Wireless Design Considerations
Wireless communication has become more and more pervasive in our modern lives. 3G, wideband CDMA (WCMDA), and GSM are becoming household names in today's cellular communication. In the near future, 802.11 wireless LAN and Bluetooth are promising to change the way we work in offices and how we enjoy our leisure time at homes. New system architectures in ASIC, systems-on-a-chip (SoC), and embedded software design are needed to address the needs of these new wireless communication systems. In addition, new design methodologies are being developed to address new design constraints and challenges. This article will examine the current state-of-the-art technology and explore future trends in wireless system design and design methodology.
With that in mind, today's wireless technologies and systems are fairly new and are still emerging on the scene. Currently, wireless technologies are comprised of infrared, UHF radio, spread spectrum, and microwave radio. These technologies can range from frequencies in the MHz (United States) or GHz (Europe) to infrared frequencies. The personal communication network (PCN) can use either code-division multiple access (CDMA) or time-division multiple access (TDMA). There is a considerable controversy among experts in the field regarding the relative merits of spread spectrum (CDMA) and narrowband (TDMA) for private communication network (PCN). The preferred technique may actually vary with the specific PCN application scenario to be addressed later in the article.
TDMA divides the radio carriers into an endlessly repeated sequence of small time slots (channels). Each conversation occupies just one of these time slots. So, instead of just one conversation, each radio carrier carry's a number of conversations at once. With the development of digital systems, TDMA is being more widely used.
The term spread spectrum defines a class of digital radio systems in which the occupied bandwidth is considerably greater than the information rate. The term code-division multiple access (CDMA) is often used in reference to spread spectrum systems and refers to the possibility of transmitting several such signals in the same portion of spectrum by using pseudorandom codes for each one. This can be achieved by either frequency hopping (a series of pulses of carrier at different frequencies, in a predetermined pattern) or direct sequence (a pseudorandom modulating binary waveform whose symbol rate is a large multiple of the bit rate of the original bit stream) spread spectrum.
As the deployment of wireless LANs grows, there is also a need for higher data rates. As a result, spectrum has been allocated for high-performance LANs (HIPERLAN) and SUPERNET activities at 5GHz, supporting connectivity of 2025Mbps. Moving to even higher frequencies (40GHz and 60GHz) with connectivity of 100Mbps is the subject of current research, although these higher frequencies are more suited to fixed-link applications.
The principal purpose of this article is to define the state of wireless communications design as it exists today and to introduce the basics behind wireless communications, as well as to provide an overview of how it all works. Economically, wireless communications are predicted to reach $4 billion in revenues by the year 2003. Overall, wireless communications spending is expected to reach $103 billion by 2003. The cost of installing and maintaining wireless communications generally is lower than the cost of installing and maintaining a traditional wired LAN, hence more and more enterprises are implementing this new wireless configuration. This article has been written with the assumption that the reader has some intermediate to advanced knowledge of networking and an understanding of telecommunications.
In wireless communications, information is transmitted from one or more data collection points to one or more data destinations. As the name implies, this is done without wires. The usual media for information interchange is sound, radio frequency, or light. Here, we will discuss both fixed and handheld microprocessor-controlled and radio frequency transmitter/receiver units. We will also cover how wireless communications systems can be configured to form a local area network (LAN) that consists of handheld communications terminals (HHCTs) that are connected to a wireless interface processor through a narrowband FM radio link. Finally, we will conduct a more in-depth discussion of the hardware, its functioning, and applications for wireless communications. Diagrams of various wireless system configurations will be included.
Furthermore, because of the wide range of services supported by Asynchronous Transfer Mode (ATM) networks, ATM technology is expected to become the dominant networking technology for both public infrastructure networks and LANs. ATM infrastructure can support all types of services, from time-sensitive voice communications and multimedia conferencing to bursty transaction processing and LAN traffic. Extending the ATM infrastructure with wireless access meets the needs of users and customers who want a unified end-to-end networking infrastructure with high performance and consistent service. Wireless ATM adds the advantages of mobility to the already great service advantages of ATM networks.
