A Mobile Ad-hoc Network (MANET) is composed of mobile nodes without any infrastructure. MANET applications such as audio/video conferencing, webcasting requires very stringent and inflexible Quality of Service (QoS). The provision of QoS guarantees is much more challenging in MANETs than wired networks due to node mobility, limited power supply and a lack of centralized control. Many researches have been done so as to provide QoS assurances by designing various MANET protocols. In recent years

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... number of QoS routing protocols with distinguishing features have been newly proposed. However, systematic performance evaluations and comparative analysis of these protocols in a common realistic environment have been performed only in a limited manner. This paper presents a thorough overview of QoS routing metrics, resources and factors affecting performance of QoS routing protocols. The relative strength, weakness, and applicability of existing QoS routing protocols are also studied and compared. QoS routing protocols are classified according to the QoS metrics used, type of QoS guarantee assured and their interaction with the medium access control (MAC) protocol.