A Survey of Mobile Cloud Computing

Xiaopeng Fan ,Jiannong Cao ,and Haixia Mao

(1.Shenzhen Institutes of Advanced Technology,Chinese Academy of Sciences;

2.Department of Computing,Hong Kong Polytechnic University;

3.Department of Land Surveying and Geo-informatics,Hong Kong Polytechnic University)

Abstract:Mobile Cloud Computing(MCC)is emerging as one of the most important branches of cloud computing.In this paper,MCC is defined as cloud computing extended by mobility,and a new ad-hoc infrastructure based on mobile devices.It provides mobile users with data storage and processing services on a cloud computing platform.Because mobile cloud computing is still in its infancy,we aim to clarify confusion that has arisen from different views.Existing works are reviewed,and an overview of recent advances in mobile cloud computing is provided.We investigate representative infrastructures of mobile cloud computing and analyze key components.Moreover,emerging MCC models and services are discussed,and challenging issues are identified that will need to be addressed in future work.

Keyw ords:mobile cloud computing;cloud computing

1 Introduction

S ince the first computer was invented in 1946,people have dreamt of using computing resources as a utility—like water,electricity,gas,or the telephone.There has been a long-held perception that computing will one day be the fifth utility.Cloud computing[1]-[6]might be the most promising way of realizing this dream.

In recent research on cloud computing,one of the most important issues has been how to sweep up data and programs from desktop PCs[1]and installthem in the compute cloud.Cloud computing refers to applications delivered as services over the Internet and also the hardware and systems software in the data centers that provide those services[2].However,whether it is called cloud computing or on-demand computing,Software as a Service(SaaS)or Internet as platform,the overriding idea is a shift in the geography of computation.

In general,cloud computing is web-based processing[7],whereby shared resources,software,and information are provided on demand to computers,smartphones[8],and other similar devices.Cloud computing is also a new style of computing in which dynamically scalable resources are provided as virtualized services[9].This allows service providers and users to adjust their computing capacity depending on how much is needed at a given time or for a given task.

Mobile computing[10]means using portable devices to run stand-alone applications and/or accessing remote applications via wireless networks.

Increasing the number of mobile applications demands greater resources and improved interactivity for better experience.Resources in cloud platforms such as Amazon EC2[11],Microsoft Azure[12]and Google App Engine[13]can remedy the lack of resources in mobile devices.But what does mobile cloud computing really mean?Different people hold different views,and there are several existing definitions of mobile cloud computing.

In[14],mobile cloud computing was first referred to as an infrastructure where data storage and processing could happen outside the mobile device,enabling a new class of applications—especially context-aware mobile social networks.Mobile cloud applications move computing power and data storage away from mobile phones and into the cloud.This brings mobile applications and computing not just to smartphone users but to a broad range of mobile subscribers.

In terms of services[7],mobile cloud computing extends processing and storage beyond devices and seamlessly integrates with other services and sensor data to open up new classes of application.

In[9],mobile cloud computing is defined as an extension of cloud computing in which foundation hardware consists at least partly of mobile devices.This definition recognizes the opportunity to harness collective sensing,storage,and computationalcapabilities of multiple networked wireless devices to create a distributed infrastructure that supports a wealth of new applications.

Therefore,our goal in this survey is to eliminate confusion by clarifying the terms and concepts,predicting state-of-the-art mobile cloud computing,discussing future trends,and identifying technicaland non-technical obstacles and opportunities.

2 Mobile Cloud Computing

Definitions of mobile cloud computing can be divided into two classes.The first refers to carrying out data storage and processing outside mobile devices[15].Mobile devices are simply terminals in cloud computing,only intended to provide a more convenient way of accessing services in the cloud.The benefit of this is that storage and computing limitations of mobile devices are avoided,and a new level of security is provided by centralizing maintenance of security-critical software.

The second class of definitions refers to computing where data storage and processing are also carried out on mobile devices[9].The infrastructure of the cloud is different from that of data-center“pay-as-you-use”cloud computing.Each node is owned by a different user and is likely to be mobile.Using mobile hardware for cloud computing has advantages over using traditionalhardware.These advantages include computational access to multimedia and sensor data without the need for large network transfers,more efficient access to data stored on other mobile devices,and distributed ownership and maintenance of hardware.

In this paper,mobile cloud computing is defined as cloud computing extended by mobility and a new ad-hoc infrastructure based on mobile devices.It provides mobile users with data storage and processing services on a cloud computing platform.Using this definition,we clarify two differences between mobile computing and cloud computing.

First,there is an overlap between the two computing paradigms.Cloud computing aims to provide services to users without letting them know where these services are hosted or how they are delivered.Mobile computing aims to support mobility so that users can access resources with wireless technology.

Second,it is possible to build up clouds within mobile devices for data storage and processing.Although mobile devices nowadays are weak in terms of energy,power,storage,and communication,this situation willbe improved dramatically with the development of technology.In[9],Hyrax shows that a platform derived from Hadoop[16]can support cloud computing on Android[17]smartphones.It demonstrates the possibility that a cloud composed of mobile devices can provide basic functions and services of cloud computing.

