Network Topology

What is Network Topology ?

Distributed computing systems have become the essential aspect of growing information technology. The performance of any distributed system is certainly influenced by the technology, which we adopt in making network interconnections. In mathematics topology is concerned with the connectedness of objects which is the most basic properties of space. In simple way network topology refers to the configuration of cables computers and other peripheral.

Each topology is suited to specific tasks and has its own advantages and disadvantages. A most simple and good example of network topology is LAN. Where it  has one or more physical links to other devices in the network. In recent days there are basically two basic categories of network topologies: Physical and Logical Topologies.

1. Physical topologies 

Physical Network Topology emphasizes the hardware associated with the system including workstations, remote terminals, servers, and the associated wiring between assets. Physical topology defines how the systems are physically connected. It means the arrangement of devices on a computer network through the actual cables that transmit data. There are eight basic topologies.

Point To Point Topology

In a point-to-point link,  two devices monopolize a communication medium.  Because the medium is not shared,  a mechanism is not needed to identify the computers.  Therefore, a simple, two-device point-to-point network has no need for addressing.

Point-to-point links can be simplex, half-duplex, or full-duplex.  When devices must engage in bi-directional communication on a half-duplex link,  some turnaround mechanisms must be in place to switch the roles of the sending and receiving devices.

Bus Topology

In a bus topology, all devices attach to the same transmission medium. The medium has a physical beginning and end. All buses are implemented using electrical cable, usually coax, and the ends of the cable must be terminated with a terminating resistor that matches the impedance of the cable. The terminating resistor prevents data reflections from coming across as data corruption. The bus is considered a multipoint system because all devices tap into the same backbone cable.

An important characteristic to remember in bus topologies is that all data signals are broadcast throughout the bus structure.  In the following diagram, if first computer sends a signal to node last computer, the signal propagates for the length of the cable and is seen by all computers as well. It is necessary to have an addressing mechanism so that each node understands which messages it is to receive and which to ignore.

Benefits of Bus topology

Bus topology has the following advantage:

• Cabling costs are minimized because of the common trunk.

Disadvantages of Bus topology

Disadvantages of bus topology are as follows:

• Difficult to trouble shoot because no central distribution points exist.
• Cable breaks can disable the entire segment because they remove the required termination from each of the two cable fragments.

Star Topology

The star topology is a popular method of connecting the cabling in a computer network. In a star, each device connects to a central point via a point-to-point link. Depending on the logical architecture used, several names are used for the central point including the following:

• Hub
• Multipoint Repeater
• Concentrator
• Multi-Access Unit (MAU)

The central hubs also can be classified in the following manner:

• Passive hub - A passive hub is a simple signal splitter. Its main function is to connect the arms of the star while maintaining the proper electrical characteristics.
  
  • A passive hub routes all traffic to all nodes. This means that a tremendous load can be created when much communication takes place between computers. Every computer has the additional burden of reading the address of each piece of information it receives to determine if the information is intended for that computer. Information containing other addresses are discarded.
• Active hub - An active hub performs the same function as a passive hub, but contains electronic circuits that regenerate and retransmit the information. Thus, active hubs can be used to extend the size of a network.
• Intelligent hub - Intelligent hubs perform the same functions as passive and active hubs; however, they can make informed path selections and perform some network management. Intelligent hubs route traffic only to the branch of the star on which the receiving node is located. If redundant paths exist, an intelligent hub can route information around normally used paths when cable problems occur.
  
  • Routers, bridges, & switches  are examples of hub devices that can route transmissions intelligently. Intelligent hubs also can incorporate diagnostic features that make it easier to troubleshoot network problems.

Benefits of Stars
Most modern cabling systems are designed in a star physical topology. The benefits of the star topology are many, including the following:

• Each device is isolated on its own cable. This makes it easy to isolate individual devices from the network by disconnecting them from the wiring hub.
• All data goes through the central point, which can be equipped with diagnostic devices that make it easy to trouble shoot and manage the network.
• Hierarchical organization allows isolation of traffic on the channel. This is beneficial when several, but not all, computers place a heavy load on the network. Traffic from those heavily used computers can be separated from the rest or dispersed throughout for a more even flow of traffic.

