In general, wireless communication has a variety of MAC protocols, which can be classified into distinct groups according to different criteria. Based on whether a central controller is involved in coordination, WSNs' MAC protocols can be categorized as centralized, distributed (decentralized), and hybrid. Actually, hybrid protocols attempt to combine the advantages of centralized and distributed schemes, but can be more complex. Figure 18.3 shows such classification .
Centralized MAC protocols include polling algorithms and controlled multiplexing (or channel partitioning) algorithms [50, 70]. A centralized controller is needed to coordinate channel access among the different nodes and collision-free operation can be achieved. Thus, energy wasted due to collisions can be eliminated. However, because of the high overhead and long delay, pure polling mechanisms are not suitable in large-scale WSNs. Depending on how bandwidth is assigned, controlled multiplexing mechanisms can be frequency division multiplexing access (FDMA), code division multiplexing (CDMA), or time division multiplexing access (TDMA). This class of protocols is preferable in WSNs , not only because it is collision free, but also because nodes can be turned off in unassigned slots, thus saving energy expenditure due to idle sensing and overhearing.
However, drawbacks exist in channel partitioning schemes. When using TDMA, the central controller will consume more energy than other nodes, and scheduling tends to be dynamic, which will lead to a more complex mechanism. Moreover, it also requires clock synchronization among all nodes, which will
also dissipate some extra energy. For FDMA, due to the limited bandwidth in the system, it is not realistic to assign a unique frequency for each individual node. Furthermore, bandwidth wastage will occur due to the low duty cycle. Similarly, when using CDMA, although all nodes can transmit at will, some overhead will result because each node must encode its data bits with its uniquely assigned code.
Centralized multiplexing access, therefore, lacks flexibility and scalability to adapt to the variation of WSN applications. Some efforts have been made to improve the performance in terms of energy efficiency. One way is to combine TDMA with other controlled multiplexing, such as self-organizing medium access control for sensor networks (SMACS) , which is a combination of TDMA/FDMA MAC protocol. Low-energy adaptive clustering hierarchical (LEACH) [32-34], on the other hand, combines TDMA and CDMA protocols, i.e., it uses TDMA protocol to prevent intracluster* collisions and CDMA to avoid intercluster collisions. Adaptive periodic threshold-sensitive energy-efficient sensor network protocol (APTEEN), described in Manjeshwar and Agrawal  and Manjeshwar et al. , also uses TDMA with CDMA; however, it adopts a modified TDMA in which the length of time slots assigned to idle nodes and sleeping nodes is different, and all the idle node slots are ordered to precede sleeping nodes. Another alternative is to apply dynamic reservation TDMA (DR-TDMA) , which is actually a hybrid approach combining TDMA and carrier sense multiple access (CSMA) mechanisms.
Distributed MAC protocols usually provide random multiple access to a wireless medium. Most prevailing MAC protocols in this category adapt carrier sensing and collision avoidance, i.e., based on CSMA/CA. Through carrier sensing, significant transmission collisions can be eliminated by deferring transmission when the channel is detected busy. To further decrease the probability of collision, some collision avoidance measures can be taken, such as a random back-off procedure; a representative example of CSMA/CA based MAC protocol is specified in IEEE 802.11 distributed coordination function (DCF) .
However, in some cases, location-dependent carrier sensing results in "hidden" and "exposed" terminal problems, which have a great impact on efficiency. A hidden terminal refers to the node within the range of the intended destination but out of range of the sender, so the hidden terminal cannot be aware of the ongoing transmission. An exposed terminal is the node within the range of the sender but out of range of the destination, so the exposed terminal will be improperly precluded from sending in order to avoid collision. Two types of CSMA/CA-based schemes have been proposed to solve these problems. In
*The concept of clusters will be explained later in the chapter. Copyright © 2005 by CRC Press LLC
DCF, the exchange of "request to send-clear to send" (RTS-CTS) control messages reserves the transmission space for subsequent data exchange, thereby eliminating hidden terminal transmission. Deng and Hass  and Hass and Deng  propose a scheme, called dual busy tone multiple access (DBTMA), that separates control and data channels to relieve the problems raised by hidden and exposed terminals by indicating the transmission or receiving status explicitly.
Other distributed MAC protocols use a jamming signal, such as elimination yield-non-preemptive priority multiple access (EY-NPMA) [3, 24], which is used in the HIPERLAN system (being developed in Europe), and black burst (BB) [84, 110], which is proposed to support prioritized data transmission in ad hoc networks.
Using distributed MAC protocols, nodes operate in a decentralized manner, so it is easy to implement and perform more flexible and scalable control mechanisms, which may fit well with the requirements of WSNs. However, they are not collision-free protocols, and the listen-before-talk scheme calls for all nodes to keep sensing the channel. This results in high energy wastage due to collisions, idle listening, overhearing, and control message overhead. In Ye et al. , a novel MAC protocol called sensor-MAC (S-MAC) is proposed and attempts to reduce all four types of energy wastage.
As discussed in previous subsections, conventional centralized and distributed MAC layer protocols cannot provide optimal results in terms of energy efficiency in WSNs. Hybrid MAC protocols attempt to integrate the controllability of centralized protocols with the flexibility of distributed protocols. A number of these protocols are discussed here.
DR-TDMA  was originally proposed for wireless ATM networks, but can be extended to WSNs. Figure 18.4 demonstrates the frame structure of the DR-TDMA. Specifically, the fixed-length frame is divided into uplink and downlink time intervals. During the contention phase of the uplink interval, a distributed collision-based MAC scheme — the framed pseudo-Bayesian priority aloha protocol — is used for nodes to transmit temporary reservation requests for next frame to the base station. Uplink data are transmitted in TDMA mode based on the time slots assigned by the base station in the preceding downlink interval. In the downlink interval, a centralized MAC protocol is used to carry out slot assignments, as well as data transmission from base station to nodes. The resource reservation and assignment are adjusted dynamically based on work load. The nodes can turn off during periods outside assigned transmission slots or contention slots. Therefore, energy wastage due to data collision, idle sensing, and overhearing can be reduced. Dynamic slot assignment can also provide flexibility to WSNs.
Fixed Length Frame
Data Slots (TDMA)
Downlink control mini-slots (allocating resource)
Uplink control mini-slots (distributed random access)
Data Slots (TDMA)
Two other hybrid TDMA-based protocols are time reservation using adaptive control for energy efficiency (TRACE)  and multihop TRACE (MH-TRACE) . Similarly to DR-TDMA, a central controller is in charge of arranging the TDMA transmission schedule according to continuing reservations and new reservation requests. Data are transmitted based on the transmission schedule, which is updated dynamically. The reservation requests are transmitted through a contention-based distributed MAC protocol. Because TRACE and MH-TRACE are dedicated for energy efficiency, these two approaches have a dynamic central controller as opposed to using a fixed-base station as the central controller in DR-TDMA.
A cluster formation scheme is used to manage the nodes. Each cluster head also plays the role of a TDMA scheduler within its cluster. By dynamically choosing cluster heads, balanced energy consumption among the nodes can be achieved. Moreover, these protocols introduce a novel control message — information summarization (IS) — to obtain information on future data transmission within the transmission range of nodes. This way energy wastage due to idle channel sensing and overhearing can be avoided. Due to their characteristics of energy efficiency, flexibility, and self-configuration, these two hybrid MAC protocols look promising for future WSNs. The issues of how to select central controllers dynamically and how to create data transmission schedules are major challenges for such protocols.
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