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The Magic WAND - Wireless ATM Network

Demonstrator System

Juha Ala-Laurila1, Geert Awater2

1

Nokia Mobile Phones, P.O. Box 68, FIN-33721, Tampere, Finland email: juha.ala-laurila@nmp.nokia.com

2

Lucent Technologies Bell Labs Utrecht (WCND), Nieuwegein, The Netherlands email: awater@lucent.com

Abstract: The primary goal of the ACTS Magic WAND project is to specify a high speed indoor wireless

ATM access network that will provide QoS support for mobile multimedia services, such as e.g. video conferencing. While most related ACTS projects focus on one specific WATM subsystem, the WAND covers the entire network ranging from the radio sub-system to user application software. This paper gives an overview of the general architecture of the WAND demonstrator and the implementation of the key components covering the explicit hardware configuration and the functional structure of the software. The selected design flow and integration framework are also briefly discussed.

Introduction

Wireless access networks are currently being enhanced to support mobile broadband multimedia with Quality of Service (QoS) guarantees. It is expected that this kind of services will have a significant business potential in the future. Wireless ATM (WATM) technology has emerged as one of the most promising technologies for implementing a high speed wireless access networks. Several WATM concepts have already been published and a few demonstrator systems have been built by corporate research projects and universities with the intention to prove the excellency of the particular system specification.

The Magic WAND (Wireless ATM Network Demonstrator) is an international research project which aims at designing a high speed indoor WATM network with a limited radio coverage and user mobility. The system is mainly targeted for customer premise networks and hot spots with a great number of people within a restricted area. The project will create a complete system specification and a technology demonstrator which will be used for proofing and evaluating the performance and the usability of the designed system. All major innovations will be also contributed to the relevant standardisation forums.

The first part of this paper gives a brief introduction of the technical requirements which should be fulfilled by the demo system. The latter part describes the detailed architecture and the implementation framework of the WAND demonstrator. The emphasis is on the design of the wireless link, i.e. radio sub-system and data link control layer.

WAND System Characteristics

The WAND system should provide a high speed seamless wireless access to the ATM network maintaining the guaranteed QoS over the radio link. User mobility and the unreliable physical medium cause a number of technical constraints which have to be solved before the system can be successfully built. The main concern should be on mobility management and effective mapping of ATM cells into the air interface with minimum overhead. Furthermore, the developed concept should be scaleable adopting different applications and air interfaces.

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The WAND terminals are designed to be operated in a microcell environment where handovers occur frequently. Mobility impacts the fixed ATM network in two ways. First, ATM connection setup and routing must be enhanced to allow user mobility. For this purpose user authentication, registration and location management procedures have to be developed. Second, the network should automatically execute handovers as the user moves out of the radio coverage area. The handovers are mobile originated and intra-switch type meaning that they occur between two access points connected to the same switch. Both backward and forward handover scenarios will be provided. The WAND system will support three ATM service classes, i.e. Constant Bit Rate (CBR), Variable Bit Rate (VBR) and Unspecified Bit Rate (UBR). In order to properly meet different cell loss and delay requirements of different traffic classes both lossless and lossy handovers have been studied [4]. Nevertheless, in order to avoid system complexity the WAND demonstrator will only implement lossy handover algorithm.

The WAND radio sub-system will operate in HIPERLAN I frequency band (5.15 - 5.3 GHz). The physical medium should offer a maximum data throughput of 20 Mbit/s under the average RF conditions using the maximum transmission power of 1W. The design focus should be on optimising the latency and overhead of the Data Link Control (DLC) and physical layers. The resulting system should provide a Bit Error Rate (BER) of 10-6 which is found satisfactory for multimedia type of services.

Demonstrator Architecture Platform Configuration

Because the project is dealing with a new high risk technology, the original system specification might have to be modified even during the final implementation phases. Therefore, the technology platform configuration was designed to be as flexible as possible. The WAND demonstrator contains five sub-systems, namely Mobile Terminal (MT), two Access Points (AP), ATM switch and control station (see Figure 1.).

Access Point ATM switch UNI Access Point MASCARA MT Wireless ATM 5 GHz 20 Mbit/s Switch API Control Station ATM network User Mobile Terminal

Figure 1: Overview of the demonstrator system

The Mobile Terminal (MT) is the end user equipment creating a 20 Mbit/s radio interface. In the

demo system the MT consists of two separate PCs. The first PC, referred to as user mobile terminal, is a portable device running Windows NT 4.0 based multimedia applications and terminal ATM signalling software. The ATM signalling is implemented by enhancing the commercial UNI 3.1 compatible Trillium signalling product. The wireless related functionality is transparent to the user, as all the enhancements to the signalling functions are implemented below the standard WinSock 2.0 Application Programming Interface (API). Consequently, the WAND user terminal supports any

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WinSock 2.0 compatible application requiring LAN emulation (LANE) based TCP/IP connections or native ATM connections.

