A Long Range View of Short Range Wireless Systems
The development of short range wireless systems, particularly Bluetooth and wireless local area networks (WLAN) has captured the industry's imagination, if not the market that was initially predicted. Bluetooth technology originated in Europe, with early research and development driven by European-based companies. In this special supplement Microwave Journal reviews current European activity, worldwide expansion and globally competing technologies to discover whether going wireless comes with strings attached.
No wires -- what an attractive proposition! Consider the savings in cabling costs and flexibility offered if an office's computers were served by a WLAN. Just imagine being able to eliminate the tangled mass of wires currently necessary to connect a PC, not just to the network, but also to its peripherals such as the keyboard, mouse and printer. Meanwhile, the mobility of cellular and cordless technology has promoted ideas for a generic short range wireless access solution for various devices.
These are all desirable aims but the interest in and development of short range wireless data networking has not just been prompted by the need to disentangle office chairs from trailing wires. The real impetus has come from the desire and expectation of individuals and companies to be able to access data and information almost anytime, anywhere, any place. Laptop-based users and broadband access in homes are more of the elements converging to drive ideas of a short range wireless access solution as well. Ally that with the prospect of vast numbers of cell phones becoming Internet enabled with users wanting to link up to laptops, headsets, hands-free kits and LAN access points, and a lucrative market is assured provided that the technology is available to implement it.
With such a large and untapped market there has been no shortage of contenders vying to provide that technology. This article looks at two of the leading contenders, Bluetooth and WLANs. Issues covered include how Bluetooth has built on its European origins and early development to capitalize on Europe's Global System for Mobile Communications (GSM) to enable it and synergize with it, together with the opportunities that 3G could offer. By mapping WLAN development and global deployment it is considered as both a competing technology and growth market in its own right.
BLUETOOTH: AN OVERVIEW
Since Ericsson originally devised the technology in 1994 Bluetooth has grabbed the imagination and most of the headlines. The company continued working on the project alone until February 1998, when it shared its research with Nokia, Intel, IBM and Toshiba to found the Bluetooth Special Interest Group (SIG). The main purpose of the SIG is to protect the integrity of the technology and control its development. It is responsible for the certification process that all devices must complete before they can be acknowledged as having a Bluetooth compliant product. Without certification, a product cannot claim to be Bluetooth-enabled or use the Bluetooth trademark. The certification process ensures that developers keep to the standard and ensure interoperability.
The commercial specification, Bluetooth 1.0, was issued in July 1999 and ratified in February of this year. The growth of activity in the technology is illustrated by the fact that there are currently some 2000 companies working on or developing products based on this specification. From its European origins -- it is named after a 10th century Norwegian King -- Bluetooth has inevitably become of global interest to both manufacturers and potential users.
The attraction is that Bluetooth can offer low cost, small physical size (single chip) and low power consumption over throughput and range. Allied to its capability to function in noisy radio environments and offer high transmission rates. These features, together with support for real-time traffic of both voice and data, make it an attractive wireless networking technology for personal digital assistants (PDA), cell phones and laptops.
Licensed spectrum is expensive, particularly in Europe (> $100 billion paid for 140 MHz). A major appeal of Bluetooth is that it operates at the internationally available unlicensed industrial, scientific and medical (ISM) 2.4 GHz frequency band, enabling worldwide compatibility. Figure 1 shows the European 3G spectrum cost vs. the WLAN spectrum (83.5 MHz in the 2.4 GHz band and 455 MHz in the 5 GHz band) at no cost. Bluetooth wireless technology operates in a multiple piconet topology (see Figure 2) that supports point-to-point and point-to-multipoint connections. With the current specification, up to seven slave devices can be set to communicate with a master radio in one device. As Figure 3 illustrates, several of these piconets can be established and linked together in ad hoc scatternets to allow communication among continually flexible configurations. All devices in the same piconet have priority synchronization, but other devices can be set to enter.
Bluetooth's baseband technology supports both synchronous connection orientated (SCO) links for voice and asynchronous connectionless (AC) links for packet data. Both utilize time division duplex (TDD) as the access technique for full duplex transmission. Voice coding is accomplished using a continuously variable slope delta (CVSD) modulation technique, under which voice packets are never retransmitted. The master unit controls the link bandwidth and decides how much bandwidth to give to each slave and slaves must be polled before transmission.
