Technology has shown significant potential in developing countries, as appropriate designs matched with real-world needs can effectively bridge information gaps, provide greater transparency, and improve communication efficiency. Unfortunately, many growing regions’ environments lack affordable network connectivity. Even where there is connectivity, networks are often characterized by frequent, lengthy, and unpredictable link outages, along with limited bandwidth and congested usage. Comparing the prevalence of access to information technology across regions, one finds marked distinctions between generally industrialized (and “wired”) countries and a large number of developing nations that lack connectivity and access to technology.
Communicating from Earth to any spacecraft is a complex challenge, largely due to the extreme distances involved. When data are transmitted and received across thousands and even millions of miles, the delay and potential for disruption or data loss is significant. Through the network, transmission has been a very easy thing at current, but it is still very difficult to transmit data in some networks that are often in delay and interrupted. Meanwhile, network interruption and delay are very common due to various reasons such as the change in network topology or harsh environment, etc. For these many researchers have proposed many solutions over the last decades.
Though these mechanisms all try to address the issue based on the conventional network protocols these mechanisms are not feasible in some specific cases, which resulted in the concept of DTN.
What is it?
A delay tolerant network is a newly emerging network, which usually deals with communications in extremely challenging environments, such as space communications and networking in sparsely populated areas, vehicular ad hoc networks, and underwater sensor networking. A Delay-Tolerant Network (DTN) is a network designed to operate effectively over extreme distances such as those encountered in space communications or on an interplanetary scale. In such an environment, long latency sometimes measured in hours or days is inevitable. However, similar problems can also occur over more modest distances when interference is extreme or network resources are severely overburdened.
Delay Tolerant Networking (DTN) is an approach to networking, which handles network disruptions and high delays that may occur in many kinds of communication networks. The primary reasons for high delay include partial connectivity of networks as can be seen in many types of ad hoc wireless networks with frequent network partitions, long propagation time as experienced in inter-planetary and deep space networks, and regular link disruptions due to the mobility of nodes as observed in terrestrial wireless network environments.
Figure 1 Example of Delay Tolerant Networking (DTN)
In a delay-tolerant network, traffic can be classified in three ways, called expedited, normal, and bulk in order of decreasing priority. Expedited packets are always transmitted, reassembled, and verified before data of any other class from a given source to a given destination. Normal traffic is sent after all expedited packets have been successfully assembled at their intended destination. Bulk traffic is not dealt with until all packets of other classes from the same source and bound for the same destination have been successfully transmitted and reassembled.
The basic features of a Delay-Tolerant Network
DTN network has the following basic features:
- Intermittent Connection: As the node’s mobility and energy are limited, DTN frequently disconnects, thus resulting in continued change in DTN topology. That is to say, the network keeps the status of the intermittent connection and partial connection so that there is no guarantee to achieve an end-to-end route.
- High delay, low efficiency, and high queue delay: End-to-end delay indicates the sum of the total delay of each hop on the route. The delay consists of waiting time, queuing time, and transmission time. Each hop delay might be very high due to the fact that DTN intermittent connection keeps unreachable for a very long time and thus further leading to a lower data rate and showing the asymmetric features in the up-down link data rate. In addition, queuing delay plays a main role in end-to-end delay and frequent fragmentations in DTN make queuing delay increasing.
- Limited Resource: Node’s computing and processing ability, communication ability, and storage space are weaker than the function of an ordinary computer due to the constraints of price, volume, and power. In addition, the limited storage space resulted in a higher packet loss rate.
- Limited Life Time of Node: In some special circumstances of the restricted network, the node commonly uses the battery power in the state of hostile environments or in harsh conditions, which will cut the lifetime of the node. When the power is off, then the node cannot guarantee normal work. That is to say, it is very possible the power is off when the message is being transmitted.
- Dynamic Topology: Note that the DTN topology is dynamically changing for some reasons such as environmental changes, energy depletion, or other failures, which results in dropping out of the network. Or, the requirements of entering DTN also make topology change.
- Poor Security: In general, DTN is vulnerable to–besides threats of wireless communication network–eavesdropping, message modification, routing spoofing, Denial of Service (DoS), and other security threats, etc, due to the lack of specialized services and maintenance in real-world.
- Heterogeneous Interconnection: DTN is an overlay network for the transmission of asynchronous messages. Introducing the bundle layer, the DTN can run on different heterogeneous network protocol stacks and the DTN gateway ensures the reliable transmission of interconnection message
How DTN Works?
DTN is a computer networking model and a system of rules for transmitting information, often referred to as a protocol suite that extends the terrestrial Internet capabilities into the challenging communication environments in space where the conventional Internet does not work well. These environments are typically subject to frequent disruptions, links that are limited to one direction, possibly long delays, and high error rates.
The DTN protocol suite can operate in tandem with the terrestrial IP suite or it can operate independently. DTN provides assured delivery of data using automatic store-and-forward mechanisms. Each data packet that is received is forwarded immediately if possible, but stored for future transmission if forwarding is not currently possible but is expected to be possible in the future. As a result, only the next hop needs to be available when using DTN.
The DTN suite also contains network management, security, routing, and quality-of-service capabilities, which are similar to the capabilities provided by the terrestrial Internet suite. Even though DTN was developed with space applications in mind, the benefits hold true for terrestrial applications where frequent disruptions and high-error rates are common. Some examples include disaster response and wireless sensor networks.
- Improved Operations and Situational Awareness: The DTN store-and-forward mechanism along with automatic retransmission provides more insight into events during communication outages that occur as a result of relay or ground station handovers and poor atmospheric conditions, and significantly reduces the need to schedule ground stations to send or receive data, which can sometimes require up to five days of planning before transmission takes place.
- Interoperability and Reuse: A standardized DTN protocol suite enables the interoperability of ground stations and spacecraft operated by any space agency or private entity with space assets. It also allows NASA to use the same communication protocols for future missions (low-Earth orbit, near-Earth orbit, or deep space).
- Space Link Efficiency, Utilization, and Robustness: DTN enables more reliable and efficient data transmissions resulting in more usable bandwidth. DTN also improves link reliability by having multiple network paths and assets for potential communication hops.
- Security: The DTN Bundle Protocol Security allows for integrity checks, authentication, and encryption, even on links where not previously used.
- Quality-of-Service: The DTN protocol suite allows for many priority levels to be set for different data types, ensuring that the most important data is received ahead of less important data
 Harminder Singh Bindra and Amrit Lal Sangal, “Considerations and Open Issues in Delay Tolerant Network’s (DTNs) Security”, Wireless Sensor Network, 2010, Volume 2, pp. 645-648
 Wei Suna, Congmin Liu and Dan Wang, “On Delay-Tolerant Networking and Its Application”, 2011 International Conference on Computer Science and Information Technology (ICCSIT 2011), IACSIT Press, Singapore
 “Disruption Tolerant networking”, available online at: https://www.nasa.gov/content/dtn
 “delay-tolerant network”, available online at: http://searchnetworking.techtarget.com/definition/delay-tolerant-network
 Demmer, Michael Joshua, “A delay tolerant networking and system architecture for developing regions”, PhD dissertation, University of California, Berkeley, 2008.