The wireless link is controlled by the second PC, nominated as MASCARA1 mobile terminalwhich comprises the DLC layer and the WAND radio modem. The mobile terminal PCs are connected back-to-back with STM-1 ATM connection. MASCARA MT uses Olicom 155Mbit/s ATM card which allows transparent cell discovery, while user MT deploys Globalcom ATM interface card supported by Trillium stack.

The Access Point (AP) creates a radio cell with a coverage radius of 50 meters. All access points are

connected to the ATM switch through a 155Mbit/s fixed ATM link utilising standard Olicom ATM cards and optical fibre. The AP hardware is identical to the MASCARA MT platform. Only the software is slightly different implementing centralised radio access link control functions, such as master scheduler.

The mobility enhanced ATM switch is a simple ATM switching matrix connected to a workstation,

called control station. The control station runs ATM network signalling software and several wireless specific control functions, such as mobility and radio resource management. The control access is provided through an ATM link and a dedicated switch control API. The WAND demonstrator deploys a VTT’s FSR ATM switch with seven 155Mbit/s ATM ports. The switch can connect up to 5 access points to the demonstrator network. The network ATM signalling software is based on Ascom’s commercial product with UNI 3.1 support. All the mobility enhancements are implemented as separate Unix processes which communicate via standard Unix Inter-Process Communication (IPC) message queues. The selected approach makes it easy to define clear explicit interfaces between various software entities thus significantly improving the reusability and testability of independent modules.

WAND Protocol Stack

The key objective of the WAND design was to maintain the compatibility between the fixed and wireless ATM systems. Therefore, the number of new protocols and functions was minimised by utilising the existing standards as much as possible and implementing only wireless specific extensions for them. Figure 2 shows the resulting WAND protocol stack [6]. All WAND specific protocol entities are shaded.

The mobility enhanced switch and MT both include separate MMC (Mobility Management Control) blocks and standard ATM functions, such as CC (Call control) and CAC (Call Admission Control). However, CC and CAC algorithms are slightly modified to meet the requirements of the radio link. The mobility specific signalling messages, related to registration and handover, are implemented by adding a dedicated entity (M signalling) in the standard ATM signalling stack.

In the target WAND system each access point hosts RRM (Radio Resource Manager) entity which maintains information on the capacity of the air interface. In the demonstrator RRM functionality is combined with wireless CAC algorithm and implemented inside the control station.

Both MT and AP contain MASCARA PC which provides a link between the ATM layer and the air interface. MASCARA PC implements both MASCARA (DLC) layer and WAND radio modem. The error correction is performed in two layers, namely in the physical (PHY) layer and in WDLC (Wireless Data Link Control) layer [3].

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User M T PC M ASC AR A PC SA A L A T M Q .2931 M LC P M M C C C A ppl. A A L5 A T M - P H Y R adio P H Y M A SC AR A A T M - PH Y W D L C L C P A A L 5 A TM R adio P H Y M ASC AR A W D LC L C P A TM - PH Y A A L 5 A TM Control Station C A C SA A L A T M Q .2931 L C P M M M C C C A A L 5 A TM - P H Y R R M N M S M ASC AR A PC 20 M bit/s 5 G Hz radio link sw itch dedicated control m sg.

M obile Term inal A ccess Point

M obility Enhanced A TM sw itch

multim ode fiber

m ultim ode fiber

Figure 2: Protocol stack of the WAND demonstrator system

Several control messages have to be passed between mobile terminal PCs in order to enable proper interworking of these two devices. A simple LCP (Layer Control Protocol) protocol was developed for carrying the management information between MT PCs and between AP and the control station. LCP is based on stop-and-wait principle utilising AAL5 frame. In the target WAND system MT will be a single machine. In that case LCP is used only for carrying control data between AP and the mobility enhanced switch.

MASCARA Platform (MASCARA MT, Access point) Overview

In order to support wireless access to fixed ATM services the WAND system includes a MASCARA protocol [2] which maps the connection-oriented ATM services into the high speed radio link still preserving the original QoS parameters. Like most WATM systems, also MASCARA is based on Time Division Multiple Access (TDMA) technique.