An asynchronous channel that transmits data can support an asymmetric link of 721 kbps in either direction and permit 57.6 kbps in return. For a symmetric link the channel can support 432.6 kbps. Since Bluetooth devices can support three voice channels operating at 64 kbps, or one data channel, they can achieve data rates of up to 1Mbps. The Bluetooth 1.0 specification calls for 1 mW transmitters with a nominal antenna power of 0 dBm to operate up to 10 m (line of sight). A higher power transmitter of 100 mW (+20 dBm) included in the specification will increase the range to 100 m, although this will require a separate PA antenna driver. The compromise is increased costs and power consumption.
Bluetooth utilizes frequency hopping spread spectrum (FHSS) technology, where the system will frequency hop 1,600 times a second, delivering short time division multiplexed packets with each hop. With spread spectrum hopping, the sequence is random and the receiver must hunt down the chosen transmission frequency after each hop. Before any connections in a piconet are created, all devices are in standby mode which allows for the device to listen on 32 hop frequencies defined for each unit, for messages every 1.28 seconds. The connection begins when one device initiates a connection and becomes the master of the piconet. A connection is made by a page message if the address is known, and if it is not then an inquiry message followed by a page message is sent. The devices synchronize and then connect. At the point of connection each device assumes a media access control (MAC) address to distinguish them.
The Bluetooth technical specification may be clear, product roll-out less so. The marketing machines did their job in creating awareness but in the process raised expectations that have yet to be fulfilled. All too quickly allegations, particularly in the media, of over hype and over elaborate market forecasts were hitting the headlines. However, last year saw a significant number of product launches together with the initial shipments of products bearing the Bluetooth logo. There has been consolidation for the first half of this year with the end of 2001 seeing significant predictions.
Frost & Sullivan forecasts global shipments of Bluetooth-enabled products to reach over 11 million units in 2001, equaling $2.5 billion in revenues, while Micrologic Research is more conservative with its estimation that the market will reach five million devices in 2001 and 1.2 billion in 2005. Such variations in figures tend to muddy the waters and emphasize the unpredictability of the market, but in such an embryonic technology this is perhaps understandable.
This is a point made by Michael Wall, research analyst at Frost & Sullivan, who has stated: "Although the delays in the development of Bluetooth are beginning to prompt a backlash from certain sections of the media, industry observers have to take the infancy of Bluetooth as an industry standard technology into consideration when assessing the status of this marketplace. Apart from Ericsson, the original pioneers, even the most progressive developers were not attracted to the project until 1998. Other mobile communications technologies such as the GSM took longer to develop than is being allowed for Bluetooth."
Semiconductor chipset development is a key element in the technology's progress, with a range of development models emerging within the Bluetooth semiconductor industry. Two distinct manufacturing routes are being taken. There are either those offering complete integrated solutions from the silicon wafer level to the consumer product level or those providing part of the sum of a chipset, that is, baseband, radio and software.
Debate continues over the most effective choice of silicon technology for Bluetooth. The diversity of silicon technologies and solutions architectures being used has emphasized the software protocol stack. It has become one of the most crucial elements of the solution, especially with regards to achieving interoperability and will become increasingly important as semiconductor companies come closer to launching their products onto the market.
Alongside some of the big names a number of smaller design services companies have entered the Bluetooth software market offering complete or partial protocol stacks to semiconductor developers. In the same vein Bluetooth has offered a number of smaller, highly innovative fabless semiconductor developers, such as Cambridge Silicon Radio and Silicon Wave, an opportunity to build early market share with fast time-to-market solutions. Amongst the larger integrated Bluetooth developers, Philips Semiconductors has been the main player to offer solutions in volume. It is expected that a large number of solutions will be on offer by the end of 2001.
