network topology

           network topology

Communication networks are designed to facilitate the sharing of resources as well as to reduce communication costs, increase throughput and decrease the delay of services. The topology (shape) of the network is an important consideration. In computer networking, topology refers to the layout of connected devices. A network topology represents its layout or structure from the point of view of data flow. In so-called bus networks, for example, all of the computers share and communicate across one common medium, whereas in a star network, all data flows through one centralized device. Common types of network topologies include bus, star, ring, and mesh. A network configuration is also called network topology. A network topology is a shape or physical connectivity of the network. The term topology is a geometrical term meaning to describe the form of something. The topology of a network refers to the way in which the network's nodes i.e. computer or other devices are linked together. Choice of topology for a computer network depends on a combination of factor such as -

1. Desired performance of the system

2. Desired reliability of the system

3. Size of the system

4. Expandability of the system

5. Cost to implement

6. Availability of communication lines Major goal

1. Provide maximum possible reliability to assure proper receipt of all traffic:

reliability refers to the ability to deliver user data correctly, without any errors, from one place to another. It entails the ability to recover from errors or lost data in the network. Reliability also refers to

 of the system, which includes day-to-day testing, relieving faulty components of their tasks and

fault isolation in the event of a problem. When components create a problem, the network should pinpoint the

error readily, isolate the fault. 2. Route the traffic across the least cost path within the network between the sending and receiving. Minimize the actual length of the channel between the components, which usually entails routing the traffic through

a few numbers of components. Provide the least expensive channel option for a particular application. 3. Give the end-user the best possible response time and throughput.

Short response time entails minimizing the delay between the transmission and receipt of data between the

DTEs and user application In a computer network computer play three roles – clients, peer and servers Client - which use network resources but do not provide them, Peers - which both use and provide network resources Server - which provides network resources.

The server runs network operating system Common topologies

1. Hierarchical topology (tree)

2. Horizontal topology (bus)

3. The star topology

4. The ring topology thus)

5. Mesh topology 1. Hierarchical topology

Hierarchical topology:

 is one of the more common networks found today. Simple to control and topology provides a concentration point for control and error resolution. In most cases, DTE at the highest order of the hierarchy is in control of the network. this network is attractive from the standpoint of the simplicity of control, it presents significant potential bottleneck problems. In some cases the uppermost DTE, typically a large scale mainframe computer, controls all traffic between DTEs. If the upper-level machines fail the network capabilities are lost completely The hierarchical topology is also called vertical network or tree network

this network resembles a tree with branches stemming from the top of the tree down to the lower level The organizational chart is of the same type. The advantages and disadvantages of the vertical communication network are much the same as those of hierarchically structured business - clear lines of authority with frequency bottleneck at the upper levels and often insufficient delegation of responsibility Advantages.

1. Hierarchical topology provides the simplicity of control on network

2. The uppermost computer control all traffic between DTES Disadvantage 1. Hierarchical topology is attractive from the standpoint of the simplicity of a control. It presents a significant. potential bottleneck problem. 2. In this also presents reliability problem.

3. In this event, the upper-level machine fails the network capability is lost completely

4. If this DTE is not fully backed up by another computer,

2. Horizontal Topology (Bus) The form of this topology is shown in fig. this arrangement is quite popular in the local area network. It is simple to control the tragic flow between and among the DTEs because the bus permits all stations to receive every transmission. That is

the

A single station broadcast to multiple stations The main drawback of horizontal topology stems from the fact that usually, only

communication channel exists to service at the devices on the network channel, the entire network is lost. Another problem with this is dit

if failure particular component tied into the bus. Advantage

ice all the devices on the network.. if failure of communication one is lost. Another problem with this is difficulty in isolating faults to anyone

1. It helps in reducing the no. of physical line 

2. The failure of the computer does not affect the communication among other computers in the network

3. Addition of new computer to the network is easy Disadvantages

1. All computers in the network must have good communication and decision-making capability. 

2. If the communication line fails the entire system break down.

3. Star Topology.

 

This topology is also used widely for a data communication system. This network was used in 1960 and early 1970s because it was easy to control - the software is not complex and the traffic flow in simple. All traffic emanates from the hub of the stars, the central site as shown in the fig. The computer is in the complete control of the DTES attached to it, it is similar to a hierarchical network, except that the star topology has limited distributed processing capabilities. Fault isolation is simple in the start network because the lines can be isolated to identify the problem. But similar to the hierarchical network, star network is subject to potential bottleneck and

failure problems at the central site. Advantage

1. Star topology has minimum line cost because n-1 lines are required for connecting n nodes.

2. Transmission delay between two nodes do not increase by adding new nodes to the network because any

two nodes may be connected via two links only.

 3. If any of the local computers fail the remaining portion of the network is unaffected. Disadvantages.

1 The system initially depends on a control node. If the host computer fails the entire network fails 

2. It creates a battle line problem.

4. Ring Topology The ring topology is popular approach to network configuration. Ring topology in B circular form. Most a time data flow is in one direction only, with one single station.

7 receiving the signal and relaying it to the next station on the ring. Each component is tasked with a straightforward job of accepting the data, sending it to the DTE attached to it or sending it out on the ring to the next intermediate component. There is one channel tying in all the components in the ring. If the channel between two nodes fails, then the entire network is lost. 

Advantage.

1. The ring network works well where there is no central site computer system 2. It is more reliable than a star network. Because communication is not dependent on a single host computer

If the link between any two computers breakdown or if one of the computer breakdown alternate routines is

possible. Disadvantages 1 In a ring network communication delay is directly proportional to the no of nodes in the network

increases the communication delay. 2. The ring network requires more complicated control software than star network 5. Mesh topology

It was used in the last few years its attraction is its relative immunity to bottleneck and failure problems. Due to the multiplicity of paths from

Es traffic can be routed around failed components of busy nodes. This is expensive but some users go for it due to the reliability need of few nodes that need to be connected.)

Traffic Control :

Traffic Control Protocols cooperate to manage the communication. One protocol is responsible for controlling the flow of the traffic on each channel, a second select the best channel for the first protocol to use. The first may be classified as a link or line protocol or data link control. The second protocol is called a switching or routing protocol. The additional protocol is also involved.

The data communication industry uses two terms to describe the event response. The term ACK denotes the positive acknowledgment, the term NAK represents a negative acknowledgment. A NAK usually occurs because disturbance in transmission may be due to channel, lightning, storm etc. It is also essential that the process shown in event 1 & 2 are completed before event 3 occurs. It places A receives an ACK in event 2, it means data have been received correctly at place B and the communication system removes the message from the queue. (application process often saves a copy on disk). The application process is responsible for the posting of the database updates. The most common method used today for error checking is Cyclic Redundancy Checking CRC

The network can be classified by its size, its ownership the distance it covers and its physical architecture. There are three primary categories of network,

Local Area Network (LAN) 2. Metropolitan Area Network(MAN) s Wide Area Network (WAN)

a) Local Area Network (LAN):

Local area network generally called a LAN'S is privately owned networking capability to a group of computers in close proximity to each other such as Network within a single building or campus up to few Kilometers in size. They are widely used to connect personnel computers or workstations in company offices and factories to share resources (eg. Printers) and exchange information LAN's are distinguishing from other kinds of the network by three categories

1. Their size-LAN is restricted in size, which means transmission time is bounded and known in advance. 2 transition technology - LAN may use a transmission technology consisting of a cable.

3. Their topology LAN often use a transmission technology considering of a single cable to which all the machine is attached. Traditional LAN's run at a speed of 10 to 100 Mbps have delayed and made few mistakes.