graduate work
1.9 Methods of data exchange in local networks
To control exchange (network access control, network arbitration) are used various methods, the features of which largely depend on the network topology.
There are several groups of access methods based on time division of the channel:
centralized and decentralized
deterministic and random
Centralized access is controlled from a network control center, such as a server. The decentralized access method operates on the basis of protocols without control actions from the center.
Deterministic access provides each workstation with a guaranteed access time (for example, scheduled access time) to the data transmission medium. Random access is based on the equality of all stations in the network and their ability to access the medium at any time to transmit data.
Centralized access to mono channel
In networks with centralized access, two access methods are used: the polling method and the delegation method. These methods are used in networks with an explicit control center.
Survey method.
Data exchange on a LAN with a star topology with an active center (central server). With a given topology, all stations can decide to transmit information to the server at the same time. The central server can communicate with only one workstation. Therefore, at any time it is necessary to select only one station broadcasting.
The central server sends requests to all stations in turn. Each workstation that wants to transmit data (the first one polled) sends a response or immediately begins transmission. After the end of the transmission session, the central server continues polling in a circle. Stations in in this case, have the following priorities: the maximum priority is for the one that is closest to the last station that completed the exchange.
Data exchange in a network with a bus topology. In this topology, perhaps the same centralized control as in the "star". One of the nodes (the central one) sends requests to all the others, finding out who wants to transmit, and then allows the transmission to whichever one of them, after the end of the transmission, reports it .
Transfer of authority method (passing token)
A token is a service package of a certain format into which clients can place their information packages. The sequence of transmitting a token over the network from one workstation to another is set by the server. The workstation receives permission to access the data transmission medium when it receives a special token packet. This method access for networks with bus and star topologies is provided by the ArcNet protocol.
Decentralized access to mono channel.
Let's consider decentralized deterministic and random methods of access to the data transmission medium. The decentralized deterministic method includes the token passing method. The token passing method uses a packet called a token. A token is a packet that does not have an address and circulates freely over the network; it can be free or busy.
Data exchange in a network with a ring topology (decentralized deterministic access method)
1. This network uses the “token passing” access method. The transmission algorithm is as follows:
a) a node wishing to transmit waits for a free token, upon receiving which it marks it as busy (changes the corresponding bits), adds its own packet to it and sends the result further into the ring;
b) each node that receives such a token accepts it and checks whether the packet is addressed to it;
c) if the packet is addressed to this node, then the node sets a specially allocated acknowledgment bit in the token and sends the modified token with the packet further;
d) the transmitting node receives back its message, which has passed through the entire ring, releases the token (marks it as free) and again sends the token to the network. In this case, the sending node knows whether its package was received or not.
For the normal functioning of this network, it is necessary that one of the computers or a special device ensure that the token is not lost, and if the token is lost this computer must create it and launch it on the network.
Data exchange in a network with a bus topology (decentralized random access method)
In this case, all nodes have equal access to the network and the decision when to transmit is made by each node locally, based on an analysis of the network state. Competition arises between nodes for network capture, and, therefore, conflicts between them are possible, as well as distortion of transmitted data due to packet overlap.
Let's look at the most commonly used carrier sense multiple access with collision detection (CSMA/CD). The essence of the algorithm is as follows:
1) a node that wants to transmit information monitors the state of the network, and as soon as it is free, it begins transmission;
2) the node transmits data and simultaneously monitors the state of the network (carrier sensing and collision detection). If no collisions are detected, the transfer is completed;
3) if a collision is detected, the node amplifies it (transmits for some more time) to ensure detection by all transmitting nodes, and then stops transmitting. Other transmitting nodes do the same;
4) after the unsuccessful attempt is terminated, the node waits for a randomly selected period of time tback, and then repeats its attempt to transmit, while monitoring collisions.
In case of a second collision, trear increases. Eventually, one of the nodes gets ahead of the other nodes and successfully transmits the data. The CSMA/CD method is often called the race method. This method for networks with a bus topology is implemented by the Ethernet protocol.
