Delay-throughput analysis of multi-channel MAC protocols in ad hoc networks
© Nieminen and Jäntti; licensee Springer. 2011
Received: 28 January 2011
Accepted: 22 September 2011
Published: 22 September 2011
Since delay and throughput are important Quality of Service parameters in many wireless applications, we study the performance of different multi-channel Media Access Control (MAC) protocols in ad hoc networks by considering these measures in this paper. For this, we derive average access delays and throughputs in closed-form for different multi-channel MAC approaches in case of Poisson arrivals. Correctness of theoretical results is verified by simulations. Performance of the protocols is analyzed with respect to various critical operation parameters such as number of available channels, packet size and arrival rate. Presented results can be used to evaluate the performance of multi-channel MAC approaches in various scenarios and to study the impact of multi-channel communications on different wireless applications. More importantly, the derived theoretical results can be exploited in network design to ensure system stability.
Multi-channel communications form the basis of various future wireless systems such as cognitive radio, next generation cellular and wireless sensor networks (WSNs). The reason for this is that the performance of a wireless network can be improved by exploiting multiple frequency channels simultaneously to ensure robustness, minimize delay and/or enhance throughput. In general, performance of multi-channel networks is heavily dependent upon used Media Access Control (MAC) protocols and efficient medium access schemes are considered as an essential part of any power-limited self-configurable wireless ad hoc network . Furthermore, delay and throughput are important Quality of Service (QoS) parameters in many applications  and hence, the performance of multi-channel MAC schemes in ad hoc networks should be investigated in detail with respect to these measures.
In the case of single-channel systems, the performances of various MAC approaches have been investigated by considering both, throughput and delay. Carrier Sense Multiple Access (CSMA) for single channel systems was first studied by Kleinrock and Tobagi in , where the authors deduced equations for delays and throughputs of CSMA and ALOHA using the busy period analysis. Later on delay distributions of slotted ALOHA and CSMA systems were derived in  for different retransmission methods. Operation of single-channel IEEE 802.11 systems was evaluated in  comprehensively using a Markov chain model to model the impact of backoff window sizes on the performance. Multi-channel MAC approaches have not been studied as widely but a performance analysis of different multi-channel protocols in a single collision domain was presented in  with respect to data rates by assuming saturated traffic conditions. However, to the best of authors' knowledge, delay-throughput characteristics of multi-channel MAC protocols have not been studied yet in case of Poisson arrivals and infinite number of users.
Contention-based multi-channel MAC protocols designed for ad hoc networks can be divided into three main classes, namely split phase, periodic hopping and dedicated control channel. In split phase-based random access approaches the operation is divided into two parts. First, during contention periods nodes reserve resources on a common control channel and afterwards, data transmissions will take place during the data period. On the other hand, the basic idea behind periodic hopping approaches is to use channel hopping on every channel to avoid availability and congestion problems of the common control channel. Moreover, dedicated control channel schemes allocate one channel as a common control channel and carry out data transmissions on other channels. Each of these approaches has specific strengths and weaknesses which will be discussed in detail.
In this paper, we derive average access delays and throughputs for different multi-channel MAC approaches in case of Poisson arrivals and analyze the performance with respect to delay and throughput. We use a similar approach as in  but extend the analysis by taking into account the effect of multi-channel communications and deduce the closed-form solutions for different multi-channel MAC schemes. Correctness of theoretical derivations will be attested by simulations. Performance of the protocols is then analyzed with respect to various critical operation parameters such as number of available channels, packet size and arrival rate. Presented results can be used to analyze the performance and suitability of different multi-channel MAC approaches for prospective wireless applications and to guide system design.
The rest of the paper is organized as follows. In Section II, we specify used system models. Next, we introduce different multi-channel approaches in Section III and derive throughputs and expected delays in Section IV for the different multi-channel protocols. Results and analyses are presented in Section V. Section VI summarizes the paper.
II. System model
In this paper, the focus is on MAC in multi-channel ad hoc networks. Since optimal FDMA/TDMA schemes introduce a lot of complexity and additional messaging, we restrict our study to random access schemes. Each device is equipped with one half-duplex transceiver which makes protocols that require an additional receiver, such as , impracticable. Throughputs and delays of different contention-based multi-channel MACs can be modeled similarly to single-channel CSMA systems with the exception that now we have multiple channels to be exploited. We presume that a common control channel (CCC) is predetermined for the protocols that require a CCC for functioning and it is always of good quality.
