home network has a strong focus on multimedia applications. For the users' ... devices are used to distribute multimedia services wirelessly within a house.
HOMEPLANE: An Architecture for a Wireless Home Area Network with Management Support for High Quality of Service Christian Schilling, Nils Langhammer, Beatriz Aznar, Ruediger Kays Communication Technology Institute Dortmund University of Technology Dortmund, Germany Abstract—This paper describes properties of a network management concept designed in the scope of the project “HOMEPLANE”. Following to the project’s vision, the future home network has a strong focus on multimedia applications. For the users’ convenience, media transfers are realized based on IEEE 802.11 WLAN. To ensure high quality of service different enhancement concepts have been developed and evaluated in hardware. This paper gives an overview on the fundamental concepts of wireless resource managing as well as current implementation results. Keywords-Home media network, WLAN, HOMEPLANE, Quality of Service, Hierarchical Network
I.
INTRODUCTION
The project “HOMEPLANE” (Home Media Platforms and Networks) is a cooperation between European Microsoft Innovation Center (EMIC), IHP (Innovations for High Performance Microelectronics), LINTEC IT AG, SIEMENS AG and Dortmund University of Technology. The overall goal of this project is to develop a homogeneous concept for the wireless integration of multimedia components in the home environment. At the end of the project, a demonstrator will be available to allow the live-presentation of results. An important task of HOMEPLANE concerns the enhancement of IEEE 802.11 based WLAN for multimedia applications. As confirmed by a user study, the wireless interconnection of multimedia devices is generally preferred. The increased flexibility of usage and the absence of additional cabling are very convenient. Users also expect the wireless network to be as reliable as cable. Nevertheless, currently available WLAN solutions are not well suited for this application scenario. Various enhancement concepts, as described e.g. in [1], [2] and [3], have been developed in the past and are now evaluated by practical implementation. In this paper, an efficient WLAN network concept is presented which is organized using ZigBee technology. Section 2 provides a short overview of the application scenario. Several aspects of HOMEPLANE are summarized in section 3, describing the interaction between different system components. In section 4 the ZigBee standard is sketched showing some of its characteristics and resulting benefits for the HOMEPLANE system. Practical results of the optimization approach implemented and evaluated on a realistic test set-up are presented in Parts of the presented work have been funded by the German Federal Ministry for Economics and Technology (BMWi) under grant 01MG509.
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section 5. Finally, section 6 concludes this paper and describes future work. II.
APPLICATION SCENARIO
Fig. 1 shows a typical home multimedia network. Different devices are used to distribute multimedia services wirelessly within a house. The devices are located in different rooms, forming a multi-dimensional arrangement. Common devices used for in-house multimedia distribution and presentation are PCs, set-top-boxes and multimedia home-servers. External networks, like DSL or coax cable can be connected via corresponding access points. These are often located in the basement of the house. Additionally, external wireless networks, like DVB-T (digital video broadcasting terrestrial) and DVB-S (digital video broadcasting satellite), are added by using an adequate receiver with wireless networking connectivity.
Figure 1. Typical home media environment.
Multimedia services are distributed via individual wireless links between two or multiple interacting devices. When using standard off-the-shelf WLAN products, a critical network situation arises if multiple high data-rate multimedia links operate simultaneously. This happens because multimedia links require a high ‘quality of service’ (QoS). The high density of communicating nodes and multimedia links, as well as the lack of suitable automatic configuration yield various problems. Interfering wireless transmissions suffer from collisions that cause the QoS requirements of media streams to be missed. The enhancement concepts developed within HOMEPLANE increase the efficiency of wireless networks based on the IEEE 802.11 standards. By improving the efficiency of each link, based on a cooperative network concept, a good overall performance of the network can be achieved. III.
BASIC SYSTEM OVERVIEW
The task of cooperative wireless resource management requires a suitable management platform. The basic system architecture of the HOMEPLANE management platform is outlined in Fig. 2. The control information exchange between applications and network management services is entirely based on the Web Services Technology, including eventing and dynamic discovery. The depicted central resource management service supervises running sessions and keeps track of available resources that might be required for further sessions. This service interfaces with the WLAN management service available on each device to gather needed statistics and to forward the application requirements. This low level WLAN management service is responsible to fulfill the requirements of all media sessions and to optimize the use of wireless resources.
