A Simulation Study of the Contention Based Access Periods of the IEEE ad Hybrid MAC

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1 A Simulation Study of the Contention Based Access Periods of the IEEE ad Hybrid MAC C. Hemanth Department of Electrical Engineering Indian Institute of Technology, Madras Chennai, India T.G. Venkatesh Department of Electrical Engineering Indian Institute of Technology, Madras Chennai, India Abstract IEEE ad is a recent Wireless LAN standard operating at 60 GHz band. In this paper a simulation based performance evaluation of the Contention Based Access Periods (CBAP) of the IEEE ad hybrid MAC protocol under Pareto arrivals is carried out. Two performance metrics namely, throughput and average delay are evaluated. Due to the hybrid nature of the protocol, CBAP packet transmissions has to be deferred when the protocol switches from CBAP to contention free period. We show that (i) this deferral of packet transmission along with (ii) CBAP bandwidth allocation scheme has significant effect on the system performance. Keywords hybrid MAC; IEEE ad; Pareto; throughput; MAC delay; simulation; I. INTRODUCTION IEEE ad is a recent (Dec. 2013) amendment to the WLAN standard, operating at 60 GHz, which promises a wireless equivalent of gigabit Ethernet [1]. Due to the availability of huge unlicensed bands at 60 GHz, larger bandwidths can be used for communication, thereby improving the data rates for multimedia applications. The 60 GHz band is very useful for short range communications with larger bandwidths. The various potential applications envisaged for the IEEE ad [2] standard are: 1) Web browsing and other best effort data transfer 2) Sync and go file transfer, that enables gigabytes of file transfer in very short time 3) Uncompressed high definition video streaming that enables wireless display of content on a remote screen 4) Wireless gaming Apart from the technical challenges, the requirement of the usages the ad network needs to support are extremely varied in nature. Applications such as wireless display, gaming etc., are very sensitive to delay and jitter and have very stringent QoS requirements [3]. On the other hand, applications such as web browsing, sync and go file transfer etc., are best effort in nature and hence do not require QoS guarantees [3]. The diverse set of requirements from the envisaged usages and applications along with the propagation characteristics in 60 GHz, creates considerable challenges for the MAC layer design. A. The Need for Hybrid MAC protocol for 60 GHz band Although attractive, the wireless communication in 60 GHz band has significant technical challenges. We know that the radiated power decays along the distance, according to the inverse square law given by Friis equation [4]. From the Friis equation, the loss between two isotropic antennas is given as L f ree = ( λ 4πr ) 2 (1) where λ is the wavelength. Converting the equation (1) in terms of frequency and expressing in units of decibels we have L f ree = log 10 f + 20log 10 r (2) where f is frequency in GHz, r is the line of sight distance between the antennas in km. From the equation (2) it can be observed that, as the frequency increases the free space loss increases. It can be observed that even for shorter distances, free space loss is high as compared to the the conventional 2.4 / 5 GHz band. For example, consider a distance of 1 m, the free space loss at 5 GHz is around 46.3 db, whereas at 60 GHz, the free space loss is db, which is almost 21 db higher [1]. Further in the case of millimetre wave signals, attenuation due to oxygen absorption also occurs since the atmospheric oxygen absorption peaks at 60 GHz [5]. This leads to further attenuation as compared to the 2.4 / 5 GHz band. Therefore due to high frequency, short wavelength and oxygen absorption spectrum characteristics, the attenuation at 60 GHz can be of the order of 20 db higher than the 2.4 / 5 GHz band operation [3], [6]. In order to compensate the 20 db loss, directional antennas can be successfully used. Due to small wavelengths, multiple antennas can be packed in a very small area, which can then be used for highly directional beamforming. In order to identify the best MAC scheme at 60 GHz, a simulation based study was carried out using the three most popular MAC schemes, namely TDMA, CSMA/CA and Polling [3]. The CSMA/CA is suitable for bursty traffic due to its efficient channel utilization, but due to omni directional communication there is a significant performance penalty. The TDMA is an efficient protocol for wireless display and large file transfers in sync and go. Polling can exploit directional communication effectively but faces a performance hit due to the high power consumption at the master node and latency involved due to polling stations having empty queues. Finally due to the pros and cons of each access scheme and diversity of requirements, a hybrid MAC scheme using these protocol on a as-needed basis was proposed. ISBN: PGNet

