performance measurements. It shows that during the download the consumption level is around 1.25 W. At the beginning the higher peak indicates when the screen backlight is on. After the 300 sec we can see that basically the consumption is zero.
It also indicates that MobTorrent does not use unnecessary energy if it does not download or upload.
Figure 6.3. Energy consumption in real environment
Considering energy consumption (Figure 6.3) and download performance (Fig-ure 6.1) meas(Fig-urements by applaying an average DA = 50kB/sec download speed and PA = 1.25W energy consumption level, an estimated energy requirement (P) can be calculated to a specific content size (CS), that is intended to download:
P = CS
DAPA (6.1)
6.6 Content Distribution via Hybrid Peer-to-Peer
provide an efficient environment for managing relationships. It can be seen clearly that the role of mobile devices is increasing rapidly in social networks.
In Chapter 4 and 5 we have shown how mobile related social networks consider mobile phones as key elements in the network. Another major and increasing functionality of social networks is content sharing. In this chapter we introduce a hybrid peer-to-peer content sharing architecture for mobile related social networks, we propose a model to compare the cost of content sharing architectures and we show that for our purposes the hybrid architecture is more efficient. One of the key differences between our contribution and the researches described in Section 3.6.4 is that our solution involves also mobile phones in the architecture.
6.6.1 Bringing Mobile BitTorrent into Content Sharing Sys-tems
Content distribution over computer networks has always been a big challenge. First of all we have to decide whether to design a client-server solution with central units or a peer-to-peer solution where the content is distributed by using the resources of the network nodes.
The general architecture becomes even more complex if we plan to support mobile phones and other types of mobile devices as well. Another important ob-jective is to create an efficient content sharing solution that does not overload the server(s) but at the same time is able to support several clients including mobile devices.
We were involved at Nokia Research Center (later Nokia Siemens Networks) in the Swarm project where the goal was to design a general hybrid peer-to-peer architecture which decreases the load of the central server, therefore it is ideal for service providers.
The final architecture is based on the efficient BitTorrent protocol [BitTorrent 1.0, 2010] which was introduced in Section 3.6.2. This protocol is known as a peer-to-peer protocol, but it has certain central elements (tracker, torrent file catalogs) that make it a good building block for a hybrid system.
In the Swarm hybrid peer-to-peer architecture a central element is responsible for the content management (which album belongs to the selected member, etc.)
and it operates as a backend seed if no other shares the specific content, however when more members try to download the same content, the BitTorrent based peer-to-peer engine decreases the load of the server.
6.6.2 Architecture Description
To design a reliable and general content distribution solution and provide a value added service to the standard BitTorrent protocol, the Swarm architecture provides the following:
• A stable tracker and reliable seed service is running at the service provider (e.g. social network server farm). This has the ability to provide a reliable service since it can operate as a backend seeder in the network.
• An XML based interface (OPML), which is used by the clients for searching the right content.
• Support for both mobile and desktop clients.
The central element of the hybrid architecture consists of different entities.
These entities can run on one server but they can also be divided to multiple servers. The central element of the Swarm hybrid architecture contains the follow-ing entities:
• Portal server which serves the content management requests via XML inter-face.
• Database for storing metadata related to shared contents and ownerships.
• BitTorrent tracker functionality.
• Backup seed functionality which is used by clients if no others share the same content.
Figure 6.4 displays the central elements of the architecture in context of mobile and desktop clients.
The main advantage of Swarm architecture from usability point of view is that it hides the technological backgrounds from the user. In order to begin a download
Figure 6.4. Objectives of the central element in hybrid architecture
we do not have to know anything about peer-to-peer technologies or BitTorrent, we just have to select the proper content via the interface of the service provider (e.g.
a social network). Figure 6.5 illustrates how a mobile client starts downloading the selected content via Swarm.
In the first step the client sends a request to the Swarm server for the selected content. Then a server entity checks whether there are other clients (desktop or mobile) who share the same content. After it, the requester connects to these clients and starts download form them. If there are no available clients, the backend seed component on the server side will serve the request.
Figure 6.5. High level architecture
6.6.3 Analysis of Peers
Swarm is able to support different type of clients, however the major goal is to support mobile clients since they usually browse the service more often [Facebook statistics, 2010] but for shorter time.
Mobile phones have several advantages like supporting several types of net-working technologies as well as short range communication technologies, which can be utilized even in content sharing systems. Another advantage is that ad-vanced software platforms of mobile phones make it possible to consume different types of multimedia content. In addition, the increased storage capacity of mobile phones allows to store more and more of our favorite video or music on our device.
Nowadays it is common to have 4, 8 or even 32 GB memory in a single mobile phone. With the evolution of mobile phones, the need for an efficient content sharing mechanism has emerged.
Applying peer-to-peer technologies on mobile phones is an open research area.
In previous sections we have introduced our research result about how to bring BitTorrent to feature phones with limited resources. We showed that these devices
are able to participate in large networks like BitTorrent. We also demonstrated our model via measurements related to performance, memory- and energy consumption as well.