WIRELESS LANs: THE DECT APPROACH Anthony Lo Centre for Wireless Communications National University of Singapore 20 Science Park Road #02-34/37 TeleTech Park Singapore Science Park II Singapore 117674 Email: cwcloa@leonis.nus.edu.sg, Fax: +65 779 5441 Abstract: This paper deals with the design and implementation of a wireless Local Area Network (LAN), called DECT LAN, providing access to a backbone wired Ethernet network. Digital Enhanced Cordless Telecommunications (DECT) standard provides only the radio interface between mobile devices and Ethernet via a base station. The DECT protocol is implemented as a combination of hardware and software components embedded in a Network Interface Card (NIC) driven by a microprocessor. The NIC is conformed to the computer industry-standard interface, i.e., PCMCIA and PCI for the mobile device and base station, respectively. The software components of the DECT protocol are designed and implemented using SDL (System Description Language). An SDL specification facilitates simulation, validation and automatic code generation using SDL support-tools, which reduces development time. Finally, some applications of DECT LAN are also described. 1. Introduction Wired Local Area Networks (LANs) have been widely used and implemented in office and industrial environments throughout the world. A large volume of information is stored on servers attached to the LANs and is growing at an accelerated pace. With the increasing number of portable devices such as Personal Digital Assistants (PDAs), or notebook Personal Computers (PCs), there is a need of accessing the information servers by these portable devices. In addition, there is also a need to extend networks quickly and flexibly as organizations expand. Wireless technologies are promised to deliver a cost-effective solution of connecting the portable devices to existing LANs without wiring and rewiring of buildings and at the same time support mobility. The European Telecommunication Standards Institute (ETSI) has standardized the Digital Enhanced Cordless Telecommunications (DECT) [1] for accessing existing wired LANs using radio frequencies in the 1.88 1.90GHz range. The DECT standard does not only define the interface to wired LANs, but it also defined interfaces to many different voice and data networks such as PSTN, ISDN, GSM, PBXs, and X.25. As such, DECT is not a network, but a network access technology [2]. Within the framework of DECT, both voice and data services can be offered over the same infrastructure. On the other hand, the wireless LAN standard developed by IEEE, 802.11 [3], was specifically designed for data applications. For data services, DECT is a scalable technology; it supports variable data rates from 24kb/s up to 552kb/s by the use of multiple timeslots for transmission in the same direction, i.e., asymmetric transmission. The paper describes the hardware and software implementation of a DECT wireless LAN prototype, called DECT LAN, which interworks to a wired backbone Ethernet LAN [4]. Finally, some applications of DECT LAN are presented. 2. DECT LAN Architecture Overview The DECT LAN architecture is shown in Figure 1 with the Ethernet as the backbone network. This architecture is conformed to the reference configuration, interworking with Ethernet LAN, described in the annex of Data Services Profile (DSP) A/B [5]. This profile specifies a packetoriented and error-corrected protocol optimized for high-speed and low latency with bursty data. It supports both symmetric and asymmetric data
Mobile Terminal Fixed Host Application Application TCP TCP IP Base Station IP IWU CSMA/CD IEEE 802.3 CSMA/CD IEEE 802.3 PHY Air Interface PHY PHY Ethernet LAN PHY Figure 1: Overview of DECT LAN Protocol Stack transfer by combining multiple time slots to achieve higher unidirectional data rates. The Base Station (BS) is configured as a LAN bridge. On one side, the BS is connected to the DECT air interface, and to the Ethernet on the other side. The Interworking Unit (IWU) of the BS is responsible for translating data link layer frame formats between the two networks, i.e., converting frame formats from DECT to CSMA/CD (IEEE 802.3) and vice-versa. In addition, it provides buffering mechanisms because Ethernet runs at a faster speed compared to DECT. The Mobile Terminal (MT) is the end user that contains the DECT protocol, the TCP/IP protocol suite and application programs (e.g., netscape, ftp, etc.). An IWU also exists in the MT (not shown) which is responsible for mapping IP service requests to service primitives and vice-versa. The communication between