Modems, Communication devices, Data transmission environments - Informatics


The modem is designed to transmit information over long distances using telephone lines and includes a modulator that converts binary information from the computer to analog signals and a demodulator that extracts encoded binary information from the received modulated signal and transmits it to the computer.

The modem is installed between the computer and the telephone line that connects the user to the Internet service provider or to the remote access server of the private network. To access the Internet or corporate network through the telephone network, the user's modem sends a call to the modem located on the Remote Access Server (RAS). A modem of any type is a sequential device, in which the data bits are transmitted one by one.

Communication Devices

There are many different communication or commuting devices, such as repeaters, bridges, hubs, routers and gateways. In Table. 9.2 shows the correspondence of the switching devices to the levels of the standard OSI network model.

Table 9.2






Application Gateway







Transport Gateway






Bridge, Switch



Repeater, hub

Considering switching devices from the point of view of the seven-level OSI model, it is possible to identify what part of the information of the original message is used by intermediate network devices to select a route during its transfer from the sender to the receiver. The data prepared by the sender (Figure 9.6) is transmitted sequentially:

Frame, Package and Headers

Fig. 9.6. Frame, package and headers

• to the transport layer, which adds its own header (for example, the TCP header of the transmission control protocol);

• The network layer, which in turn also adds its header (packet), resulting in a network layer package (for example, an IP package);

• The channel layer where the frame is formed by adding another header (frame) and a trailer as a checksum (CRC code);

• The physical layer for transporting over the network.

Let's look at the features of switching devices and find out how they relate to packets and frames.

Repeaters (Repeaters) are communication devices of the lowest, physical level. The simplest repeater is a two-port analog device for physically connecting different segments of the LAN cable to increase the overall network length (Figure 9.7, a). Each port has its own transceiver, consisting of a transmitter and a receiver. The repeater improves the quality of the transmitted signal: restores the amplitude and power of the output signal, reduces the length of the edges, and so on. In the network

Fig. 9.7. Switching devices

Ethernet, four repeaters can be installed, allowing you to extend the cable length to 2500 m.

Concentrators (Concentrator); or hubs, like repeaters, operate at the physical level, but differ from them in that they have several electrically connected inputs/outputs ( ports), to which the transmission lines are connected. All lines must work at the same speed. In Fig. 9.7, b the electrical connection inside the switch is indicated by a large point. The frames arriving at any line (input) are transmitted to all other lines (outputs). If two frames arrive simultaneously on different lines (inputs), then because of the presence of an electrical connection in the concentrator, a collision (collision) occurs.

Ethernet hubs have between 8 and 72 ports. The transceiver of each port, in addition to the transmitter and receiver, contains a collision detector with which it is possible to provide access to the network and isolate the port if there are continuous errors (collisions) on it.

Logical network structuring is performed using bridges, switches, routers and gateways. Consider bridges and switches working at the link layer.

Bridges connect two (see Figure 9.7, in) or more local networks, also called subnets, network segments or collision domains. The main function of the bridge is to retransmit the data (frame) from one segment of the network to another. A bridge, unlike a repeater or concentrator, analyzes the destination address of the frame, if:

• the destination address of the incoming frame belongs to the same segment, then the frame is ignored by the bridge;

• the destination address is known to the bridge and belongs to another segment, then the bridge translates this frame to the corresponding port;

• The destination address is not yet known to the bridge, then the frame is broadcast to all ports, except where it came from, and the unknown address is stored for later use, i.e. in the course of work the bridge is self-learning. After self-learning, the bridge transmits frames only to the destination segment, thereby reducing the total amount of data transmitted over the network.

Broadcast and multicast frames are also broadcast to all ports. The bridge allows you to change the logical structure of the network while maintaining the physical location of nodes and the connections between them. Logical subnetting increases the security of data by limiting access to individual users.

Modern bridges, like concentrators, are equipped with network cards, usually designed for four or eight inputs of a certain type. If there are several boards, the bridge is able to work with different types of networks.

Switches (Switch) are advanced bridges and also use frame addresses for routing. Each switch is equipped with a dedicated processor, so that the overall performance of the switch exceeds the performance of a traditional bridge that has one processor unit. However, unlike bridges connecting entire networks, switches are most often used to connect individual computers (see Figure 9.7, d). Therefore, switches have much more connectors for network cards than bridges. Each port is a region of collisions (collisions). To prevent them, each switch port is equipped with a buffer for storing incoming frames. Therefore, collisions can occur only when the buffer overflows. To prevent collisions, modern switches begin sending frames right after receiving their headers, i.e. they do not use protocols with expectation. Such switches are called pass-through. The hardware implementation of the algorithm without waiting is most often used, whereas in bridges there is traditionally a processor that implements routing with expectation.

Router (Router) refers to the network layer of the OSI model and have significant differences from bridges and standard hubs. The main function of the router is to read the headers of packets of network protocols and to decide on the next route of the packet. A packet formed by the network layer arrives on the router (see in Figure 9.6 in dark color), in which there is no frame header and a trailer (CRC). The packet is transferred to the software of the router that analyzes the header of the packet and, in accordance with it, selects the next path of the packet.

The appearance of routers is due to the limitations of bridges and switches in connection topology and other indicators. Due to the use of composite numeric addresses (with subnet numbers, computers and own ports), routers more reliably and effectively isolate the traffic of separate parts of the network from each other. In addition to traffic localization, routers are able to perform many other useful functions, for example they can work in a closed loop network, while choosing a rational route from several possible ones, and also to connect to a single network subnets built using different network technologies, for example, Ethernet and X .25.

Transport Gateways are used to connect computers that use different connection-oriented transport protocols, such as TCP/IP and ATM. In this case, the transport gateway can copy packets, simultaneously leading them to the desired format.

Application Gateways work with the formats and content of packages at a higher level. For example, the E-Mail gateway can translate emails into SMS messages for mobile phones.

Communication Environments

The main component of telecommunication networks is the medium (Medium) or the medium of data transmission, over which signals are transmitted. As such a medium, a coaxial cable, a twisted-pair cable, a fiber optic cable, and a wireless medium (free space) are used.

thematic pictures

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