What are the topologies of computer networks?

The topology of a computer network refers to the physical structure of nodes and lines formed by online computers or equipment and transmission media. It mainly includes star topology, bus topology, ring topology, and tree topology. structure, hybrid topology, mesh topology, switching power supply topology; among them, star network topology is the most widely used network topology.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

Computer Network Topology refers to the distribution and connection status of devices among a network composed of computers. Draw these two on the map to form a topology map. Generally, the location of the equipment, the name type of the equipment, and the type of connection medium between the equipment should be indicated on the diagram. It is divided into two types: physical topology and logical topology.

The topology of a computer network

The topology of a computer network refers to the nodes and connections formed by online computers or equipment and transmission media. The physical pattern of lines. There are two types of network nodes: one is the transfer node that converts and exchanges information, including node switches, hubs, and terminal controllers; the other is the access node, including computer hosts and terminals. Lines represent various transmission media, both tangible and intangible.

Composition

Every network structure is composed of nodes, links and pathways.

1. Node: also called network unit, it is various data processing equipment, data communication control equipment and data terminal equipment in the network system. Common nodes include servers, workstations, lines, switches and other equipment.

2. Link: The connection between two nodes can be divided into two types: physical link and logical link. The former refers to the actual communication line, and the latter refers to the logical link. Network access.

3. Path: refers to a series of nodes and links from the node that sends information to the node that receives information, that is, a series of nodes to nodes established through the communication network. chain.

Common types of topology

Computer networks are divided into star topology, bus topology, and ring topology according to topology. , tree topology, hybrid topology, mesh topology, switching power supply topology.

Star topology

The star topology is composed of a central node and various sites connected to the central node through point-to-point communication links. The central node implements a centralized communication control strategy, so the central node is quite complex, while the communication processing burden of each site is very small. The switching methods used in star networks include circuit switching and message switching, with circuit switching being more common. Once a channel connection is established in this structure, data can be transmitted between the two connected sites without delay. The popular PBX (Private Branch exchange) is a typical example of star topology.

Advantages of star topology:

• (1) Simple structure, convenient connection, relatively easy management and maintenance, and strong scalability.

• (2) The network delay time is small and the transmission error is low.

• (3) Support multiple transmission media within the same network segment. Unless the central node fails, the network will not be easily paralyzed.

• (4) Each node is directly connected to the central node, faults are easy to detect and isolate, and faulty nodes can be easily eliminated.

Therefore, the star network topology is the most widely used network topology.

Disadvantages of star topology:

• (1) The cost of installation and maintenance is higher

• (2) The ability to share resources is poor

• (3) A communication line is only used by the central node and edge node on the line, and the utilization rate of the communication line is not high

• (4) The requirements for the central node are quite high. Once the central node fails, the entire network will be paralyzed.

Star topology is widely used in situations where network intelligence is concentrated on the central node. Judging from the trend, the development of computers has developed from centralized host systems to a large number of powerful microcomputers and workstations. Under this situation, the use of traditional star topology will be reduced.

Bus topology

The bus topology uses a channel as the transmission medium. All stations are directly connected to this public transmission medium through the corresponding hardware interface. The transmission medium is called a bus. The signal sent by any station propagates along the transmission medium and can be received by all other stations.

Because all stations share a common transmission channel, only one device can transmit signals at a time. A distributed control strategy is usually used to determine which station can send. When sending, the sending station divides the message into packets, and then sends these packets one by one, sometimes alternately with packets from other stations for transmission on the media. When the packets pass through each station, the destination station will recognize the destination address carried by the packets and then copy the contents of these packets.

Advantages of bus topology:

• (1) The bus structure requires a small number of cables, a short cable length, and is easy to wire and maintain.

• (2) The bus structure is simple, it works from the source, and has high reliability. The transmission rate is high, up to 1~100Mbps.

• (3) Easy to expand, it is convenient to add or reduce users, simple structure, easy networking, convenient network expansion

• (4) Multiple nodes share a transmission channel, and the channel utilization rate is high.

Disadvantages of bus topology:

• (1) The transmission distance of the bus is limited and the communication range is restricted.

• (2) Fault diagnosis and isolation are difficult.

• (3) Distributed protocols cannot guarantee the timely transmission of information and do not have real-time functions. The site must be smart and have media access control functions, which increases the site's hardware and software overhead.

Ring topology

In the ring topology, each node is connected through a ring interface in a closed ring communication line connected end to end. , any node on the ring can request to send information. Once the request is approved, information can be sent to the ring. Data in the ring network can be transmitted in one direction or two directions. Since the ring is public, the information sent by a node must pass through all the ring interfaces in the ring. When the destination address in the information flow matches the address of a node on the ring, the information is received by the ring interface of the node, and then the information continues to flow to the next loop interface until it flows back to the loop interface node that sent the information.

Advantages of ring topology:

• (1) Short cable length. The cable length required for a ring topology network is similar to that for a bus topology network, but much shorter than for a star topology network.

• (2) When adding or reducing workstations, only a simple connection operation is required.

• (3) Optical fiber can be used. The transmission rate of optical fiber is very high, which is very suitable for unidirectional transmission of ring topology.

Disadvantages of the ring topology:

• (1) Node failure will cause the entire network to fail. This is because data transmission on the ring must pass through every node connected to the ring. Once a node in the ring fails, it will cause a failure in the entire network.

• (2) Fault detection is difficult. This is similar to the bus topology, because it is not centralized control and fault detection needs to be performed at each node on the network, so it is not very easy.

• (3) The media access control protocols in ring topology all use token passing. When the load is very light, the channel utilization is relatively low.

