Traffic Differentiation in IEEE 802.15.4 Networks: an Analysis over the Contention Window

Mauricio González Telecommunications Engineering

Department University of Medellin

Medellin, Colombia magonzalez@udem.edu.co

Natalia Gaviria Gómez Department of Electronic

Engineering University of Antioquia

Medellín, Colombia ngaviria@udea.edu.co

Germán Urrego Giraldo Department of Systems

Engineering University of Antioquia

Medellín, Colombia gaurrego@udea.edu.co

Andrés Gil Molina Department of Electronic

Engineering University of Antioquia

Medellín, Colombia agil@udea.edu.co

Abstract – The increasing interest in developing solutions for applications, such as industrial automation, body sensor networks, natural risks management and prevention; without the need of a wired media has encouraged the development of a low data rate and low power consumption wireless communication systems, commonly known as Wireless Sensor Networks (WSN). Although many proprietary solutions have been proposed, the standard IEEE 802.15.4 is about to become the global standard for WSN, since utilizes energy-efficient schemes to access the channel, and to transmit information at low speeds. However, this standard does not have a strategy for information differentiation, in which packets from nodes with a higher priority need to be sent sooner than packets from other nodes. Consequently, this paper evaluates, by means of numerical simulations, the assumption of an information differentiation scheme, which consists in modifying the Contention Window (CW) in the slotted IEEE 802.15.4, considering that nodes with lower CW values have higher probabilities to access the channel than nodes with higher CW values. Partial results show that this information differentiation scheme increases the channel access probability, when two or three levels of CW are implemented within the network.

Keywords – IEEE 802.15.4, Differentiated Traffic, Contention Window.

I. INTRODUCTION

Standards for Wireless Sensor Networks (WSN) have to deal with different challenges in order to ensure a reliable and energy-efficient communication. One of the most important challenges is related to energy-consumption, due to the absence of external energy sources for some nodes within the network. When nodes do not have the possibility to be re-charged for long periods of time, a really efficient energy-consumption scheme should be suggested in order to avoid the loss of data from discharged nodes. Thus, the IEEE 802.15.4, considered as the global standard to provide the design guidelines for low data rate and energy-efficient devices [1], is based on the philosophy of saving power, by means of turning off the nodes when they do not have frames to transmit. In

addition, the power saving is bigger than the achieved in other suggested medium access controls. These are surveyed in [2] and analyzed in [3].

So far, power saving has been constrained due to the fact that the power is unnecessarily wasted when a certain node tries to access the channel when it is in a busy state. Therefore, it is mandatory to design energy-efficient schemes reducing the possibility of occurrence of this situation [3]. Furthermore, in many applications, there might be nodes with more relevant information than other nodes within the network, and this information should be transmitted with a higher priority. Unfortunately, an information differentiation scheme is not explicitly available in the IEEE 802.15.4 and modifications of the standard are not easy to be performed because they require an intense research work to be added by different manufacturers before to deliver a draft over it. Consequently, the current tools offered by the standard have to be used and optimized in order to obtain a more energy-efficient performance, while not disturbing the nature of the protocol. This can be achieved by modifying protocol parameters within the nodes of the network. Several works [1, 4-6] have evaluated the effects of modifying different protocol parameters on the channel performance, and have tried to explain these parameters modifications using Markov Chains.

In this paper, we evaluate, by means of numerical simulations, the influence of the modification of the Contention Window (CW) parameter on the system performance. The simulation scenario consists on a network based on the IEEE 802.15.4 standard. We performed three sets of simulations: the first set is a traditional network with CW=2; the second set consists on a network with two levels of priority, i.e. nodes with CW=1 and CW=2. Finally, the third set is a network with three levels of priority (CW=1, CW=2 and CW=3). We show, in the three scenarios, nodes with the highest priority have higher probabilities to access the channel.

The rest of the paper is organized as follows: in the Section II we make a brief review of the MAC protocol in CSMA/CA slotted mode of the IEEE 802.15.4. Section III describes the conditions of the simulations, the tools and node implementation. Section IV shows the results in the

experiment and the analysis over them. We finally conclude in section V.

