Mobile Networks | Opportunistic Networks – A Survey
INTRODUCTION
The widespread availability of mobile devices worldwide has been seen recent technological advancements in the field of networking. The advancement in this field has made opportunistic networks one of the most renowned and promising technology for the next-generation mobile applications [1]. Opportunistic networks (OPPNETs) are the emerging networking paradigm where communication is done on the fly whenever the communication links become available. Even in the absence of direct network routing, the communication between two mobile devices can be performed through sporadic connectivity [1]. OPPNETs are the future of communication technologies where the networking paradigms rely on heterogeneous networking. Due to recent advancement in the wireless communication area, the OPPNETs seem to open promising dimensions for future mobile applications. OPPNETs and delay tolerant networks (DTN) are often used interchangeably [2].
OPPNET is easily incorporated networking paradigm that can extend the existing wireless networks including 3G and Wi-Fi [3]. The data is communicated between two devices whenever they are in mutual wireless transmission range. OPNET allow dissemination of data in peer-to-peer manner just like humans when they interact and cooperate with each other while staying in mobility. According to Chaintreau et al. (2007), the major motivation for OPPNET comes from the emerging trend of pervasive computing where different mobile devices manipulate the communicational environment for disseminating the data. Several nodes are present for OPPNET data communication that are equipped with sensing capability, short-range radio transmission functionality, large memory and wireless connection presence [4].
In OPPNETs, the nodes are the mobile devices that can be carried by the users easily. These nodes contain wireless properties and can change their locations whenever and wherever networks are present. The routing decisions in OPPNETs are made based on knowledge availability of a particular node [3]. Hence, the complete communication network topology or path in OPPNETs does not exist. Hop by Hop packets are utilized by nodes in OPPNETs for transferring data to the other node for delivering information to its destination in the network [5]. In OPPNETs, the communication devices can be easily carried by vehicles, animals and people. The connection between different nearby nodes can make up a small mobile ad hoc network that can dynamically change with time and destination [3].
WORKING OF OPPNETs
Under OPPNETs, the nodes are usually connected wirelessly. These nodes can be either mobile, stable or even be fixed with finite communication rage for dissemination of data. However, the nodes can only pass on the information to other nodes when the range allows them to connect with each other [6]. The data/information to be disseminated stays stored in the nodes until another node comes into range. The message reaches its destination by travelling through different nodes after staying connected within a given range. In real life, the OPPNETs allow the individuals to send email messages through connecting nodes [6]. The figure 1 below shows how a message from one computer travels through different nodes i.e. through nearby bus WiFi to a person’s mobile who is travelling on bus and then through that mobile to a cyclists’ mobile and then through cyclists’ mobile to a person who is passing by the office and then through that person to the final intended receivers’ laptop.
Figure 1: How OPPNETs Work in Real Life
Source: (Kaur and Mathur, 2016)
Opportunistic networks rely on the opportunities for delivering and passing on the messages i.e. whenever the nodes get the opportunity for sending the message, they do. Whenever the intermediate nodes come into the range, the nodes pass on the message until the message reaches its final destination [6]. However, the activation and deactivation of the neighboring nodes can change the overall topology of the network. In figure 2 below, it can be seen that source node i.e. S wants to send message to recipient node i.e. R. During the whole communication process, nodes 1 and 2 work together as neighboring nodes and help in transferring message from node S to node R. At first, node S sends the message to node 1 which is stored in node 1 until other nodes come into range. When nodes 3 and 4 appear in communication range of node 1, the message is instantly passed on to the neighboring node i.e. node 3 (as it is nearest than node 4). After node 3 gets the message, it stores the message until node R comes into range. Whenever node R moves into range of node 3, the message is sent to destination. However, until node R is in range, the data is stored in node 3 or moved to another node whenever it gets an opportunity.
Figure 2: Message Forwarding Between Intermediate Nodes. Source Node and Receiver Node
Source: (Kaur and Mathur, 2016)
TYPES OF OPPNETs
There are broadly four types of OPPNETs i.e. Sensor Networks, Pocket Switched Networks, Vehicular Networks and Amorphous Opportunistic Networks (see figure 3 below).
