The network topology refers to the physical layout of devices connected by transmission media. There are multiple ways to connect the various devices involved in LAN work with media. In fact, there are only several ways to fit LAN work. If a network only connects several devices, the easiest way is to directly connect them together. This connection is called a point-to-point connection. The network formed in this way is called a fully interconnected network, as shown in.
There are 6 devices in the figure. In the case of full interconnection, 15 transmission lines are required. If there are n devices to be connected, the required lines will reach N (n-1)/2! Obviously, this method is available only when the geographical scope is small and the number of devices is small. Even in this environment, it is not used in LAN technology. The topological structure we call is because this wan interconnection technology is possible when multiple LANs need to be interconnected through an interconnected device (such as a router. Currently, most networks use three types of topology: 1 star-row topology, 2 ring-row topology, and 3 bus-type topology knot;
1,
Star Topology
The star structure is the oldest connection method. The telephones used every day belong to this structure, as shown in. Figure (a) shows the star structure of the telephone network. Figure (B) shows the most common Ethernet star structure. The network device in the center is called a hub, the English name is hub. (A) Star row structure of the telephone network (B) hub-centered structure for centralized control, because the communication between end users must pass through the Central Station. This feature makes it easy to maintain and secure. When the end user equipment is shut down due to a fault, the communication between other end users will not be affected. However, this structure is very unfavorable because the central system must have high reliability because once the center system is damaged, the entire system is paralyzed. The central system usually uses dual-host hot backup to improve system reliability. An extension of this network topology is the star tree, as shown in. The connection between each hub and the end user is still a star, and the hub is connected to form a tree. However, it should be pointed out that the number of hub-level connections is limited and changes with the vendor's differences. It should also be pointed out that although the network structure composed of hub is in star layout, the mechanism for accessing media used by it is still the bus mode of shared media. 2. The ring network topology is mostly used in LAN. In this structure, the transmission media goes from one end user to another end user until all end users are connected to a ring ,. This structure obviously removes the dependence on the central system when the end user communicates. The feature of the loop structure is that each end user is connected to two adjacent end users, so there is a point-to-point link, but it is always operated in one way. As a result, there is a name for upstream and downstream users. In this example, user n is the upstream user of user n + 1, and user n + 1 is the downstream user of user n. If the n + 1 end needs to send data to the N end, it takes almost one week to bypass the loop to reach the N end. Any message transmitted on the ring must pass through all endpoints. Therefore, if a certain point of the ring is disconnected, the communication between all ends of the ring will be terminated. To overcome the weakness of this network topology, each endpoint is connected to a secondary ring in addition to a ring. When the primary ring fails, it is automatically forwarded to the secondary ring. 3. bus topology the bus structure is a way to connect all end users using the same media or cable. That is to say, the physical media used to connect end users is shared by all devices, as shown in. One problem that must be solved when using this structure is to ensure that the end user cannot conflict when using the media to send data. In point-to-point link configuration, this is quite simple. If this link is a half-duplex operation, you only need to use a simple mechanism to ensure that the two end users work in turn. In the one-to-multiple-point method, the access to the line is determined by the query at the control end. However, in a LAN environment, the above mechanism cannot be adopted because all data stations are equal. In this paper, a media access method used in a shared bus network is studied: a carrier with Collision Detection listens for multiple accesses, abbreviated as CSMA/CD. This structure has the advantages of low cost, flexible data-end user access, and failure of a site or a certain end user does not affect the communication between other sites or end users. The disadvantage is that only one end user can send data at a time. Other end users must wait until they get the sending permission. The mechanism for obtaining media access is complex. Despite some of the shortcomings above, due to simple wiring requirements, easy expansion, terminal user failure, addition and deletion do not affect the work of the whole network, so it is the most common use of network technology.