Market success may be determined by a chicken and egg combination of chipset supply. Observers have warned that restrictions in the supply of chipsets to smaller product developers may cause delays in the time-to-market of new innovative applications that will provide future revenue streams for chipset suppliers. Despite such words of caution Frost & Sullivan forecasts that the total shipments of Bluetooth chipsets will be over 956 million in 2006, and the total market for these chipsets is predicted to be over $2.3 billion in 2006. Further up the value chain from chipsets the early Bluetooth offerings are fairly generic wireless network access products, such as PC cards and other add-on devices, together with access points (AP).
Also, in Europe, a significant number of Bluetooth mobile phones were launched at the CeBIT exhibition in Germany in March 2001 with many more expected over the summer. However, the market cocktail has become more intriguing because of 3G market developments. At a time when the huge cost of 3G licenses is impacting on the telecoms stock market and the equipment required to roll-out Universal Mobile Telecommunication System (UMTS) networks has not yet come to fruition, many of the services planned for 3G mobile could be delivered by currently available technologies which operate in unlicensed (free) frequency bands.
Mobile operators who have 3G license debts to service are under pressure to maximize revenue of existing data services, and demonstrate that the market has the appetite for 2.5G and 3G services. Bluetooth mobile phones could be one solution by allowing users access to the Internet on their PDA using the phone as a wireless gateway. Ericsson, for instance, is promoting the Bluetooth Local Information Point (BLIP), which provides Bluetooth access to the Internet, within range of a BLIP access point. Such developments will continue to keep Bluetooth in the headlines and the public eye.
WLANs are emerging from the wings as a strong contender to rival Bluetooth. WLANs enable the Ethernet cable from the wall outlet to a device (such as a PC) to be replaced by a wireless link between an access point and a wireless interface card that is either part of the wireless device or plugged into it. The technology is in no way a newcomer, however. In fact, it was back in 1990 when, in the US, the IEEE 802.11 Wireless Local Area Networks Standards Working Group was formed with the task of developing a global standard for radio equipment and networks operating in the 2.4GHz unlicensed frequency band for data rates of 1 and 2 Mbps.
Over a decade ago what the original 802.11 standard did, to a degree, was to help unify a confused WLAN marketplace, which was crowded with proprietary solutions. Although the original specification supported three different transmission media frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS) and infrared (IR) the major area of development has been for DSSS. DSSS spreads the signal over several frequencies, can switch channels to avoid interference and also makes the signal harder to intercept than standard wired Ethernet.
The IEEE 802.11 standard was adopted in 1997. The modulation scheme used when operating at the 1 Mbps rate is binary phase shift keying (BPSK) where each symbol carries one bit and one million symbols per second (1 Msps) are transmitted. Thus, with each symbol storing one bit, the bit-rate achieved is 1 Mbps. Quadrature phase shift keying (QPSK) is the modulation scheme used to yield 2 Mbps. With this technique the system is able to transmit two channels simultaneously, and although the symbol rate is still 1 Msps with QPSK mapping two bits per symbol, the result yields 2 Mbps. However, these data rates of 1 Mbps and 2 Mbps are significantly slower than the wired LAN equivalents. This aligned with questions over interoperability and price, limited take up and acceptance of the standard as a viable option.
That all changed in September 1999 when the IEEE ratified a new high rate standard for WLANs IEEE 802.11b, which also goes under the various guises of WiFi (Wireless Fidelity) and high rate wireless Ethernet. It is significant because it offers a top-end data rate of 11 Mbps. Each access point can support dozens of connections, although they all must share 11 Mbps of capacity. There can be three access points working in the same area, and each typically has an indoor range of 90 m at 1 Mbps and 25 m at 11 Mbps. To achieve this higher data rate the IEEE 802.11b specifies complementary code keying (CCK) as the modulation scheme. The technique maps four bits per symbol to achieve 8 Mbps, which allied to an increased rate of 1.375 Msps yields a bit rate of 11 Mbps. Therefore, while the number of symbols sent per second hardly varies from the symbol rate used for IEEE 802.11 LANs, more bits per second are sent. Also, as CCK is a DSSS technique, 802.11b is backward-compatible with products that meet the original 802.11 specification, enabling 802.11b standard products to interoperate with 802.11 compliant DSSS products by falling back to 1 Mbps or 2 Mbps operation.
