QoS Support in Multi-hop Ad-hoc Networks

Background

QoS is a term which has been widely used in modern telecommunications. QoS is the ability to provide different priorities to different applications or flows to guarantee a certain level of performance. QoS guarantees are especially important when the network capacity is insufficient or the network is exposed to congestion. QoS is most commonly measured by the following metrics: bit rate, delay, variation of delay (jitter), packet dropping probability and bit error rate (BER). In multi-hop ad-hoc networks providing QoS is particularly difficult because of the challenges at the following layers:

• •

  Physical layer. The use of wireless technologies makes links susceptible to fluctuations in the radio channel. As a result such factors as fading or interferences may lead to low bit rates and high BERs. The physical layer should quickly respond in such situations to prevent high frame error rate (FER) at the data link layer. Furthermore, random movement of mobile nodes introduces unpredictable link failures which lead to network reconfiguration. Additionally, mobile nodes are usually limited by their battery power. Power consumption can be one of the QoS attributes, because it has a strong influence on all QoS metrics.

• •

  Data link layer. With the help of adequate MAC protocols, nodes need to support service guarantees for multiple traffic classes and efficiently share a common radio channel with their neighbors. Additionally, traffic scheduling schemes for real-time traffic should be used to avoid starvation of best effort traffic. The protocol should also promptly react to transmission errors and collisions. The automatic repeat request (ARQ) or adaptive error correction methods should also be used when transmission quality degrades on the data link layer.

• •

  Network layer. Nodes can move in a random way. Therefore, the network topology changes unpredictably and routing protocols need to quickly adjust. Additionally, there should be a signaling protocol responsible for admission control, resource reservation, reaction to congestion and negotiation of QoS parameters.