In the LAN, computers, terminals and other devices are regarded as "nodes", and communication lines are regarded as "lines". The topological connection mode formed by these nodes and lines is called network topology.
The topology of the network reflects the structural relationship of the network cabling, which has an important impact on the performance, reliability and cost calculation of the network.Therefore, the network topology is not onlyWebsite DesignIn addition, it plays a very important role in network construction and network management.
An important difference between LAN and WAN is that they cover different geographical ranges.The main goal of LAN design is to build a "limited geographical range" of a company, a school, a building or even several buildings. It has chosen the "shared media" mode and the "exchange" mode in terms of the basic communication mechanism. Therefore, to build a LAN, what kind of physical topology, what kind of media access control methods are used, and their respective characteristics,We will introduce them one by one below.Common LAN physical topologies include bus topology, star topology, ring topology and tree topology derived from star topology.
Bus topology
Bus topology, all nodes are connected to the passive public bus through the corresponding hardware interface (three-way connector), and a 50 ohm terminal resistor (called "impedance matcher" - to suppress interference caused by signal reflection) is installed at both ends of the bus. CSMA/CD communication control mode is adopted,That is, digital signals sent by any node will be transmitted along the bus and listened by other nodes on the bus. Its transmission direction is from the sending node to both ends of the bus. It is a broadcast structure, as shown in Figure 5-6.
In the network with this structure, the adapter (network card) on each node has a receiver/transmitter, and the data will be sent out in "frame" unit. Only the node that matches the destination MAC of the frame can receive.
The bus structure has the advantages of simple installation, easy expansion, low cost and high reliability.For example: 1OBase-5,1OBase-2 standard Ethernet.
With the emergence of Fast Ethernet, this kind of traditional, low-speed bus structure network has been eliminated, but as a kind of LAN structure, it just needs to be understood.
1. Features of traditional bus Ethernet
(1) In the broadcast channel, all nodes have equal status, and there is no central node control. The transmission of data frames always spreads from the sending node to both ends, and other nodes can listen to the data frame.
(2) The communication medium is coaxial thin cable, which is connected with the node through a three-way connector;The transmission rate is 10Mbps, and the maximum transmission distance is 185 meters.
(3) The media access method adopts the CSMA/CD protocol of Ethernet.
2. Insufficient traditional bus Ethernet
(1) Shared bus, network communication efficiency is very low under high load.
(2) The network is sensitive to bus failure. Once the bus has a problem, the whole network will be paralyzed.
(3) Because of the data conflict problem in the network with this structure, it is not suitable for applications with high real-time requirements.