Quality of Service Support in IEEE 802.16 Networks

Claudio Cicconetti, Luciano Lenzini, and Enzo Mingozzi, University of PisaCarl Eklund, Nokia Research Center

IEEE Network April 2006

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Outline

Introduction QoS Support in IEEE 802.16 Performance Evaluation

Residential Scenario SME Scenario

Conclusions

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Introduction

According to the WiMAX forum, 802.16 technology is attractive in a wide variety of environments, including high-speed Internet access, WiFi hotspot backhaul, cellular backhaul, public safety services, and private networks.

However, in [3], it is envisaged that the first 802.16-compliant products to be deployed will very likely be aimed at providing last-mile Internet access for residential users and small and medium-sized enterprises (SMEs).

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Introduction

The challenge for BWA networks is in providing quality of service (QoS) simultaneously to services with very different characteristics.

QoS support in wireless networks is a much more difficult task than in wired networks, mainly because the characteristics of a wireless link are highly variable and unpredictable, both on a time-dependent basis and a location dependent basis.

To cope with such issues, QoS in wireless networks is usually managed at the medium access control (MAC) layer.

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Introduction

In [4] the author performed a hybrid analytical-simulative analysis of the effect on the system performance of several MAC mechanisms, including the fragmentation of service data units (SDUs) and the padding of OFDM symbols.

The performance with the time-division duplex (TDD) mode was partially analyzed in [5].

Finally, in [6] the authors analyzed the performance of expected WiMAX compatible systems, from a physical-layer perspective.

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Introduction

At here review and analyze the mechanisms for supporting QoS at the IEEE 802.16 MAC layer.

Then analyze by simulation the performance of IEEE 802.16 in two application scenarios, which consist of providing last-mile Internet access for residential and SME subscribers, respectively.

This analysis is aimed at showing the effectiveness of the 802.16 MAC protocol in providing differentiated services to applications with different QoS requirements, such as VoIP and Web.

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QoS Support in IEEE 802.16

The 802.16 standard specifies two modes for sharing the wireless medium: point-to-multipoint (PMP) and mesh (optional).

With PMP, the BS serves a set of SSs within the same antenna sector in a broadcast manner, with all SSs receiving the same transmission from the BS.

Transmissions from SSs are directed to and centrally coordinated by the BS.

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QoS Support in IEEE 802.16

On the other hand, in mesh mode, traffic can be routed through other SSs and can occur directly among SSs.

Access coordination is distributed among the SSs. The PMP operational mode fits a typical fixed BWA

scenario, where multiple service subscribers are served by one centralized service provider so that they can access external networks (e.g., the Internet) or services (e.g., Digital Video Broadcasting — DVB).

In this study we focus on the PMP mode alone.

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QoS Support in IEEE 802.16

In PMP mode, uplink (from SS to BS) and downlink (from BS to SS) data transmissions occur in separate time frames.

In the downlink subframe, the BS transmits a burst of MAC protocol data units (PDUs).

Since the transmission is broadcast, all SSs listen to the data transmitted by the BS.

However, an SS is only required to process PDUs that are addressed to itself or that are explicitly intended for all the SSs.

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QoS Support in IEEE 802.16

In the uplink subframe, on the other hand, any SS transmits a burst of MAC PDUs to the BS in a time-division multiple access (TDMA) manner.

Downlink and uplink subframes are duplexed using one of the following techniques, as shown in Fig. 1: Frequency-division duplex (FDD) is where downlink and uplink subframes occur simultaneously on separate frequencies, and time-division duplex (TDD) is where downlink and uplink subframes occur at different times and usually share the same frequency.

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QoS Support in IEEE 802.16

However, based on the above mentioned functions and mechanisms, the 802.16 MAC specifies four different scheduling services in order to meet the QoS requirements of multimedia applications: unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS), and best effort (BE).

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QoS Support in IEEE 802.16

UGS is designed to support real-time applications (with strict delay requirements) that generate fixed-size data packets at periodic intervals, such as T1/E1 and VoIP without silence suppression.

rtPS is designed to support real-time applications (with less stringent delay requirements) that generate variable-size data packets at periodic intervals, such as Moving Pictures Expert Group (MPEG) video and VoIP with silence suppression.

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QoS Support in IEEE 802.16

Unlike UGS and rtPS scheduling services, nrtPS and BE are designed for applications that do not have any specific delay requirement.

The main difference between the two is that nrtPS connections are reserved a minimum amount of bandwidth (by means of the minimum reserved traffic rate parameter), which can boost performance of bandwidth-intensive applications, such as File Transfer Protocol (FTP).

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Performance Evaluation

We assume that all SSs have full-duplex capabilities, and that the frame duration is 10 ms.

Implement DRR as the SS scheduler, because the SS knows the sizes of the head-of-line packets of its queues.

The simulation scenarios consisted in several SSs located at various distances from the BS.

We assumed that the SSs employ the following modulations: QPSK 3/4, 16-QAM 3/4, and 64-QAM 3/4, which are evenly partitioned among SSs.

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Residential Scenario

We considered three cases, which differ in terms of the number of SSs served by the BS (3, 12, and 24 SSs).

Packet sizes are drawn from a Pareto distribution with cutoff (shape factor = 1.1, mode = 4.5 KB, cutoff threshold = 2 MB), while packet interarrival times are distributed exponentially (average = 5 s), which yields an average load of 25 KB/s.

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SME Scenario

We now consider the SME scenario, which involves a BS providing several enterprise customer premises with three different types of services: VoIP, videoconference, and data.

We assume that each SS has four VoIP sources multiplexed into an rtPS connection, two videoconference sources multiplexed into an rtPS connection, and a BE connection loaded with data traffic at 25 KB/s.

We assume that the BS grants a unicast poll to each VoIP and videoconference connection every 20 ms.

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Conclusions

The first one (residential scenario) dealt with data (non-QoS) traffic only, and was thus managed by the BE scheduling service.

Our results have shown that the average delay of the uplink traffic is higher than that of the downlink traffic.

In the second scenario (SME scenario), on the other hand, we have shown the service differentiation, in terms of delay, between data (served via BE) and multimedia traffic (served via rtPS).

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Conclusions

This is achieved because scheduling in 802.16 is controlled by the BS in both the downlink and uplink directions.

Therefore, it is possible to employ scheduling algorithms that have been proposed for wired environments, which are able to provide QoS guarantees.

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View point

在这篇文章简略的介绍了WiMax，以及针对其 QoS四个通讯等级中第二优先等级的 rtPS作一些仿真。

结果显示，虽然通讯排程是由 BS决定，在SS数量超过 48个用户时， Uplink的效能会比 Downlink来的好。

但很明显的，在这个模拟的结果之下，当使用者过多时 (48)，无论是 Uplink或是 Downlink皆无法提供及时的传输效率 (根据其它学者的研究 Delay应小于 50ms)。