Functioning of Mobile Ad Hoc Cloud

The proposed framework [30] has basically three functions: service discovery, offloading decision, and seamless service. The services provided by the system and the interaction between modules in client and peer devices for the purpose of providing service to the users are depicted in Figure 5.23. The service discovery module helps the client device to find different devices accessible in the mobile ad hoc cloud [30]. This module is connected between the client and peer device so as to discover various services offered by the peer device. The seamless service modules of the client and the peer interact to decide on con- tinuing the service based on the accessibility of connections. Applications and data can be offloaded either to the mobile ad hoc cloud or to the default cloud for execution. Offloading decisions are influenced by the bandwidth, device, and application characteristics [30]. The offloading module interacts with the service discovery and seamless service modules to take the appropriate offloading decision.

TABLE 5.12

Formation of Mobile Ad Hoc Cloud

1. A mobile device X might be connected with its peer Y by means of one network connection. Y might be connected with another device Z, which is not in the vicinity of X.

2. Devices X and Y form one domain, while Y and Z form another domain. 3. Thus connecting the device between X and Z is Y.

4. If device X requires service of Z, the connecting device Y can be used for communication between the devices providing the service.

5. In this manner, many devices can be connected together to avail the service. The client device can know about the devices of the other domains with the help of the discovery module.

Cloud Module 1: service discovery Module 1: service

discovery Module 1:offloading discussion Module 2: seamless service Module 2: seamless service Module 3: offloading discussion FIGURE 5.23

The three basic functions of a mobile ad hoc cloud are considered to be modules, as described in the following sections [30].

5.8.2.1 Module 1: Service Discovery

Every device in a domain has various services to offer. The services provided by a device are advertised using a discovery message. Through service discovery, a device in a domain can know about the availability of the service in another domain. When a device enters a mobile ad hoc cloud, the discovery message is advertised. The message is received by all other devices, and they update their service discovery table. The structure of the discovery table is shown in Table 5.13.

The parameters of the table are as follows [30]:

Unique ID: It identifies a particular service in mobile ad hoc cloud.

Service offered: It specifies the service that is ready to be offered by the device.

Computational ability: It specifies the processing capability of the device such as the type of processor, memory capacity, and so on.

Network characteristics: It specifies the medium through which it is connected to another network and bandwidth details.

Distance: It specifies the hop count to reach the device when it is advertising. The initial value is set to 0. This value is updated by every device once the message is received. If the received hop count is zero, the device updates the distance by 1. It helps to know how many hops are needed to reach the device.

Available energy: It specifies the energy required to provide service to the other devices in order to assign weights to the devices while deciding whether to offload the contents.

Every device circulates the service discovery table to the devices with which it is con- nected. On receiving the table, the recipient device compares it with its own table and updates the information accordingly. This table is used by the device to make an appropri- ate decision on selecting the device for availing the service.

5.8.2.2 Module 2: Seamless Services

To provide seamless service, it is required to keep track of the signal strength of devices in a mobile ad hoc cloud. The received signal strength indication (RSSI) is a measure to do so, and a threshold RSSI is maintained.

The intermediate mobile devices help the client to obtain services from other devices. When the intermediate devices find that the signal strength has fallen below the threshold, it breaks the service and passes it to a nearby device with a similar configuration. If such a device is not found, the interim result is passed to the client for further action. Figure 5.24 shows when to avail service from the cloud or from a peer device. This threshold policy is executed on the client device that requests for service [30].

TABLE 5.13

Service Discovery Table

Unique ID Service offered Computation

5.8.2.3 Module 3: Offloading Decisions

Offloading applications help to execute them in parallel or in a distributed manner. The total computation cost of an application is the sum of the cost of offloading data into the cloud (Ccloud), offloading modules to various peers (Cpeers), and calculating a part of the

application within the device itself (Cmobile) [30]. In order to provide energy conservation of

the mobile device, this total computation cost of using peers, cloud, and the mobile device must be less than the computation cost of all the modules if computed within the mobile device itself. The total computational cost is given by Ravi and Peddoju [30]

Totalc Ccloud Cpeer Cmobile i n = + + =

å

Using Equation 5.22, the total cost for computation is determined.