Why do we need mobile cloud computing?There are at least three possible explanations.

First,mobility support is very important in allowing users access to cloud services.Service availability is considered the first obstacle[9]for cloud computing.Mobile cloud computing exploits information about a user's location,context,and requested services and significantly evolves the heterogeneous access management schemes for traditional heterogeneous access scenarios.This is called“intelligent access”[15].It can noticeably improve user experience.

Second,the capacity of mobile devices has increased dramatically with the growth of mobile users,especially with the prevalence of smartphone users.Each mobile device has storage,computing,sensing,and power resources.By using these resources,applications or services can be easily accessed by users inside and outside the mobile network.

Third,there are still some obstacles to implementing cloud computing,and mobile computing technology can help overcome these obstacles.For example,long WAN latencies are a fundamental obstacle when a mobile device executes a resource-intensive application on a distant high-performance server or cluster.A solution is to instantiate customized service software on a nearby cloudlet[18]and then to use that service over a wireless LAN.

3 Existing Works on Mobile Cloud Computing

According to our definition of mobile cloud computing,there are two key issues:How to build up a mobile cloud using mobile devices and how to access services provided by clouds in a mobile way.Thus,existing work on mobile cloud computing can be classified as that based on the cloud platform,or that based on access schemes.For the former,we investigate existing work on mobile cloud platforms consisting of mobile devices.The Hyrax[9]platform is derived from Hadoop[16]and supports cloud computing on Android[17]smartphones.To the best of our knowledge,it is the only platform comprised of mobile devices.For the latter,we investigate existing access schemes in mobile cloud computing.Mobile cloud computing is still in its infancy,and we investigate Virtual Machine(VM)-based cloudlets[18]and intelligent access schemes[15].

3.1 Hyrax

The goal of Hyrax is to develop a mobile cloud infrastructure that enables smartphone applications with distributed data and computation.Hyrax allows applications to conveniently use data and execute computing jobs on smartphone networks and heterogeneous networks of phones and servers.Research has been focused primarily on implementation and evaluation of mobile cloud computing infrastructure based on Map Reduce[19].Map Reduce,Hadoop,and Android should be introduced before describing Hyrax infrastructure.

Map Reduce is considered a programming model for processing and generating large data sets.Auser specifies a map function that processes a key/value pair in order to generate a set of intermediate key/value pairs.It also has a reduce function that merges all intermediate values associated with the same intermediate key.The Map Reduce runtime system splits input data,schedules map and reduce tasks,and transfers input and output data to machines running the tasks.There is a master to manage jobs,assign tasks to slave machines,and provide locations of intermediate values to reduce tasks.

▲Figure 1.Hyrax hardware and software layers.

Apache Hadoop is an open source implementation of Map Reduce that is used by many organizations for large-scale data processing.It is designed to operate data stored in a distributed file system,Hadoop Distributed File System(HDFS).There are four types of processes in Hadoop instances:NameNode,Job Tracker,DataNode,and TaskTracker.There is one NameNode and one Job Tracker in a Hadoop cluster.NameNode schedules jobs and coordinates sub-tasks among TaskTrackers.A DataNode instance and a TaskTracker instance both run on each worker machine.DataNode stores and provides access to data blocks,and TaskTracker executes tasks assigned to it by Job Tracker.Clients access files by first requesting block locations from NameNode and then requesting blocks directly from these locations.The hardware and software layers of Hyrax are described in Fig.1.

Hyrax ports Hadoop to the Android platform.In Hyrax,each machine runs one instance of NameNode and one instance of Job Tracker.DataNode and TaskTracker are run on each phone in separate Android service processes within the same application.Android applications may consist of multiple processes,some of which run as background services.Since DataNode and TaskTracker are run as Android services,they can run in the background of other applications.

A thread is also spawned to record information about the system load—including power leveland CPU,memory,network,and disk I/O statistics—into the local file system.Within the application,a server is run to allow external scripts to control data uploading,kill the program,and check the program status.Fig.2 illustrates the data interactions between all the software components on each phone.

Hyrax is implemented in a real testbed consisting of a cluster of 10 Android G1(HTC Dream)phones and 5 HTC Magic phones,each running Android 1.5 Cupcake.Since Android does not support peer-to-peer networking yet,the phones communicate with each other on an isolated 802.11g network via a Linksys WRT54G wireless router with no firmware modifications.NameNode and Job Tracker processes are run on a desktop machine connected behind this router via Ethernet.The phones are connected via USBto a controller that executes experiment scripts.These scripts are used to install Hyrax,initialize the cluster,run benchmarks,and collect and post process data.To determine the advantages and drawbacks of Hyrax,an application was developed on it.The Hyrax multimedia search and sharing application,HyraxTube,allows users to browse videos and images stored on a network of phones and search by time,location,and quality.Quality ratings based on sensor data are generated by periodically executing a Map Reduce job.Requests are serviced by reading results generated by the Map Reduce job from HDFS.The client interface is implemented as a web application so that it can be used on mobile devices and desktop machines.

