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The basic information about LAN network topologies is about the main basic structure that we should know in order to design a network. It acts as a backbone of a network which holds the overall structure of a network. It interprets the interconnection that being used between various nodes in the network. In order to create a network, the performance of LAN topologies analysis should be done to get the best information of the best topologies to be used. This article will describe the difference of analysis between 3 types of network LAN topologies which are Bus, Ring and Star topology.
Local area network (LAN) is a computer in the network that connects fews computers and technologies devices in a specific geographical area which is in limited space such as school, home, office building, computer laboratory, and any close groups of buildings. The most common type of local area network is an Ethernet LAN. The smallest home LAN can have exactly two computers while a larger LAN can include many thousands of computers. LANs are known which will provide a few benefits such as a private communications facility, the services over a relatively limited geographic area, a high data rate for computer communications, and common access to a wide range of devices and services.
A local area network usually provides a high bandwidth communication with inexpensive transmission media. Just like any other data communication network, a local area network includes of three basic hardware elements. There are transmission medium, often twisted pair, coaxial cable, or fiber optics, a mechanism for control of transmission over the medium, and an interface to the network for the host computers or other devices. A LAN is useful for sharing a simple basic resources like files, printers, games or other applications . In addition, a LAN in turn often connects to other LANs, or to the Internet and finally to other WAN . Most local area networks are built with relatively inexpensive and a simple hardware such as Ethernet cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware options also exist.
2.0 Local Area Network Transmission Method
LAN data transmissions fall into three classifications. There are unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to each node.
With unicast transmissions, a packet in the transmission is sent from the source to each destination on a network. The source addresses the packet by using the network address of the destination node. The client in the network then will receive packet which being forwarded from source . Figure 2.1 below is an example of a unicast network.
Figure 2. : Diagram of Unicast Network
In multicast transmission, a data packet will be forwarded to the nodes in the network with their own subset. By using multicast addresses, all the packets being sent directly from the source to the final node destination . Figure 2.2 is an example of a multicast network.
Figure 2. : Diagram of Multicast Network
A broadcast transmission involves the transmission of packet to all nodes located in the network. In this case, the source broadcast the packet to all final nodes destination by using broadcast address . Figure 2.3 below shows the diagram of broadcast transmission.
Figure 2. : Diagram of Multicast Network
Multimedia broadcast traffic is different than data broadcast. Multimedia broadcasts consists of several megabit in size, thus, this will cause of high consuming of network and bandwidth resources. Data broadcast are important in LAN, even they have a minimal impact due to the small data broadcast frames being traversed in the network.
3.0 Network Topology
Network topology is a unique diagram of a basic network design. It acts as a backbone of a network which hold the overall structure of a network. It interprets the interconnection that being used between various nodes in the network. The topologies can be physical or logical design. Physical topology describes the physical design of a network including the location, devices and cable installation. Logical topology on the other hand, describes the transmission of data in a network as referred to its physical design .
Basically, a Local Area Network (LAN) is one example of network that applies the both basic physical and logical topology describe above. The design involves any nodes in LAN which have more links that connected to each other. There are 3 basics types of network topologies, which are Bus topology, Ring topology, and Star topology. These 3 types of topologies describe many different structure and architecture. It might be different in term of transmission rate, collision of data, and Ethernet types used. All of those differentiations will totally effects the performance of LAN.
3.1 Bus Network Topology
Bus topology or sometimes may also be defined as linear-bus topology. It is a very simple basic design of network that fully utilizes a single length of cable. Besides that, it is also include the medium that directly attached to LAN stations. Bus is able to connect multiple clients. However, some problems might occur when the two clients are having transmission process at the same time on the same bus. In that case, any systems that are using the architecture of Bus Network will have collision handling and collision avoidance ready for any transmission interruption .
In Bus topology, all stations are sharing the same cable segment and able to see the transmission at each node on the segment. It is call a broadcast medium . Figure 3.1 below shows the example of Bus topology.
Figure 3.1: Diagram of Bus Topology
Based on the figure above, each workstation is connected to a single cable. The printer and peripheral device are connected to server which is also connected to the single cable.
3.1.1 Bus LAN Transmission Logic
Bus LAN transmission is actually passive. The computer on the Bus which indicates the LAN node simply will listen for a signal and listen to the Bus for traffic. When there is no traffic detected after listening to a Bus, then it will transmit the data. Otherwise, if the traffic busy is detected, the computer at each node got to wait for a few period of time before starting the transmission again. This will be reoccurring and thus repeated attempts will also be made until the bus is found free .
3.1.2 Collision of Data
In Bus topology, collision of data can be occur easily since each workstation is being connected to each other only via one single medium, cable. Thus, there is a tendency of two workstations that might find the bus is free at the same time. When this thing occurs, both stations would transmit at the same time and will cause the collision of data. Both workstations will then wait for a random period of time before attempting for other transmission. However, the system with Bus topology will be provided with some scheme of collision handling or collision avoidance for communication on the bus. It often using Carrier Sense Multiple Access/ Collection Detection (CSMA/CD) or in the other hand with the presence of a bus master that controls access to the shared bus. Carrier Sense is applied when each machine on the network examines the cable first before get sending the data packet. If the other machines is transmitting the data, the traffic being detected by the machines and waits until the cable is free  .
