4. LIMITATION AND CHALLENGES IN OPPORTUNISTIC NETWORK
In an opportunistic network, when nodes move away or need to turn off their power to save up energy, links may be disturbed or shut down intermittently. These events result is in intermittent connectivity. When there is no communication path existing between the source and destination, the network partition occurs. The possibility of the data being sent by sender might be not received or semi-received by the receiver. Therefore, nodes need to communicate with each other opportunistic contacts through Store-Carry-Forward (SCF) operation. The specific challenges in opportunistic network are the contact opportunity and the node storage.
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This scheme achieves the optimal delay with unlimited relay buffers. However, such a multiple-copy scheme generally incurs significant overhead on storage constraint. Another theory from other researcher is a buffer-management strategy, RRFS-with-RandomDrop, to avoid head-of-line blocking in the FIFO case. They showed that the proposed strategy can reduce the deprivation of average data delivery delay performance.
Another challenge in opportunistic network is data integrity. Data integrity is a part of data security, also an important element to establish secure communication. Digital signatures can be used to guarantee the integrity of data. But they are too costly investment for weak devices (like mobile phones, PDAs etc.) which also running on a limited battery power. Hence, alternatives should be devised to guarantee the integrity of data packets. Also, packet sizes may vary when it travels through an opportunistic network. Suppose that a packet is sent from a mobile phone to the base station through a PC connected to the Internet. In this case, the packet size when it travels from the mobile phone to the PC will be different from the packet size when it travels from the PC to the base station. If the packet fragmentation and aggregation process cannot be performed securely, the end-to-end security mechanisms could fail.
5. RESEARCH PROJECTS ON OPPORTUNISTIC NETWORKS
The Haggle project has been developed by researcher from Department of Information Technology, Uppsala University in Sweden. It is an innovative concept for Autonomic Opportunistic Communication Research project funded by the European Commission under the Information Society Technologies Programme of the 6th Framework between January 2006 and June 2010. This application supports many platforms, but mainly targets on mobile phones. The example of platforms include Google Android, iPhone OS, Windows Mobile, Mac OS X, Linux, etc. The programming languages used are a mix of C++ and C. But the applications themselves can be written in other languages as well, like C# and Java.
The Haggle allows mobile devices to exchange content directly between them when they connected in close range contact. A Haggle-enabled application can exchange photographs or songs mostly are multimedia data between mobile phones. The content exchange happens according a subscribe model, where users state their interests via keywords and then receive content items from others based on to how well they match the interests. Haggle application also supports both Bluetooth and Wi-Fi connectivity. Figure 7 below show Haggle architecture and how it works in content transmission.
This function of application internally used a concept of opportunistic network technology in their data dissemination. Haggle provides a search-based data dissemination framework for mobile opportunistic communication environments, making it easy to share any type of media directly between intermittently connected mobile...