With an industry body to verify interoperability and the interoperability of 802.11b cards being assured, due to there being just two silicon manufacturers worldwide using a similar MAC layer specification, that deficiency in the WLAN offering has been addressed. The increased bit rate of 11 Mbps has also dealt with the performance issue with 802.11b being able to match standard Ethernet for speed. This has resulted in a renewed interest in, and perhaps more importantly, investment in the development of 802.11b products by large players who did not view any involvement in 1 to 2 Mbps products as a viable option.
Now, the benefits that WLANs offer in terms of mobility and flexibility, allied to increased speed and the falling costs of PC cards, has made it an attractive option for the home market where broadband access is growing for small businesses and particularly for the enterprise customer. Typical applications include the creation of ad hoc LANs, the linking of portables into a wired infrastructure, WLAN bridging and in peer-to-peer networks where PCs with wireless cards can exchange data directly. Alternatively, an access point allows PCs to communicate with fixed Ethernet topologies via an Ethernet hub or switch port. Although WLAN cards are still far more expensive than ordinary cable-based Ethernet cards, having a standard means that all manufacturers move to the same technology and prices come down. Today there are cards at around the $200 mark.
The key to the progress of WiFi is its wide and global deployment, and without any hype it has begun. Airports as far afield as Europe, Japan, Hong Kong and the US have installed 802.11b networks, with hotels and conference centers also being prime areas of development. Additionally, with the increased use of laptops, the natural synergy between their mobility and the mobility offered by WLANs is propelling the growth of 802.11b. Offering mobility is going to be the key to success of WiFi. For instance, when users have a notebook, they want to be able to use it in the office, at home and on their travels without having to swap cards. Only a wide deployment of 802.11b will facilitate that.
Mobile operators also see WLANs as a cheap and easy way to provide high speed access to densely populated areas. Because they rely on very short-range transmissions, users see improved battery life, and with health risks being a concern there is the added advantage of lower power usage. Again, at CeBit there were a large number of equipment suppliers showing WiFi components in the form of PC cards, universal serial bus (USB) devices, access points and home gateways. However, at present the Wireless Ethernet Compatibility Alliance (WECA) only recognizes one test house in the US for certification of WiFi products with plans for a European test house to be recognized soon. Such expansion is vital for the technology to be viewed as truly global in terms of development.
The key factor in the growth and development of the WLAN market has been the increased data rate of 11 Mbps being afforded by the 802.11b standard. However, in October last year the IEEE Standards Board approved P802.11g, a new project within the IEEE 802.11 WLAN Working Group to enhance the data rate of WLANs operating in the 2.4GHz frequency band. The expectation is that the data rates will be increased to greater than 20 Mbps and the mission of the task group is to review proposals. Areas of development currently being undertaken which could afford this 'doubled' data rate include a new modulation technology that improves the robustness of RF data transmissions. It not only overcomes much of the background RF noise and other sources of interference but also provides better performance against multipath interference.
On the receiver side, advanced equalizer technology used in concert with these new modulation algorithms will act to reduce the need to retransmit data packets. This is important because when interference in WLANs causes unrecoverable corruption of a reflected data stream or noisy signals are discarded and are retransmitted which slows the data rate and interrupts the data flow, the system is less reliable for real time transmission. With advanced equalizer technologies, reflected or noisy signals are not simply discarded or filtered out. Forward error correction (FEC) algorithms can take corrupted signals and reconstruct them, significantly reducing retransmits.
Data rates of over 20 Mbps will open up new applications for the industry to exploit. As might be expected, interest will most likely be led by leisure applications. Faster transmission speeds will enable streaming video for high definition television and graphics for interactive gaming while also providing the headroom to accommodate new applications when they come on stream. Businesses and enterprises are always screaming out for the means to transmit large amounts of data quickly. Home automation will be another avenue by facilitating the interaction of heating, lighting, air conditioning and security systems.
