Mobile ad-hoc networks (MANETs) are well known by their flexibility and usefulness, being an ideal technology to support ubiquitous computing environments. Such environments are expected to support a plethora of applications, including real-time video and voice communications. In terms of applications, this technology can be used whenever there is a lack of infrastructure for support, which typically occurs in rescue missions, areas affected by natural disasters, remote areas, war scenarios, and also in the underground. The use of real-time voice and video communications could allow, for example, firemen rescue teams to communicate seamlessly and for the head officer to remotely supervise their activity using different video channels. The deployment of real-time services over mobile adhoc networks requires QoS (quality of service) support at different network layers. QoS support is understood as the network ability to offer some guarantees about the traffic being delivered. Within the scope of QoS we often define performance in terms of availability (uptime), bandwidth (throughput), latency (delay), delay jitter, and error rate. Offering QoS support in mobile ad-hoc network environments is, nevertheless, quite difficult due to the innate complexity of these networks. The problems that impact mobile ad-hoc networks can be split according to the network layer affected. At the physical layer, frequent topology changes?in conjunction with channel contention and unstable radio links?make real-time services support in such networks very hard to achieve (Georgiadis, Jacquet, & Mans, 2004). At the Medium Access Control (MAC) layer, channel access is typically distributed, provoking the well-known hidden and exposed node problems, which complicate bandwidth reservation. At the network layer, routing protocols have to deal with frequent topology changes and simultaneously discriminate among the available paths to meet QoS requirements. At the application layer, awareness of the type of networks and technologies being used allows applications to adapt themselves according to path conditions and so improve performance. This article discusses the aforementioned issues related to QoS challenges and solutions in mobile ad-hoc networks. It first includes some background information on the history of QoS support in computer networks. It then refers to the problematic of QoS support in mobile ad-hoc networks by referring MAC and routing layer solutions, along with QoS architectures for ad-hoc networks. To conclude the article there is reference to future trends in terms of QoS support in ad-hoc networks.
The first attempts at providing significant QoS support improvements in computer networks took place on the Internet in the early 1990s. The main problem faced by engineers was that the Internet was initially created to handle best-effort traffic alone. This means that its infrastructure was not designed considering QoS-related functionality such as resource reservation, and so all users compete for bandwidth. For this reason the Internet protocol (IP) is connectionless, requiring no set-up “signaling” for admission control.
When enhancements in terms of available bandwidth and a terminal’s capabilities brought up the need for supporting new services on the Internet, preliminary evaluation studies showed that the performance of these new services was very poor due to the best-effort policy. There was, therefore, a need to enhance the Internet infrastructure in order to allow performing resource reservations in a similar fashion to telephony networks. The RSVP protocol (Braden, Zhang, Berson, Herzog, & Jamin, 1997) was created to fulfill this need as part of the Internet’s integrated services (IntServ) architecture (Braden, Clark, & Shenker, 1994). RSVP follows a receiver-based model since it is the responsibility of each receiver to choose its own level of reserved resources, initiating the reservation and keeping it active. The actual QoS control, though, occurs at the sender’s end. The sender will try to establish and maintain resource reservations over a distribution tree. If a particular reservation is unsuccessful, the correspondent source is notified.
Key Terms in this Chapter
DiffServ: Abbreviation that refers to Internet’s Differentiated Services architecture.
Media Access Control (MAC): A communications protocol sub-layer that provides addressing and controls channel access to allow several terminals to communicate over a shared medium.
Mobile Ad-hoc NETwork (MANET): An autonomous type of network where terminals participate both as clients and routers, eliminating the need for any type of support infrastructure.
Resource Reservation Protocol (RSVP): A transport layer protocol designed to perform resource reservation on the Internet; one of the key elements in the integrated services architecture proposed for the Internet.
CSMA/CA: Stands for Carrier Sense Multiple Access with Collision Avoidance. The distributed channel access mechanism adopted by the IEEE 802.11 standard for operation.