ATM is a cell-based data transfer technique in which channel demand determines packet allocation. ATM offers fast packet technology, real-time, demand-led switching for efficient use of network resources. It is also the generic term adopted by ANSI and the ITU-TS to classify cell relay technology within the realm of broadband WANs, specifically B-ISDN. In ATM, units of data are not time-related to each other and, as part of the B-ISDN standard, are specified for digital transmission speeds from 34Mbps to 622Mbps. IBM currently offers ATM at a nonstandard 25Mbps format. ATM will be the high-bandwidth networking standard of the decade.
Wireless ATM: Wide-Area Interconnection of Heterogeneous Networks
ATM has been advocated as an important technology for the wide-area interconnection of heterogeneous networks. In ATM networks, the data is divided into small, fixed-length units called cells. The cell is 53 bytes. Each cell contains a 5-byte header. This header contains the identification, control priority, and routing information.
The other 48 bytes are the actual data. ATM does not provide any error-detection operations on the user payload inside the cell, and it also offers no retransmission services.
ATM switches support two kinds of interfaces: User Network Interface (UNI) and Network Node Interface (NNI). UNI connects ATM end systems (hosts, routers, etc.) to an ATM switch, while an NNI may be imprecisely defined as an interface connection between two ATM switches. The International Telecommunication Union Telecommunication (ITU-T) recommendation requires that an ATM connection be identified with connection identifiers that are assigned for each user connection in the ATM network.
The ITU-T is an international body that develops worldwide standards for telecommunications technologies. The ITU-T carries out the functions of the former CCITT.
At the UNI, the connection is identified by two values in the cell header: the virtual path identifier (VPI) and the virtual channel identifier (VCI). Both the VPI and the VCI can combine together to form a virtual circuit identifier. Figure 1 shows the UNI and NNI interface to a wireless ATM network 25.
Figure 1 Wireless ATM reference architecture.
In any event, there are two fundamental types of ATM connections: permanent virtual connections (PVC) and switched virtual connections (SVC). First, a PVC is a connection set up by some external mechanism, typically network management. In this setup, switches between a source and destination ATM are programmed with the appropriate VPI/VCI values. PVCs always require some manual configuration. On the other hand, an SVC is a connection that is set up automatically through a signaling protocol. SVCs do not require the manual interaction needed to set up PVCs and, as such, are likely to be much more widely used. All higher-layer protocols operating over ATM primarily use SVCs.
Reasons for Wireless ATM
Since the beginning, the concept of ATM is for end-to-end communications (in a WAN environment). The communication protocol will be the same (ATM), and enterprises will no longer have to buy extra equipment (like routers or gateways) to interconnect their networks. Also, ATM is considered to reduce the complexity of the network and improve the flexibility while providing end-to-end consideration of traffic performance. That is why researchers have been pushing for an ATM cell-relay paradigm to be adopted as the basis for next-generation wireless transport architectures.
There are several factors that tend to favor the use of ATM cell transport for a personal communication network:
Flexible bandwidth allocation and service type selection for a range of applications
Efficient multiplexing of traffic from bursty data/multimedia sources
End-to-end provisioning of broadband services over wireless and wired networks
Suitability of available ATM switching equipment for intercell switching
Improved service reliability with packet-switching techniques
Ease of interfacing with wired B-ISDN systems that will form the telecommunications backbone 25
In general, interworking may always be seen as a solution to achieve wireless access to any popular backbone network, but the consequence, in this case, is a loss of the ATM quality of service characteristics and original bearer connections. The more interworking there is in a network, the less harmonized the services provided will be. Therefore, it is important to be able to offer appropriate wireless extension to the ATM network infrastructure.
One of the fundamental ideas of ATM is to provide bandwidth on demand. Bandwidth has traditionally been an expensive and scarce resource. This has affected the application development and even the user expectations. So far, application development has been constrained because data-transmission pipes cannot support various quality of service parameters, and the maximum data transmission bandwidth that the applications have to interface with is relatively small or simply insufficient. Finally, ATM has removed these constraints. Bandwidth has become truly cheap, and there is good support for various traffic classes. A new way of thinking may evolve in application development.