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Topic 1.3: Open systems and the OSI model
Topic 1.4: Basics of local networks
Topic 1.5: Basic technologies of local networks
Topic 1.6: Basic software and hardware components of a LAN
Local networks
1.5. Basic technologies of local networks
1.5.2. Methods of data exchange in local networks
To control the exchange (network access control, network arbitration), various methods are used, the features of which largely depend on the network topology.
There are several groups of access methods based on time division of the channel:
- centralized and decentralized;
- deterministic and random.
Centralized access is controlled from a network control center, such as a server. The decentralized access method operates on the basis of protocols without control actions from the center.
Deterministic access provides each workstation with a guaranteed access time (for example, scheduled access time) to the data transmission medium. Random access is based on the equality of all stations in the network and their ability to access the medium at any time to transmit data.
Centralized access to mono channel
In networks with centralized access, two access methods are used: the polling method and the delegation method. These methods are used in networks with an explicit control center.
Survey method
Data exchange on a LAN with a star topology with an active center (central server). With a given topology, all stations can decide to transmit information to the server at the same time. The central server can communicate with only one workstation. Therefore, at any time it is necessary to select only one station broadcasting.
The central server sends requests to all stations in turn. Each workstation that wants to transmit data (the first one polled) sends a response or immediately begins transmission. After the end of the transmission session, the central server continues polling in a circle. Stations, in this case, have the following priorities: the maximum priority is for the one that is closest to the last station that completed the exchange.
Data exchange in a network with a bus topology. This topology may have the same centralized control as a star. One of the nodes (the central one) sends requests to all the others, finding out who wants to transmit, and then allows the transmission to whichever one of them reports it after the end of the transmission.
Transfer of authority method (passing token)
A token is a service package of a certain format into which clients can place their information packages. The sequence of transmitting a token over the network from one workstation to another is set by the server. The workstation receives permission to access the data transmission medium when it receives a special token packet. This access method for networks with bus and star topologies is provided by the ArcNet protocol.
Decentralized access to mono channel
Let's consider decentralized deterministic and random methods of access to the data transmission medium.
Decentralized deterministic access method
The decentralized deterministic method includes the token passing method. The token passing method uses a packet called a token. A token is a packet that does not have an address and circulates freely over the network; it can be free or busy.
Data exchange in a network with a ring topology (decentralized deterministic access method). This network uses the “token passing” access method.
The transfer algorithm is as follows:
- A node that wants to transmit waits for a free token, upon receiving it, it marks it as busy (changes the corresponding bits), adds its own packet to it, and sends the result further into the ring.
- Each node that receives such a token accepts it and checks whether the packet is addressed to it.
- If the packet is addressed to this node, then the node sets a specially allocated acknowledgment bit in the token and sends the modified token with the packet further.
- The transmitting node receives back its message that has passed through the entire ring, releases the token (marks it as free) and sends the token back to the network. In this case, the sending node knows whether its package was received or not.
For the normal functioning of this network, it is necessary that one of the computers or a special device ensure that the token is not lost, and if the token is lost, this computer must create it and launch it into the network.
Data exchange in a network with a bus topology (decentralized random access method)
In this case, all nodes have equal access to the network and the decision when to transmit is made by each node locally, based on an analysis of the network state. Competition arises between nodes for network capture, and, therefore, conflicts between them are possible, as well as distortion of transmitted data due to packet overlap.
Let's look at the most commonly used carrier sense multiple access with collision detection (CSMA/CD).
The essence of the algorithm is as follows:
- A node that wants to transmit information monitors the state of the network, and as soon as it is free, it begins transmission.
- The node transmits data and simultaneously monitors the state of the network (carrier sensing and collision detection). If no collisions are detected, the transfer is completed.
- If a collision is detected, the node amplifies it (transmits some more time) to ensure detection by all transmitting nodes, and then stops transmitting. Other transmitting nodes do the same.
