The existing research on QoS routing for ad hoc networks can be divided into two categories: QoS route information and QoS route computation. QoS route information provides the QoS information over the path it constructs using traditional best-effort routing algorithms. QoS route computation calculates feasible routes based on various QoS requirements.
For the QoS route information, Chen et Al. [?] propose a bandwidth-constrained routing algorithm. Each node calculates the available bandwidth over the wireless links to the destination. Such bandwidth information is piggybacked in the Destination Sequence Distance Vector (DSDV) routing algorithm [?]. Lin and Liu [?] have a similar approach using DSDV. Focusing on bandwidth control, bandwidth information is embedded in the nodes' routing tables and sent to the neighbors. Upon receiving a routing table from a neighbor, a node updates its own routing table and the path bandwidth information. With the bandwidth information, a node can decide whether or not it should accept a new connection request based on the bandwidth requirement of that connection. Kazantzidis and Gerla [?] have developed an architecture that supports accurate permissible throughput explicit feedback to multimedia transports and call admission applications. The motivation is for the architecture to base a cost/benefit analysis of end-to-end measurements versus lower level explicit feedback in last mile 802.11 networks. They tackled common measurements effectively, by dealing with them on a link-by-link source-destination specific basis. After making available the bottleneck permissible throughput at endpoints of a multi-hop ad hoc network, they compared the network feedback adaptation to end-to-end.
For the QoS route computation, Wang and Crowcroft [?] consider a number of issues in QoS routing. They first examine the basic problem of QoS routing, namely, finding a path that satisfies multiple constraints, and its implications on routing metric selection. They present a centralized algorithm that is suitable for source routing and two distributed algorithms that are suitable for hop-by-hop routing based on bandwidth and delay constraints. Chen [?] proposed a heuristic algorithm for the delay-cost constrained routing problem which is NP-Complete. The main idea of this algorithm is to first reduce the NP-Complete problem to a simpler one which can be solved in polynomial time, and then solve the new problem by either using an extended Dijkstra algorithm or extended Bellman-Ford algorithm. Al-Fawaz and Woodward [?] propose a routing algorithm to find the shortest path between one source and one destination node while considering the criteria of multiple metric constraints. They have three goals to achieve, 1) sort the QoS metrics according to the requested service, 2) find the possible routes between the source and the destination and 3) speed up the path determination by using sliding window and hierarchical clustering techniques. In [?] authors propose and analyze the performance of a distance-vector QoS routing algorithm that takes into account three metrics: propagation delay, available bandwidth and loss probability. Classes of service and metric combination are used to make the algorithm scalable and as on a two-metric scheme. Cheng and Nahrstedt [?] give an algorithm to find a path that meets two requirements in polynomial time. The algorithm reduces the original problem to a simpler one by modifying the cost function, based on which the problem can be solved using an extended shortest path algorithm. The shortcoming of this method is that the algorithm has to use high granularity in approximating the metrics, so it can be very costly in both time and space, and it cannot guarantee that the simpler problem has a solution if the original problem has.