If a packet transmission fails for some reason, retransmission of the packet will be attempted until successful transmission takes place, i.e. packets will not be discarded in any case. For the analysis, we divide the operation into multiple discrete time slots and assume fixed packet sizes along with perfect time synchronization among the nodes. The length of a time slot τ is defined to correspond to the maximum propagation delay of resource request and acknowledgement messages. Channel sensing time is equal to the maximum propagation delay as well and we neglect channel switching penalty for the sake of simplicity. We only consider slotted systems with an infinite number of users.
where N(t) is the number of occured events up to time t. All new packets will try to access the channel in the following time slot immediately after generation. Furthermore, we assume fixed packet sizes with transmission time T and define 2τ < T. Packet transmission time T also includes the acknowlegment message from the receiver. All the nodes in a network are awake constantly and have identical channel conditions.
III. Multi-Channel MAC protocols in ad hoc networks
Research efforts in the field of access mechanisms for single-channel ad hoc networks have been extensive. For example, a multiplicity of single-channel MAC protocols has been proposed for WSNs . Moreover, various multi-channel MAC protocols have been designed for different wireless systems as well. In this section, we briefly introduce operation principles of the most popular multi-channel MAC approaches for which average access delays and throughputs will be derived in Section IV. We divide random access multi-channel MACs into three main categories based on the nature of operations: split phase, periodic hopping and dedicated control channel. The categories include several protocols designed for different purposes of use such as Cognitive Radio Network (CRN), WSN and Wireless Local Area Network (WLAN). We will choose only one protocol from each category for a detailed study. In all of the considered cases resource reservations and negotiations are based on the IEEE 802.11 RTS/CTS message exchange. The main problem of multi-channel systems is the multi-channel hidden node problem which occurs if the channel usage of neighbor nodes is not known and nodes choose to transmit on a busy channel.
Periodic hopping protocols hop on all the channels according to a hopping pattern to avoid availability and congestion problems of the common control channel. Nodes may obey a common hopping pattern or have individual hopping patterns. In multi-channel WLANs the common hopping approach is used for example in Channel-Hopping Multiple Access (CHMA) which was introduced in . In addition, in the context of CRNs at least SYN-MAC  uses this approach and similar approach has been proposed for WSNs as well, called Y-MAC , which starts hopping only in the case of congestion. McMAC  and Slotted Seeded Channel Hopping (SSCH)  are examples of protocols which employ individual hopping patterns. Since the delay-throughput performance of various periodic hopping protocols is similar, we select SYN-MAC and evaluate its performance in this paper. Functioning of SYN-MAC is depicted in Figure 1b.
Dedicated control channel approaches use one channel only for distributing control information. The idea was first presented in , where the basic operation of IEEE 802.11 was extended for multiple channels simply by allocating data transmissions to different channels. However, the multi-channel hidden node problem is completely ignored in the design. A protocol which considers the multi-channel hidden node problem in this class is CAM-MAC . CAM-MAC requires all neighbors that hear a resource request message to verify availability of the proposed data channel. Consequently, channel reservations consume a lot of resources.
A dedicated control channel approach which consumes less resources than CAM-MAC while considers the multi-channel hidden node problem is Generic Multi-channel MAC (G-McMAC) . Thus, we choose G-McMAC for the analysis from this class. The protocol is designed especially for multi-channel WSNs. G-McMAC is a hybrid CSMA/TDMA protocol in which contention and data periods are merged to minimize delays. In general, the operation of the protocol is divided into two segments: Beacon Period (BP) and Contention plus Data Period (CDP). Activities of G-McMAC are illustrated in Figure 1c.
Each beacon includes the following information: preferable channel list, send time stamp, channel schedules, hierarchy level, beacon interval length. Gateway node (GW) of the WSN is on level 1 on the synchronization hierarchy and starts the beaconing process by sending the first beacon. All the receivers synchronize to the time reference provided by the GW and set their level as 2. After this, the nodes on level 2 will broadcast beacons as well and so forth. After a node has received beacons from all its neighbors, it can start the data negotiation process. If a node has a packet to send it first senses the wanted data channel to acquire the latest channel information and after this the node will send a Resource Request (RsREQ) message to the intended receiver which includes the desired data channel and transmission time, if the channel is free. The proposed frequency-time block will be chosen by utilizing the receiver's and transmitter's preferable channel lists and schedules. After receiving a RsREQ message, the intended receiver will sense the desired data channel and respond with a Resource Acknowledgment (RsACK) message on the common control channel if the proposed channel is available. Afterwards, the nodes will carry out the data transmission on the chosen channel at the agreed time.
IV. Throughputs and expected delays
Since, we assume that packet arrivals follow Poisson process performance evaluation of random access schemes can be carried out by exploiting the busy period analysis , where the average busy time and average idle time are used for determining the characteristics of various schemes. In the appendices, we derive the following probabilities for different multi-channel protocols using the busy period analysis: Ps is the probability of successful transmission, Pc is the probability of collision and Pb is the probability that the channel is sensed busy. In this section, we derive closed-form solutions for average access delays and throughputs of various multi-channel MAC schemes individually by exploiting derived probabilities. Theoretical results are confirmed by simulations. Examined protocols are G-McMAC, MMAC and SYN-MAC. In the case of G-McMAC and SYN-MAC, we derive the theoretical results rigorously. On the other hand, since MMAC uses finite contention windows, only approximations can be found in case of MMAC which are then justified by simulation results.