- After the unsuccessful attempt is terminated, the node waits for a randomly selected period of time tback and then repeats its attempt to transmit, while monitoring for collisions. In case of a second collision, trear increases. Eventually, one of the nodes gets ahead of the other nodes and successfully transmits the data. The CSMA/CD method is often called the race method. This method for networks with a bus topology is implemented by the Ethernet protocol.
Sooner or later, when working with a local network, users need to transfer files from computer to computer. As a rule, there are no special problems with sending files over a local network: you can use standard tools in the Windows/Linux operating system or use additional software.
How to transfer files over a local network between 2 computers?
To send files over a local network using the operating system, the so-called . To do this, you need to open the folder properties (where the required file) and in the “Access” section allow you to use the computer on the network, view the folder and/or change and copy files.
This is not exactly transferring files from computer to computer, but the principle is similar: you will give the user access to the necessary files, and he will be able to open or copy the document he needs.
In addition, to share files over a local network, you can create one or all network computers“shared folder” into which the user will upload files through the “Network Neighborhood”/“Network” section, etc. (depending on the operating system used).
And anyone using a computer on the local network, if necessary, will be able to copy the necessary files to/from this folder.
A program for sharing files over a local network
For more advanced “users” (who do not want to use standard means OS or wants to receive additional features when transferring files over a local network) third-party software has been developed.
For example, the wonderful HTTP File Server program, which does not require installation, is suitable for exchanging files on a local network.
The main task of this program is the creation (or better said, imitation), which acts as a file hosting service.
After launching the program (it will be minimized to tray) the following will be presented:
Information about the “local IP address” - it will be indicated in address bar and it is also the server address; 
- - menu with available functionality;
- - number of the listening port.
The program is only available on English language, but the interface is quite simple and understanding it will not be difficult.
So, to transfer a file from computer to computer via a local network, you need to:
In the "Virtual" window File System» click right click mouse and select “Add File” or “Add folder from disk” (to select one file or a folder as a whole);
- - by these actions you placed files on the server and opened them for downloading to other users;
- - now to download this file from another computer you need to go to the address indicated at the top of the address bar (in the example it is “192.168.1.3”), select the desired file and download it.
The file hosting service is ready: local network users can add their files and folders through the program menu.
In addition, you can add accounts to restrict access to the file hosting service for third-party users (in this case, you will need to specify a login and password to download a file) or create local groups (so as not to “mix” all the files in one heap, but to structure them as needed).
The main goal that is pursued when connecting computers into a network is the ability to use the resources of each computer by all network users. In order to realize this feature, computers connected to the network must have the necessary means of interaction with other computers on the network.
Separation problem network resources includes solving many problems - choosing a method for addressing computers and coordinating electrical signals when establishing electrical communication, ensuring reliable data transmission and processing error messages, generating sent messages and interpreting received messages, as well as many other equally important tasks.
The usual approach to solving a complex problem is to break it down into several subproblems. A certain module is assigned to solve each subtask. At the same time, the functions of each module and the rules for their interaction are clearly defined.
A special case of task decomposition is a multi-level representation, in which the entire set of modules that solve subtasks is divided into hierarchically ordered groups - levels. For each level, a set of query functions is defined, with which modules at a given level can be accessed by modules at a higher level to solve their problems.
This set of functions performed by a given layer for a higher layer, as well as the message formats exchanged between two neighboring layers during their interaction, is called an interface.
The rules for interaction between two machines can be described as a set of procedures for each level. Such formalized rules that determine the sequence and format of messages exchanged between network components located at the same level, but in different nodes, are called protocols.
Harmonized set of protocols different levels, sufficient to organize internetworking, is called a protocol stack.
When organizing interaction, two main types of protocols can be used. In connection-oriented network service (CONS) protocols, before exchanging data, the sender and recipient must first establish a logical connection, that is, agree on parameters of the exchange procedure that will only apply within of this connection. After completing the dialogue, they must terminate this connection. When a new connection is established, the negotiation procedure is carried out again.
The second group of protocols are protocols without prior connection establishment (connectionless network service, CLNS). Such protocols are also called datagram protocols. The sender simply transmits the message when it is ready.
