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The term topology refers that way in which the end points, or stations, attached to the network are interconnected or it is the arrangements of systems in a computer network. It can be either physical or logical. The physical topology refers that, how a network is placed in a physical way and it will include the devices, installation and location. Logical topology refers that how a data transfers in a network as opposed to its design.
The network topology can be categorized into bus, ring, star, tree and mesh.
Hybrid networks (They are the complex networks, which can be built of two or more topologies together).
A bus topology is characterized by the use of a multi point medium. A long and single cable acts as a backbone to connect all the devices in a network. In a bus topology, all computers are stations attaching through the tap (an interfacing hardware to connect to the network) and it connects directly to the bus network. Data’s are transmitting and receiving to the bus, by the duplex actions between the tap and the device. Devices in the bus topology send a broadcast message to the other device for communications. But the proposed device can only accepts and processes the messages.
- Bus topology can install very easily on a network.
- Cabling will be less compare to other topologies because of the main backbone cable laid efficiently in the network path.
- Bus topology suited for a small network.
- If one computer fails in the network, the other computers are not affected they will continue to work.
- It is also less expensive than star topology.
- The cable length will limited and there by limits the number of stations.
- The main cable (backbone cable) fails, and then the entire network will fail.
- It is very difficult to trouble shoot.
- Maintenance cost is very high in a long run.
- Terminators are required for both the ends of the cable.
The ring topology is the network consists of dedicated point to point connection and a set of repeaters in a closed loop. Signals passing through ring in a single direction until they reach to its final destination. It may be clock wise or anti clock wise. Data’s are transmitted in the form of frames. These topologies are used in school campuses and some office buildings.
- It performs better than star topology under heavy work load
- For managing the connection between the computers, there is no need for the network server.
- It is cheaper than star topology because of less wiring.
- By adding the token ring in the network, can create large network.
- Very order network because all the devices has a access to the token ring and opportunity to transmit.
- A failure or break in the ring, it can disable the entire network.
- It is much slower than an Ethernet network with under normal load.
- Any moves, changes and ads of the devices can affect the network.
- Network connection devices like (Network adapter cards and MAU) are much more expense than Ethernet cards.
Star topology is the network in which each station is directly connected to a central connecting node called hub. In star topology all the devices are not directly connected to one another. All the devices are connecting to the central server (switching hub). This topology does not enable the direct traffic between the devices in the network. A controller act as the interface between the devices. A star topology feature, each device needs only one link and one input/output port to connect the number devices in the network. This type of topology is used in local area networks (LAN) and sometimes high speed LAN often uses a star topology with central hub.
- If anyone connection is fails in the network, it will not affect the entire network. Only that connection or link affected.
- It is easy to identify the fault and fault isolation.
- Easy to expand the network in the star topology.
- No failure to the network when connecting or removing devices.
- It is very easy to manage because of its simplicity in the function.
- In a star topology, if the central connecting device goes down, the entire network will fails.
- It requires more cable length compared to the linear bus topology.
- Star topology is more expensive than bus topology because o the connection ports like hub.
Tree topology is the generalized form of the bus topology. It integrates the multiple star topologies together on to a bus. The data transmission of the tree topology, through the cables with closed loops. The transmission medium is a branching cable with no closed loops. The layout of the tree topology is beginning at the head end. These layouts have many branches and these are quite complex layouts in the topology. Any transmission from the device is going through the medium and it can receive by all other devices in the tree topology network. Tree Topology will give the expansion of the existing network.
- Tree topology is well supported by the hardware and software vendors.
- Point to point wiring for each and every segments of the network.
- It is the best topology for the branched networks.
- It is more expensive because more hubs are required to install the network.
- Tree topology is entirely depends upon the backbone line, if it fails then the entire network would fail.
- It is very difficult to configure and wire than other network topologies.
- In a tree topology, the length of network depends on the type of cable being used.
In a mesh topology, every device has connected to each other or a dedicated point to point link to every other device. (Dedicated term means that the traffic links only between the two devices it connects). To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to other node. Node 1 must be connected to n-1nodes, node 2 must be connected to n-1nodes, and finally node n must be connected n-1 nodes. If each physical link in the network can allow the communication in both directions, we can divide the number of links by 2.In other words we can say that in a mesh topology, we need n (n-1)/2.
Suppose if we are connecting 15 nodes in a mesh topology, then the number of cables required;
CN = n (n-1)/2 CN = Number of cables
= 15 (15 – 1)/2 n = Node
Therefore, the total number of cables required for connecting 15 nodes = 105.
- There is no traffic problem because of the dedicated link in the mesh network.
- Mesh topology is very strong. If any link becomes not active it does not deactivate the entire system.
- Point-to-point links make full identification and fault isolation easy.
- Security or privacy for data travels along the dedicated line.
- Network can be expanded without any disruptions to the users.
- Installation and reconnection are difficult.
