obile ad hoc networks (MANETs) are distinguished from other types of networks by their physical characteristics, organizational format, and dynamic topology: • Physical characteristics: Wireless channels are inherently error-prone, due to effects such as multipath fading, interference, and shadowing; causing unpredictable link bandwidth and packet delay. • Organizational format: The distributed nature of MANETs means that channel resources cannot be assigned in a predetermined way. • Dynamic topology: As hosts in a MANET are mobile, links are created and destroyed in an unpredictable way. Therefore, the network status can change quickly, causing hosts to have imprecise knowledge of the current network state. Because of device mobility in MANETs and the shared nature of the wireless medium, offering guaranteed quality of service (QoS), such as bandwidth, delay, delay jitter, and packet delivery ratio, is not practical. Therefore, soft QoS and QoS adaptation are proposed instead. Soft QoS implies that failure to meet QoS is allowed, for example, when routes break or the network becomes partitioned [1] . However, if a network changes too fast to propagate the topology status information, it is challenging to offer even soft QoS. Therefore, combinatorial stability -which means that given a specific time window topology changes occur sufficiently slowly to allow successful propagation of all topology updates as necessary [2] -must be met in order to provide QoS. Certain applications, such as real-time applications that can optimize their performance based on feedback about network resource availability, can benefit from QoS adaptation. For example, layered coding allows enhanced layers of different quality levels to be transmitted, provided a minimum bandwidth is guaranteed for transmitting the base layer. By providing feedback to the application about available resources, the application can alter its coding strategy to provide the best quality for the current resource limitations. Routing is used to set up and maintain routes between nodes to support data transmission. Early MANET routing protocols focused on finding a feasible route from a source to a destination, without any consideration for optimizing the utilization of network resources or for supporting specific application requirements. To support QoS, the essential problem is to find a route with sufficient available resources to meet the QoS constraints and possibly to incorporate optimizations, such as finding the lowest cost or most stable of the routes that meet the QoS constraints. Given these goals, the following are the basic design considerations for a QoS-aware routing protocol. • Resource estimation: To offer a resource-guaranteed route, the key concept is to obtain information about the available resources from lower layers. This information helps in performing call admission and QoS adaptation. Most existing techniques focus on bandwidth and/or delay QoS constraints, and thus, the bandwidth available to a node or link and/or the delay must be estimated. In MANETs, hosts share the bandwidth with their neighbor hosts, and thus, the bandwidth available to a node varies and is dynamically affected by the traffic of its neighbors. Therefore, the two key problems in bandwidth estimation are: how exactly to estimate the available bandwidth and how frequently to estimate it. In general, the trade-off between the benefit from using resource estimation and the cost in terms of overhead and computing resources used for resource estimation is a key issue. • Route discovery: There are two main approaches to routing in MANETs: reactive routing and proactive routing. Reactive routing reduces overhead at the expense of delay in finding a suitable route; whereas, the reverse is true for proactive routing. For QoS-aware routing, another issue is determining the combination of reduced latency and reduced overhead that is best for supporting QoS. • Resource reservation: As previously stated, the bandwidth resources are shared by neighboring hosts in MANETs. Therefore, another challenging issue is how to allocate these shared resources, the type of resource reservation Lei Chen and Wendi B. Heinzelman, University of Rochester The explosive growth in the use of mobile devices coupled with users' desires for real-time applications has provided new challenges in the design of protocols for mobile ad hoc networks. Chief among these challenges to enabling real-time applications for mobile ad hoc networks is incorporating support for quality of service (QoS), such as meeting bandwidth or delay constraints. In particular, it is important that routing protocols incorporate QoS metrics in route finding and maintenance to support end-to-end QoS. This article extensively and exclusively studies the issues involved with QoS-aware routing and presents an overview and comparison of existing QoS-aware routing protocols. In addition, the open issues that must be addressed to fully support QoS-aware routing are discussed.
For this reason, most existing QoS-aware routing protocols in the literature assume that the available bandwidth is known REF .
A Survey of Routing Protocols that Support QoS in Mobile Ad Hoc Networks
IEEE Network
IEEE Network
Computer Science