2 Applications such as web browsing and sync and go file transfers can use the contention based access while TDMA protocol is ideal for applications like video streaming. In this paper we analyse the performance of the Contention Based Access Periods (CBAP) of the IEEE ad MAC protocol using simulations. The remainder of the paper is organized as follows. IEEE ad MAC layer is described in detail in Section II. Section III describes the IEEE ad CBAP protocol and highlights the differences from the conventional DCF operation. Literature related to the performance analysis of IEEE ad is presented in section IV. The performance evaluation using simulation is presented along with results and discussions in Section V. Finally the paper is concluded in Section VI. II. DESCRIPTION OF THE IEEE AD MAC LAYER In this section, description of the IEEE ad MAC standard [2] is presented which will form the basis for our simulations in the subsequent sections. A. Network Architectures Due to the characteristics of the wireless medium in 60 GHz and the envisaged usages, the conventional architectures like Basic Service Set (BSS) and Independent Basic Service Set (IBSS) of the legacy networks are unsuitable for ad networks [2]. Based on the above factors, a new architecture called as Personal Basic Service Set (PBSS) has been proposed for IEEE ad. A PBSS network behaves in an adhoc manner, with one of the station (STA)/node assuming the role of PBSS central point (PCP)/Access Point (AP) to provide basic timing, synchronization and allocation of Data Transmission Time (DTT). Even though a new architecture is defined for IEEE ad, the conventional architectures of IBSS and infrastructure BSS are also supported. This is to ensure the backwards compatibility of the system. B. Medium Access Channel access by a STA occurs during beacon intervals (BI) and is coordinated using a schedule. A STA operating as a PCP/AP generates the schedule and communicates it to STAs at frequent intervals called Beacon Intervals (BI) using Beacon and Announce frames [2]. Fig. 1 illustrates the structure of a BI where the channel access is divided into Beacon Header Interval (BHI) and Data Transmission Time (DTT). The access during BHI includes the transmission of beacon frames (during BTI), beamforming training (during A-BFT) and request-response based management access periods (during AT). The Beacon Time Interval (BTI) is a time period during which the PCP/AP transmits one or more beacon frames in different directions. In addition to network management information carried in the beacon, the beacon frame is also used to bootstrap the beamforming procedure between the PCP/AP and a receiving station. A station willing to join the network scans for a beacon, continues the beamforming Fig. 1. Example structure of IEEE ad Beacon Interval process with the PCP/AP in the Association Beamforming Training (A-BFT) time, and finally associates with the PCP/AP. The A-BFT is a time period to perform initial beamforming training between the station and the PCP/AP. The A-BFT follows the BTI to provide continuity to the beamforming process that was bootstrapped through the beacon transmission in the preceding BTI. The structure of the A-BFT is slotted, which allows for multiple stations to do beamforming with the PCP/AP concurrently in the same A-BFT. Fig. 2. Illustration of request response based frame transfer during AT of IEEE ad Beacon Interval During the Announcement Time (AT) period, request response based management frames are exchanged between the PCP/AP and STAs. Apart from various other management frame exchanges, the Announcement Time (AT) period of the BI is used to reserve the slots for SP. This reservation process in AT is depicted in Fig. 2. The PCP/AP initiates the frame transmission in AT. During the AT, the PCP/AP polls all the nodes or a subset of nodes. All the nodes associated with the PCP/AP which are in need of SP, send a Service Period Request (SPR) as a reply to the poll sent by a PCP/AP. Based on these requests and QoS requirements, the PCP/AP allocates SPs to the different nodes in the subsequent frames. The PCP/AP keeps count of all the SP requests received from the individual nodes. The amount of outstanding request is reduced as the SP is allocated to each node. To support the wide variety of applications and usages envisioned, the MAC supports both random access and scheduled TDMA access. The DTT of the BI is divided into Service Periods (SP) and Contention Based Access Periods (CBAP) which provides transmission opportunities for STAs that are part of the network [2]. Channel access during the SPs is scheduled and assigned to specific STA and the access in CBAP is based on IEEE Distributed Coordination Function (DCF).

