Computer network topology

The term computer network topology describes a way of organizing physical connections between computers and other network components. The choice of a particular topology affects the composition of the necessary network equipment, the possibility of expanding the network, and the way the network is managed. All networks are built on the basis of basic topologies: bus, star, ring, mesh, which are not complicated, but in practice quite complex combinations of them are often found.

The term computer network topology describes a way of organizing physical connections between computers and other network components

The bus (Fig. 1, a). This topology is often called a linear bus. It is the simplest of all topologies. It uses one cable, called a trunk, along which all computers are connected.

In a network with a bus topology, data in the form of electrical signals is transmitted to all computers in the network, but it is received by one whose address matches the recipient's address, encrypted in these signals. Moreover, at each moment in time, only one computer can transmit. Therefore, the performance of such a network depends on the number of computers connected to the bus. The more computers waiting for data transfer, the slower the network. Network performance is also affected by: type of hardware for network computers; the frequency with which computers transmit data; type of working network applications; type of network cable; distance between computers on the network.

The bus is a passive topology of computer networks: computers only listen to the data transmitted over the network, but do not move them from the sender to the receiver. Therefore, the failure of one or more computers out of order does not affect the operation of the network.

Star (Fig. 1, b). In star topology, all computers are connected via cable segments to a central device called a hub. The signals from the transmitting computer go through the hub to all the others.

Currently, the hub has become one of the standard network components. In networks with star topology, for example, it serves as a central node. Hubs are divided into active and passive. Active regenerate and transmit signals in the same way as repeaters. Active hubs should be connected to the mains. Passive hubs include mounting or switching panels. They simply pass the signal through themselves, without amplifying or restoring it.

The disadvantages of this topology are: additional cable consumption, installation of a hub. The main advantage of this topology over the bus is its higher reliability. The failure of one or more computers does not affect the operation of the network.

Ring (Fig. 1, c). Computers connect to a cable looped into a ring. Signals are transmitted in a ring in one direction and pass through each computer. Unlike the passive bus topology, here each computer acts as a repeater (repeater), amplifying the signals and transmitting them to the next computer. Therefore, the failure of at least one computer leads to a drop in the network.

The method of transmitting data in a ring is called token transfer. A token is a special sequence of bits transmitted over a network. Each network has only one token. The marker moves in a ring sequentially from one computer to another until it is captured by the computer that wants to transfer data. The sending computer adds to the marker the data and address of the recipient, and sends it further along the ring. Data passes through each computer until it is at the one whose address matches the address of the recipient. Then the receiving computer sends the message to the transmitter, in which it confirms the fact of reception. After receiving confirmation, the sending computer restores the token and returns it to the network.

Cellular topology (Fig. 1, d). A network with a mesh topology is highly redundant and reliable, since each computer in such a network is connected to each other individual cable.

The signal from the sending computer to the receiving computer can go through different routes, so a cable break does not affect the network’s performance. The main disadvantage is the high cost of laying the cable, which is offset by high reliability and ease of maintenance.

In addition to the basic topologies of computer networks, there are combinations of these - combined topologies. Most often, two combined topologies are used: star-bus and star-ring.