The main physical way of implementing the transportation operation is the use of local networks and data transmission networks. In the development and use of networks for compatibility, a number of standards are used that are combined into a seven-level model of open systems, adopted worldwide and defining the rules for the interaction of network components at a given level (level protocol) and rules for the interaction of components of different levels (inter-level interface) (4, 36 ). International standards in the field of network information exchange are reflected in the reference seven-level model, known as the OSI model (Open System Intercongtction - connection of open systems) (Figure 4.3). This model was developed by the International Organization for Standardization (ISO Standards). Most manufacturers of network software and hardware tend to adhere to the OSI model. But in general, it is not possible to achieve full compatibility yet.
The physical layer implements physical control and refers to a physical circuit, such as a telephone, over which information is transmitted. At this level, the OS1 model defines the physical, electrical, functional and procedural characteristics of the communication circuits, as well as the requirements for network adapters and modems.
The link layer. This layer manages the link of the network (the channel) and sends blocks (a set of bits) of information over the physical link. It performs such control procedures as determination of the beginning and end of the block, detection of transmission errors, addressing of messages, etc. The link layer defines the rules for sharing network hardware by network computers.
The network layer refers to a virtual (imaginary) chain that does not have to exist physically. Using the interface provided by this layer, you can hide complexity of transmission control at the physical level. The software of this level provides the definition of the route of packet transmission in the network. Routers that provide the search for the optimal route based on the analysis of address information, function at the network level of the OSI model. As the simplest routing device between network segments
Fig. 4.3. Open Systems Communication
or different local networks, there may be a device operating on the lower channel level of the OSI model, called a bridge.
Transport layer. The first three layers form a common network, in which many users can collectively work. At the transport level, the order of the message packets and their belonging are controlled. Thus, in the process of exchange between computers, virtual communication is maintained, similar to telephone switching.
Session level. In some cases, it is difficult to organize the process of interaction between users because of the plethora of ways of such interaction. To eliminate these difficulties, the processes of establishing a session, controlling the transmission and reception of message packets, and terminating the session are coordinated and standardized at this level. At the session level, virtual communication is established and terminated between computers using the same principle as for voice telephony.
View management. The software of this level performs data transformations from the internal format of the sending computer to the internal format of the receiving computer, if these formats differ from each other (for example, IBM PC and DEC). This level includes functions related to the character set used, the data encoding, and how data is presented on the display or print screens. In addition to converting formats at this level, the data being compressed is compressed and unpacked.
The application layer refers to functions that support the user at a higher application and system level, for example:
• Organization of access to common network resources: information, disk storage, software applications, external devices (printers, streamers, etc.);
• general network management (configuration management, sharing access to shared network resources, recovery from failures and failures, performance management);
• the transmission of electronic messages, including e-mail;
• the organization of electronic conferences;
• high-level dialog functions.
The OSI model is a standardized framework and general recommendations, while the requirements for specific components of network software are specified by protocols.
The protocol is the standard in the field of network software and defines the set of functional and operational requirements for any of its components that the producers of this component adhere to. The protocol requirements may differ from the requirements of the OSI reference model.
The International Institute of Electrical and Electronics Engineers (IEEE) has developed standards for data transfer protocols in local networks. These standards, which describe the methods of access to network data channels, are called IEEE 802.
Network communication protocols can be classified by the degree of proximity to the physical medium of data transmission. These are the protocols:
• The lower layer, propagated to the channel and physical layers of the OSI model;
• middle tier distributed to network, transport, and session OSI layers;
• The top level that is propagated to the presentation layer and application layer of the OSI model.
Each implementation of the protocols of higher levels uses the implementation of the protocols of the subordinate levels.
Low-level OSI protocols correspond to the level of network hardware and the lower layer of network software. Among the most common standards of this level, we distinguish the following [11, 12, 30]:
• NDIS (Network Driver Interface Specification), developed jointly by Microsoft and 3Com;
• ODI (Open Datalink Interface) standard, developed jointly by Novell and Apple Computer.
