Quality of Service

1. introduction
for supporting multimedia applications, it is desirable that an ad hoc network has a provision of quality of service (qos). however, providing the qos in a mobile ad hoc network is a challenging task.
`quality of service (qos) means that the network should provide some kind of guarantee or assurance about the level or grade of service provided to an application. the definition for qos and the qos parameter may be considered different for different applications, which purely depends upon specific requirements of an application.
for example, an application that is delay sensitive may require the qos in terms of delay guarantees. some applications may require that the packets should flow at certain minimum bandwidth. in that case, the bandwidth will be a qos parameter. the other application may require a guarantee that the packets are delivered from a given source to a destination reliably then, reliability will be a parameter for qos.

2. challenges
the characteristics of an ad hoc network pose several challenges in the provision of qos. some of these challenges are as follows:
• dynamically varying network topology. since the nodes in an ad hoc wireless network do not have any restriction on mobility, the network topology changes dynamically. hence the admitted qos sessions may suffer due to frequent path breaks, thereby requiring such sessions to be re-established over new paths. the delay incurred in re-establishing a qos session may cause some of the packets belonging to that session to miss their delay targets/deadlines, which is not acceptable for applications that have stringent qos requirements.
• imprecise state information. in most cases, the nodes in an ad hoc wireless network maintain both the link-specific state information and flow-specific state information. the link specific state information includes bandwidth, delay, delay jitter, loss rate, error rate, stability, cost, and distance values for each link. the flow-specific information includes session id, source address, destination address, and qos requirements of the flow (such as maximum bandwidth requirement, minimum bandwidth requirement, maximum delay, and maximum delay jitter). the state information is inherently imprecise due to dynamic changes in network topology and channel characteristics. hence, routing decisions may not be accurate, resulting in some of the real-time packets missing their deadlines.
• lack of central coordination. unlike wireless lans and cellular networks, mobile ad hoc networks (manets) do not have central controllers to coordinate the activity of nodes. this further complicates qos provisioning in manets.
• error-prone shared radio channel. the radio channel is a broadcast medium by nature. during propagation through the wireless medium, the radio waves suffer from several impairments, such as attenuation, multipath propagation, and interference
(from other wireless devices operating in the vicinity).
• hidden-terminal problem. the hidden-terminal problem is inherent in manets. this necessitates retransmission of packets, which may not be acceptable for flows that have stringent qos requirements. the rts/cts control packet exchange mechanism adopted in the ieee 802.11 standard reduces the hidden-terminal problem only to a certain extent.
• limited resource availability. as manets have limited resources such as bandwidth, battery life, storage space, and processing capability, they have to be utilized in a very
efficient way. out of these, bandwidth and battery life are considered as very critical resources, the availability of which significantly affects the performance of the qos provisioning mechanism. hence, efficient resource management mechanisms are required for optimal utilization of these scarce resources.
• insecure medium. security in a wireless channel is considerably less, due to the broadcast nature of the wireless medium. hence, security is an important issue in manets, especially for military and tactical applications. manets are susceptible to attacks such as eavesdropingping, spoofing, denial of service, message distortion, and impersonation. without sophisticated security mechanisms, it is very difficult to provide
secure communication guarantees.

3. design choices for providing qos support
hard-state versus soft-state resource reservation: in any qos framework, qos resource reservation is a very important component. it is responsible for reserving resources at all intermediate nodes along the path from the source to the destination as requested by the qos session. in hard-state resource reservation schemes, resources are reserved at all intermediate nodes along the path from the source to the destination throughout the duration of the qos session. if such a path is broken due to network dynamics, these reserved resources have to be released explicitly released by a deallocation mechanism. such a mechanism not only introduces additional control overhead, but also may fail to release resources completely in case a node previously belonging to the session becomes unreachable. due to these problems, soft-state resource reservation mechanisms, which maintain reservations only for small time intervals, are used.
stateful versus stateless approach: in the stateful approach, each node maintains either global state information or only local state information in the case of the stateless approach, no such information is maintained at the nodes. state information includes both the topology information and the flow-specific information. the source node can use a centralized routing algorithm to route packets to the destination if the global state information is available. the performance of the routing protocol depends on the accuracy of the global state information maintained at the nodes. significant control overhead is incurred in gathering and maintaining global state information. on the other hand, if mobile nodes maintain only local state information (which is more accurate), distributed routing algorithms can be used. even though control overhead incurred in maintaining local/state information is low, care must be taken to obtain loop-free routes. in the case of the stateless approach, neither flow-specific nor link specific state information is maintained at the nodes. though the stateless approach solves the scalability problem permanently and reduces the burden (storage and computation) on nodes, providing qos guarantees becomes extremely difficult.
hard qos versus soft qos approach: the qos provisioning approaches can be broadly classified into two categories: hard qos and soft qos approaches. if qos requirements of a connection are guaranteed to be met for the whole duration of the session, the qos approach is termed as a hard qos approach. if the qos requirements are not guaranteed for the entire session, the qos approach is termed as a soft qos approach.

4. classification of qos solutions
based on the interaction between the routing protocol and the mac protocol, qos approaches can be classified into two categories: independent and dependent qos approaches. in the independent qos approach, the network layer is not dependent on the mac layer for qos provisioning. the dependent qos approach requires the mac layer to assist the routing protocol for qos provisioning. finally, based on the routing information, updating mechanisms are employed.



4.1 mac layer solutions
the mac protocol determines which node should transmit next on the broadcast channel when several nodes are competing for transmission
on that channel. ex, cluster tdma.

4.2 network layer solutions
the bandwidth reservation and real-time traffic support capability of mac protocols can ensure reservation at the link level only hence, the network layer support for ensuring end-to-end resource negotiation,
reservation, and reconfiguration is very essential. to assist qos routing, the topology information can be maintained at the nodes of awns. the topology information needs to be refreshed frequently by sending link-state updating messages, which consume precious network resources such as bandwidth and battery power. otherwise, the dynamically varying network topology may cause the topology information to become imprecise. this trade-off affects the performance of the qos routing protocol. as path breaks occur frequently in awns compared to wired networks where a link goes down very rarely, the path satisfying the qos requirements needs to be recomputed every time the current path gets broken. the qos routing protocol should respond quickly in the case of path breaks and recompute the broken path or bypass the broken link without degrading the level of qos.