ISO/OSI model
Just because a protocol is an agreement between two interacting entities, in this case two computers working on a network, does not mean that it is necessarily a standard. But in practice, when implementing networks, they tend to use standard protocols. These can be branded, national or international standards.
The International Standards Organization (ISO) has developed a model that clearly defines the different levels of interaction between systems, gives them standard names, and specifies what work each level should do. This model is called the interaction model open systems(Open System Interconnection, OSI) or ISO/OSI model.
In the OSI model, communication is divided into seven layers or layers (Fig. 1). Each level deals with one specific aspect of interaction. Thus, the interaction problem is decomposed into 7 particular problems, each of which can be solved independently of the others. Each layer maintains interfaces with the layers above and below.
The OSI model only describes system tools interactions without touching end-user applications. Applications implement their own communication protocols by accessing system facilities. It should be borne in mind that the application can take over the functions of some of the upper layers of the OSI model, in which case, if necessary, internetworking it accesses directly the system tools that perform the functions of the remaining lower layers of the OSI model.
An end-user application can use system interaction tools not only to organize a dialogue with another application running on another machine, but also simply to receive the services of a particular network service.
So, let's say an application makes a request to an application layer, such as a file service. Based on this request, the application level software generates a standard format message, which contains service information (header) and, possibly, transmitted data. This message is then forwarded to the representative level.
The presentation layer adds its header to the message and passes the result down to the session layer, which in turn adds its header, and so on.
Finally, the message reaches the lowest, physical layer, which actually transmits it along the communication lines.
When a message arrives on another machine over the network, it moves up sequentially from level to level. Each level analyzes, processes and deletes the header of its level, performs the corresponding this level function and passes the message to the higher level.
In addition to the term "message", there are other names used by network specialists to designate a unit of data exchange. ISO standards for protocols of any level use the term “protocol data unit” - Protocol Data Unit (PDU). In addition, the names frame, packet, and datagram are often used.
ISO/OSI Model Layer Functions
Physical level. This layer deals with the transmission of bits over physical channels, such as coaxial cable, twisted pair or fiber optic cable. This level is related to the characteristics of physical data transmission media, such as bandwidth, noise immunity, characteristic impedance and others. At the same level, the characteristics of electrical signals are determined, such as requirements for pulse edges, voltage or current levels of the transmitted signal, type of coding, signal transmission speed. In addition, the types of connectors and the purpose of each contact are standardized here.
Physical layer functions are implemented in all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port.
Data link level. One of the tasks of the link layer is to check the availability of the transmission medium. Another task of the link layer is to implement error detection and correction mechanisms. To do this, at the data link layer, bits are grouped into sets called frames. The link layer ensures that each frame is transmitted correctly by placing a special sequence of bits at the beginning and end of each frame to mark it, and also calculates a checksum by summing all the bytes of the frame in a certain way and adding the checksum to the frame. When the frame arrives, the receiver again calculates the checksum of the received data and compares the result with the checksum from the frame. If they match, the frame is considered correct and accepted. If the checksums do not match, an error is recorded.
The link layer protocols used in local networks contain a certain structure of connections between computers and methods for addressing them. Although the data link layer provides frame delivery between any two nodes on a local network, it does this only in a network with a very specific connection topology, precisely the topology for which it was designed. Typical topologies supported by LAN link layer protocols include shared bus, ring, and star. Examples of link layer protocols are Ethernet, Token Ring, FDDI, 100VG-AnyLAN.
Network layer. This level serves to form a unified transport system that unites several networks with different principles for transmitting information between end nodes.
Network layer messages are usually called packets. When organizing packet delivery at the network level, the concept of “network number” is used. In this case, the recipient's address consists of the network number and the computer number on this network.
In order to transmit a message from a sender located on one network to a recipient located on another network, you need to make a number of transit transfers (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers through which a packet passes.
The problem of choosing the best path is called routing and its solution is main task network level. This problem is complicated by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transmission along this route; it depends on the capacity of communication channels and traffic intensity, which can change over time.
