Medium Access Control protocols for ad hoc wireless networks: A survey
Introduction
Back in the 1970s, the Defense Advanced Research Projects Agency (DARPA) was involved in the development of packet radio networks for use in the battlefields. Around the same time, the ALOHA [1] project used wireless data broadcasting to create single hop radio networks. This subsequently led to development of the multi-hop multiple-access Packet Radio Network (PRNET), which allowed communication coverage over a wide area. The term multi-hop refers to the fact that data from the source needs to travel through several other intermediate nodes before it reaches the destination. One of the most attractive features of PRNET was rapid deployment. Also, after installation, the whole system was self-initializing and self-organizing. The network consisted of mobile radio repeaters, wireless terminals and dedicated mobile stations. Packets were relayed from one repeater to the other until data reached its destination.
With the development of technology, devices have shrunk in size and they now incorporate more advanced functions. This allows a node to act as a wireless terminal as well as a repeater and still be compact enough to be mobile. A self-organizing and adaptive collection of such devices connected with wireless links is now referred to as an Ad Hoc Network. An ad hoc network does not need any centralized control. The network should detect any new nodes automatically and induct them seamlessly. Conversely, if any node moves out of the network, the remaining nodes should automatically reconfigure themselves to adjust to the new scenario. If nodes are mobile, the network is termed as a MANET (Mobile Ad hoc NETwork). The Internet Engineering Task Force (IETF) has set up a working group named MANET for encouraging research in this area [2].
Typically, there are two types of architectures in ad hoc networks: flat and hierarchical [3], [6]. Each node in an ad hoc network is equipped with a transceiver, an antenna and a power source. The characteristics of these nodes can vary widely in terms of size, processing ability, transmission range and battery power. Some nodes lend themselves for use as servers, others as clients and yet others may be flexible enough to act as both, depending on the situation. In certain cases, each node may need to act as a router in order to convey information from one node to another [4], [5].
Coupled with global roaming capabilities and seamless integration with existing infrastructure, if any, ad hoc wireless networks can be used in many new applications [6], [8]. In case of natural or other disasters, it is possible that existing communication infrastructure is rendered unusable. In such situations, an ad hoc wireless network featuring wideband capabilities can be set up almost immediately to provide emergency communication in the affected region. In mobile computing environments, mobile wireless devices that have the capability to detect the presence of existing networks can be used to synchronize data with the user’s conventional desktop computers automatically, and download appointment/schedule data. A user carrying a handheld Personal Digital Assistant (PDA) device can download Context sensitive data in a shopping mall or museum featuring such wireless networks and services. The PDA would be able to detect the presence of the network and connect itself in an ad hoc fashion. Depending on the user’s movement, the PDA can poll the network for relevant information based on its current location. For instance, if the user is moving through the clothing section of the shopping mall, information on special deals or pricing can be made available. Similarly, ad hoc networks can be used in travel-related and customized household applications, telemedicine, virtual navigation, etc.
There are several important issues in ad hoc wireless networks [3], [6], [7], [8], [70]. Most ad hoc wireless network applications use the Industrial, Scientific and Medical (ISM) band that is free from licensing formalities. Since wireless is a tightly controlled medium, it has limited channel bandwidth that is typically much less than that of wired networks. Besides, the wireless medium is inherently error prone. Even though a radio may have sufficient channel bandwidth, factors such as multiple access, signal fading, and noise and interference can cause the effective throughput in wireless networks to be significantly lower. Since wireless nodes may be mobile, the network topology can change frequently without any predictable pattern. Usually the links between nodes would be bi-directional, but there may be cases when differences in transmission power give rise to unidirectional links, which necessitate special treatment by the Medium Access Control (MAC) protocols. Ad hoc network nodes must conserve energy as they mostly rely on batteries as their power source. The security issues should be considered in the overall network design, as it is relatively easy to eavesdrop on wireless transmission. Routing protocols require information about the current topology, so that a route from a source to a destination may be found. However, the existing routing schemes, such as distance-vector and link-state based protocols, lead to poor route convergence and low throughput for dynamic topology. Therefore, a new set of routing schemes is needed in the ad hoc wireless context [5], [8].