The progress toward ATM transport in fixed networks has already started, and the market push is strong. It can be expected that new applications will evolve that fully exploit all the capabilities of the ATM transport technology. The users will get used to this new service level and require that the same applications be able to run over wireless links. To make this possible, the wireless access interface has to be developed to support ATM quality-of-service parameters.
The benefits of a wireless ATM access technology should be observed by a user as improved service and improved accessibility. By preserving the essential characteristics of ATM transmission, wireless ATM offers the promise of improved performance and quality of service not attainable by other wireless communications systems, such as cellular systems, cordless networks, or wireless LANs. In addition, wireless ATM access provides location independence that removes a major limiting factor in the use of computers and powerful telecom equipment over wired networks. Figure 2 shows a typical ATM network 25.
Figure 2 Normal ATM network.
Wireless ATM Architecture
The architecture proposed for wireless ATM communications is composed of a large number of small transmission cells called pico cells. Each pico cell is served by a base station. All the base stations in the network are connected via the wired ATM network. The use of ATM switching for intercell traffic also avoids the crucial problem of developing a new backbone network with sufficient throughput to support intercommunication among large number of small cells. To avoid hard boundaries between pico cells, the base stations can operate on the same frequency.
Reducing the size of the pico cells has major advantages in mitigating some of the major problems associated with designing and building wireless LANs. The main difficulty encountered is the delay due to multipath effects and the lack of a line-of-sight path resulting in high attenuation. Pico cells can also have some drawbacks as compared to larger cells. There are a small number of mobiles, on average, within range of any base station, so base-station cost and connectivity is critical. As cell size is reduced, handover rate also increases. By using the same frequency, no handover will be required at the physical layer. The small cell sizes also gives us the flexibility of reusing the same frequency, thus avoiding the problem of running out of bandwidth.
The mobile units in the cell communicate with only the base station serving that particular cell, not with other mobile units. The basic role of the base station is to interconnect the LAN or WAN and the wireless subnets, and also to transfer packets and convert them to the wired ATM network from the mobile units.
In traditional mobile networks, transmission cells are colored using frequency-division multiplexing or code-division multiplexing to prevent interference between cells. Coloring is wasteful of bandwidth because, in order for it to be successful, there must be areas between reuse that are idle. These inactive areas could potentially be used for transmission. Figure 3 shows a typical ATM-to-base station connection 25.
Figure 3 Normal ATM-to-base-station connection.
Wireless ATM research has been active for some time now. There are many papers and books written on wireless ATM, and, there are even announced wireless ATM prototypes such as RATM (Radio ATM) by Olivetti research laboratory 1. Yet, the most important type of activity has been missing from the wireless ATM scene. For enterprises with enterprise interests, the main objective is often to implement only equipment/systems conforming to standards. Thus, the wireless ATM communications subject has been brought to two different standardization forums, namely the European Telecommunications Standards Institute 2 Society for Technical Communications 3 Remote Execution Service 10 (ETSI STC RES10) and the ATM forum 4.
Currently, there are three standard bodies that have defined the physical layer in support of ATM: American National Standards Institute (ANSI) 5, International Telecommunication Union's Telecommunications (ITU-T) 6, and the ATM forum. None of these forums has considered the wireless ATM interface. The ETSI RES10 subtechnical committee is the first standardization body to start working on wireless multimedia, ATM compatibility, and standardization. RES10 committee has already been engaged with the HIPERLAN (High-Performance Radio Local Area Network) standardization, and the wireless ATM group is working on this subject. The initial work has concentrated on possible usage scenarios and specific requirements. Also, the search for available spectrum in the 5.2GHz range for wireless ATM system is crucial and, therefore, was one of the first tasks of RES10.
The ATM forum is not an official standardization body, but it plays a significant role in the standardization arena because of its strong industrial participation and support. Wireless ATM activity has now been officially approved in the ATM forum.
One wireless ATM activity solution that was approved divided the standardization of wireless ATM between the ATM forum and RES10. Nevertheless, it would probably be wise to let the ATM forum concentrate on the fixed network side and RES10 focus on the wireless interface. The main focus of the ATM forum should be that the ATM physical layer is not necessarily always a reliable medium and that terminals may be mobile. Both of these facts are due to the fact that ATM/Broadband Integrated Services Digital Network (B-ISDN) connections may be stretched over the wireless links in the future and should be independent of the specific wireless interface.
