Mobile Ad Hoc Networks (MANETs) for Multimedia Transmission

Background

Wireless Local Area Networks (WLANs) are one of the most popular and ubiquitous forms of wireless connectivity between different types of equipment. WLAN interfaces are embedded in many common electronic devices: laptops, PDAs, smart phones etc. IEEE 802.11 (IEEE, 2007) is the dominant standard for WLANs. Networks can be deployed using available COTS (Commercial Off-The-Shelf) equipment supporting 802.11 for both home and enterprise scenarios. There is also a multitude of available hardware and software tools for many OS platforms, making them easy to administer. Furthermore, WLANs provide mobility, high-speed transmission, and distributed topologies.

The most common topology of IEEE 802.11 WLAN is infrastructure mode. Stations (STA) connect to an Access Point (AP) using a radio link. The AP is connected to the external network through a wired link (Ethernet being most common). This is a centralized approach, however, more robust topologies can be found in the 802.11 standard.

In ad-hoc mode presented in Figure 1, there is no central station (STA). All STA within range can communicate directly; otherwise a multi-hop packet-forwarding connection is used. This approach provides reliability and robustness to the connection. There is no single point of failure (provided there are multiple paths in the network) which makes them a feasible choice for ad-hoc networks deployments. For ad-hoc mode networks the IEEE 802.11 standard defines physical layer (PHY) and medium access layer (MAC) while network layer with routing protocols are defined mainly by the International Engineering Task Force (IETF).

Figure 1.

IEEE 802.11 ad-hoc network

MANETs are based on ad-hoc technology. However, current MANETs built with IEEE 802.11 technology lack scalability and Quality of Service (QoS). Mobility is a unique feature but it results in a very dynamic topology in which routing can become a very complicated task. There is also a need for fast scanning and fast roaming to support real-time applications. The scanning process must consume as little energy as possible. Fast mobility must be supported to serve traffic in MANETs. During handoffs, except for QoS, security is of crucial importance. The routing protocols that have been designed for wireless ad-hoc networks directly affect the performance of the serving applications. Each protocol has its own routing strategy that is used in order to discover a routing path between two ends. The performance varies, depending on network conditions like the density of nodes in a specific area, their speed and direction. It is obvious that the selection of the proper routing protocol for a specific network topology plays a critical role.