Regrouping algorithm to alleviate the hidden node problem in 802.11ah networks

doi:10.1016/j.comnet.2016.05.011

Computer Networks

Volume 105, 4 August 2016, Pages 22-32

https://doi.org/10.1016/j.comnet.2016.05.011 Get rights and content

Abstract

An IEEE 802.11ah network has been designed to service a wide range of sensor network applications where a single access point (AP) covers a transmission range of up to 1 km and needs to support more than 8000 nodes. As a result, it has much more hidden node pairs compared to conventional 802.11a/b/g/n/ac networks. In addition, since nodes in power saving mode wake up simultaneously to send frames after receiving a beacon frame from the AP, the hidden node problem can become worse, resulting in frequent packet collisions and performance degradation. The 802.11ah standard proposes a group-based contention scheme to resolve the performance degradation problem, but it cannot resolve it appropriately.

In this paper, we analyze the harmful impact of the hidden node problem on network performance and propose a new grouping algorithm to alleviate the performance degradation. Our proposed hidden matrix based regrouping (HMR) algorithm first finds hidden node pairs, and generates a hidden node matrix accordingly. Then it regroups hidden nodes to alleviate the hidden node problem using the hidden node matrix. Through extensive simulations, we show that our HMR algorithm eliminates most of hidden node pairs, thereby improving the performance of the 802.11ah network significantly in terms of the number of hidden node pairs and power-save poll (PS-Poll) transmission end time.

Introduction

An IEEE 802.11 Wireless Local Area Network (WLAN) is one of most popular wireless communication systems in use. 802.11 WLAN systems operate in industrial, scientific, and medical (ISM) bands, such as 2.4 GHz and 5 GHz bands, which are error prone due to the interference from other devices that use the same band. In addition, as many 802.11 WLAN systems are deployed at hot-spot areas, they suffer from severe congestion, and the backward compatibility is a big issue whenever a new WLAN system is introduced.

The IEEE 802.11ah Task Group (TGah) has defined a new WLAN standard, which operates at sub 1 GHz ISM bands. It aims to service a wide range of sensor network applications such as smart grid, and supports an efficient power save mode. Due to the superior propagation characteristic of a low frequency spectrum, the 802.11ah provides a much longer transmission range, i.e., 1 km, compared to 802.11a/b/g/n/ac WLANs, enabling an access point (AP) to support more than 8000 nodes [1].

One of the most important challenges in the 802.11ah is that it suffers from the severe hidden node problem [2]. The probability that any two nodes become hidden from each other increases up to 41% in the case when the network is randomly deployed [3]. For the deployment of 8000 nodes, the expected number of hidden node pairs reaches 1,311,836. In addition, this problem can be exacerbated especially when the power saving mode is active. Since most of nodes in an 802.11ah network operate in power save mode, they wake up and listen to the AP’s beacon at the same time, and attempt to transmit power-save poll (PS-Poll) frames, resulting in many collisions.

To minimize such collisions, the 802.11ah proposes a group based contention scheme [4] where each node is allocated to a group, and contends with other nodes within the same group. Previous studies have focused on improving the performance of the group based contention scheme [5], [6], [7]. They adjust the number of contention groups or the duration of a restrict access window (RAW) size to achieve optimal performance. In these works, it is assumed that the network is fully connected, i.e., so no hidden node problem occurs. This means that currently available group-based contention mechanisms are not able to solve the hidden node problem properly although the performance enhancement of the 802.11 access scheme is important research area [8], [9], [10].

Extensive researches on solving the hidden node problem in 802.11 networks have been performed, and most of these works have been designed to find hidden node pairs first in a given network [11], [12]. In [11], the authors have proposed a hidden node pair detection method that uses a clear channel assessment (CCA), but its effectiveness is limited due to the imperfect CCA information. In [12], a time difference-based detection mechanism that needs the help from neighboring APs has been proposed, but having such a configuration is not practical in 802.11ah networks.

In this paper, we first analyze the performance degradation due to the hidden node problem in 802.11ah networks, and propose a new grouping algorithm, termed hidden matrix-based regrouping (HMR) algorithm, to alleviate this problem. Our proposed algorithm running at the AP first detects hidden node pairs by using the time difference of PS-Poll transmission times. Then it creates a hidden node matrix where each element value is easily obtained from PS-Poll transmission times. Finally, it moves nodes experiencing the hidden node problem into another group one by one, in the order of experiencing hidden nodes in number. Through extensive simulations, we confirm that our proposed algorithm significantly reduces the total number of hidden nodes, leading to performance improvement in terms of PS-Poll transmission end time and the number of retransmissions.

The rest of the paper is organized as follows. In Section 2, we briefly describe the 802.11ah standard and the system model in Section 3. We analyze the PS-Poll transmission end time in Section 4, and propose our hidden matrix based regrouping (HMR) algorithm in Section 5. After evaluating the proposed scheme in Section 6, we conclude our paper in Section 7.

Section snippets

IEEE 802.11ah: overview

The IEEE 802.11ah standardization project has been in progress and its fifth draft was published in 2015 [4].