At the network level, two types of protocols are defined. The first type refers to the definition of rules for transmitting end node data packets from the node to the router and between routers. These are the protocols that are usually meant when people talk about network layer protocols. The network layer also includes another type of protocol, called routing information exchange protocols. Using these protocols, routers collect information about the topology of internetwork connections. Network layer protocols are implemented by operating system software modules, as well as router software and hardware.
Examples of network layer protocols are the TCP/IP stack IP Internetwork Protocol and the Novell IPX stack Internetwork Protocol.
Transport layer. On the way from the sender to the recipient, packets may be corrupted or lost. While some applications have their own error handling, there are others that prefer to deal with a reliable connection right away. The job of the transport layer is to ensure that applications or the upper layers of the stack - application and session - transfer data with the degree of reliability that they require. The OSI model defines five classes of service provided by the transport layer.
As a rule, all protocols, starting from the transport layer and above, are implemented software end nodes of the network - components of their network operating systems. An example of transport protocols is TCP protocols and UDP of the TCP/IP stack and the SPX protocol of the Novell stack.
Session level. The session layer provides conversation management to record which party is currently active and also provides synchronization facilities. The latter allow you to insert checkpoints into long transfers so that in case of failure you can go back to the last checkpoint, instead of starting all over again. In practice, few applications use the session layer, and it is rarely implemented.
Presentation level. This layer provides assurance that information conveyed by the application layer will be understood by the application layer in another system. If necessary, the presentation layer converts data formats into some common presentation format, and at the reception, accordingly, performs the reverse conversion. In this way, application layers can overcome, for example, syntactic differences in data representation. At this level, encryption and decryption of data can be performed, thanks to which the secrecy of data exchange is ensured for all application services at once. An example of a protocol that operates at the presentation layer is the Secure Socket Layer (SSL) protocol, which provides secure messaging for the application layer protocols of the TCP/IP stack.
Application layer. The application layer is really just a set of various protocols through which network users access shared resources such as files, printers, or hypertext Web pages, and also organize their collaboration, for example, using the protocol Email. The unit of data that the application layer operates on is usually called a message.
There is a very wide variety of application layer protocols. Let us give as examples at least a few of the most common implementations of file services: NCP in operating system Novell NetWare, SMB in Microsoft Windows NT, NFS, FTP and TFTP included in the TCP/IP stack.
Application interaction protocols and transport subsystem protocols
Functions at all layers of the OSI model can be classified into one of two groups: either functions that depend on a specific technical implementation of the network, or functions that are oriented to work with applications.
The three lower levels - physical, channel and network - are network-dependent, that is, the protocols of these levels are closely related to the technical implementation of the network and the communication equipment used.
The three upper levels - session, presentation and application - are application-oriented and have little dependence on the technical features of network construction. The protocols at these layers are not affected by any changes in network topology, hardware replacement, or migration to another network technology.
The transport layer is the intermediate layer, it hides all the details of the functioning of the lower layers from the upper layers. This allows you to develop applications independent of technical means directly involved in the transport of messages.
Figure 2 shows the layers of the OSI model at which the various network elements operate.
A computer with a network OS installed on it interacts with another computer using protocols of all seven levels. Computers carry out this interaction through various communication devices: hubs, modems, bridges, switches, routers, multiplexers. Depending on the type, a communication device can operate either only at the physical layer (repeater), or at the physical and link level (bridge and switch), or at the physical, link and network layer, sometimes including the transport layer (router).
Functional compliance of types of communication equipment with levels of the OSI model
The best way to understand the differences between network adapters, repeaters, bridges/switches, and routers is to think of them in terms of the OSI model. The relationship between the functions of these devices and the layers of the OSI model is shown in Figure 3.
A repeater, which regenerates signals, thereby allowing you to increase the length of the network, operates at the physical level.
The network adapter operates at the physical and data link layers. The physical layer includes that part of the functions of the network adapter that is associated with the reception and transmission of signals over the communication line, and gaining access to the shared transmission medium and recognizing the MAC address of the computer is already a function of the link layer.