Figure 3: Opportunistic Networks Applications and Types
Source: (Wahid et al., 2014)
Sensor networks are those type of OPPNETs that rely on weakly-controlled connectivity. This system allows data dissemination at lower cost within the given time frame and target environment [3]. As compared to ubiquitous sensing device infrastructure, this network allows large scale sensing power for carrying on networking in a given environment. Sensor networks include MULEs (mobile ubiquitous LAN extension), BIONETs (BIOlogically inspired automatic NETworks and services) and wildlife monitoring. Pocket switch networking (PSN) is another type of OPPNET that does not rely on any type of infrastructure for disseminating content between mobile devices [3]. PSN mainly relies on human mobility to establish connection between different devices so that information can be transferred easily. Local connectivity is also utilized by PSN for transferring data between different users’ devices [3]. Under PSN, the data can be transferred between two devices even when no direct network connectivity is available due to which PSN only targets those devices that are always on and are never switched off i.e. mobile phones and other wireless devices [3]. PSN includes MANETs and mobile social networking.
The third type of OPPNET is a vehicular network that allows nodes for communicating in challenging environments that contains high delays and unstable links. Under these kinds of environments, the communication might be challenging because of varying link conditions that result in link error [3]. However, under such networks, the spreading of connecting nodes across networks might reduce the linking errors by establishing secure links throughout the network. It includes delay tolerant network (DTN), deep space internet and rural region internet systems. The fourth type of OPPNET is called amorphous opportunistic network. It is often named as overlay network that is established whenever the data sharing is enabled under insecure group [3]. Gnutella and Freenet opportunistic systems under amorphous opportunistic network have grabbed the spotlight in recent research. Amorphous networking system inter-connects the small internet nodes due to which the internet insecurities gets proliferated across network [3].
REAL WORLD APPLICATION OF OPPNETs
Various kinds of real-life applications of OPPNETs can be found. Mainly the renowned ones are wildlife monitoring and rural area networking. Under wildlife monitoring, OPPNETs are used for tracking the wild species’ behaviors and way of interaction [6]. The OPPNETs in this monitoring system contains special sensors that can easily predict animal behavior and examine the interaction of different animals with each other. A popular networking software used for wildlife monitoring is Zebranet and Shared Wireless Infostation model. Under the former, the system tracks the zebras and their behavior while in the later whales’ behavior and interaction is studied [6].
Under rural area networking system, internet is provided to the people residing in rural areas without infrastructure. Saami Network and DakNet networking are two major examples in which e-commerce, internet connectivity, email sending and receiving and audio/video messaging is supported without presence of infrastructure in the area [6].
CHALLENGES OF OPPNETs
OPPNETs are envisioned to provide communication means in absence of infrastructure which makes it appropriate way to regulate communication in urban scenarios [7]. However, like all other networks, OPPNETs are also prone to security and privacy challenges. The key issues and challenges faced by OPPNETs include heterogeneity, intermittent connectivity, delay tolerance, secure routing, data integrity, data privacy and denial of service (DoS) [6]. All of these challenges can cause delays in messages and disrupt the smooth communication process of OPPNETs.
OPPNET is a heterogeneous network in which several communication devices (e.g. mobiles, cameras, sensors etc.) are connected to each other due to which interoperability issue can rise. Apart from this, the high mobility of nodes can cause issues with data transference. This is because the data is transferred to neighboring nodes without having previous network information about the nodes due to which intermittent connectivity issues can arise [6]. The store and forward technique of OPPNETs allow the nodes to store data and forward on opportunity basis. This might reduce the overall transfer time of message as the data can only be transferred when the neighboring nodes become available. Until the neighboring nodes come into range, the data stays stored in the previous node which can cause delays in the communication process [6].
During the whole process, the secure routing of message has to be maintained by keeping a list of trusted devices used during communication [6]. The message has to pass through different nodes that are present in different devices owned by different institutions like government offices, universities, police stations etc. In order to make the whole process risk free, only trusted devices must be used during the process which is very challenging for OPPNETs. However, secret keys or digital signatures can be used for making the whole process trustworthy [6]. The key issues like data privacy and data integrity also arise in OPPNET due to challenge of maintaining trusted devices. A secure mechanism like data encryption and cryptography is required for broadcasting the message otherwise a malicious device can disrupt the whole secure message transferring process under OPPNET. Moreover, for maintaining the data integrity, digital signatures can be used which can although be expensive [6].
