Quality of Service for Mobile Ad-hoc Networks : an Overview

Mobile Ad-hoc NETworks (MANETs) are multi-hop ad-hoc wireless networks in which there are no backbone infrastructure. Each Mobile Node (MN) acts either as a host generating flows or being the destination of flows from other MN or as a router forwarding flows directed to other MN. Due to the unpredictable location and mobility of MNs in MANETs, classical routing protocols used on wired networks are not suitable for MANETs. Protocols defined for ad hoc networks are classified as reactive protocols and proactive protocols. Reactive protocols are characterized by MNs acquiring and maintaining routes on demand, while proactive protocols are characterized by all MNs maintaining routes to all destinations all the time. Examples of reactive protocols are DSR (Dynamic Source Routing) [5], and AODV (Ad hoc On-demand Distance Vector) [3]. Examples of proactive protocols are OLSR (Optimized Link State Routing Protocol) [17], and TBRPF (Topology Dissemination Based on Reverse-Path Forwarding) [15].All these protocols have been analyzed and compared in several papers. The main conclusion on these comparisons is that none of them is the best for all environments. Depending on several aspects such as mobility, load of the network, diameter of the network, etc, a protocol may behave better than other. For obtaining QoS (Quality of Service) on a MANET, it is not sufficient to provide a basic routing functionality. Other aspects should also be taken into consideration such as bandwidth constraints due generally to a shared media, dynamic topology since MNs are mobile and the topology may change and power consumption due to limited batteries. For wired networks there are two approaches to obtain QoS: an over-provisioning and network traffic engineering. Over-provisioning consists of the network operator offering a huge amount of resources such that the network can accommodate all the demanding applications. Instead, network traffic engineering classifies ongoing connections and treats them according to a set of established rules. Two proposals belonging to this class has been done inside the IETF: Integrated Services (IntServ) [2] and Differentiated Services (DiffServ) [1]. IntServ is a reservation-oriented method where users request for the QoS parameters they need. The Resource reSerVation Protocol (RSVP) [14] has been proposed by IETF to setup resource reservations for IntServ. Opposite to IntServ, DiffServ is a reservation-less method. Using DiffServ, service providers offer a set of differentiated classes of QoS to their customers to support various types of applications. IPv4 TOS octet or the IPv6 Traffic Class octet is used to mark a packet to receive a particular QoS class. In general, the specific aspects of MANETS make the wire-based QoS models not appropriate for MANETs. Over-provisioning, for instance, may not be possible because resources are scarce. IntServ/RSVP may require unaffordable storage and processing for MNs, and signaling overhead. Diffserv on the other hand, is a lightweight overhead model that may be more suitable for MANETs.