Bridges do most of their work at the data link layer. For them, the network is represented by a set of MAC addresses of devices. They extract these addresses from headers added to packets at the data link layer and use them during packet processing to decide which port to send a particular packet to. Bridges do not have access to network address information related to more high level. Therefore, they are limited in making decisions about possible paths or routes for packets to travel through the network.
Routers operate at the network layer of the OSI model. For routers, a network is a set of device network addresses and a set of network paths. Routers analyze all possible paths between any two network nodes and choose the shortest one. When choosing, other factors may be taken into account, for example, the state of intermediate nodes and communication lines, line capacity or the cost of data transmission.
In order for a router to perform the functions assigned to it, it must have access to more detailed information about the network than that available to the bridge. In addition to the network address, the network layer packet header contains data, for example, about the criteria that should be used when choosing a route, about the lifetime of the packet in the network, and about which upper-level protocol the packet belongs to.
Thanks to the use additional information, a router can perform more packet operations than a bridge/switch. Therefore, the software required to operate the router is more complex.
Figure 3 shows another type of communication device - a gateway, which can operate at any level of the OSI model. A gateway is a device that performs protocol translation. A gateway is placed between communicating networks and serves as an intermediary, translating messages coming from one network into the format of another network. The gateway can be implemented either purely by software installed on a regular computer, or on the basis of a specialized computer. Translating one protocol stack into another is a complex intellectual task that requires maximum complete information about the network, so the gateway uses the headers of all broadcast protocols.
IEEE 802 Specification
Around the same time that the OSI model was introduced, the IEEE 802 specification was published, which effectively extends the OSI networking model. This expansion occurs at the data link and physical layers, which determine how more than one computer can access a network without conflicting with other computers on the network.
This standard details these layers by breaking the data link layer into 2 sublayers:
– Logical Link Control (LLC) – logical link control sublevel. Manages connections between data channels and defines the use of logical interface points, called Services Access Points, which other computers can use to pass information to higher layers of the OSI model;
– Media Access Control (MAC) – device access control sublayer. Provides parallel access for multiple network adapters at the physical level, has direct interaction with network card computer and is responsible for ensuring error-free data transfer between computers on the network.
By protocol stack
A protocol suite (or protocol stack) is a combination of protocols that work together to provide network communication. These protocol suites are usually divided into three groups, corresponding to the OSI network model:
– network;
– transport;
– applied.
Network protocols provide the following services:
– addressing and routing of information;
– checking for errors;
– retransmission request;
– establishing rules of interaction in a specific network environment.
Popular network protocols:
– DDP (Delivery Datagram Protocol). The Apple data transfer protocol used in AppleTalk.
– IP (Internet Protocol). Part of the TCP/IP protocol suite that provides addressing and routing information.
– IPX (Internetwork Packet eXchange) and NWLink. A Novell NetWare networking protocol (and Microsoft's implementation of this protocol) used for routing and forwarding packets.
– NetBEUI. Developed jointly by IBM and Microsoft, this protocol provides transport services for NetBIOS.
Transport protocols are responsible for ensuring reliable transport of data between computers.
Popular transport protocols:
– ATP (AppleTalk Transaction Protocol) and NBP (Name Binding Protocol). AppleTalk session and transport protocols.
– NetBIOS/NetBEUI. The first one establishes a connection between computers, and the second one provides data transfer services for this connection.
– SPX (Sequenced Packet exchange) and NWLink. Novell's connection-oriented protocol used to provide data delivery (and Microsoft's implementation of this protocol).
– TCP (Transmission Control Protocol). Part of the TCP/IP protocol suite responsible for reliable data delivery.
Application protocols responsible for the interaction of applications.
Popular application protocols:
– AFP (AppleTalk File Protocol). Protocol remote control Macintosh files.
– FTP (File Transfer Protocol). Another member of the TCP/IP protocol suite, used to provide file transfer services.
– NCP (NetWare Core Protocol – NetWare Basic Protocol). Novell client shell and redirectors.
– SMTP (Simple Mail Transport Protocol). A member of the TCP/IP protocol suite responsible for transmitting electronic mail.