II. IEEE 802.15.4. SLOTTED MODE REVIEW

Fig. 1 summarizes the CSMA/CA slotted mode in IEEE 802.15.4 with default parameters. These parameters are detailed in the next subsection, and the principle of operation of the CSMA/CA slotted is covered in subsection B.

FIGURE 1. IEEE 802.15.4 CSMA/CA SLOTTED ALGORITHM

a. Parameters

In order to achieve a better understanding of the protocol, we detail the parameters involved into the CSMA/CA slotted:

- aUnitBackoffPeriod: duration of one backoff slot (default value = 3.2 ms).

- NB: the Number of Backoffs, which represents to the number of non-successful attempts to transmit.

- CW: the Contention Window is defined as the number of times each node has to sense the channel in idle state before transmitting (default value = 2).

- BE: the Backoff Exponent is used to define a random time each node has to wait before transmitting. This time is defined as the BackoffPeriod.

- macMinBE: The lowest value for BE (default value = 3). - macMaxBE: The highest value allowed for BE. - macMaxCSMABackoffs: This is the highest value NB can

reach, i.e. when NB reaches this value, the packet is dropped.

b. Principle of Operation CSMA/CA slotted

When a package arrives to the output buffer, before transmission, the node initializes three parameters as follows: NB is set to 0 (indicating that have been no attempts to send information), CW is set to 2 (indicating that the node has to sense the channel twice before transmitting) and BE is set to the value of macMinBE. This procedure starts when a central controller device, known as the PAN coordinator, sends a beacon in order to synchronize all the nodes within the network. Thereafter, a BackoffPeriod is calculated randomly. This period acts as a counter and corresponds to a number uniformly distributed between 0 and 2BE-1. This BackoffPeriod is decremented by one after each aUnitBackoffPeriod for each Backoff slot, regardless of the channel state. Once the time is up (i.e. BackoffPeriod drops to zero), the node performs the carrier sensing via Clear Channel Assessment (CCA) to evaluate whether the channel is idle. In this case, the CW is decremented by one and the node performs another CCA after a Backoff slot or aUnitBackoffPeriod. Thus, the node will start the transmission, only if, the channel is found to be idle during consecutive Backoff slots until CW reaches zero. On the other hand, if the channel is found to be busy after performing the CCA, the node will enter into the next Backoff stage and will increment NB and BE by 1, the CW will be reset in 2 and a new BackoffPeriod will be calculated. While this process is repeated, the BE could be incremented by 1 just until it reaches MacMaxBE. The whole process is finished when the frame is transmitted or NB reaches the value of MacMAxCSMABackoffs and a Channel Access Failure is declared, thus the package is dropped [7].

III. NODE MODELLING

According to the literature [1, 3, 8-10], the Carrier Sensing (CS) process performed in IEEE 802.15.4 can be modeled by means of Markov chains. Accordingly, we have used the models in [1] for values of CW =1 and 2 respectively. In addition, Figure 2 depicts the Markov chain states diagrams for values of CW = 3.

Following we will explain the notation used in the Markov chains:

- pn,1 : Probability of receiving information to transmit according to Poisson Arrival Rate [1].

- pi: Probability associated to the random number of backoff slots geometrically distributed.

- pci: Probability of available channel.

- pci|i: Conditional probability of available channel if the

channel was already found available. - BOi: State of waiting in backoff. - CSij: State in which nodes sense the channel. Subindex i

indicates the number of attempts to access the channel (up to MacMAxCSMABackoffs). Subindex j indicates the number of CS performed in the same attempt.

- TX: State of transmission.

FIGURE 2. MARKOV CHAIN DIAGRAM FOR NODES WITH CW = 3.

The natural state a node is idle. This means that the node has no information to transmit in a given time. When a package needs to be transmitted (with a probability pn,1), the possibility to perform a CS is associated to the combined probability between p1 and pn,1, so the node immediately performs a CS (going to the state CS11) or goes to BO1 (waiting the number of random backoff slots by means of a geometric distribution). Depending on the priority of the node, the carrier sensing is performed once, twice or three times, always taking into account that the CS is made in discrete slots (going to states C1j). If the process is successful (determined by pc

i|i) the package can be transmitted (thus, the probability pc

i|i to transmit is the same in the states Ci2 and Ci3 preserving the property of memoryless of markovian processes). In any case, if the channel is busy, the node calculates and waits a new Backoff time until the package transmission (going to BOi or Ci1). Finally, if the number of attempts reaches the counter macMaxCSMABackoffs, the package is dropped with a probability = 1 - pi

c.