3.1.3 Solution for Bus LAN Network Topology
The most important cause of low performance in Bus topology is the occurrence of collision domains. Collision domains are present when there is a use of hub in the network. Thus, by changing to switches, this can eliminate the collision domain. This is because of the function of switch itself which is more intelligent than hub. Switch is able to control the collision that might be occur compared to hub that only receive any in coming data without controlling the transmission . The figure 3.2 below shows the used of switch in the network.
Figure 3.2: Replacing Hub with Switch
Overall, the use of switches in the network can reduces the congestion of network traffic and thus increases the Bus network topology utilization.
3.2 Ring Network Topology
A Ring network is a network topology whereby each workstation is connected to other two workstations in a loop of single pathway. However the loop itself is vulnerable to a single point of failure. Thus the two loops are often implemented in a pattern known as counter rotating ring. The data flow through each workstation to other workstation with each station handling the packet transmitted. Ring topology involves only one pathway between other stations, thus it is exposed to any disrupted failure to a single link. However, the creation of Ring pattern is to ensure if the primary ring faced a cable failure, it can cause the two stations on the either side detects the fault and do the retransmission of data to the secondary ring .
Besides that, all stations in ring topology are considered as repeaters. The repeaters are a function of attachment of stationâ€™s network interface card (NIC). The NIC itself is capable of recognizing and handling any interrupted repeater. Other than that, Token Ring (IEEE 802.5) also is best in representing a ring topology. Although token ring provides an alternative to Ethernet, it can also provide a higher speed and higher performance in load . Figure 3.3 below shows the Ring topology diagram.
Figure 3. 3: Diagram of Ring Topology
Based on the figure above, each work stations are connected to each other in a circle ring with the server. While the network printer and peripheral devices are connected to server each.
3.2.1 Ring LAN Transmission Logic
The data path of the Ring is connected the workstations and the servers. The transmission logic is based on a token that is able around the ring. The token carries the data which is going to be transferred along the path. The most popular protocol applied is IEE 802.5 .
3.2.2 Collision of Data
The access to the network is controlled and being known as a technique of token passing. Compared to Bus topology before, it is based on CSMA/CD system used. The transmission of data only occurs when there is a receiving of token that will be passed through the work stations. Suppose, there will be no collisions that can be occurred. Thus, the used of token ring technique in transmission of data operates more efficiently than the used of CSMA/CD, hence, this will totally increase the performance of Ring topology in LAN rather than Bus topology .
3.2.3 Solution for Ring LAN Topology
A work station failure and an interruption of cable can cause isolation to every work stations attach to the ring. In order to overcome this problem, fiber Data Distributed Interface (FDDI) is being applied. FDDI is the fullest implementation of a true ring topology. Basically, FDDI is randomly being used in LAN because of the high cost of installation and maintenance. However, the application of FDDI is good.
FDDI send data on a clockwise and counterclockwise ring in order to maintain the availability of path in the network. Some of the key features of FDDI are providing 100Mbps by using the primary ring to the network. It is also a fiber optic cabling and thus is useful for long distance (2km with single mode fiber, longer distances with multimode fiber). It is able to provide redundancy by two counter rotating rings .
3.3 Star Network Topology
Now a day, most of the network structures being built are using star network topology. Star network has been popular topology to be used due to its simple and systematic structure which consists of a central switch, hub or computer that involves in data transmission.
Basically, there are several different cable types that can be used in the process of point to point link. Some example of the cable types are unshielded twisted pair (UTP), and fiber-optic cable. Since there is no cable segment is a single point of failure which might affect the entire network. This is part of better management of the LAN. When there is any one of the cables faced and creates a problem, only the LAN which attached station is affected. But, the other none effected stations will remain operational. The figure 3.4 below shows the diagram of Star network topology .
Figure 3. : Diagram of Star Topology
Based on the diagram above, it demonstrates a star topology with two workstations, server, and printer connected to the hub. From here, it is clear that if any cable to one of the workstations is being interrupted, the other devices of the network are unaffected unless there is a requirement access to any disconnected device.
3.3.1 Star LAN Transmission Logic
Any data which located on Star network will pass through the hub or switch until they reach the destination. Through all the way, the hub or switch will manage and control the data transmission. Besides that, hub for sometimes will act as a repeater in the data flow process. This commonly involves with twisted pair cable. Other than that, coaxial cable and optical fiber cable can be used .
3.3.2 Collision of Data
By connecting all of the network systems to a central node, Star topology is reducing the possibility of data collision in the transmission process. Besides that, the network performance is better in Star topology. In Star topology, there is a prevention of passing the data packets through many network stations in a time. Basically, there are only 3 devices and 2 links which involved in transmission process in the network. Even though there is a huge overhead at the central hub, it can give a full utilization by one device without affecting others. Besides that, the isolation of devices makes the disconnection between devices. Thus, this will avoid any failure occurred from affected the network .
As a conclusion, there are a lot of network topologies that can be built in LAN. Three basic topologies which are Bus, Ring and Star topologies consist of their own characteristics that will totally affect the performance of LAN network. Even though there are some constrain, or limitation to reach the optimization of the best performance, there are also some solution that can be used in order to enhance the quality of the performance.