A key difference of 802.11ah from conventional WLANs is the band in which it operates. While conventional 802.11 WLANs operate in 2.4 GHz and 5 GHz bands, 802.11ah operates in sub 1 GHz license-exempt bands. Owing to the good channel propagation characteristics of the sub 1 GHz bands, the transmission range of an 802.11ah AP is larger than those of conventional 802.11 WLAN APs [13]. For

System model

We consider an 802.11ah network with N nodes and a single AP. The AP is plugged into an electric outlet while all the nodes are battery powered. We assume the nodes operate in power save mode, and they are uniformly distributed within the AP coverage with no mobility.

To investigate the hidden node problem, we consider PS-Poll frame transmissions (i.e., uplink) that are normally sent in the first RAW. Each node periodically wakes up at TBTT to check whether the AP has buffered packets for it.

PS-Poll transmission end time analysis

This section analyzes the PS-Poll transmission end time with and without hidden nodes in an 802.11ah network. In the 802.11ah network, most of nodes are in power save mode and many of them may be hidden from each other, and nodes can only transmit packets during their allocated RAWs. Therefore, the PS-Poll transmission end time is important in allocating a proper time period to each RAW.

Hidden Matrix based Regrouping (HMR) algorithm

The existing grouping algorithm for 802.11ah reduces the frequency of collisions by simply lowering the allocated number of nodes in each group. The algorithm, however, does not solve the hidden node problem properly since each node is randomly allocated to a group member. To reduce the number of nodes suffering the hidden node problem, we propose a new grouping algorithm that aims to alleviate the hidden node problem using hidden node relation observed during the network operation. Our HMR

Simulation results

In this section, we evaluate the performance of the proposed HMR algorithm against the 802.11ah standard, i.e., random grouping, with respect to the number of hidden node pairs, the PS-Poll transmission end time, and the number of retransmissions.

Conclusion

The IEEE 802.11ah standard has been proposed to have a much longer transmission range and an enhanced power save mode to service a wide area for sensor networks applications. However, the hidden node problem can be exacerbated especially when many nodes in sleep mode are activated simultaneously. As a solution, the 802.11ah standard proposed a group-based contention algorithm, but it is not a satisfactory solution to the problem.

To evaluate the performance degradation, this paper has analyzed

Acknowledgement

This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A01008240) and in part by ICT R&D program of MSIP/IITP (B0126-15-1017, Spectrum Sensing and Future Radio Communication Platforms).

Sung-Guk Yoon received B.S. and the Ph.D degree from Seoul National University, Seoul Korea, in 2006 and 2012, respectively. From 2012 through 2014, he was with the same university as a post-doctoral researcher. In 2014, he joined the school of electrical engineering at Soongsil University as an assistant professor. His research interests include next generation wireless networks, power line communications, and smart grid.

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Jung-O Seo received B.S. and the M.S. degree from Seoul National University, Seoul Korea, in 2012 and 2014, respectively. Since 2014, he has been with TMAX Soft, Gyeonggi-do, Korea. His research interests include Wi-Fi.

Saewoong Bahk is a professor at Seoul National University (SNU). He served as Director for the Institute of New Media and Communications during 2009-2011. Prior to joining SNU, he was with AT&T Bell Laboratories as a member of technical staff from 1991 to 1994 where he had worked on network management.He received KICS Haedong Scholar Award in 2012. He has been leading many industrial projects on 3G/4G/5G and IoT connectivity supported by Samsung Electronics, LG Electronics, SK Telecom, etc and published more than 200 technical papers and holds 72 patents.He was TPC Chair for IEEE VTC-Spring 2014 and general chair of JCCI 2015. He is co-EIC of IEEE/KICS Journal of Communications and Networks (JCN), and was on the editorial board for Computer Networks Journal (COMNET) and IEEE Transactions on Wireless Communications (TWireless). He is an IEEE senior member and a member of Whos Who Professional in Science and Engineering.He received B.S. and M.S. degrees in Electrical Engineering from Seoul National University (SNU) in 1984 and 1986, respectively, and the Ph.D. degree from the University of Pennsylvania in 1991.

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Regrouping algorithm to alleviate the hidden node problem in 802.11ah networks

Abstract

Introduction

Section snippets

IEEE 802.11ah: overview

System model

PS-Poll transmission end time analysis

Hidden Matrix based Regrouping (HMR) algorithm

Simulation results

Conclusion

Acknowledgement

IEEE 802.11ah: a long range 802.11 WLAN at sub 1 GHz

J. ICT Standard.

Adaptive approaches to relieving broadcast storms in a wireless multi-hop mobile ad hoc network

IEEE Trans. Comput.

Performance analysis of group-synchronized DCF for dense IEEE 802.11 networks

IEEE Trans. Wireless Commun.

WLAN power save with offset listen interval for machine-to-machine communications

IEEE Trans. Wireless Commun.

Enhancement of IEEE 802.11ah MAC for m2m communications

IEEE Commun. Lett.

Formal modeling and verification of an enhanced variant of the IEEE 802.11 CSMA/CA protocol

J. Commun. Netw.

Multichannel random access in OFDMA wireless networks

IEEE J. Sel. Areas Commun.

Resolving 802.11 performance anomalies through qos differentiation

IEEE Commun. Lett.

Novel hidden station detection mechanism in IEEE 802.11 WLAN

IEEE Commun. Lett.

Detecting hidden and exposed terminal problems in densely deployed wireless networks

IEEE Trans. Wireless Commun.