IV. SIMULATION

a. Simulation conditions

Considering the fact that the standard IEEE 802.15.4 does not differentiate the types of traffic each node handles, and taking into account that some kinds of information demand a higher priority than others, we performed an analysis based on the CW parameter. This analysis is made by means of numerical simulation, and is focused on determining whether the variation of the CW parameter affects positively the probability of channel access in the nodes with higher priority.

The simulation tool used for the simulation was the well-recognized OMNeT++ IDE freeware in conjunction with the plugin INETMANET. We performed three sets of simulations with a total of 10 nodes within the network for each set: first, a traditional network with CW=2 for all nodes; second, a network with two levels of priority, 5 nodes with CW=1 (high priority) and 5 nodes with CW=2 (low priority); finally, a network with three levels of priority, 3 nodes with CW=1

(high priority), 3 nodes with CW=2 (intermediate priority) and 4 nodes with CW=3 (low priority). We noticed, from the three scenarios, that nodes with the highest priority (i.e. CW=1) have greater probabilities to access the channel. The simulations show the probability of channel access per node, as a function of the Packet Arrival Rate (PAR).

To generate traffic, we implemented an UDP application with 64 bytes in each node, that is, when a package has to be sent, the buffer has a size of 64 bytes. The package in each node is generated randomly in time according to a uniform distribution (these are the default parameters of INETMANET plugin).

b. Node implementation

OMNeT is a framework based on C++, which supports simulations of telecommunications scenarios. It allows the use of object-oriented programming including classes, methods and attributes. Figure 3 depicts the node implementation. The node configuration includes some packages by default, which have not been modified in this simulation, and considers other features, such as: display interface, battery performance, routing tables (in which information is addressed among nodes according some constraints) and an interface table (to register the network interfaces available). In the notification board is possible to record the events in a timeline by means of a console application that is embedded in the simulator.

Other important packages are also considered as

mechanisms to generate traffic. They consist on UDP and TCP applications (both of them use an additional package in order to instantiate the same application in different and independent applications). Further, a ping application is considered in the node model, although it does not have the possibility to be instantiated several times. As it was said above, we used the UDP application sending 64 bytes randomly generated in time. The reason to choose UDP instead TCP is based on the fact that UDP is not connection based, so when a node needs to transmit some information, an acknowledge is not required to be sent from the receiver side [11] (our goal is to assess the probability to access the channel, and we assume that all the information will be received satisfactorily).

The main part of our model is located inside the WLAN

component in which the algorithm to access the channel is implemented. Figure 4 depicts the configuration of this component. The component is formed by three sub-components: the IFQ sub-component, which handles queues in the network interface cards; the MAC sub-component, which implements the IEEE 802.15.4 slotted standard; and the PHY sub-component, in which issues of the physical layer are considered. We have focused our work over the MAC sub-component in order to evaluate probability of accessing the channel for each node. In this work, we have performed code

modifications in order to include a new BackoffPeriod calculation method according to a geometric distribution, as suggested in [1].

FIGURE 3. NODE MODEL FOR IEEE 802.15.4

FIGURE 4. WLAN COMPONENT

In the IEEE 802.15.4 standard, the calculation method of the number of Backoff cycles is based on a uniform distribution [7], which is used to know how many cycles have to be waited before a CCA is performed in each node. Considering that the uniform distribution is useful to describe a random variable which has constant probability in a given interval, it is suggested to use the geometric distribution (as the discrete counterpart of the uniform distribution) [8]. As it is known, the geometric distribution allows the probability

calculation in which a number k of experiments is repeated until having success. Thus, we realized that this is the same scenario in which a node is immersed to access the channel (evidently each node repeats the CCA until finding the channel available). Since the mean number of Backoffs in IEEE 802.15.4 is 3.5, it is mandatory to use p=1/4.5 in the geometric distribution, in order to keep the same meaning of the uniform distribution [8-10].