Disadvantages of Star topology
Star topology has the following disadvantages:

• Because point-to-point wiring is utilized for each node, more cable is required.
• Hub failures can disable large segments of the network.

Ring Topology

The ring topology is a physical, closed loop consisting of point-to-point links. In the diagram, you can see how each node on the ring acts as a repeater. It receives a transmission from the previous node and amplifies it before passing it on.

Benefits of Ring topology

Ring topology has the following advantage:

• Each repeater duplicates the data signals so that very little signal degradation occurs.

Disadvantages of Ring topology

Ring topology has the following disadvantages:

• A break in the ring can disable the entire network. Many ring designs incorporate extra cabling that can be switched in if a primary cable fails.
• Because each node must have the capability of functioning as a repeater, the networking devices tend to be more expensive.

Mesh Topology

In this type of topology each device is interconnected with one another, allowing for most transmissions to be distributed even if one of the connections goes down. A major disadvantage is high chances of redundancy in many of the network connections and overall cost is too high compared to any other network topology. 

Benefits of Mesh topology

•  Enhance for error tolerance provided by redundant links.
•  Easy to troubleshoot.

Disadvantages of Mesh topology

• Difficult to install and maintain.
• Expensive.

Tree Topology

Tree Structure suits best when the network is widely spread and vastly divided into many branches. Tree topology is a combination of two or more bus and the Star Topology connected together. Each star network is a local area network (LAN) in which there is a central computer or server to which all the connected nodes directly linked. The central computers of the star networks are connected to a main cable called the bus. 

A major disadvantage is the length of the network depends on the type of cable that is being used and tree topology network is entirely dependent on the trunk which is the main backbone of the network. If that has to fail then the entire network would fail. 

Benefits of Tree topology

• This topology is easy to control.
• The root provides centralised management and monitoring.

Disadvantages of Tree topology

• If the backbone cable breaks, the entire segment goes down.
• The tree topology is difficult to configure, wire, and maintain, especially in extensive networks.

Hybrid Topology

Hybrid topology is a network topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies or a type of network topology that is composed of one or more interconnections of two or more networks that are based upon the same physical topology, but where the physical topology of the network resulting from such an interconnection does not meet the definition of the original physical topology of the interconnected networks. 

A major disadvantage is typically more expensive than other networks since it exploits the features of its component topologies. It requires more cabling between its hardware devices than other types of network topologies. Hybrid networks are difficult to set up and troubleshoot. 

Benefits of Hybrid topology

Hybrid topology has the following advantages:

• One company can combine the benefits of several different types of topologies.
• Workgroup efficiency and traffic can be customized.

Disadvantages of Hybrid topology

The disadvantages of hybrid topology include the following:

• Devices on one topology cannot be placed into another topology without some hardware changes.

1. Logical Topologies

Logical Network Topology emphasizes their presentation of data flow between nodes. It means logical topology is associated with the arrangement of devices on a computer network and how they communicate with one another. The main role of logical topology is to communicate across the physical topologies among different systems. There are two categories of logical topologies: Shared media topology and token-based topology.

Shared Media Topology

In shared media topology the systems have unrestricted access to the physical media that is all the systems in a network have the ability to access the physical layout whenever they need it. Collision is the main disadvantage of this topology as more than one system send information out on the wire at the same time, the packets collide and as a result this collision kills the packets. Ethernet is an example of a shared media topology. As a remedy some huge networks are broken down into smaller networks. Some ethernet uses Carrier Sense Multiple Access protocol to reduce the number of collisions. 

Token Based Topology

In token based topology a token is used which travels around the network to access the physical media. If any node wants to send a packet to another one it should wait for the token which is traverse within the network either clockwise or anti-clockwise direction. After getting the token a node can send the packet towards the network and all the nodes within the path from sender node to destination node, and all the intermediate nodes should check the destination address, if it matches to anyone it should accept the packet and generate an acknowledgement packet. Acknowledgement packet should follow the reverse path to acknowledge the sender node that the packet is received by the destination node.