This task requires several components highlighted by the dotted box in Fig. 2. Statistical data like spectral usage and attenuation of links to neighboring stations is acquired from the monitoring interface of the WLAN module. The retrieved information is recorded inside a local management database. Based on this information and received control messages, the WLAN management service determines the most reliable and efficient configuration. When an update is required, the connected WLAN module is properly reconfigured. Control messages distribute such updates to other stations in order to synchronize related management information. The control protocol also allows safeguarding critical operations like channel changes. This inter WLAN management communication is not based on Web Services, in contrast to all high level services developed in HOMEPLANE. If the available wireless resources are insufficient, admission control is taken into account. For this purpose data describing the amount of available capacity is passed to the central resource management. Further middleware components, outside the focus of this paper, concern the user interface for system configuration and interaction, as well as the integration of a home automation network. Due to the three-dimensional distribution of media sources the ‘independent basic service set’ (IBSS), as defined in IEEE 802.11, appears suitable for data transmission and is in the main focus of the current work. The IBSS (also known as ad-hoc network) allows a fair access to the wireless media from separate devices like internet router, media storage or TV receivers. It does not require a central coordinator like an ‘access point’ (AP). The WLAN management service implements the enhancement concepts that have been proposed in [1], [2] and [3] to achieve efficient transmissions that fulfill all QoS requirements. The ‘efficiency’ indicates that a transmission is performed at a low level of resource consumption. Thus, collisions and interferences have to be avoided. By means of ‘dynamic frequency selection’ (DFS) and ‘transmit power control’ (TPC) it is possible to decrease the possibility of interference with neighboring networks and links. To augment the potential of DFS, links of the same network may be simultaneously operated on non-overlapping channels. It is improbable for real systems to always select a channel without any interferer. For this reason a modified MAC procedure has been developed. It allows a simple prioritization of limited rate traffic like media traffic, which additionally keeps the number of collisions at a low level. Section 5 summarizes the current implementation status as well as the driver interfaces that have been defined for this purpose.
Figure 2. Diagram showing the WLAN management embedded into the overall system.
As introduced a control protocol is necessary to coordinate the use of the described measures. Native IEEE 802.11 usually employs in-band signaling to exchange control information. Considering DFS and TPC, a dedicated control network is a powerful alternative. Such a network assures a robust and ubiquitous connectivity. Additionally, the control network is capable to transport other protocol payload next to the control messages required for WLAN management. Together, both
Station 1
Station 3
Station 2
Station 4
high rate traffic via WLAN low rate/signalling traffic via control network • WLAN coordination • remote procedure calls (UPnP, Web Service) • ARP, DHCP, …
Figure 3. Hierarchical approach: the control network handles low bandwidth traffic and WLAN only handles demanding traffic.
network types form a hierarchical transmission system. While WLAN links are exclusively used for demanding high data rate traffic, the control network is employed for low rate traffic. Examples are low effort application traffic and well known protocols like ARP and DHCP. The hierarchical approach is sketched in Fig. 3. The chosen control network is described in the following section. IV.
ZIGBEE SUPPORTED NETWORK ORGANIZATION
A. Advantages Several technologies have been evaluated concerning their ability to provide an adequate control network. The decision to select ZigBee as the control network has been taken due to its advantages compared to other technologies. ZigBee is an open standard optimized for home and industrial sensor and control applications. The work on the ZigBee standard started in 1998 with the ZigBee Alliance. In December 2004, the first version of the ZigBee specification was released followed by enhanced versions in 2006 and 2007 [5]. Based on the IEEE 802.15.4 PHY and MAC Layer, ZigBee provides a long-range, highly reliable and stable transmission of monitoring and control information. This is achieved by using an offset-QPSK modulation combined with ‘directsequence-spread-spectrum’ (DSSS) technology. The ZigBee specification defines a fully featured Network and Application layer on top of the IEEE 802.15.4 PHY and MAC [6]. By offering techniques for multi-hop-routing and meshing, the ZigBee Network layer provides methods to reliably interconnect nodes of the network. Additionally, multi-hop-routing increases the maximum range of the network. These benefits come along with low power requirements provided by techniques of the IEEE 802.15.4 MAC. A further important aspect is the fact that ZigBee transceivers can easily be implemented without high costs and a broad range of products is already available on the market. Due to the fact that ZigBee is designed for industrial and home control applications, an easy and future proof integration into existing home-automation-networks seems to be possible. B. Control Protocol Overview The absence of a single control instance requires a cooperative interaction between all devices of the network. Single stations have to make decisions in the context of the entire network. This is achieved by aligning the local management databases of all stations in the network by means of an appropriate control protocol.