MAC layer, sometimes also referred to as a sub-layer of the ‘Data Link’ layer, involves the functions and procedures necessary to transfer data between two or more nodes of the network. It is the responsibility of the MAC layer to perform error correction for anomalies occurring in the physical layer. The layer performs specific activities for framing, physical addressing, and flow and error controls. It is responsible for resolving conflicts among different nodes for channel access. Since the MAC layer has a direct bearing on how reliably and efficiently data can be transmitted between two nodes along the routing path in the network, it affects the Quality of Service (QoS) of the network. The design of a MAC protocol should also address issues caused by mobility of nodes and an unreliable time varying channel [6], [7], [8].
The popular Carrier Sense Multiple Access (CSMA) [9] MAC scheme and its variations such as CSMA with Collision Detection (CSMA/CD) developed for wired networks, cannot be used directly in the wireless networks, as explained below.
In CSMA-based schemes, the transmitting node first senses the medium to check whether it is idle or busy. The node defers its own transmission to prevent a collision with the existing signal, if the medium is busy. Otherwise, the node begins to transmit its data while continuing to sense the medium. However, collisions occur at receiving nodes. Since, signal strength in the wireless medium fades in proportion to the square of distance from the transmitter, the presence of a signal at the receiver node may not be clearly detected at other sending terminals, if they are out of range. As illustrated in Fig. 1, node B is within the range of nodes A and C, but A and C are not in each other’s range. Let us consider the case where A is transmitting to B. Node C, being out of A ’s range, cannot detect carrier and may therefore send data to B, thus causing a collision at B. This is referred to as the ‘hidden-terminal problem’, as nodes A and C are hidden from each other [10], [11].
Let us now consider another case where B is transmitting to A. Since C is within B’s range, it senses carrier and decides to defer its own transmission. However, this is unnecessary because there is no way C’s transmission can cause any collision at receiver A. This is referred to as the ‘exposed-terminal problem’, since B being exposed to C caused the latter to needlessly defer its transmission [11]. MAC schemes are designed to overcome these problems.
The rest of the paper is organized as follows. A classification of ad hoc network MAC schemes is given in Section 2. Details of various MAC schemes in each class are discussed in Sections 3 Review of non-QoS MAC protocols, 4 QoS-aware MAC protocols. The summary and future research directions are described in Section 5, followed by conclusion in Section 6.
Section snippets
Classification
Various MAC schemes developed for wireless ad hoc networks can be classified as shown in Fig. 2. In contention-free schemes (e.g., TDMA, FDMA, CDMA), certain assignments are used to avoid contentions [6]. Contention based schemes, on the other hand, are aware of the risk of collisions of transmitted data. Since contention-free MAC schemes are more applicable to static networks and/or networks with centralized control, we shall focus on contention-based MAC schemes in this survey.
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Review of non-QoS MAC protocols
In particular, we shall discuss several important contention based MAC schemes in the single channel, receiver initiated, power-aware, and multiple channel categories. Due to space limitation, we will only briefly discuss other categories. However, it should not mean that these other categories are less important.
QoS-aware MAC protocols
With the growing popularity of ad hoc networks, it is reasonable to expect that users will demand some level of QoS from them. Some of the QoS related parameters that may be quantified are end-to-end delay, available bandwidth, probability of packet loss, etc. However, the lack of centralized control, limited bandwidth, error-prone wireless channels, node mobility, and power or computational constraints makes it very difficult to provide effective QoS in such networks [3], [72], [73], [74].
When
Summary and future directions
Due to space constraints and the large number of MAC schemes reviewed in this paper, it is difficult to compare their quantitative performance. We briefly discuss below qualitative performance of some of these schemes.