THE WLAN MARKET
Such applications may be some way off but the WLAN is a growing market as the statistics show. According to the latest figures from IDC worldwide WLAN equipment revenue jumped 80% in 2000, breaking the $1 billion mark. IDC predicts that by the end of 2005 the market will be approaching $3.2 billion. Demand, especially in the US, has been particularly strong in vertical industries such as education, retail and health care. In the coming years, the market will see increased use of WLANs in the home and small- to medium-sized business (SMB) segments together with the growth of broadband. Despite the optimistic outlook for the overall market, particularly in the US, Western Europe and Japan, IDC believes vendors will have to overcome several obstacles, including resolving standardization issues, educating their partners, improving security and reducing prices so that WLANs are affordable for mainstream segments.
The chipset market for 2.4 GHz WLAN products is set to continue to expand, although growth will not be as high as for Bluetooth chipsets. Frost & Sullivan anticipates direct sequence 802.11b chipsets to be in great demand, predicting that the market for them will be worth over $1.3 billion in 2006. This demand will be driven by the growth in mobile computing and by falling product costs.
Bluetooth and WLANs may have differing profiles in terms of marketing and publicity but it is clear from the market statistics and investment in technical development that both are technologies that are becoming established and set to grow. However, can they coexist technically? Interference has been a topic of debate and concern since the early stages of Bluetooth development and to a certain extent it has become a fear of the unknown. What is known is that interference between 802.11b and Bluetooth devices can occur. In the US the Federal Communications Commission (FCC) requires every device operating in unlicensed bands to have a label stating that it can cause interference. However, what is not known is the potential of the problem. The fact that the devices operate in an unlicensed band and projections of mushrooming market growth for Bluetooth and 802.11b is fueling concerns.
Although the level of concern may turn out to be unwarranted, it has at least grabbed the attention of wireless standards groups, regulatory bodies and wireless industry participants. They are all well aware that if users do experience interference problems it will damage user confidence in the technology. With so much investment it is a risk that manufacturers, in particular, cannot take. Global technical development work is being carried out and standards are being addressed to limit interference. In the US the IEEE 802.15.2 Task Group is coordinating efforts, and the FCC has also put together a set of rules that allow multiple devices to share the spectrum, providing room for considerable innovation in building radios that can resist interference.
Consequently, extensive research to monitor the effect that WiFi and Bluetooth devices operating in the same vicinity have on one another is under way. Results do vary and Figures 4 and 5 are examples of a particular study to illustrate the effect. However, what is generally accepted is that if the antennas of the Bluetooth and WiFi devices are kept over 2m apart, then there will be graceful degradation of the two protocols, which will only be noticed by very sensitive users. Move the two antennas within a meter, however, and there can be significant interference.
Interference really becomes a serious issue when both radios are integrated into the same device with the antennas close together. Examples of when the two devices are collocated (that is, separated by less than 10cm) are in a combination PC card and laptops or Internet appliances enabled with both technologies. Also, it is believed that collocated products will play an important role in devices such as notebook PCs. An example is a notebook that has a Bluetooth radio integrated for connection to a PDA or cell phone and at the same time has a WiFi radio integrated for LAN access.
Coexistence is a major issue for such applications and one which the industry is striving to address with standards bodies and wireless companies starting to develop and lobby for a variety of coexistence approaches. These vary from regulatory intervention and special standards task forces such as IEEE 802.15.2 to various technical approaches ranging from simple device 'collocation without any coexistence mechanisms' to integrated silicon solutions covering the entire wireless sub-system.
Mobilian Corporation, together with industry partners, is a company working on developing a solution and has categorized these various technical approaches into a performance and user experience hierarchy, as shown in Figure 6, with each having their strengths and limitations. 'Collocation without a coexistence mechanism' is relatively controversial. It does have the advantage of being a rapid time-to-market approach which provides a single-card reference design only. The close proximity of the two radios with no coexistence mechanism will likely produce worst-case scenarios, and can consequently result in significant degradation to both radios' performance.
Dual-mode radio switching does not require changes to the silicon, and could be relatively quick to market. It incorporates a coexistence mechanism that requires that while one radio is operational, the other is completely suspended. The operation can be implemented primarily in two ways. In the first, the system simply shuts the non-operating radio off with no signaling to other nodes in its network. This can result in difficulties for the network and can drop performance levels below that of simple 'collocation without a coexistence mechanism.' The second method does signal other network nodes that it is suspending one of its radios. Performance will still be 60 percent lower than that of unhindered radios because of its modal nature (one on/one off), but is better than simply shutting the radios off. Neither method supports switching while Bluetooth voice (SCO) links are in operation.