These standards allow the implementation of middle-tier protocols regardless of network hardware and ensure the joint functioning of various types of middle-tier protocols. The universal link-layer interface is shown in Fig. 4.4. Manufacturers of network hardware tend to develop drivers that meet both standards.
Fig. 4.4. Universal link layer interface
The network adapter driver is the latest software component in front of the physical layer of the OSI model and is called the Media Access Control (MAC) sublayer. The MAC sublayer is oriented to performing functions such as direct access control to the transmission medium, checking message packets for errors.
The LLC (Logical Line Control) sublayer is considered independent of the characteristics of the physical transmission medium and the methods used to access the data channels. Standards for developing interfaces for linking implementations of middle-tier OSI protocol protocols to network hardware drivers relate primarily to the LLC sublayer.
The middle-tier protocols extend to the network, transport, and session layers of the reference model. By the type of intercomputer exchange, these protocols can be classified as follows:
• session protocols (virtual connection protocols);
• datagram protocols.
Session protocols define the organization of information transfer between computers on the so-called virtual channel in three stages:
• setting up a virtual channel (setting up a session);
• the implementation of direct information exchange;
• Destroying the virtual channel (disconnection).
In session protocols, the sequence of packets during transmission corresponds to their original order in the message, and the transfer is performed with the delivery confirmation, and in case of loss of the sent packets, they are retransmitted.
When datagram protocols are used, message packets are transmitted by so-called datagrams independently of each other, therefore the order of delivery of packets of each message may not correspond to their original order in the message. In this case, message packets are transmitted without confirmation.
Thus, from the point of view of authenticity, session protocols are more preferable, but the transfer rate when using datagram protocols is much higher.
Any medium-level protocol provides for the following stages of the implementation of intercomputer exchange:
• initializing the connection;
• direct information exchange;
• the end of the exchange.
The most commonly used sets of middle-tier protocols are:
• a set of SPX/IPX protocols used on local networks running a NetWare network operating system;
• NetBIOS and NetBEUI protocols supported by most network operating systems and used only on local networks;
• TCP/IP protocols, which are the standard for the global Internet, used in local networks and supported by most network operating systems.
The set of SPX/IPX protocols is used in Novell's NetWare network operating system.
The IPX (Internetwork Packet Exchange) protocol is a datagram protocol and corresponds to the network layer of the reference model. It is used to perform addressing functions when exchanging message packets.
The SPX protocol (Sequenced Packet Exchange) is a session protocol and corresponds to the transport and session layers of the reference model. By the degree of proximity to the lowest level of the reference model, the SPX protocol is above the IPX protocol and uses this protocol.
The driver that implements the SPX protocol uses a driver that implements the IPX protocol during its operation. The IPX protocol is faster than the SPX protocol.
An important drawback of the SPX and IPX protocols is the incompatibility with the TCP/IP protocols used in the global Internet. To connect a local NetWare network to the Internet, use one of the following methods:
• direct installation on each network computer of drivers implementing a set of TCP/IP protocols;
• connection of the local network to the Internet through the IPX-IP gateway.
The NetBIOS and NetBEUI protocols are developed by IBM and are intended only for local computer networks.
The NetBIOS protocol (Network Basic Input/Output System) corresponds to the network, transport, and session levels of the reference model. The implementation of this protocol provides the application interface used to create network software applications.
The NetBEUI protocol (NetBIOS Extended NetBIOS) is a modification of the previous protocol and applies only to the network and transport layers.
Implementations of the NetBIOS and NetBEUI protocols provide the solution of the following tasks: name support, support for session and datagram interaction, obtaining information about the network status.
Advantages of NetBIOS and NetBEUI protocols: convenient addressing, high performance, self-tuning and good error protection, economical use of RAM.