IP and IWU is through the PC bus (formerly, PCMCIA). The DECT protocol consists of three layers 1 : Physical, Medium Access Control (), and Data Link Control (). The Physical layer is responsible for segmenting the radio transmission medium into physical channels using a Time Division Multiple Access (TDMA) scheme. Ten carriers are allocated in the frequency band 1.88 1.9GHz. Each carrier contains a TDMA frame of 24 time slots, which 1 Note that, the Network layer is defined in the DECT standard [1], but is not required in wireless LAN applications. See [4] for more details. normally provide 12 duplex channels, i.e., 12 slots for sending and 12 for receiving. The layer is responsible for effective allocation of physical resources and for multiplexing user and signalling data on to a physical channel. It uses the multi-bearer provision to time slots to achieve higher data rates. For instance, a data rate of 552 kb/s is attainable by combining 23 slots in a unidirectional data transfer. The Data Link Control () provides flow control, frame sequencing, and segmenting and reassembly of IP packets. 3. The DECT LAN Design and Implementation The DECT, and Physical layer [1] are implemented as a combination of software and hardware components embedded in a wireless Network Interface Card (NIC). One side of the wireless NIC connects to the computer s internal bus. The other side acts as the interface to the transmission medium. The following subsections describe the design and implementation of the wireless NIC for the mobile terminal and the base station. 3.1 DECT LAN Mobile Terminal The mobile terminal consists of a portable computer (such as a notebook PC running Windows 95) and a wireless NIC with PC bus interface. The wireless NIC is designed using a 16-bit microprocessor which controls the NIC as
P D C R M I C V I E A R PCMCIA Bus depicted in Figure 2. The DECT processor provides a complete implementation of the DECT Physical layer, and a partial implementation of the layer. The DECT processor interfaces to the 1.152Mbps radio link. The NIC card is equipped with 128 KB and 256 KB of RAM and, respectively. 3.2 DECT LAN Base Station The base station comprises an Intel-x86 PC, a wireless NIC, and an Ethernet NIC that interfaces to the Ethernet LAN. Figure 3 shows the block diagram of the wireless NIC. The design of the wireless NIC is similar to the mobile NIC except the base station NIC interfaces to the PCI bus. Data flowing in and out of the NIC is managed by the PCI controller. 3.3 DECT LAN Protocol Software The DECT Physical layer and partial layer P D RAM 128KB µprocessor 256KB Bus DECT Processor Figure 2: Block Diagram of the Mobile Wireless NIC RF Interface are implemented in hardware (i.e., DECT Processor), while the rest of the layer and the layer are implemented in software. The strategy used in the software development is based on a standardized formal description language, SDL (System Description Language) [6]. Formal description techniques improve the correctness of specifications by avoiding ambiguities, and through formal verification and simulation using support tools. In addition, formal specifications allow automatic code generation, which significantly reduces the time required to produce protocol implementations compared to the traditional coding technique. The SDL supporting tool-set used in the software development was the Telelogic SDT (SDL Design Tool) [7]. The software development strategy adopted is illustrated in Figure 4, which is a three-step process: 1. Formal specification in SDL; 2. Simulation and validation; and 3. Automatic code generation. In step 1, an SDL formal specification of the DECT and is developed. The specification is derived from DSP A/B [5]. Each C R 2KB RAM 128KB 256KB I I Bus V E R PCI Controller µprocessor DECT Processor RF Interface PCI Interface PCI Bus Figure 3: Block Diagram of the Base Station Wireless NIC
Traditional Coding ETSI DECT Standard:, SDL Specification Manual Refinement (Step 1) Simulation & Validation (Step 2) simulation. Testing by simulating the specification helps the protocol implementor to locate logical errors. The external behaviour is examined by sending signals from the environment to the SDL system and then observing the resulting output. The internal behaviour is also examined by checking active processes, current variable values, signal parameter values, etc. The SDT Validator is employed to detect any abnormal behaviour in the specification, i.e., deadlocks, signal race conditions, and data processing errors. C Code Generator C Code (Step 3) Figure 4: Protocol