Driver-level transmit switching generally describes an approach in which application transmit requests are mediated at the driver level, thereby avoiding simultaneous transmission. Intuitively, this approach degrades throughput by some measure simply due to its modal transmit structure. More important, though, are its limitations in avoiding collisions with incoming packets. The resulting transmission of one protocol during reception of the other causes loss of received packets, interference and potential user difficulties. This is caused by the technique's dependence on the host operating system, which is generally non-deterministic in its response time (non-real-time). Again, this approach does not switch quickly enough to support Bluetooth SCO links, and will also have difficulties mitigating the interference from Bluetooth piconet master/slave polling activities.
While Bluetooth adaptive hopping certainly improves simultaneous performance under limited penetration scenarios, its widespread adoption will likely require intervention from regulatory organizations and standards bodies. Even under a fast-track program, this can be a time-consuming process. This time-delay exacerbates the problem that the technique's effectiveness is compromised with higher penetrations of WiFi systems and unmodified Bluetooth devices. Adaptive hopping will likely be initiated as an optional Bluetooth profile, indicating that modified devices will not use the functionality in piconets with unmodified devices. Further, in the presence of more than one Bluetooth piconet or WiFi network, adaptive hopping can be counter productive to coexistence.
MAC-level switching is the most effective of the modal/switching style approaches, and provides performance levels approaching those in no-interference scenarios. It is a collaborative technique accomplished by exchanging information between the two protocols at the MAC level and managing transmit/receive operations accordingly. Because MAC-level switching is performed in the baseband, it is able to switch between protocols at a much faster rate than driver-level approaches. Consequently, it is able to mitigate many of the problems that driver-level switching cannot. MAC-level switching does not suffer from transmitting signals into incoming receptions, Bluetooth polling or operating system latency. However, it is susceptible to adjacent-channel interference and does suffer noticeable degradation. Also, because it is located in the baseband, it has a longer development cycle than driver-level approaches.
Simultaneous operation provides the ability to automatically detect all available wireless networks, select the ones needed and connect to them seamlessly. By providing coexistence in a highly integrated two-chip solution an analog front-end chip and a digital baseband chip it allows simultaneous operation of the two protocols while eliminating interference and maintaining reliability and performance. Interference is a genuine concern and, as has been illustrated, there are measures that can be taken and innovative initiatives under development to provide coexistence particularly for collocated devices. The potential market is too large and too lucrative for every effort not to be made to ensure smooth operation.
BLUETOOTH vs. WLAN APPLICATIONS
Bluetooth and WLAN may be competing in the same frequency band but are they competing for the same applications? Due to its simplicity in not having to be configured, low power, short range and low cost Bluetooth will be focused on small devices such as PDAs and cell phones. To provide access and synchronization of those personal devices there will also be the need for Bluetooth radios to be incorporated in access points and notebooks.
Another possibility that Bluetooth affords is the deconstruction of devices into individual components, allowing for new form factors and device types. For instance, by having a separate headset there is no longer the need to include one in a cell phone, which simply becomes a cellular receiver/transmitter interacting with the cellular network, PDAs and laptops. More long-term, a so-called killer application for Bluetooth could well be public access. It is all very well to have synchronization between the notebook, PDA or cell phone but, when in an airport or shopping mall, access to the Internet or information about the local area would be valuable. For that to happen, though, there is the chicken and egg situation where a company is not going to deploy Bluetooth enabled access points unless there are significant numbers of devices in the marketplace to use them and vice versa. The same goes for the providers of the information that users will be seeking. Nevertheless, this is an area actively being developed.
Public access is a definite application for WLAN and, as has been mentioned, systems are already being globally deployed in airports. Their high data rate being comparable to the wired Ethernet makes them particularly suitable for the enterprise sector for computer networking between PCs and to take advantage of the trend towards laptop mobility. Simplicity, low cost and the facility for expansion also make WLAN suitable for small office home office (SoHo) implementation and the expansion of the home broadband access market, particularly in the US, also opens up opportunities.