Tree topology

Tree topology can be considered to be composed of a multi-level star structure, but this multi-level star structure It is distributed in a triangle from top to bottom, just like a tree, with fewer branches and leaves at the top, more in the middle, and the most branches and leaves at the bottom. The bottom of the tree is equivalent to the edge layer in the network, the middle part of the tree is equivalent to the aggregation layer in the network, and the top of the tree is equivalent to the core layer in the network. It adopts a hierarchical centralized control method, and its transmission medium can have multiple branches, but it does not form a closed loop. Each communication line must support two-way transmission.

Advantages of tree topology:

• (1) Easy to expand. This structure can extend many branches and sub-branches, and these new nodes and new branches can be easily added to the network.

• (2) Fault isolation is easier. If a node or line on a certain branch fails, it is easy to isolate the failed branch from the entire system.

Disadvantages of tree topology:

• Each node is too dependent on the root. If the root fails, the entire network will not be able to function normally. Work. From this point of view, the reliability of the tree topology is somewhat similar to that of the star topology.

Hybrid topology

Hybrid topology is a topology that mixes two single topologies and takes the advantages of both.

One is the "star-ring" topology that is a mixture of star topology and ring topology, and the other is a "star-total" topology that is a mixture of star topology and bus topology.

These two hybrid structures have similarities. If the two endpoints of the bus topology are connected together, it will become a ring topology.

In a hybrid topology, the aggregation layer devices form a ring or bus topology, and the aggregation layer devices and access layer devices form a star topology.

Advantages of hybrid topology:

• (1) Fault diagnosis and isolation are more convenient. Once a network failure occurs, you only need to diagnose which network device is faulty and isolate the network device from the entire network.

• (2) Easy to expand. When you want to expand users, you can add new network devices, or during design, leave some spare connection ports in each network device that can be plugged into new sites.

• (3) Easy to install. The main link of the network only needs to connect to the aggregation layer devices, and then connects the aggregation layer devices and access layer devices through branch links.

Disadvantages of hybrid topology:

• (1) Intelligent network equipment needs to be selected to realize automatic diagnosis of network faults and isolation of faulty nodes. The cost of network construction is relatively high.

• (2) Like the star topology, the cable installation length from the aggregation layer device to the access layer device will increase significantly.

Network topology

Network topology. This structure has been widely used in wide area networks, and its advantage is that it is not affected by bottleneck problems and failure problems. Because there are many paths between nodes, appropriate routing can be chosen for the transmission of data flows, bypassing failed components or overbusy nodes. Although this structure is relatively complex, the cost is relatively high, and the network protocol that provides the above functions is also relatively complex, it is still welcomed by users because of its high reliability.

One application of network topology is in the BGP protocol. In order to ensure the connectivity between IBGP peers, a fully connected relationship, that is, a mesh network, needs to be established between IBGP peers. Assuming that there are n routers within an AS, the number of IBGP connections that should be established is n(n-1)/2.

Advantages of mesh topology:

• (1) There are many paths between nodes, reducing collisions and blocking.

• (2) Local faults do not affect the entire network and have high reliability.

Disadvantages of network topology:

• (1) The network relationship is complex, the network is difficult to build, and it is not easy to expand.

• (2) The network control mechanism is complex and routing algorithms and flow control mechanisms must be used.

Switching power supply topology

With the continuous development and improvement of PWM technology, switching power supplies have been widely used due to their high cost performance. There are many circuit topologies for switching power supplies. Commonly used circuit topologies include push-pull, full-bridge, half-bridge, single-ended forward and single-ended flyback. Among them, in the half-bridge circuit, current flows through the transformer primary throughout the entire cycle, the magnetic core is fully utilized, and there is no bias problem. The power switch tube used has lower voltage withstand requirements, and the saturation voltage drop of the switch tube is reduced. At the minimum, the voltage requirements for the input filter capacitor are also lower. Due to many of the above reasons, half-bridge converters are widely used in high-frequency switching power supply designs.

There are about 14 basic topologies commonly used in switching power supplies.

Each topology has its own characteristics and applicable occasions. Some topologies are suitable for offline (grid-powered) AC/DC converters. Some of them are suitable for low power output (<200W), some are suitable for high power output; some are suitable for high voltage input (≥220V AC), some are suitable for 120V AC or lower input situations; some are suitable for high voltage DC output (>~200V) Or multiple groups (more than 4 to 5 groups) have advantages in output situations; some use fewer devices at the same output power or have a better compromise between the number of devices and reliability. Smaller input/output ripple and noise are also factors often considered when selecting a topology.

Some topologies are more suitable for DC/DC converters. When choosing, you also need to consider whether it is high power or low power, high voltage output or low voltage output, and whether it requires as few components as possible. In addition, some topologies have their own flaws and require additional circuits that are complex and difficult to analyze quantitatively to work.

Therefore, to choose the topology appropriately, it is very important to be familiar with the advantages, disadvantages and applicable scope of various topologies. The wrong choice can doom a power supply design to failure from the start.

Commonly used topologies of switching power supplies:

buck switching regulator topology, boost switching regulator topology, reverse polarity switching regulator topology, push-pull topology, forward converter topology, double-ended forward Exciter converter topology, interleaved forward converter topology, half-bridge converter topology, full-bridge converter topology, flyback converter, current mode topology and current fed topology, SCR resonant topology, CUK converter topology

A collection of various switching power supply topologies first gives six basic DC/DC converter topologies

The order is buck, boost, buck-boost, cuk, zeta, sepic converter

Disadvantages of tree topology:

• Each node is too dependent on the root.

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