Abstract—Over the last decade, a technology which has beenmaking a great impact in the IT industry is Cloud Computing.Cloud Computing can be seen as Distributed computingenvironment which will be helpful in providing the On-DemandServices to various users, which will deploy on the basis of theircomputing needs in a virtualized environment without the priorneed of knowledge of technical infrastructure. Due to its variousproperties, such as low bandwidth, reliability, high performanceand scalability, many industries are running their applications onthe cloud. The Cloud Service Providers are providing variouscloud services to the users or organizations who are registeredwith them on the basis of payment mode across the globe. Thereare various cloud services which can be termed as “X-AAS”, thatis, X-As-A-Service. Here X can be represented as Software,Storage, Infrastructure and Platform. These services can bedeployed to the user on the basis of user’s requirement and theSLA(Service Level Agreement). The main role of cloud serviceprovider besides providing the cloud services to the users is thatthe security they provide to the user by deploying cloud services.The main objective behind this paper is to provide a crystal clearidea about the cloud services models and outline the issuesrelated to security in the service models.Keywords—Cloud Computing, Service models, SAAS, PAAS,IAAS, Security Issues)I. INTRODUCTIONCloud Computing can be treated as one of the mostsignificantly achieved development in the IT industry. Mostcompanies are running their applications in the cloud due torapid advancement in communication network. It is nextgeneration computing platform which will help users acrossthe globe to access and share the resources throughcommunicating mediums. We can say that, in simpler terms,cloud computing can be defined as a distributed computingenvironment that enables the users to access and exchangetheir resources(data and applications ) remotely and provideservices to the user to use the remote hardware and softwarewithin a network without the knowledge of technologicalinfrastructure. The figure-1 given below will help inunderstanding the infrastructure requirements for clouddeployment.Figure-1 The Infrastructure of CloudII. ADVANTAGES OF CLOUD COMPUTINGA. Broad Network AccessWe can use the cloud services capabilities which areavailable over the network and it can be accessed throughvarious standard mechanisms that can be used in any of theheterogeneous platforms like PDA’s, laptops, mobile devices.B. Elasticity or FlexibilityUsers have the advantage that the can access the servicesand computational resources whenever and where ever theywant and for this they don’t have to depend on humaninteractions. Capabilities can be provides rapidly andelastically and in some of the cases it can be providedautomatically too.C. Location IndependentUsers have the advantage that the Cloud interfaces arehaving a top notch that they are location independent and theycan be accessed by any of the available interfaces like Webbrowsers or web services, so this will not give anyoneinformation about the exact location of the user. It gives highlevel of abstraction to the user’s data also.D. ReliabiltyIt improves the use of multiple sites which makes cloudcomputing suitable for business continuity and disasterrecovery.E. Easy Access to InformationOnce the user is registered to the services, than the user canaccess the information from anywhere in the world, providedthat the user is having an active internet connection.F. SustainabilityIt improves resource utilization and makes cloudenvironment more efficient.G. Unlimited StorageStoring information in the cloud gives almost unlimitedstorage capacity. Here no more need to worry about runningout of storage space or increasing current storage spaceavailability.H. Backup and RecoverySince all of the users data is stored in the cloud , so backing upof data and restoring back the same will become more easierfor the user than doing the same on the physical device. Mostof the cloud service providers are competent enough forhandling all the recovery of information. Hence cloud makesthe whole process of backup and recovery much easier andsimple than the traditional methods of storing of data.III. CLOUD COMPUTING MODELSBased on the usages of data and the software, cloudcomputing services can be broadly classified as three differenttypes known as Cloud Service Models.Figure-2 shows the different types of cloud services whichcloud service provider will provide.Figure-2 Cloud Service modelIV. CLOUD COMPUTING SERVICE MODELSA. Infrastructure As A ServiceInfrastructure As A Service (IAAS) is one of the threefundamentals service models of cloud computing. A layers ofcloud computing model and their infrastructural requirementsis shown in figure-3. In this model the users are allocated withcomputing resources in order to run their applications. Thecomputing services are provided in a virtualized environmenti.e in cloud by using a communication network. The bestknown example of IAAS is Amazon Cloud Formation,Amazon EC2. It can be implemented by utilizing the conceptslike Enterprise infrastructure, Cloud hosting, and Virtual DataCenters (VDC). Network as a service (NAAS) is a category ofcloud infrastructure services where the user can use thenetwork connectivity as a services. NAAS involves theoptimization of resource allocations and resource computingin the network. VPN, and bandwidth on demand are thecommon example of NAAS.Advantages Of IAAS1. Resources are available on demand as and when the userrequires it. That means the user will not have to worry aboutthe Infrastructure required to run the application. so thisscalable.2. In IAAS the Infrastructure i.e the virtualized environment isset up and maintained by the cloud provider. So no investmentin hardware for the users. As it saves the implementation costand time of execution.3. The service can be accessed on demand and the client onlypays for the resource or application used not for theInfrastructure4. The service can be accessed from any location 24X7provided there is an internet connection. So IAAS is locationindependent.5. Physical security of user’s data is the responsibility of thecloud provider. So the time required to give security to data issaved.6. The chance of system failure is less. Any case of failurewill be smoothly handled by the service provider. So it is faulttolerant.