THE 5 GHZ FREQUENCY BAND
Even if just a fraction of these applications for Bluetooth and WLAN come to fruition, the narrow (80 GHz) 2.4 GHz band will soon become congested. In anticipation of this, spectrum will play a crucial role in the deployment of next-generation, high speed WLANs and has prompted licensing authorities globally to allocate large blocks of license free spectrum in the 5 GHz band. As Figure 7 shows, in Europe, a total of 455 MHz is available in the two blocks from 5.15 to 5.35 GHz and from 5.470 to 5.725 GHz. Similarly, the US has allocated a total of 300 MHz in the two blocks of spectrum at 5.15 to 5.35 GHz and 5.725 to 5.825 GHz. In Japan, one 100 MHz block at 5.15 to 5.25 GHz is being considered.
Again two different 5 GHz standards are being developed on either side of the Atlantic with both specifications offering data rates of up to 54 Mbps, and therefore well placed to provide high speed communication services. Originating in the US the IEEE 802.11a standard was ratified in 1999. The physical layer (PHY) is based on orthogonal frequency division multiplexing (OFDM) and shares a common MAC layer with all IEEE 802.11 standards including 802.11b.
Alternatively the European Telecommunications Standards Institute (ETSI) is developing high performance radio LAN (HIPERLAN) standards as part of its Broadband Radio Access Network (BRAN) initiative. Under its remit is the development of four standards -- HIPERLAN1, HIPERLAN2, HIPERLink (designed for indoor radio backbones) and HIPERAccess (intended for fixed outdoor use to provide access to a wired infrastructure).
The HIPERLAN1 standard, which is based on the well-established technique of Gaussian minimum shift keying (GMSK) modulation, is complete and was ratified in 1997. HIPERLink and HIPERAccess, on the other hand, are at the early stages of development. It is HIPERLAN2 where current activity is focused.
The physical layers of both 802.11a and HIPERLAN2 use OFDM modulation to achieve high speed transmission rates. This multichannel spread spectrum modulation technique allows individual channels to maintain their distance (or orthogonality) to adjacent channels, enabling data symbols to be reliably extracted and multiple subchannels to overlap in the frequency domain for increased spectral efficiency. For instance, in the spectrum allocation for Europe, HIPERLAN2 channels will be spaced 20 MHz apart for a total of 19 channels.
Both IEEE 802.11a and HIPERLAN2 specify an OFDM physical layer that splits the information signal across 52 separate sub-carriers. 48 provide separate wireless pathways for parallel data transfer, while the remaining four are used as a reference to disregard frequency or phase shifts of the signal during transmission and provide synchronization. Synchronization enables coherent (in-phase) demodulation. The two standards may have this similarity but differ above the physical layer with 802.11a generally viewed as simpler and less complex, while HIPERLAN2 is more sophisticated (or complicated depending on your viewpoint) with wider scope.
For HIPERLAN2, mobile terminals such as a laptop or handheld devices communicate with access points. To provide continuous coverage, these access points must have overlapping coverage areas. Coverage typically extends 30 m indoors and 150 m in unobstructed environments. By utilizing automatic frequency allocation (AFA) access points monitor the HIPERLAN radio channels around them and automatically select an unused channel. A mobile terminal, after association, will only communicate with one AP at each point in time, but if it receives a better signal strength it can request to be connected to another. When a mobile terminal roams from the coverage area of one access point to another, it automatically initiates a handoff to the new access point. The APs involved in the handover ensure that established connections over the air interface as well as security associations are transparently shifted from the old to the new. Security support includes both key negotiation, authentication (conventions such as the network access identifier (NAI) and X.509 can be used), as well as encryption using DES or 3-DES.