Now let's take a look at some ongoing projects in the area of wireless communications.
The following are some of the ongoing projects in the area of wireless ATM communications:
- Wireless ATM Network Demonstrator
- ATM Wireless Access Communication System
- International joint ventures
Wireless ATM Network Demonstrator
The objectives of this project are:
To specify a wireless, customer premises, access system for ATM networks that maintains the service characteristics and benefits of the ATM networks to the mobile user
To promote the standardization of wireless ATM access
To demonstrate and carry out user trials and test the feasibility of a radio based ATM access system 25
For example, the Magic WAND project (Wireless ATM Network Demonstrator) 7 covers the whole range of functionality from basic (wireless) data transmission to shared multimedia applications in Europe. The primary goal of the project is to demonstrate that wireless access to ATM (capable of providing real multimedia services to mobile users) is technically feasible. The project partners have chosen to use the 5GHz frequency band for the demonstrator and to perform studies on higher bit-rate operation greater than 50 Mbps in the 17GHz frequency band.
The aim of the user trials is to verify a wireless access system for ATM networks that maintains the service characteristics and benefits of ATM networks in the 5GHz range allocated to wireless high-speed data transmission. The feasibility of a radio-based ATM access system has also been demonstrated by the user trials with selected end-user groups in hospital (medical consultation) and office environments.
The medical consultation shows an advanced scenario fully exploiting the wireless ATM service capabilities in the hospital environment. The Joint Video Telecommunication Operating System (JVTOS) is being used with an x-ray viewing application, using both native audio and video services over ATM. In this scenario, doctors are equipped with a mobile terminal while visiting patients. With the help of a wireless ATM connection, doctors are able to retrieve patient information from the network, consult expert doctors, and share documents. The setup is shown in Figure 4 25.
Figure 4 The Magic WAND setup.
Wireless ATM extends all the benefits of the ATM and, therefore, also the ATM signaling and virtual channels/paths into the mobile terminal, raising important issues that have to be solved both in the wireless access interface and in the supporting customer premises ATM network. In the air interface, the wireless ATM transmission is subject to the problems associated with the radio medium, and, therefore, special radio design measures are required in order to offer users an adequate level of service. These measures constitute some of the major technical challenges of this project.
The main result of the project is a Wireless ATM Access Network Demonstration system that serves as a proof of concept for the developed technology and help the wireless ATM standardization work. The current achievements of the project include the complete functional system specification on the demonstrator that has been specified with the Specification and Description Language (SDL) and verified with the simulation model. In addition, the project has defined the exact demo platform setup and, therefore, has enabled the basis for the implementation work that has been started on all parts of the system.
Besides demonstrator work, the project has been active in its liaison and standardization activities. The stochastical radio channel model for channel simulations was developed and verified by measurements on 5GHz and 17GHz frequency bands. The model has been given as an input (for signal level 1 [SIG1] work). Furthermore, the project has been active in the standardization forum by contributing and harmonizing the work between the ATM forum and ETSI RES10.
The Magic WAND project has continued the work on gaining knowledge on the wireless ATM radio design and its medium access-control functions as well as wireless ATM-specific control and signaling functions. These results have been and will continue to be contributed to the ETSI and ATM forum in order to influence all of the relevant standards for wireless ATM systems.
Wireless Access Communication System
The objectives or goals of the ATM Wireless Access Communication System (AWACS) project are the development of a system concept and testbed demonstration of public access to B-ISDN services. The system offers low-mobility terminals operating in the 19GHz band with a support of user bit rates up to 34Mbps with radio transmission ranges of up to 100 meters. The demonstrator of ATM Wireless Access (AWA) pre-prototype equipment provides immediate propagation data, basic encoding rules (BER), and ATM performance at 19GHz. Based on this information, enhancement techniques for AWACS support cellular as well as spectrum- and power-efficient radio-access technologies associated with HIPERLAN type 4 specifications.
Basic encoding rules are rules for encoding data units, as described in the ISO ASN.1 standard.
