwidth after establishing a connection (this is important when transmitting voice, image, managing objects in real time);
• fairly wide scope. Circuit-switched networks are well suited for switching data streams of constant speed, when the switching unit is a long-term synchronous data stream between interacting subscribers.
Switching with intermediate storage. The end-to-end channel between the sender and the receiver is not established. The calling entity, by means of dialing or via a leased line, communicates only with the nearest network node and transmits information bits to it. Each node has a switch, built on the basis of a communication computer with a storage device (RAM). The transmitted information must be stored at each node along the way to the destination, and the storage delay will typically be different for the nodes. The presence of a memory in intermediate communication nodes prevents the loss of transmitted information, as a result of which the systems that implement the considered switching methods belong to the class of systems without loss of service requests. One of the indicators of these methods is the possibility of matching data transfer rates between the points of origin and destination, which is ensured by the presence of effective decoupling in the network, implemented by the creation of buffer memory in communication nodes. Finally, for store-and-forward networks, bit transparency is a requirement.
Message switching. This method of data transmission was predominant in the 1960s and 1970s. and is still used in some areas (e-mail, electronic news, teleconferences, teleseminars). Like all store-and-forward switching methods, message switching technology is a store-and-send technology. In addition, message switching technology usually provides for a master-slave relationship. The switch (communication computer) in the message switching center (MCS) performs registration and selection when managing incoming and outgoing flows. Interactive and real-time operation are not covered here, but data can be transmitted through the switch at very high speeds with appropriate priority levels for different types of data streams. High-priority threads stay in the service queue for a shorter time than low-priority threads, which allows for interactive applications.
It is important to note that when switching messages, a message, regardless of its length (the spread in the length of messages can be quite large), completely retains its integrity as a single object in the process of its passage from one node to another up to destination. Moreover, the transit node cannot start further transmission of a part of the message if it is still being received. In terms of its effect on latency, this is equivalent to a low level of network resource utilization.
Thus, message switching is designed to organize the interaction of users in the off-line mode, in which no immediate response to the received message is expected.
Disadvantages of the message switching method:
• the need to implement sufficiently serious requirements for the capacity of buffer memories in communication nodes to receive large messages, which is due to the preservation of their integrity;
• Insufficient opportunities for the implementation of the dialogue mode and work in real time when transmitting data;
• Failure of the entire network if the switch fails, since all data flows pass through it (this is typical for the “master-slave” structure);
• the message switch is a potential bottleneck in terms of throughput;
• Data links are used less efficiently than other store-and-forward switching methods.
Advantages of the method:
• no need for early (before the start of data transmission) establishment of an end-to-end communication channel between subscribers;
• the possibility of forming a route from individual sections with different throughput;