- Mesh topology required more cabling and the number input/output ports comparing with other network topologies.
- Sheer bulk of the wiring can be greater than the available space can accommodate.
- The hardware required to connect each link can be prohibitively expensive.
A network can be hybrid, which uses two or more network topologies together in a network. An example of hybrid technology is the star ring network.
OSI Model in the Network
The OSI model was developed by the ISO (International Organization for Standardization) in 1947, as a model for a computer protocol architecture and as a frame work for developing protocol standards. The purpose of the OSI model is show how the communications are going through in a network between different systems. The OSI model is not a protocol; it is a model for understanding a network architecture .This model is flexible, very strong and interoperable. The OSI model is a layered frame work for the design of network systems that allows communication between all types of computer systems. OSI model contains of seven layers, each of which defines a part of the process moving information across a network.
The OSI reference model has been divided into two categories according to their specific functions in the network architecture; upper layers and lower layers. The lower layers are used to allow traffic through the network to the other system. The other four layers are used to complete the process to the other system.
The Advantages of the OSI model are given below
- It helps the user to understand the whole features involved in networking
- OSI model gives a better understanding to the users that how a software and hardware components working together in the networks.
- Troubleshooting in the OSI model is easier as it breaks the network into usable layers
- The basic functional relationship terms are defined so that it can be used by network professional on another network
- The users can understand new technologies as they develop in this model
Upper Layers of the OSI Models are;
- Application layer
- Presentation layer
- Session layer
The upper layers have act as an interface between the end user and the applications. It consists of all the issues about the application, presentation and sessions and these layers are applied in software only. The application layer is more close to the end user.
Examples of upper layer technologies in the OSI model are SNMP, FTP, and WWW etc.
Lower Layers of the OSI Model
- Transport Layer
- Network Layer
- Data link Layer
- Physical Layer
These layers provide network specific functions like data transport issues (flow control, addressing and routing). The bottom layers in the OSI model (physical layer and data link layer) can use in software and hardware also. TCP, UDP, IP, IPX are some examples for the lower layers.
The application layer act as the interface to the end users can access the network. Application layer is the layer the user sees in contest of loading an application such as e-mail, supporting the file transfer, ability to print on a network, surfing the World Wide Web etc. These layer have another specific functions like network virtual terminal, access and management, mail services and directory services. Protocols used in this layer are SMTP, Telnet, FTP, and SNMP.
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In the presentation layer, the application formats the data to be sent out on the network depending on the presentation layer. The presentation allows applications to read and understand the data or message sent.The presentation layer is also responsible for the translation, compression and encryption. Messages are sending between the layers. Presentation layer will translate data into understandable for the transmission. This layer also handles the encryption, decryption, data compression and decompression.
The session layer is the network dialog controller. It establishes, maintains and synchronizes the interaction among communicating systems. This layer is responsible for opening, using and closing session. It handles the remote procedure calls. Session layers are communication through the gateways and application interfaces. Session layer will also places check points in the data flow. Examples for the session layer are SQL, ASP, and RPC etc.
The transport layer offers end to end communication between the two end networks devices in a network. The transport layer is also responsible for the delivery of a message from one process to another. A process is an application program running on a host. The transport layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Transport layer gives a type of address to the data called port address. Transport layer also have the responsibilities of detecting transmission error and the proper sequence. This layer divides the messages into smaller packets and it controls the data flow.
A Network layer provides the end to end (source to destination) delivery of a packet, across the multiple networks (links). This layer ensures that each packet gets from its point of origin to its final destination. Network layer gives logical addressing, so that endpoint (receiver) can be identified. During a packet delivery process it send messages and report errors. This layer also defines the routing works and how routers can learn how a packet can deliver. Examples for network layer protocols are IP, IPX, and TCMP etc.
Data link layer
A Data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer. This layer divides the data’s into frames and gives the physical address. It uses the MAC address for define a hardware or data link address .Data link layer handles the flow control, error control and the access control. This layer can identify the specific computer on the network.
Data link layer contains two sub layers; Logical Link Control and Medium Access Control. LLC is the upper layer and which maintain and establish the communication links to the device. And it also responsible for the frame error control and addressing.MAC is the lower sub layer of the data link layer. It controls how the devices sharing the media channel.
A Physical layer is used to define the connector and interface conditions, also the medium requirements which are cables. The Electrical, Mechanical, Procedural and Functional conditions are also defined by sending a bit stream on the network. This layer defines the characteristics and different types of medium. Physical layer defines the data rate i.e. the duration of a bit or how long they will exist. Physical layer will tell the transmission mode between two devices. Examples for the physical layers are Cables, hubs, switches, repeaters etc.
Network topologies helps the user to understand the whole features involved in networking and also how the software & hardware components together in networks. Network topology provides easy to trouble shooting the OSI models. Users can understand very well about the new technologies in the OSI model.
I have searched various websites and books for gathering information about the network topologies and OSI model.
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