Furthermore, the AWACS technical approach is centered on a testbed and associated trial campaign program. Trials are conducted using the existing ATM wireless access platform made available to the project by one of its partners. An associated program of work is directed on enhancing this current state-of-art system toward the final target features of the emerging ATM wireless specifications; in particular, HIPERLAN type 4 is currently being defined by ETSI-RES10. These enhancements to the existing demonstrator are considered in the following areas:
Application of source/channel coding and intelligent antennas
Optimization of link-layer protocols to match ATM bearer types
Feasibility of 40GHz radio frequency (RF) technology for ATM wireless LAN applications
Mobility-management techniques together with the impact on the radio bearer appropriate for high-bit-rate communications 25
Radio frequency is a generic term referring to frequencies that correspond to radio transmissions. Cable TV and broadband networks use RF technology.
The AWACS field trial covers the concept of virtual office trials. This includes three potential cases, depending on the technical capabilities of the demonstrator:
Wireless multimedia communication link between an engineer at the production site and an expert at this office
Video communication in meetings between physically separated sites
Visual, wireless network access to virtual office facilities at a partner's location
The objectives of these trials are summarized as follows:
Improves communication between physically separated offices by telepresence technologies
Reduces the need to travel between the geographically separated offices
Improves the response time of expert advice in problem solving by visual communications
Frees staff from fixed office hours 25
The key issues to be considered include:
The performance evaluation of a 19GHz ATM-compatible modem
Identification of the strengths and weaknesses of the existing ATM wireless experimental demonstrator
Investigation of possible enhancement to the ATM-compatible modem
AWACS field trials with the concept of virtual office, which aims to improve the communication between physically separated offices by telepresence technologies 25
The AWACS demonstrator based on ATM in packet-transmission schemes supports limited, slow-speed mobility as it is in line with expected use of high-data services. Therefore, the project generally covers the following directions, which are open to developers of mobile communication systems for the future: construction of a wireless system providing seamless service in connections to hardwired systems (quality-oriented system) and services making the most of the excellent mobility and portability of mobile communication systems (mobility-oriented system).
The AWACS trials indicate the capacity of the available system in a real user environment. The results of the trials contributed to the development of common specifications and standards such as ETSI-RES10 (for HIPERLAN type 4 specifications), ITU, Telecommunication Technology Committee (TTC), and Association of Radio Industries and Businesses (ARIB)8 in Japan.
The Telecommunication Technology Committee (TTC) was established as a private standardization organization in October 1985 to contribute to further activation of the field of telecommunications, in which the free-market principle was introduced based on implementation of the Telecommunication Business Law in 1985, and to respond to the Japan/U.S. Market Oriented Sector Service (MOSS) Conference, which was held in the same year.
International Joint Ventures
Wireless ATM has started, and there is a worldwide effort to unify and standardize its operation. The Public Communication Networks Group of Siemens AG 9, Newbridge Networks 10, and Broadband Networks, Inc., (BNI) 11 recently announced an extensive joint research and development program to address the digital wireless broadband networks market. The three enterprises will focus on integrating BNI's broadband wireless technology with the Siemens/Newbridge Alliance's MainStreetXpress family of ATM switching products to develop wireless network base stations that are fully compatible with wireline services.
BNI has already deployed terrestrial wireless networks that provide wireless cable in a digitally compressed MPEG2 (Motion Pictures Experts Group) format, delivering laser diskquality transmissions with the capacity for hundreds of channels. The Siemens/Newbridge Alliance offers carriers the most comprehensive suite of ATM products and the largest ATM core infrastructure switch, scaleable up to 1Tb and beyond. The introduction of ATM into the broadband wireless environment will enable network operators to cost effectively deploy high-capacity access services such as high-speed data, broadcast (cable) distribution, and Internet access in the 28GHz range. By incorporating both MPEG2 and ATM into the broadband wireless environment, the network solution provided by BNI and the Siemens/Newbridge Alliance ensures high-speed, high-quality, and high-capacity video, voice, and data transmissions. It also represents an effective bandwidth allocation that ensures sufficient capacity for additional innovative residential and commercial services as they evolve.
Let's take a look at wireless communications hardware in the form of its functioning and applications for wireless communications. Diagrams (Figures 5 to 8) of various system configurations are also available.