Elsevier

Computer Networks

Volume 52, Issue 1, 18 January 2008, Pages 96-115
Computer Networks

Scheduling solution for the IEEE 802.16 base station

https://doi.org/10.1016/j.comnet.2007.09.021Get rights and content

Abstract

The IEEE 802.16 standard defines a wireless broadband access network technology called WiMAX. It introduces several advantages, one of which is the support for QoS at the MAC level. To ensure meeting the QoS requirements, the 802.16 base station must run some algorithm to allocate slots between connections. This algorithm is not defined in the 802.16 specification but rather is open for alternative implementations. We propose a simple, yet efficient, solution that is capable of allocating slots based on the QoS requirements, bandwidth request sizes, and the 802.16 network parameters. To test the proposed solution, we have implemented the 802.16 MAC and PHY layers in the NS-2 simulator. Several simulation scenarios are presented that demonstrate how the scheduling solution allocates resources in various cases. According to the simulation results, the proposed scheduling solution ensures the QoS requirements of all 802.16 service classes. The solution shares free resources fairly and demonstrates work-conserving behaviour.

Introduction

IEEE 802.16 is a standard for wireless broadband access network [1]. The main advantages of 802.16 when compared to other network access technologies, such as 802.11, are the longer transmission range and more sophisticated support for Quality-of-Service (QoS) at the MAC level. Various application and service types can be used in 802.16 networks and the MAC layer is designed to support this convergence. The standard defines two basic operational modes: point-to-multipoint (PMP) and Mesh. While a subscriber station (SS) can communicate with other stations and with the base station (BS) in the Mesh mode, it is allowed to communicate only through the BS in the PMP mode. It is anticipated that providers will use the PMP mode to connect customers to the Internet [16]. In this case, a provider can control the environment to ensure the customers’ QoS requirements. A good overview of the key 802.16 features is given in [11]. Recently, the IEEE 802.16e standard [2] defined mobile extensions which opened a possibility to build mobile wireless VoIP terminals and embed 802.16 into mobile phones and personal data assistants.

An important principle of 802.16 is that it is connection oriented. This means that an SS must register with the base station before it can start to send or receive data. During the registration process, an SS can negotiate the initial QoS requirements with the BS. These requirements can be changed later, and a new connection may also be established on demand. The QoS requirements may be either per connection (GPC) or per subscriber station (GPSS) [11]. In this paper, we do not take into account which one of these modes is used because in GPSS, it is the responsibility of an SS to collect its service requirements into one connection.

The basic approach for providing the QoS guarantees in the 802.16 network is that the BS does the scheduling for both the uplink and downlink directions. In other words, an algorithm at the BS has to translate the QoS requirements of SSs into the appropriate number of slots within the 802.16 frame. When the BS makes a scheduling decision, it informs all SSs about it by using the UL-MAP and DL-MAP messages at the beginning of each frame. These management messages explicitly allocate slots to each SS in the uplink and downlink directions. However, the scheduling policy, i.e., an algorithm to allocate slots, is not defined in the 802.16 specification, but rather is open for alternative implementations.

There are several research works on 802.16 QoS scheduling that present architectures and scheduling disciplines to guarantee QoS. However, in [25], [8] the authors have focused mainly on the scheduling issues and components of the QoS architecture without presenting any exact method. Several research works [6], [26], [19] propose using complex schedulers, such as earliest deadline first (EDF), deficit round robin (DRR) [24], weighted fair queueing (WFQ) [20], worst-case weighted fair queueing (W2FQ) [5], and even a hierarchy of schedulers. However, it is a challenging task to use a hierarchy of schedulers because the per connection QoS requirements must be translated into the scheduler configuration at each level. Furthermore, it is not enough to calculate the scheduler configuration only once when an SS joins or leaves the network. As SSs send data, their request sizes change all the time. As a result, the scheduler at the BS should reassign slots for every 802.16 frame to achieve an accurate and fair resource allocation. For instance, if there are 400 FPS (frames per second) [1], then the BS must make 400 scheduling decisions per second. The OFDMa PHY specification allows the sending of up to 500 FPS. This is precisely reason why we suggest to use one level with a simple scheduling mechanism that is based conceptually on the round robin (RR) approach. A simpler solution is better because there is not much time for the scheduling decision.