– SNMP (Simple Network Management Protocol). TCP/IP protocol used to manage and monitor network devices.
Methods of data exchange in local networks
To control the exchange (network access control, network arbitration), various methods are used, the features of which largely depend on the network topology.
There are several groups of access methods based on time division of the channel:
centralized and decentralized
deterministic and random
Centralized access is controlled from a network control center, such as a server. The decentralized access method operates on the basis of protocols without control actions from the center.
Deterministic access provides each workstation with a guaranteed access time (for example, scheduled access time) to the data transmission medium. Random access is based on the equality of all stations in the network and their ability to access the medium at any time to transmit data.
Centralized access to mono channel
In networks with centralized access, two access methods are used: the polling method and the delegation method. These methods are used in networks with an explicit control center.
Survey method.
Data exchange on a LAN with a star topology with an active center (central server). With a given topology, all stations can decide to transmit information to the server at the same time. The central server can communicate with only one workstation. Therefore, at any time it is necessary to select only one station broadcasting.
The central server sends requests to all stations in turn. Each workstation that wants to transmit data (the first one polled) sends a response or immediately begins transmission. After the end of the transmission session, the central server continues polling in a circle. Stations, in this case, have the following priorities: the maximum priority is for the one that is closest to the last station that completed the exchange.
Data exchange in a network with a bus topology. This topology may have the same centralized control as a star. One of the nodes (the central one) sends requests to all the others, finding out who wants to transmit, and then allows the transmission to whichever one of them reports it after the end of the transmission.
Transfer of authority method (passing token)
A token is a service package of a certain format into which clients can place their information packages. The sequence of transmitting a token over the network from one workstation to another is set by the server. The workstation receives permission to access the data transmission medium when it receives a special token packet. This access method for networks with bus and star topologies is provided by the ArcNet protocol.
Decentralized access to mono channel
Let's consider decentralized deterministic and random methods of access to the data transmission medium.
The decentralized deterministic method includes the token passing method. The token passing method uses a packet called a token. A token is a packet that does not have an address and circulates freely over the network; it can be free or busy.
Data exchange in a network with a ring topology
1. This network uses the “token passing” access method. The transfer algorithm is as follows:
a) a node wishing to transmit waits for a free token, upon receiving which it marks it as busy (changes the corresponding bits), adds its own packet to it and sends the result further into the ring;
b) each node that receives such a token accepts it and checks whether the packet is addressed to it;
c) if the packet is addressed to this node, then the node sets a specially allocated acknowledgment bit in the token and sends the modified token with the packet further;
d) the transmitting node receives back its message, which has passed through the entire ring, releases the token (marks it as free) and again sends the token to the network. In this case, the sending node knows whether its package was received or not.
For the normal functioning of this network, it is necessary that one of the computers or a special device ensure that the token is not lost, and if the token is lost, this computer must create it and launch it into the network.
Data exchange in a network with bus topology
In this case, all nodes have equal access to the network and the decision when to transmit is made by each node locally, based on an analysis of the network state. Competition arises between nodes for network capture, and, therefore, conflicts between them are possible, as well as distortion of transmitted data due to packet overlap.
Let's look at the most commonly used carrier sense multiple access with collision detection (CSMA/CD). The essence of the algorithm is as follows:
1) a node that wants to transmit information monitors the state of the network, and as soon as it is free, it begins transmission;
2) the node transmits data and simultaneously monitors the state of the network (carrier sensing and collision detection). If no collisions are detected, the transfer is completed;
3) if a collision is detected, the node amplifies it (transmits for some more time) to ensure detection by all transmitting nodes, and then stops transmitting. Other transmitting nodes do the same;
4) after the unsuccessful attempt is terminated, the node waits for a randomly selected period of time tback, and then repeats its attempt to transmit, while monitoring collisions.
In case of a second collision, trear increases. Eventually, one of the nodes gets ahead of the other nodes and successfully transmits the data. The CSMA/CD method is often called the race method. This method for networks with a bus topology is implemented by the Ethernet protocol.