3.2 VM-Based Cloudlets

Satyanarayanan et al.[18]present a new vision for mobile cloud computing.They foresee a new world in which mobile computing seamlessly augments users'cognitive abilities via intensive capabilities such as speech recognition,natural language processing,computer vision and graphics,machine learning,augmented reality,planning,and decision making.

▲Figure 2.Hyrax worker application componentinteraction diagram.

The means of achieving this vision is to let a mobile user exploit VM technology to rapidly instantiate customized service software on a nearby cloudlet.That service is then used over a wireless LAN.A cloudlet is a trusted,resource-rich computer or cluster of computers that is well-connected to the Internet and available for use by nearby mobile devices.Mobile devices act as a thin client in a 3-layer cloud computing architecture.

This solution mainly addresses the resource poverty of mobile devices,and long WAN latency is a fundamental obstacle.The predominant view is that WAN latency can hurt usability by degrading the crispness of system response,and it is unlikely to be improved.Bandwidth-induced delays also hurt user experience.The transmission of large data items should occur within a tight user-machine interaction loop.

Real-time interactive response can be achieved through low-latency,one-hop,high-bandwidth wireless access to the cloudlet.Fig.3 illustrates cloudlets and how they work.A cloudlet resembles a data center in a box;it is self-managing and requires little more than power,Internet connectivity,and access control for setup.Such simplicity of management is similar to an appliance model of computing resources and makes cloudlets simple to deploy in a business premises.Internally,a cloudlet resembles a cluster of multicore computers,with gigabit internal connectivity and high-bandwidth wireless LAN.

Cloudlet infrastructure is deployed in much the same way as Wireless Fidelity(Wi-Fi)access points.The key challenge is to simplify cloudlet management,and a proposed solution is transient customization of cloudlet infrastructure using hardware VM technology.Pre-use customization and post-use cleanup ensure that cloudlet infrastructure is restored to pristine state after each use,without manual intervention.A VM cleanly encapsulates and separates the transient guest software environment from the cloudlet infrastructure's permanent software environment.Therefore,a VM-based approach is less brittle than alternatives such as process migration or software virtualization.It is also less restrictive and more general than language based virtualization approaches that require applications to be written in a language such as Java or C#.

▲Figure.3.The cloudletconcept.

3.3 Intelligent Access Schemes

Mobile cloud computing relies on permanent connectivity.Intelligent access schemes[15]try to meet this requirement by specific information such as the user's location,context,and request services provided by the so-called mobile cloud controller.

Efficient network access management across different Radio Access Technologies(RATs)—Heterogeneous Access Management(HAM)—is necessary.Radio resources are limited due to physical,technology-specific,and regulatory constraints.Context provided by terminals,network nodes,or sensors deployed in the user's environment are used to significantly reduce wasting scarce radio resources and to efficiently manage wireless access across heterogeneous RATs.

The proposed solution is designed for a heterogeneous access scenario with a wide range of different radio access technologies and context information.The proposed Intelligent Radio Network Access(IRNA)concept takes into account the characteristics and status of each RATwhile simultaneously trying to accommodate users who face different environmental conditions in order to provide the best possible end-to-end performance.IRNA is shown in Fig.4.

The proposed Context Management Architecture(CMA)acquires,processes,manages,and delivers context information.AContext Quality Enabler(CQE)is incorporated into the architecture.The CQEcontrols the provision of context information according to the requirements of the Mobile Cloud Controller.A Context-Aware Radio Network Simulator(CORAS)is proposed to model context availability,accuracy,and delay,and enable an evaluation to be made on the impact of context relevance,confidence,and quality on simulation results.

4 Conclusions

Wireless and mobile computing technologies provide more possibilities for accessing services conveniently.Mobile devices will be improved in terms of power,CPU,and storage.Mobile cloud computing has emerged as a new paradigm and extension ofcloud computing.

▲Figure 4.The conceptof Intelligent Access.

In this survey,terminologies and concepts are clarified,and a definition of mobile cloud computing is provided based on an understanding of underlying technologies and applications.Existing work has also been surveyed and two classes of architecture framework are described for mobile cloud computing.

For future research,there are still some challenging issues related to the models of mobile cloud computing and services.They are:

(1)Mobile devices are constrained in storage and processing capacity.How can efficient use be made of the limited resources for cloud computing?

(2)There are several operating systems for mobile devices,especially mobile smartphones.These include Android,Symbian,iOS[20],Chrome,and MeeGo[21].Is it possible to provide a general access platform for mobile cloud computing on top of these various OSplatforms?

(3)From Hyrax and Cloudlet,mobile devices act not only as clients but also as components of clouds.What is the impact of mobility of mobile devices on performance of the cloud?What are the benefits and disadvantages?

(4)Users in mobile clouds are still exposed to security threats both inside and outside the cloud.What is the best solution for such a mobile environment?

Demand and applications of mobile cloud computing will grow rapidly,and researchers and developers should pay more attention to this area of cloud computing research and development.