3 III. REVIEW OF THE LITERATURE ON THE PERFORMANCE ANALYSIS OF IEEE AD There are many papers on general IEEE ad and the PHY aspects of it. Not much work has been done to analyse the MAC performance. In this section we present a few litereture related to the performance evaluation of MAC layer of the IEEE ad. The authors in [6] give an overview of the current IEEE ad task group activities, and the expected enhancements to The IEEE ad standard was briefly explained and the MAC and PHY amendments were discussed by Cordeiro et al [7]. The survey paper [8] highlights the PHY/MAC enhancements and QoS mechanisms for the high throughput WLANs namely IEEE n, IEEE aa, IEEE ac and IEEE ad. In this paper many work related to the above mentioned standards are compared and contrasted for their contribution. This survey paper emphasizes the need to analyse the performance of IEEE ad hybrid MAC analytically. The authors in [1] highlight the needs of multi gigabit WiFi. They also highlight the challenges in implementation of multi gigabit WiFi using the examples of IEEE ad and IEEE ac. The authors in [9] discussed the possibility of a dual band architecture for ad based networks where the 2.4 / 5 GHz band could be used for contention mechanism and later 60 GHz band could be used for data transmission. The performance of CSMA/CA at 60 GHz was evaluated using NS- 2 simulations in [10]. The impact of antenna directivity and CSMA/CA operation modes on the throughput was analyzed. The results showed that a 90 antenna pattern at both PCP/AP and the stations exhibited the maximum throughput under the scenarios considered. The quality of service requirements and potential solutions for supporting variable bit rate traffic along with data traffic was investigated in [11]. The authors finally concluded that a combination of MAC schemes like TDMA with either CSMA or polling would be efficient for QoS guarantees along with efficient channel utilization. In [12] the authors design a random access MAC protocol with directional and cooperative communication, D-coop MAC for the IEEE ad. They use a 3 dimensional Markov chain model to analyse the performance of the proposed MAC scheme and claim that D-coop MAC outperforms the conventional IEEE ad MAC. To evaluate the performance of IEEE ad MAC, they have considered only the CBAP and neglected the effect of SP on CBAP in the analysis. In [13] the authors have presented analytical modelling and analysis of the CBAP of the IEEE ad using a Markov chain and evaluated the throughput and delay performance. For the analysis and simulations, Poisson arrival model was considered. Even though Poisson arrival process are easier to analyse analytically, best effort / bursty traffic are well modelled by Pareto distributions and not by Poisson distribution. In this paper, we aim to do a performance evaluation by considering arrival models as close to real world traffic using Pareto arrivals. IV. PROTOCOL DESCRIPTION In this section we describe the access mechanism of CBAP in detail, which is needed to understand the performance of the system. The system is assumed to be slotted. The access during CBAP is based on IEEE DCF, which is basically a carrier sense multiple access with collision avoidance (CS- MA/CA) with binary exponential backoff scheme. However, due to hybrid nature of the MAC protocol, the DCF mechanism has been adapted as follows [13]. In the basic access mechanism of the DCF, if the channel is sensed idle for a DIFS duration, the packet is transmitted. If the channel is sensed busy, the node waits for the channel to be idle for a DIFS duration. A random backoff counter value is then generated from U[0,CW min 1], where CW min is the minimum backoff window size. The counter value is decremented at the end of every idle slot and frozen whenever the channel is sensed busy. In case of IEEE ad networks, channel can be busy either due to packet transmission is CBAP or during SPs. The node attempts to transmit the packet whenever the counter reaches zero if sufficient time remains in the current CBAP duration. If the transmission is successful, the process is started for the next packet if available. On the other hand, if the transmission encounters a collision, the backoff window is doubled and the backoff process is continued. If the remaining time in the current CBAP is not sufficient for the packet transmission, the packet transmission is deferred. Upon deferral of the packet transmission due to lack of sufficient time, a random count is generated from the current backoff window and the backoff process is continued. In the above mentioned case of transmission deferral, the backoff window size is not doubled since the deferral was due to lack of time in the current CBAP period and not due to collision [13]. The doubling of the backoff window is continued until the backoff window reaches its maximum stipulated value of CW max. Upon further collision, the backoff window is not doubled, but the backoff process continues till the maximum retrial limit is reached. The packet is dropped when a collision occurs after reaching the max retrial limit. A. Simulation Setup V. PERFORMANCE EVALUATION The simulations have been carried out using MATLAB [14]. A single hop network of N functionally identical STAs, transmitting the data using the CBAP of the IEEE ad MAC is assumed. We assume a scenario where all the N STAs are transmitting the packet to a PCP/AP through a common wireless channel. There are no hidden STAs present in the network. We consider the basic access mechanism of the DCF. We consider web browsing as an application that uses CBAP of the IEEE ad. In order to model the above scenario, we assume that the inter arrival times are Pareto distributed. All nodes are assumed to have an infinite buffer. We assume the structure of BI as shown in Fig. 1 for our model and analysis. We perform simulations for different CBAP fractions. CBAP fraction (denoted as α) is defined as the ratio of CBAP duration to the length of DTT of the BI.