The complete MASCARA frame with variable length is composed of a frame header, downlink, uplink and contention periods. The contention period is reserved for asynchronous service requests, registration and other control messages. Each MASCARA frame begins with a frame header in which access point describes how the uplink and downlink periods and slots are allocated between various mobile terminals. The master scheduler inside the access point composes the frame structure taking into account the traffic demands and QoS parameters of active mobile terminals. The main duty of the scheduler is to guarantee that the admitted connections and QoS parameters are maintained over the air interface.

MASCARA Implementation

The functionality of MASCARA has been specified and implemented by using Specification and Description Language (SDL92). The Telelogic SDT tool was used to create the formal SDL description, verify correctness of the developed protocol and for automatic code generation, which turned formal SDL specification into the working code.

The MASCARA code is run in PC environment. PC platform was selected as it offers an easily accessible, flexible and cheap development environment for which high-quality compilers and debuggers are available. In addition it provides standard interfaces for a wide range of different extension cards. The MASCARA platform includes two PCI -bus I/O adapters. One custom-built adapter, called Frame Processor, provides the interface between MASCARA and the radio modem,

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while the other is a commercially available Olicom 155Mbit/s ATM adapter that establishes a link to the mobility enhanced switch (AP) or to the user MT.

Due to the high speed radio link and a large number of internal control signals the MASCARA model becomes rather complex and time critical. The size of a single TDMA slot is 21.6 µs, within which MASCARA layer should be capable of moving one ATM cell from ATM layer to the air interface. During this process MASCARA should modify ATM cell header, schedule incoming cells, calculate CRCs and create frame headers and complete MASCARA frames. According to the initial performance evaluation results made with the reduced MASCARA, the capacity of 200 MHz Pentium/MMX PC machine seems to be enough.

PC software functions are asynchronous while the WAND air interface is synchronous. This creates a serious dilemma, because CPU has difficulties in scheduling tasks at specific time instants, with sub-microsecond accuracy. The frame processor is required for controlling the radio in real time. It connects to the CPU (MASCARA software) to the radio modem through a standard PCI bus. The Direct Memory Access (DMA) approach is used to transmit data between the frame processor and the CPU. The CPU sends time stamped commands to the frame processor. The time stamp tells the frame processor in which slot of the current frame it shall execute the command.

To control the adapters, dedicated ATM and frame processor drivers were developed. The frame processor driver simply relays time stamped commands over the PCI bus between host memory and frame processor hardware. The ATM driver is more involved as it extends the original AAL5 service by cell-transparent operation and manages the transmission of AAL5 and AAL0 frames. The drivers are running on a PC with the MS-DOS 6.22 operating system and a 32-bit DOS extender. The MASCARA code uses its own run time kernel which schedules its software processes and supports signal exchange between processes. Further information on MASCARA implementation can be obtained from [3].

WAND Radio Sub-system Overview

The WAND radio physical layer deploys the unlicensed HIPERLAN band. In this frequency range the radio propagation is far more efficient compared to 40 or 60 GHz bands which are utilised by a few competitive demo systems. The used modulation technique should support effective transmission of short ATM cells with minimum physical overhead and radio turnaround time. In addition the selected approach should minimise the power consumption and allow cost-efficient hardware implementation.

The design of WAND radio [1] utilises a multi-carrier modulation, to obtain immunity against inter-symbol interference due to multi-path transmission. Orthogonal Frequency Division Multiplexing (OFDM) modulation technique was considered as the best alternative because it will not require the complex equaliser and the multipath resistance as well as channel separation are better than with Gaussian Medium Shift Keying (GMSK). The demonstrator system deploys the OFDM scheme with 16 carriers while the target commercial WAND system would probably use OFDM with 32 (or 64) carriers.

The main characteristics of the WAND radio are: OFDM with 16 carriers, 24 bits per symbol, symbol time 1.2 µs and guard time 0.16µs. The resulting gross bit rate is 20 Mbit/s with the maximum accommodated delay spread of 100 ns. The system provides a coverage range of 50 meters with omni-directional antenna. Further information of the radio design can be obtained from [1].

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The WAND demo radio is composed of separate transmit and receive modems with own antennas. A single HP Vectra desktop PC is deployed for housing both MAC engine and the radio. The same hardware is applied both in access points as well as in mobile terminals for providing the air interface.

