What are computer networks? Basic concepts.

Description

Computer network - a communication system between two or more computers. In the broadest sense, a computer network is a communication system through a cable or wireless medium, computers of various functional purposes and network equipment.

Various physical phenomena can be used to transmit information, as a rule - various types of electrical signals or electromagnetic radiation.

Transmission media in computer networks can be telephone cables and special network cables: coaxial cables, twisted pairs, fiber optic cables, radio waves, light signals.

Understanding layers and protocols

Network protocol - a set of rules for interaction between computers. Protocols define format, synchronization, sequencing, and error control. Without these rules, the computer will not see the meaning in the stream of input bits.

OSI (Open System Interconnection) model - includes a set of protocols defining and standardizing the data communication process. The model divides this process into 7 groups called layers.

Network layers:

• Application layer (Program) - ensures interaction between the software running on the computer and the network.
• Presentation layer (presentational) - performs code conversion and data format change (syntax translation). It is a network translator that checks that the data is in the correct form for the application that will receive it.
• Session layer (Sessional) - decides when to enable or disable communication between two computers.
• Transport layer (Transport) - checks whether data is successfully sent and received at the two end nodes. If the data is sent incorrectly, this layer is responsible for resending the data.
• Network layer (Network) - establishes, maintains and interrupts logical and physical connections between many intermediate networks. Responsible for translation (conversion) of logical addresses or names into physical addresses. It performs the functions of network routing and flow control through the computer-network interface.
• Data-Link - handles physical transmission, framing (assembly of data into a single block), flow control, and error control during a single data transfer.
• Physical layer (Physical) - provides an electrical or mechanical interface for a network cable. This layer gives the Data-Link layer the ability to transmit a stream of data bits between two interacting systems. Transmits bits of data to be moved.

Understanding of the TCP/IP model and protocols

Generalized groups of network layers:

• Application layer (Program) - provides programs for network troubleshooting, file transfer, remote control and Internet activity. It also supports APIs (Application Programming Interfaces), which allow programs written for certain operating systems to access the network.
• Transport layer (Transport) - provides flow control, error control and authentication services for the Internet. Serves as an interface for network applications.
• Internet layer (Internet) - provides logical, hardware-independent addressing so that data can be transferred between subnets with different physical architectures. Provides routing to reduce traffic and supports network delivery. Associates physical addresses (used at the Network Access layer) with logical addresses.
• Network Access layer (Network access) - provides an interface with the physical network. Formats the data for transmission and addresses the data for the subnet based on the physical hardware addresses. Provides error control for data transmitted over a physical network.

Understanding of physical layer protocols and devices

Physical level determines the electrical, time and other characteristics of networks through which bits of information are transmitted in the form of electrical signals. The performance of data transmission channels (their bandwidth, delay time, and error rate) is determined by various properties of physical carriers.

Understanding of software layer protocols (HTTP, FTP, Telnet)

Software layer is on top of the TCP/IP protocol suite. Communicates with lower layers via TCP or UDP ports.

Understanding HTTP and WWW

HTTP (Hypertext Transfer Protocol) is used for communication between the browser and the web server. NTTR supports the transfer of HTML documents. To establish a connection, the TCP transport protocol is used.

NTTR is responsible for the following:

• Establishes a connection between the browser and the server
• Setting parameters for the session
• Provides orderly transmission of HTML content
• Closing the connection with the server
• After establishing a TCP connection, the browser uses the HTTP GET command to retrieve the Web page from the server.

UDP (User Datagram Protocol) - a protocol in the TCP/IP stack, which, unlike the TCP protocol, works without establishing a connection. Exchanges messages without confirmation and delivery guarantee. When using the UDP protocol, the responsibility for error handling and data retransmission is assigned to the protocol layer above. Effective for servers sending small responses to a large number of clients.

Tools for tracking and solving problems (ICMP, ping, traceroute)

Ping - a utility that tests the network connection.

Traceroute - a utility that tracks the path of a datagram (message) between networks.

ICMP (Internet Control Message Protocol) - a network protocol used to transmit messages about errors that occurred during data transmission. The operation of utilities is based on this protocol ping and traceroute .

Client-server model

Client-server - a network architecture in which the task or network load is divided between service providers (servers) and customers (clients).

Sockets, IP and port addressing

Network socket - the final abstract point of connection in a computer network.

API socket - a software interface for controlling and using network sockets.

Socket address - combination of IP address and port number.

Use of proxy servers

Proxy server - a server (computer or program) that allows you to perform indirect (and sometimes proxy) requests to network services. It is used for access of computers from the local network to the Internet, data caching, data compression, protection of the local network from external access, restriction of Internet access and traffic control, for anonymous access.

File transfer services: FTP, TFTP

FTP (File Transfer Protocol) - client/server utility and protocol used to transfer files between two computers in a TCP/IP network. Can also create and delete directories and display the contents of directories.

TFTP (Trivial File Transfer Protocol) - a client/server utility and protocol based on UDP and used for simple file transfer operations.

Name conversion services: DNS, whois

DNS (Domain Name System) - a system for naming resources in TCP/IP networks.

WHOIS - a network protocol/service (based on the TCP protocol) that makes it possible to determine the owner of a domain name/IP address.

Remote access services: Telnet, SSH, rdesktop, VNC

Telnet - a set of components that provide terminal access to a remote computer. A Telnet session requires a Telnet client and a Telnet server. Telnet is also a protocol, a system of rules that defines the interaction between the Telnet server and the client.