This paper presents a scheduling solution for the 802.16 base station. When compared to previous research, our solution supports all the 802.16 service classes. We propose to allocate slots based on the scheduling class, the QoS requirements, either bandwidth request size (uplink connection) or the queue state (downlink connection). Furthermore, we propose a policy to allocate slots so that the BS can perform polling. The previous research works on 802.16 scheduling are based on simulations that were run in simple environments, such as MATLAB. We have tested our scheduling algorithm in the NS-2 simulator that provides a significantly better environment to simulate realistic network topologies, traffic characteristics, and behaviour of the transport protocols. To run simulation scenarios, we have implemented the 802.16 MAC and PHY layers.

The rest of the article is organized as follows. Section 2 presents the theory behind our scheduling proposal. That section presents calculations on how to convert the QoS requirements into a number of slots, how to allocate free slots, how to order slots, and how to estimate the 802.16 MAC overhead. Next, Section 3 describes the simulation environment that is used to test the proposed scheduler. Several simulation scenarios are presented and the simulation results are analysed. Section 4 concludes the article and presents the further directions for our research work.

Section snippets

802.16 QoS architecture

Fig. 1 presents a basic 802.16 QoS architecture, which is similar to the one considered in [9]. The 802.16 BS allocates a separate queue for each downlink connection to track resource demands. Also, it keeps so-called uplink virtual queues that correspond to the uplink resource demands of SS. While the downlink queue state is updated whenever a packet arrives there or leaves it, the uplink virtual queue state is updated when the BS receives a bandwidth request from an SS.

Environment

This section presents the simulation results for the proposed scheduling solution. For testing, we have implemented the 802.16 MAC and PHY layers, and the scheduling mechanism in the NS-2 simulator. The MAC implementation contains the main features of the 802.16 standard, such as downlink and uplink transmission, management and transport connections, PDUs, packing and fragmentation, the contention and ranging periods. We have also implemented the most important MAC signaling messages, such as

Conclusions

In this paper, we have presented a scheduling solution for the 802.16 base station working in the PMP mode. Our solution is based conceptually on the round robin scheduling which makes it fast and simple in implementation. Furthermore, such an approach simplifies the translation of the QoS requirements into a number of slots in the 802.16 frame. Our solution takes into account connection parameters, such as the minimum/maximum bandwidth requirements, traffic priority, class type, the bandwidth

Acknowledgement

We express our acknowledgments to Juha Karhula (Telecommunication laboratory/University of Jyväskylä) for his great contribution to the implementation of the 802.16 MAC level in the NS-2 simulator. We would like to give special thanks to Jani Lakkakorpi (Nokia) and Jani Moilanen (Nokia Siemens Networks) for their valuable discussions on the scheduler design and implementation.

Alexander Sayenko has obtained the B.Sc degree from the Kharkov State University of RadioElectronics (Ukraine) in 2001. He has obtained the M.Sc degree from the University of Jyväskylä (Finland) and the PhD degree from the same university in 2002 and 2005, respectively. Currently, he works for the Nokia Research Center, where he is responsible for the resource and power management solutions. His research interests are QoS, resource management and scheduling in the wireless networks.

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  • Cited by (0)

    Alexander Sayenko has obtained the B.Sc degree from the Kharkov State University of RadioElectronics (Ukraine) in 2001. He has obtained the M.Sc degree from the University of Jyväskylä (Finland) and the PhD degree from the same university in 2002 and 2005, respectively. Currently, he works for the Nokia Research Center, where he is responsible for the resource and power management solutions. His research interests are QoS, resource management and scheduling in the wireless networks.

    Olli Alanen is a researcher working in University of Jyväskylä, Finland. He has been studying QoS issues in WiMAX and other IP based networks for the past years. He received his M.Sc. in Computer Sciences from University of Jyväskylä in 2004. Currently he is finalizing his PhD studies at the same faculty.

    Timo Hämäläinen received the B.Sc in automation engineering from the Jyväskylä Institute of Technology in Finland on 1991 and the M.Sc and Ph.D degrees in telecommunication from Tampere University of technology and University of Jyväskylä, Finland in 1996 and 2002, respectively. Currently, he is Professor of Telecommunications at the University of Jyväskylä. His current research interests include traffic engineering and Quality of Service in wired and wireless networks.

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