Disadvantages of NetBIOS and NetBEUI are related to the attitude to global networks: lack of support for routing functions and low performance.
The TCP/IP family of protocols was designed to integrate various computer networks into one global network, called the Internet.
The TCP/IP protocol family includes protocols related to both the middle and other levels of the OS1 model:
• application layer and presentation layer - file transfer protocol (FTP), email protocols (SMTP, POP3, IMAP4), remote access protocols (SLIP, PPP, Telnet), network file system protocol (NPS), network management protocol (SNMP), Hypertext Transfer Protocol (HTPR), and others;
• session and transport layers - TCP and UDP protocols;
• Network layer - IP, ICMP, IGMR protocols
• Link layer - ARP, RARP protocols.
The Internet Protocol (IP) is the primary for the network layer and provides the routing of transmitted message packets.
The Internet Control Message Protocol (ICMP) is responsible for exchanging error messages and other important information with network-level software on another computer, router or gateway.
The Internet Management Protocol (IGMP) is used to send IP packets to multiple computers on the network.
TCP (Transmission Control Protocol) is a network layer protocol and provides reliable data transfer between two computers by organizing a virtual exchange channel and using it to transfer large data sets.
User Datagram Protocol (UDP) implements a much simpler transfer service, providing reliable delivery of data without establishing a logical connection.
Top-level protocols correspond to the level of users and applications and extend to the presentation layer and the application layer of the reference network interaction model. The most common are the following high-level protocols:
• redirecting requests and messaging (SMB, NCP);
• Network management (SNMP);
• Network File System (NFS);
• calling remote procedures (RPC);
• Increase the efficiency of using middle-level TCP/IP protocols (DNS, DHSP);
• remote access to computer resources (SLIP, PPP, Telnet);
• file transfer (FTP);
• Hypertext transfer (HTTP);
• E-mail (SMTP, POP3, IMAP4);
• Organization of electronic conferences and news system (NNTP).
The SMB (Server Message Blocks) protocol, developed jointly by Microsoft, Intel and IBM, is used in the network operating systems Windows NT, Lan Manager, LAN Server. This protocol defines the series of commands used to transfer information between network computers.
The NCP (NetWare Core Protocol) protocol was developed by Novell and used in NetWare network operating systems.
Simple Network Management Protocol (SNMP) provides flexible and complete network management, while the administrator assumes the following functions: configuration management, access to common network resources, performance, preparation for recovery, recovery. In this case, any of the management functions should provide the solution of three basic problems:
• getting information about the state of the managed object;
• analysis of information received and development of control actions;
• transfer of control actions to execution.
The NFS (Network File System) protocol is designed to provide a universal file interface for various operating systems, network architectures and middle-tier protocols.
The Remote Procedure Call (RPC) protocol is intended for organizing inter-program interactions for the client-server network and provides a link between the client processes and the server processes implemented on different computers on the network.
The DNS (Domain Name System) protocol is used to establish the correspondence between the symbolic names and IP addresses of computers.
DHCP (Dynamic Host Configuration Protocol) allows you to automatically assign IP addresses to computers connected to the network and change them when moving from one subnet to another.
The SLIP (Serial Line Internet Protocol) protocol ensures the operation of TCP/IP protocols with a dial-up connection.
The Point-to-Point Protocol (PPP) provides the establishment of a connection and the realization of direct information exchange, and also allows to solve the following tasks in comparison with SLIP:
• configuration and quality control of communication;
• Authentication (authentication) of the remote user;
• dynamic assignment of IP addresses and management of these addresses;
• detection and correction of errors, etc.
The Point-to-Point Tunneling Protocol (PPTP) is designed to support multi-protocol virtual private networks (VPNs) and allows remote users to have secure access to corporate networks on the Internet.
The Telnet protocol is a universally accepted standard for remote Internet remote control that allows you to run and run programs on a computer with a remote connection in command line mode.
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