The implementation of the physical layer is a multiple PC board based approach consisting of 3 ISA cards and 3 PCI extension cards [5]. The radio boards are lodged in PCI and ISA slots, purely for mechanical robustness. They are connected to the MASCARA via the Frame processor card interface. The Frame Processor controls the data transmission and ensures that the transmitter and receiver parts are never simultaneously switched on, so they can use the same carrier frequency. The transmitter is composed of two boards, a baseband board and a Radio Frequency (RF) part. The baseband board converts the incoming data stream (supplied by the Frame processor) into OFDM symbols using lookup tables. It also creates training sequences to the MAC PDUs to render packets which can be transmitted over the air. This board operates at 40 MHz rate at which the baseband signal samples must be produced. The RF part consists of a shielding case which accommodates two printed circuit boards, one with a set of local oscillators end one with the up-converter and power amplifier.

The WAND radio receiver includes three discrete boards, namely RF part, Frequency detection, synchronisation and FFT board and Phase correction and decoding unit. The RF part includes a 5 GHz front end, the remaining 5 GHz functions, and the IF/baseband processing. The Frequency detection, synchronisation and FFT Board controls the analogue part of the receiver, establishes frequency synchronisation, and performs an FFT to extract the ODFM carrier phases from the incoming baseband signal. Custom designed FPGAs and few standard (rotator) chips are used. The Phase correction and decoding unit, corrects the phase errors on the individual carriers and decodes the ½ rate Forward Error Correcting code. This unit is implemented on an off-the-shelf DSP board from ALEX Corporation, containing 7 AD SHARC DSPs.

Demonstrator Design Flow and Time Schedules

The demonstrator design is divided into several successive phases with clearly defined outputs. In the first stage the functional descriptions of their components were specified using SDL. Next the formal SDL models were combined into a complete system model, which was simulated with SDT tool to verify the correctness of the functionality. The explicit interface and signal definitions of the SDL model were applied as the basis of the implementation specification.

The demonstrator integration can be considered as a two dimensional process consisting of vertical and horizontal integration tasks. During the horizontal integration various components are integrated and tested in the “peer-to-peer basis” e.g. radio link between access point and mobile terminal. The vertical integration aims at combining tested discrete components into complete sub-systems, such as mobile terminal and access point. The final demonstrator with full functionality is scheduled to be available by 03/98.

Conclusion

Due to the still on-going implementation phase it is impossible to present any explicit performance evaluation results for the specified WAND system. So far the project has provided a complete technical specification of the demonstrator system which has been successfully verified with SDL simulations. A few initial software and hardware prototypes have been constructed in order to carry out early performance evaluations with reduced functionality. The performed tests have proved the applicability of the selected design approach. Currently it appears that the specified WAND system concept can be turned into a working demo system within the given time frame. The resulting demonstrator platform shall be publicly introduced during the user trials that will take place simultaneously in Finland and Germany in the mid of 1998.

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Acknowledgements

This work has been performed in the framework of the project ACTS AC085 The Magic WAND, which is partly funded by the European Community and the Swiss BBW (Bundesamt für Bildung und Wissenschaft). The author(s) would like to acknowledge the contributions of his colleagues from Nokia Mobile Phones, Lucent Technologies, Tampere University of Technology, Technical Research Centre of Finland, Ascom Tech AG, University of Lancaster, Robert Bosch GmbH, University of ULM, Compaginie IBM France, IBM Zürich Research Laboratory, Eurecom Institute, ETH Zürich, Intracom Hellenic Telecommunications and University of Athens.

References

[1] Aldis James, et al., “Physical Layer Architecture and Performance in the WAND User Trial System”, ACTS Mobile Communications Summit, Granada, Spain, Nov 1996.

[2] Bauchot Fréderic, et al, “MASCARA, a MAC protocol for wireless ATM”, ACTS mobile Communications Summit, Granada, Spain, Nov 1996

[3] Awater Geert, et al, “Magic WAND: The DLC implementation”, ACTS mobile Communications Summit, Aalborg, Denmark, Oct 1997

[4] Hansen Harri, et al, “Description of the Handover Algorithm for Wireless ATM Network Demonstrator (WAND), ACTS Mobile Communication Summit, Granada, Spain, Nov. 1996 1997

[5] Aldis James, et al, “Magic into reality, building the WAND modem”, ACTS mobile Communications Summit, Aalborg, Denmark, Oct 1997

[6] Mikkonen Jouni, et al, “The Magic WAND: a wireless ATM access system”, ACTS mobile Communications Summit, Granada, Spain, Nov 1996

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