Software Development Process of the and protocol entity is modelled as a block specification in SDL. An SDL block in turn consists of a number of processes (extended finite state machines) which specify the behaviour of the protocol entity. For example, the block comprises three processes. A process consists of a number of states and a number of transitions connecting the states. A transition is initiated by a signal (message) sent by the process itself or an external process via logical communication channels, signal routes. In step 2, the SDL specification is simulated and validated. The SDT Simulator is used to perform Real Time Operating System (RTOS) Run Time Library (System Dependent) System independent Figure 5: DECT LAN Implementation Environment Once the specification has been thoroughly validated, the specification is translated into C- code using the SDT C-code generator (step3). The generated C-code is then compiled and linked with the SDL run time library to produce an executable program on the target system which is running a Real Time Operating System (RTOS), see Figure 5. Unlike the generated C- code, the run time library is hand-coded and includes functions that are system dependent. The functions are as follows: Memory management; Timer management; Communications management; and Interrupt handling management. 4. DECT LAN Applications in On The Move The Advanced Communications Technologies and Services (ACTS) AC034 On The Move project [8-10] is in the process of developing a Mobile Applications Support Environment (MASE) based on Universal Mobile Telecommunications System (UMTS). MASE supports multimedia applications over different types of mobile terminals and heterogeneous networks (see Figure 4). An important service of the MASE is Quality Of Service (QoS) negotiations (e.g., dynamic bandwidth adaptation). The variable bandwidth capability of DECT LAN allows the user to control and vary bandwidths, and tests the dynamic bandwidth QoS. In addition, MASE also supports roaming between networks which DECT LAN can be used, e.g., GSM and DECT.
Applications Hotel Guide & Navigator Video Conference Newspaper Personal World Wide Web Mobile Application Programming Interface Mobile Application Support Environment (MASE) UMTS Networks B-ISDN DECTLAN GSM UMTS Figure 6: On The Move s Mobile Application Support Environment 5. Conclusions and Future Work In this paper, we have presented the design and implementation of a DECT wireless LAN prototype with multi-slot capabilities. Presently, the prototype can support symmetric data transfer up to a maximum of 264 kbit/s (11 slots) in each direction. This is very useful to the multimedia applications and testing of the MASE services developed in the OnTheMove project. In the next phase, the DECT LAN prototype will be improved to support asymmetric data transfer with variable data rates up to a maximum of 552 kbits/s (23 slots) and 24 kbits/s (1 slot) in the opposite direction. To provide multimedia applications, the DECT LAN system will be extended to support Generic Access Profile (GAP) [11] using the same infrastructure. Acknowledgements This research work was supported by grants from the National Science and Technology Board (NSTB) of Singapore. The author would like to thank his colleagues for their helpful and constructive comments. The author would also like to acknowledge the support of Wind River Systems, Inc. and Telelogic AB. References 2. A. Elberse, DECT for Multimedia, DECT 97 Conference, London, UK, 1997. 3. IEEE: Wireless LAN Medium Access Control () and Physical Layer (PHY) Specifications, Draft Std 802.11, D3.1, 1996. 4. J. E. Goldman, Local Area Networks, John Wiley & Sons, 1997. 5. ETSI: Digital European Cordless Telecommunications (DECT): Data Services Profile (DSP); Base Standard including interworking to connectionless networks (service type A & B, Class1), 1996. 6. ITU-T: Specification and Description Language, Z.100, 1993. 7. SDT 3.1: Telelogic, SDT User s Manual, Telelogic AB, 1996. 8. E. Geulen, et al., Client/Server Architectures for Mobile Users: The ACTS On The Move Project, ACTS Mobile Communications Submit, Granada, Spain, 1996. 9. T. Kassing, et al., OnTheMove Field Trials (Testbeds and Results), In Proc. ACTS Mobile Communications Submit, Granada, Spain, 1996. 10. J. Harmer, The OnTheMove Project, MoMuC-3, 1996. 11. ETSI: Digital European Cordless Telecommunications (DECT); Generic Access Profile (GAP), ETS 300 444, 1996. 1. ETSI: Digital European Cordless Telecommunications (DECT); Common Interface (CI); Part 1 to 9, ETS 300 175-1 to ETS 300 175-9, 1996.