The developed control protocol is optimized for the distributed architecture and is therefore able to keep the distributed management databases consistent. This is achieved by using concepts described in IEEE 802.11k with enhancements. The control information is transmitted via the ZigBee network by utilizing some of the system features like end-to-end reliability and broadcast transmissions. One characteristic of the control protocol is the improvement of the network discovery. Every station that joins the network informs all other stations about its presence and the individual settings. Hence, all stations are aware of each other and can easily align their behavior in the network context. In the scenario of the concurrent wireless medium access, the control protocol is used to coordinate the selection of the ‘arbitration inter frame space’ (AIFS) periods. The task is to assign short but unique AIFS periods to each of the highprioritized multimedia links. The proposed modified MAC procedure uses a deterministic channel access function. In contrast to the standardized DCF or EDCA procedure, specified in IEEE 802.11e, the AIFS period is not extended by a random backoff period. This random backoff period usually yields a fair and optionally weighted fair channel access for all competing stations. Because multimedia traffic claims an approximately constant and limited bandwidth, a fixed and short AIFS period assures a high access priority without flooding the channel. But it is absolutely necessary to employ unique AIFS periods for neighboring links when the usual backoff mechanism is deactivated. Firstly, the used AIFS periods from the current links are communicated throughout the network to update the management database of all stations. Secondly, the control protocol is able to re-sort the assigned AIFS periods. This is necessary if a high-prioritized multimedia link is started that claims a lower AIFS period. In such a situation, the lower prioritized links have to increase their own AIFS period for benefit of the higher prioritized link. In addition to the concurrent wireless medium access the control protocol provides mechanisms for DFS and TPC. In the context of DFS the protocol can be used to assign multimedialinks on non-overlapping channels. C. ZigBee Transport Protocol In addition to the network control traffics, the protocol provides an encapsulation to transport packets of other network traffic like IP, ARP, etc. with low data rates. It is accomplished with the use of a special protocol frame format. Therefore, both control traffic and additional network traffic can be multiplexed and transmitted via a single ZigBee endpoint. This is needed since most of the commercially available ZigBee modules offer only one endpoint for data transmission. Furthermore, the protocol supports segmentation and reassembly because the ZigBee maximum transmission unit (MTU) is smaller than the MTU of other protocols. This is necessary because the ZigBee PRO specification, which offers segmentation, is not widespread and therefore not implemented in many modules at this time.
V.
STATUS OF IMPLEMENTATION
1) Hardware Platform The selected WLAN hardware for the first tests is based on a commercially available chip set. In the course of the project, a specific wireless chip set, provided by the project partner IHP, will be integrated to optimize the system’s performance. In the current demonstrator a CardBus card software driver was extended with required features. The chip sets used in the demonstrator are supporting QoS according to the standard IEEE 802.11e [4]. Therefore, four different access classes are defined as data buffers: Best Effort, Background, Video and Voice. The main modifications affect the MAC parameter configuration for these data buffers in the hardware, e.g. CWmin, CWmax, AIFS and TxOpLimit. By default it is not intended to set up these values in ad-hoc mode. As explained before, shorter AIFS periods are used to prioritize limited bandwidth traffic, like multimedia streams. As proposed, the backoff procedure can be disabled for the modified MAC procedure by setting CWmin and CWmax to 0. Furthermore, in the modified driver each single transmission descriptor contains several configurable parameters like data rate, transmit power or antenna. Several interfaces have been implemented to facilitate their settings. Further features have been developed related to change the channel during a transmission enabling four different policies to deal with the buffered packets. This is necessary for the implementation of other complementary optimization technique like DFS. Additionally, to enable communication between the driver and the WLAN management service, a socket based interface, Netlink, has been created to transfer information. Examples of events that trigger a Netlink dispatch are packet drops due to lifetime or retry count excesses or crossings of lower or upper thresholds in the data buffers. The other mode to send a message via Netlink is ‘on demand’, e.g. for statistics or interference measurements on the channel. All these mentioned WLAN driver modifications establish an adequate test bench to evaluate the performance of the proposed method. Furthermore, ZigBee modules are used to set up the control network. B. Measurement Results As mentioned in section 3, the proposed modified MAC procedure has been evaluated in a first step. The practical evaluation of DFS and TPC will be performed in a later step. A basic DFS implementation will also be done based on the current platform, but the best results are expected with the specially customized WLAN module designed by IHP within the HOMEPLANE project. 1) Evaluation Setup Fig. 4 sketches the setup used to evaluate the performance of the proposed modification of the DCF MAC procedure. This setup consists of three laptops using the mentioned WLAN module and the extended driver. A generic DFS mechanism
link 1 highest priority
Station 1
In order to evaluate the performance of the optimization approach, several experimental tests have been carried out, which are described in this section.
link 3 lowest priority
Station 2 link 2 medium priority
Station 3
Figure 4. The evaluation setup showing stations and transmissions. TABLE I.