4 The parameters used for the simulation are taken from the standard [2] and it is given in Table I. The simulation was carried out for 2x10 8 slot times. We assume a fixed BI length of 5 ms for the simulations. A constant CBAP packet payload of 7500 octets is used for simulation purposes. The parameters used for Pareto distribution are location = 0.38, shape = 0.85 [15]. TABLE I. Parameter NETWORK PARAMETERS FOR THE ANALYSIS OF CBAP Value Channel Bit Rate 2Gbps Slot time(σ) 5 µs DIFS 13 µs SIFS 3 µs Propagation Delay(δ) 100 ns CW min 15 CW max 1023 T c, Te D Header+L+DIFS+δ T s, Te A Header+L+SIFS+δ +ACK+DIFS+δ Max MPDU 7995 octets Throughput (Gbps) Poisson Pareto Number of Stations (N) Fig. 3. Overall CBAP throughput versus the number of stations (N) for different arrival distributions. α =1, Poisson average inter arrival time =50µs. B. Results and Discussions 1) Overall CBAP Throughput Versus Number of Stations: In Fig. 3 we compare the throughput variation with respect to the number of stations for both Pareto and Poisson arrivals. We observe that as the number of nodes increases, due to an unsaturated nature of the system the overall system throughput initially increases and then saturates for both Pareto and Poisson arrival processes. Once the system reaches saturation, the per node throughput would decrease with a further increase in the number of stations. It can also be observed that, due to the hybrid nature of the protocol, the maximum channel utilisation obtained from CBAP is 40%. From the Fig. 3, we can observe that the throughput of Poisson arrival is larger than the throughput of Pareto arrivals. Hence most of the existing analysis in the literature are over estimating the realistic throughput. 2) Overall CBAP Throughput Versus Arrival Rate: The variation of throughput with respect to the arrival rate for different CBAP fractions is shown in Fig. 4. We observe that, as the arrival rate increases, the throughput initially increases and then gets saturated. When the CBAP fraction is reduced from 1 to 0.8, intuitively the throughput should scale down by a factor of 0.8 as the role of SP is just to freeze the CBAP activity. However we observe that the throughput scales down by a factor less than 0.8. This could be attributed to two opposing factors namely, (a) deferral of packet transmission due to the unavailability of channel in the current CBAP would lead to decrease in throughput and (b) when the system is frozen due to SP, the arrivals to the queue maintained by the node continues, which in turn reduces the time the system is idle due to queue being empty. This would have an effect of moving the system towards saturation and thereby increase the throughput. In this case we observe that factor (b) dominates and thereby, the throughput reduction is less than a factor of 0.8. Throughput (Gbps) CBAP fraction = 1 CBAP fraction = 0.8 CBAP fraction = Arrival Rate (pps) 3 4 x 10 5 Fig. 4. Overall CBAP throughput versus arrival rate for different CBAP fractions for N=10. 3) Overall CBAP Throughput Versus Packet Length: The variation of throughput with respect to the packet length is shown in Fig. 5. It can be seen that for all CBAP fractions, for smaller packet lengths (till 20 kb), the throughput increases with an increase in packet length. The reason behind this would be that as the packet length increases, the relative MAC overhead decreases, thereby increasing the throughput. For all values of N, CBAP fraction, with an increase in packet length above 90 kb, the throughput decreases. This could be due to the fact that with an increase in the packet size, the possibility of deferrals due to insufficient time would increase. Further, due to the alternating nature of the hybrid