The CSMA based MAC schemes are not suitable in ad hoc networks due to multi-hop transmission and hidden/exposed terminal problems. The MACA scheme [13] was proposed to solve these problems with the help of two relatively short RTS/CTS control packets. The MACAW scheme [14] adds an
Conclusion
This study has presented a broad overview of the research work conducted in the field of ad hoc wireless networks with respect to MAC protocols. We have discussed many schemes and identified their salient features. In particular, we have looked at issues of collision resolution, power conservation, multiple channels, advantages of using directional antennas and QoS.
We have discussed the characteristics and operating principles of several MAC schemes. While some of them are general-purpose
Dr. Sunil Kumar received B.E. (Electronics Engineering) degree from S.V. National Institute of Technology, Surat (India), in 1988 and the M.E. (Electronics and Control Engineering) and Ph.D. (Electrical and Electronics Engineering) degrees from the Birla Institute of Technology and Science (BITS, Pilani, India) in 1993 and 1997, respectively. He also served as a Lecturer in the Electrical and Electronics Engineering department at BITS from January 1993 to July 1997. From August 1997 to August
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Dr. Sunil Kumar received B.E. (Electronics Engineering) degree from S.V. National Institute of Technology, Surat (India), in 1988 and the M.E. (Electronics and Control Engineering) and Ph.D. (Electrical and Electronics Engineering) degrees from the Birla Institute of Technology and Science (BITS, Pilani, India) in 1993 and 1997, respectively. He also served as a Lecturer in the Electrical and Electronics Engineering department at BITS from January 1993 to July 1997. From August 1997 to August 2002, he was a postdoctoral researcher and adjunct faculty in Signal and Image Processing Institute, Integrated Media Systems Center and Electrical Engineering department at the University of Southern California, Los Angeles, USA. From July 2000 to July 2002, he was also an expert consultant in industry on JPEG2000 and MPEG4-based projects and participated in JPEG2000 standardization activities. Since July 2002, he is an assistant professor with the Electrical and Computer Engineering department at Clarkson University, Potsdam, NY, USA.
He is a Senior Member of IEEE. He has authored more than 60 technical publications in international conferences and journals as well as a book on Radio Resource Management for Multimedia QoS Support in Wireless Networks (Kluwer Academic Publishers, 2003). He is a Guest Editor of Special Issue of Journal of Visual Communications and Image Representations on ‘Emerging H.264/AVC Video Coding Standard’ to be published during June–October 2005.
His research interests include QoS support for multimedia traffic in wireless cellular, ad hoc and sensor networks, Error resilient multimedia compression techniques, MPEG-4, H.264/AVC and JPEG2000 image/video compression standards.
Vineet S. Raghavan received his Bachelors degree in Architecture from the School of Planning and Architecture, New Delhi, India in 1999. After two years of working as an architect and self-taught software developer, he obtained his M.S. degree in Computer Science from Clarkson University, Potsdam, NY in 2003. He is now with the embedded software for digital televisions group at ATI Research Inc. in Marlborough, MA, USA.
Dr. Jing Deng received the B.E. and M.E. degrees in Electronic Engineering from Tsinghua University, Beijing, P. R. China, in 1994 and 1997, respectively, and the Ph.D. degree from Cornell University, Ithaca, NY, in 2002.
He was a teaching assistant from 1998 to 1999 and a research assistant from 1999 to 2002 in the School of Electrical and Computer Engineering at Cornell University. From 2002 to 2004, he was a research assistant professor with the CASE center and the Department of Electrical Engineering and Computer Science at Syracuse University, Syracuse, NY, USA, supported by the Syracuse University Prototypical Research in Information Assurance (SUPRIA) program. He is currently an assistant professor in the Department of Computer Science at the University of New Orleans, LA, USA. His research interests include mobile ad hoc networks, wireless sensor networks, wireless network security, energy efficient wireless networks, and information assurance.