The following are four topologies commonly used in local area networks.
1. star topology
A star topology consists of a central node and sites connected to the central node through point-to-point links.
(1) working mode
It is quite complicated for the central node to realize centralized communication control strategy; And the communication processing burden of each site is very small.
The popular PBX is a typical example of star topology.
⑵ Advantages of star topology
(1) central node realizes centralized control, which can provide services and reconfigure conveniently.
② Only one device is connected to each connection, and the failure of the connection point will not affect the whole network.
③ Because each site is directly connected to the central node, the fault can be easily detected and isolated, and the fault site can be easily removed from the system.
④ The access protocol is simple.
(3) Disadvantages of star topology
① Because each site is directly connected to the central node, a large number of cables and cable trenches are needed. The installation and maintenance of cables are prone to problems.
② Too much dependence on the central node. When the central node fails, the whole network cannot work, so the reliability of the central node is required.
2. Bus topology
The bus topology uses a single transmission line as the transmission medium, and all stations are directly connected to the transmission medium (bus) through corresponding hardware interfaces.
(1) working mode
Data sent by any site can be transmitted through the medium. Usually, the destination address has been encoded in the message information so that the station matching the address in the message can receive the information.
Because all nodes share a common data transmission link, it can only be occupied by one device at any time. In order to make the work orderly, a distributed control strategy (carrier sense multiplexing protocol with collision detection) is usually adopted to decide which station can send data next time.
⑵ Advantages of bus topology
① Short cable length, easy wiring, easy maintenance and low installation cost.
② Simple structure, passive components and high reliability.
(3) Easy expansion: new stations can be directly added at any position of the bus; In order to increase the length of network segments, additional network segments can be added through repeaters.
(3) Disadvantages of bus topology
Difficulties in fault diagnosis and isolation: the bus structure is not centralized control, and fault detection is needed at all sites on the Internet. If the fault occurs at a station, the station should be removed from the bus. If the transmission medium fails, the whole bus should be cut off. It can't isolate the fault simply by removing the connection of a site like a star structure.
3. Ring topology
This network consists of point-to-point links in a closed loop.
(1) working mode
Each repeater is connected to two links. It receives data from one link and transmits it to another link at the same speed without buffering. All links are given the same sending and receiving directions, so data is transmitted circularly on the ring.
Since multiple devices * * * share a ring, distributed control is adopted to decide which station can put packet data on the ring when.
⑵ Advantages of ring topology
① Short cable length: The cable length required by ring topology is similar to that of bus topology, but much shorter than that of star topology.
② Various transmission media can be used:
H Because the ring network is point-to-point, twisted pair can be used in the building and optical cable can be used in the outdoor backbone network to solve the problems of transmission rate and electromagnetic interference.
H Because the ring topology is unidirectional transmission on each ring, it is very suitable for optical fiber transmission media with high transmission rate.
4. Tree topology
Tree topology is evolved from bus topology. It has a branched root, and it can extend several branches. Tree topology usually uses coaxial cable as transmission medium and adopts broadband transmission technology.
The comparison between tree topology and bus topology is as follows:
(1) The main difference between tree topology and multi-segment bus topology lies in the existence of roots. When the message data sent by the node is received by the root, it can be relayed to the whole network.
⑵ Tree topology is easy to isolate faults, which is incomparable to bus topology. Other advantages are the same as bus topology.
⑶ The disadvantage of tree topology is that it relies too much on roots. If the root fails, the whole network will not work properly. The reliability of this network is similar to that of a star topology.