5 MAC, for packet lengths greater than the CBAP durations, the probability of packet transmissions becomes zero and thereby the throughput goes to zero. For packet lengths between kb depending on the CBAP fraction, the critical packet length beyond which the throughput starts reducing changes. For higher CBAP fractions (=1), the critical packet length (70 kb) is larger as compared to smaller CBAP fraction (=0.5) (critical packet length= 50kb). Here we also observe that for a given CBAP fraction (=1), as the number of nodes is increased, the throughput reduces under basic access mechanism. This could be attributed to the fact that as the number of nodes increases, the chances of collisions increases (under the basic access mechanism) and hence the throughput reduces. Average Delay (ms) CBAP fraction =.1 CBAP fraction = 0.5 CBAP fraction = 1 Throughput (Gbps) N = 10, CBAP fraction = 1 N = 30, CBAP fraction = 1 N = 10, CBAP fraction = 0.5 N = 10, CBAP fraction = Packet Length (kb) Fig. 5. Overall CBAP throughput versus the packet length (in kb) for different number of users (N) and different CBAP fractions. 4) Average Delay versus Number of Stations: The variation of average CBAP delay with respect to number of stations is presented in Fig. 6. Here we can observe that for a given CBAP fraction, as the number of stations increases, the average delay increases. This can be attributed to the fact that as the number of stations increases, the possibility of collision increases and thereby the average duration between two successful transmission of the packet increases. Further we can observe that as the CBAP fraction reduces, the average delay increases. This can be attributed to the fact that as the CBAP fraction decreases, the duration for which SP operates increases, thereby increasing the average time taken to transmit the packet. Apart from that, the possibility of the packet transmission being deferred due to insufficient time in the current CBAP increases and hence the increase in average delay. VI. CONCLUSION In this paper we have analysed the performance of the Contention-Based Access Periods of the IEEE ad hybrid Number of STAs (N) Fig. 6. Average delay versus Number of Stations for different values of CBAP fractions for N=10. MAC using simulations. The simulations are carried out for the case of Pareto arrivals. Throughput and delay are evaluated as the performance metrics of the protocol. The results indicate that the Poisson traffic over estimates the system performance as compared to the Pareto traffic. The CBAP throughput scales down and average delay increases with an decrease in CBAP fraction. It can also be observed that packet length plays an important role in deciding the maximum throughput that can be achieved by the CBAP. REFERENCES [1] L. Verma, M. Fakharzadeh, and S. Choi, Wifi on steroids: ac and ad, Wireless Communications, IEEE, vol. 20, no. 6, pp , December [2] IEEE standard for information technology telecommunications and information exchange between systems local and metropolitan area networks specific requirements-part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications amendment 3: Enhancements for very high throughput in the 60 GHz band, IEEE Std ad-2012 (Amendment to IEEE Std , as amended by IEEE Std ae-2012 and IEEE Std aa-2012), pp , Dec [3] C. Cordeiro, Evaluation of medium access technologies for next generation millimeter-wave WLAN and WPAN, in International Conference on Communications Workshops, ICC IEEE, 2009, pp [4] T. S. Rappaport, Wireless Communications: Principles and Practice. Dorling Kindersley, [5] S. Yong, P. Xia, and A. Valdes-Garcia, 60 GHz Technology for Gbps WLAN and WPAN: from Theory to Practice. Wiley, [6] E. Perahia, C. Cordeiro, M. Park, and L. L. Yang, IEEE ad: defining the next generation multi-gbps Wi-Fi, in Consumer Communications and Networking Conference (CCNC), th IEEE. IEEE, 2010, pp [7] C. Cordeiro, D. Akhmetov, and M. Park, IEEE ad: introduction and performance evaluation of the first multi-gbps wifi technology, in Proceedings of the 2010 ACM international workshop on mmwave communications: from circuits to networks. ACM, 2010, pp. 3 8.

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