MAC PARAMETER SET USED WITHIN EVALUATION SETUP
Link Number and Priority
AIFSN [slot time]
CWmin [slot time]
CWmax [slot time]
1 2 3
2 4 6
1 1 1
1 1 1
desired data rate [Mb/s] 10.3 10.3 10.3
The desired data rate column describes the data rate of the WLAN frame payload (1500 Byte). The AIFSN, CWmin and CWmax values are given in units of a slot time. This is the duration of a single backoff slot used by the standardized contention algorithm.
was employed for the selection of a channel without interferers by the participating laptop. This assures easy reproducibility. This generic DFS mechanism will not switch the channel while measurements or transmissions are running. All devices have been located within the same room with a distance of approximatly three meters. The physical data rate used for this frame transmission experiment was fixed to 54 Mb/s. During the measurement three UDP links were running with a desired data rate of approximately 10.3 Mb/s. The data flow of this experiment is show in Fig. 4. Table I summarises the MAC parameter set that has been established for this setup. Based on the priority, increasing AIFS periods have been assigned to the frames associated with each link. The AIFS period can be calculated according to the following equation.
AIFS = aSIFSTime + AIFSN ⋅ aSlotTime
(1)
aSIFSTime (= 16 μs) and aSlotTime (= 9 μs) are values defined in the IEEE 802.11 standard. Multiples of the slot time have to be applied to keep the carrier sense mechanism functioning. Concerning the driver implemented on each station, one of the four available data buffers has been configured to the settings shown above. The corresponding traffic is directed into this data buffer. During this experiment the remaining three data buffers stay unused. The AIFSN values shown in table I have an increment of 2. This is necessary since the backoff mechanism cannot be disabled completely in the current chipset. The chosen AIFS period will be randomly extended by one backoff slot duration. 2) Evaluation Results Both, the standardized and the modified MAC procedure are compared in order to demonstrate the efficiency of the proposed approach. Fig. 5 a) shows the actual data rate that can be reached by each of the three links when using the standardized MAC procedure. The parameters set used in this case is
the concept outlined in section 3, this situation could be circumvented by means of DFS or even by admission control as a last resort. To maintain network stability, the capacity available on a single channel must not be exceeded only by multimedia traffic. In the case of this example as a real situation, link 3 should be terminated by means of admission control. The modified MAC procedure meets the hard QoS requirements of multimedia applications. The deterministic channel access function increases the successful access probability. Therefore the data rate of concurrent best-effort traffic is automatically decreased by this prioritization scheme. VI. CONCLUSION Implementation results and fundamental concepts of the HOMEPLANE architecture have been presented in this paper. The HOMEPLANE platform for future high data-rate wireless home media networks builds on enhancements of IEEE 802.11 based WLAN. In addition to the multimedia network, a second network based on ZigBee technology is used to transmit the necessary control traffic. The HOMEPLANE system uses a middleware, offering easy integration by using flexible Web Services Technology. Several WLAN optimization techniques like DFS, TPC and a modified MAC are applied to the HOMEPLANE WLAN. The coordination of WLAN enhancements is handled via the ZigBee control network as described in section 4. Figure 5. Comparison of actual data rates reached when using the standardized MAC procedure (a) and the modified MAC procedure (b).
AIFSN = 2, CWmin = 15 and CWmax = 1024. Clearly, the cumulative rate of 30.9 Mb/s can hardly be fulfilled by an IEEE 802.11a/g system due to the overhead. No link is capable to meet the desired data rate of 10.3 Mb/s. All three links operate with an actual rate between 8 and 9 Mb/s. This is due to the fair channel access policy of conventional WLAN systems. Considering multimedia applications, every stream will fail to display correctly. The input buffer of every video decoder will frequently run empty. Fig. 5 b) shows the resulting actual data rate when using the modified MAC procedure. It can be seen easily that the actual data rate of at least two high-prioritized links meets the desired data rate of 10.3 Mb/s. Solely, the link that has been assigned the longest AIFS period (link 3) fails in this scenario. Station 3 has no chance to access the channel, when the stations 1 and 2 have frames in their data queues. These stations are able to access permanently the channel prior to station 3. For this reason, the data rate of link 3 is automatically limited. Similarly, the data rate of the best-effort traffic is reduced. By means of the modified MAC procedure two media sessions are possible in this scenario, even when the desired overall data rate exceeds the maximum capacity of the IEEE 802.11a/g based transmission system. This scenario involves an overload situation in order to prove the effect of the modified MAC procedure. Considering
The first WLAN enhancement implementation in the project concerns the modified MAC protocol to provide a prioritized medium access. A short overview of the hardware platform and the needed driver adjustments has been given in section 5. Additionally, the results of a practical measurement have been presented. The measurement results prove the efficiency of the underlying concepts used in the HOMEPLANE WLAN. Next steps will be the implementation and integration of DFS and TPC into the HOMEPLANE System. The availability of the control network will be the basis for an efficient DFS and TPC implementation. REFERENCES [1]
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