FIGURE 5. PROBABILITY OF ACCESSING THE CHANNEL VS. PACKET ARRIVAL

RATE FOR THE FIRST SIMULATION SET WITH DEFAULT VALUE OF CW = 2

V. RESULTS ANALYSIS

As stated before, the CW was varied in the traditional IEEE802.15.4 scheme in order to provide different levels of priority. We used a metric based on the probability of accessing the channel by measuring how many times a node can transmit successfully information compared to complete number of attempts, as shown in equation (1). = (1)

The probability of accessing the channel is given as a function of the packet arrival rate, i.e. the number of packets per packet duration ( or number of packets per unit time). Figure 5, Figure 6 and FIGURE 7 show this probability for the three simulation sets. We stopped performing simulations based on the standard deviation computation (as it is suggested in [12]); thus, as we obtained values lesser than 0.042 it is possible to conclude that each set of simulations is statistically valid. Such scenario (with an acceptable standard deviation) was achieved after performing 240 simulations.

Thus, in the first simulation set (Fig. 5), the CW parameter has been set equal to 2 for all 10 nodes within the network as described in section II for the default configuration of the IEEE 802.15.4 slotted mode. We can observe from Fig. 5 that at higher values of arrival rate, the probability of accessing the channels decreases. This can be explained by the fact that as more packets are arriving to be transmitted, nodes would try to access the channel more constantly, thus it is less

probable to find the channel in idle state during the CCA for CW = 2 consecutive times.

FIGURE 6. PROBABILITY OF ACCESSING THE CHANNEL VS. PACKET ARRIVAL

RATE FOR THE SECOND SIMULATION SET WITH TWO LEVELS OF PRIORITY, CW=1 (HIGH PRIORITY) AND CW=2 (LOW PRIORITY)

In the second simulation set, we divided the group of 10 nodes into two sub-groups of five nodes, one of them preserved the value of the CW equal to 2 and a value of CW equal to 1 was given to the second sub-group, considering these nodes as nodes with higher priority information. Fig. 6 shows how nodes with a higher priority (CW = 1) have a higher probability to access the channel for certain values of packet arrival rate. This is due the fact that nodes with CW=1 just have to found once the channel in idle state in order to transmit a packet.

FIGURE 7. PROBABILITY OF ACCESSING THE CHANNEL VS. PACKET ARRIVAL RATE FOR THE THIRD SIMULATION SET WITH THREE LEVELS OF PRIORITY,

CW=1 (HIGH PRIORITY), CW=2 (INTERMEDIATE PRIORITY) AND CW=3 (LOW PRIORITY)

We also performed a simulation set with three levels of priority. In this case we divided the group of ten nodes into three sub-groups as follows: 3 nodes with CW=1 (high priority), 3 nodes with CW=2 (intermediate priority) and 4 nodes with CW=3 (low priority). Fig. 7 shows that until a certain value of the packet arrival rate the probability of accessing the channel is lower for as the priority of the nodes

decreases, i.e. the value of CW increases. In addition, we can notice from Fig. 9 that the probability of accessing the channel for nodes with low priority (CW=3), surprisingly, increases above the value of 0.45 of packet arrival rate. This can be explained due to the difference of the number of nodes for this sub-group, which is higher than the other two sub-groups with higher priorities.

VI. CONCLUSIONS

In this paper we have summarized the results after evaluating, by means of numerical simulations, the standard IEEE 802.15.4 when a traffic differentiation scheme is performed in order to provide some kinds of priorities, according to the importance of the information to be transmitted. Thus, we performed an analysis over the Contention Window parameters by means of a simulation made in the OMNET framework for three scenarios: first, assessing a network without modifications in CW; second, using two values for CW; and third, representing three kinds of nodes equivalent to CW=1, CW=2 and CW=3. We noticed that the lower the value of CW, the higher the probability to access the channel is obtained. We conclude that this traffic differentiation scheme does not imply considerable changes in the IEEE 802.15.4 standard, instead it is enough to vary the CW parameter in order to differentiate which nodes should have higher priority for transmission. Furthermore, the probability to perform a Clear Channel Assessment decreases, thus power consumption is enhanced as well.

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