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Top1. Introduction
A Mobile Ad hoc NETwork (MANET) is a wireless ad-hoc network made up of radio nodes structured in a mesh topology (Sarkar et al., 2013). In particular, a MANET contains autonomous nodes that form a dynamic, multi-hop radio network in a decentralized way (Conti & Giordano, 2014). MANETs are deployed mainly in emergency situations like battlefield and natural disasters (e.g. for detection of earthquakes and floods) as there is no need to deploy any infrastructure to make nodes to communicate with each other (Basagni et al., 2014). Nodes themselves implement network management in a cooperative fashion. Such cooperation requires detecting routes and forwarding data packets. In ad-hoc mode, all nodes participate in both data processing and routing tasks. The network also relies on the multi-hop type of routing for the data transmission, since the destination node is often out of the radio-range of the source node, and some nodes can act as a router to forward data (Kanellopoulos, 2017).
MANETs pose the following design challenges:
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Problems related to hidden and exposed terminals;
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Constraints on resources: MANET devices work with limited CPU processing capabilities, limited battery life, limited bandwidth support, limited storage etc.;
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Error-prone shared broadcast channel: The transmissions by a node are broadcast in nature, and MAC layer algorithms try to control access to the shared broadcast channel. The wireless links have also a higher error rate, fading, interference of signals etc.;
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Nodes mobility: The dynamic nature of MANET results frequently in changing network topology and link breaks. It makes routing more difficult because of the frequent route change/route break leading to loss of connectivity. New challenges for video transmission are also imposed as the mobility of nodes adds an extra overhead (Kanellopoulos, 2017). The routes must be updated frequently.
Providing quality of service (QoS) support to video streaming applications over a MANET is a crucial problem due to nodes limited resources and nodes mobility (Rath et al., 2016). To solve this problem, some topology-based control routing algorithms for MANETs have been proposed (Yakine & Idrissi, 2015; Zhang et al., 2015). However, these routing solutions are insufficient because they solve the problem partially (the problem of QoS support remains). Moreover, MANETs have unique characteristics that complicate the design of congestion control inside the Transport Layer protocol (OSI-RM). The standard TCP congestion control mechanism is not able to handle the special properties of a shared wireless multi-hop channel well. The frequent changes of the network topology and the shared nature of the wireless channel pose significant challenges. For this reason, various congestion control approaches for MANETs have been proposed. Kanellopoulos (2019) discussed TCP enhancements for wireless links. He analyzed the design challenges for an enhanced transport protocol, and presented congestion control schemes for MANETs.
Actually, real-time multimedia applications need an enhanced transport protocol over MANET that should reliably handle packet loss, minimize errors, manage network congestion and transmit efficiently. Recently, new enhanced transport protocols have been designed such as Stream Control Transmission Protocol (SCTP) (Stewart, 2007), and TCP-Friendly Rate Control (TFRC) (Handley et al., 2008). Both protocols are still under research whether they can be used for real-time applications over MANETs practically. Actually, the choice of what transport protocol will be used depends on the QoS requirements imposed by the multimedia application on hand. Additionally, the advent of heterogeneous networks magnifies the volatility of network conditions and imposes greater challenges for multimedia delivery.