A. Generic multi-channel MAC (G-McMAC)
First, we derive equations for the throughput and average access delay of G-McMAC . We exclude the beacon period from this analysis since beaconing may be used by other protocols as well, such as MMAC, or periodic beaconing may be required for time synchronization, routing, etc. For example, many routing protocols use broadcast messages to distribute routing information  and hence, require a beacon period in practice to avoid transmission of a routing packet many times. Moreover, it is presumed that beacon periods are carried out rarely such that the impact of the period is negligible to the packet arrival process.
Average access delay of G-McMAC depends on two issues. First, the contention process on the CCC and possible collisions induce some delay. Second, if all data channels are occupied an extra delay will be added as well. We deduce the delay of the contention process first and the impact of occupied channels will be taken into account while deriving the probability of successful transmission in Appendix A.
B. Multi-channel MAC (MMAC)
C. Synchronized MAC (SYN-MAC)
Resource reservation interval is divided into multiple small time slots (τ) and to avoid collisions, each transmitter chooses a random backoff value from a given fixed window ω. In other words, SYN-MAC exploits UB. Length of the contention period is Ts = ωτ and we set Ts = 10 since this should give good results in general according to . Consequently, to validate the assumption of Poisson arrivals, retransmitted packets are delayed over several contention windows randomly in simulations. Moreover, we denote the total length of a cycle by Tc = T + Ts.
V. Results and analysis
In this section, we analyze the performance of different multi-channel MAC approaches with respect to throughput and average access delay using previously deduced analytical results which were confirmed by simulations. First, we focus on delay analysis and consider the impact of arrival rate, number of channels and packet sizes on the expected delays. Second, we evaluate the performance of the protocols in terms of total throughput with respect to the same critical system parameters. Finally, we consider stability of the different approaches since it is of significant importance to understand what is the maximum traffic load that a MAC protocol can handle.
A. Delay analysis
Moreover, SYN-MAC offers relatively constant delays with different packet sizes and approaches G-McMAC when we get closer to the stability point of G-McMAC. However, with small packets the difference is remarkable and SYN-MAC introduces over twice as large delays as G-McMAC. Furthermore, performance of MMAC is significantly worse already with small packet sizes and access delay increases linearly when the packet size grows. As the packet size is increased, delay of MMAC grows constantly and the difference compared with other protocols enhances. In this case, the number of channels does not have any impact on the delay of MMAC since Tatim ≤ 2ω. As the results imply, delay of MMAC is heavily affected by the chosen packet size whereas G-McMAC and SYN-MAC offer relatively constant delays with different packet sizes. To summarize, G-McMAC achieves the best performance in general while SYN-MAC performs better with large packet sizes since it does not suffer from stability problems as quickly.
B. Throughput analysis
We conclude that G-McMAC achieves highest throughputs in case of small or moderate packet arrival rates while packets are small. On the other hand, SYN-MAC outperforms other approaches in case of small packets and high packet arrival rates. Finally, MMAC provides the best performance with respect to throughput when the packets are large. However, MMAC causes very high latencies in general. Our delay analysis undoubtedly shows that G-McMAC outperforms other protocols clearly in terms of delay.
C. System stability
In this paper, the performance of multi-channel MAC protocols in ad hoc networks was studied with respect to two important QoS parameters, delay and throughput. We deduced average access delays and throughputs for different multi-channel MAC approaches in closed-form by considering Poisson arrivals. Theoretical results were verified by simulations for each of the considered protocols. Throughput and delay analyses were given in terms of critical system parameters such as number of available channels, arrival rate and packet sizes. We conclude that Generic Multi-channel MAC (G-McMAC) consistently outperforms other protocols with respect to delay. G-McMAC also achieves higher throughputs in some cases compared with other approaches, whereas, in some cases other approaches will achieve better throughput. Moreover, the low stability point of G-McMAC may be a problem for some applications and in those cases other approaches should be used. Presented results can be exploited to study the performance and suitability of different multi-channel MAC approaches for different wireless applications and to guide system design.
APPENDIX A: Probabilities for G-MCMAC
APPENDIX B: Probabilities for MMAC
APPENDIX C: Probabilities for SYN-MAC
This research work is supported by TEKES (Finnish Funding Agency for Technology and Innovation) as part of the Wireless Sensor and Actuator Networks for Measurement and Control (WiSA-II) program.
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