OFDM modulation can supply transmission rates of 54 Mbps but this can be dynamically adjusted to a lower rate by using different modulation schemes depending on the prevalent radio conditions. All traffic is transmitted on connections, bi-directional for unicast traffic and uni-directional towards the mobile terminals for multicast and broadcast traffic. This approach makes support for quality of service (QoS), implemented through time slots, straightforward. QoS parameters include bandwidth, bit error rate, latency and jitter. The original request by a mobile terminal to send data uses specific time slots that are allocated for random access. The access point grants access by allocating specific time slots for a specific duration in transport channels. The mobile terminal then sends data without interruption from other mobile terminals operating on that frequency. A control channel provides feedback to the sender, indicating whether data was received in error and whether it must be retransmitted. The QoS delivered depends on how the HIPERLAN2 network interoperates with the fixed network; for example, if it is via packet-based Ethernet, cell-based ATM or IP.
HIPERLAN2 operates as a seamless extension of other networks, so wired network nodes see HIPERLAN2 nodes as other network nodes. All common networking protocols at layer 3 (IP and IPX, for example) will operate over HIPERLAN2, permitting all common network-based applications to operate, making the technology both network and application independent. Interoperation with Ethernet networks is supported from the beginning, but easy extensions also provide support for ATM, PPP, IP and UMTS. The standard has been specified with the clear objective of achieving interoperability and the industry consortium, HIPERLAN2 Global Forum (H2GF), aims to run tests to verify interoperability among products from member companies.
The most obvious application for HIPERLAN2 will be in the enterprise LAN environment but networks can also be deployed at 'hot spot' areas such as airports and hotels, supplying remote access and Internet services to business people. Its ability to act as an alternative access technology to 3G cellular networks is also a key application. As the high throughput and QoS features of HIPERLAN2 support the transmission of video streams in conjunction with datacom applications, HiperLAN2 has potential applications in the home by creating a wireless infrastructure for home devices (for connecting home PCs, VCRs, cameras and printers, for example).
HIPERLAN2 almost sounds too good to be true and price-to-market is a concern. For instance, the higher cost of silicon for OFDM operation could stall reasonably priced implementation. At present, costs remain relatively high for 5 GHz OFDM systems, mainly due to the high linearity demands that it places on the power amplifier in the transmitter and the low noise amplifier in the receiver. Consequently, HIPERLAN2 products will likely cost more than lower speed alternatives. Also, some view the fact that HIPERLAN2 is sophisticated and able to support a wide range of applications not necessarily as a selling point but as overkill that comes at a price.
On the other hand, IEEE 802.11a, due to its simplicity and maturity, represents lower costs and a faster time-to-market. However, although 802.11a and HIPERLAN2 have a near identical physical layer, they differ in the MAC layer. Deficiencies include built-in quality of service, guaranteeing performance in work environments and when streaming home video. Therefore, efforts to close the MAC gap are a priority. Moreover, whereas the IEEE 802.11a and HIPERLAN2 both meet US regulatory spectrum requirements, HIPERLAN2 is currently the only 5 GHz WLAN that meets European interference avoidance restrictions. Conversely, HIPERLAN2 must restrict the frequency range and power for the US to comply with FCC rules.
The danger is obvious with the possibility that the US and Europe will embrace two different standards. The consequence that the corporates' inability to use one standard and benefit from lower acquisition and support costs could delay deployment of 5GHz wireless LANs significantly. It is a serious issue for global development because they are two incompatible WLAN standards. Thus, if 802.11a and HIPERLAN2 wireless terminals were operating in the same area, there would be interference, no coexistence and no interworking. Also, no roaming would be possible if different access points were deployed in different public areas. The end user will be forced to make a standards choice and the 5 GHz WLAN market is in danger of being fragmented if different industry players adopt different standards.
To avoid this several industry partners have started a 5 GHz industry advisory group. In the HIPERLAN2 ETSI BRAN group and 802.11a Forum there are sub groups specifically looking at what is required to get to one standard. At present, there is much work to be done.
Over the last few years the short range wireless data networking headlines have been dominated by Bluetooth, resulting in unreasonably high expectations. What tends to be forgotten is that, in relation to the development of similar technologies, Bluetooth is still embryonic. It is also a victim of its own potential. Articles on the subject wax lyrical about the possibility of consumer appliances being Bluetooth-enabled to have the capacity to 'talk' to each other and the merits of so-called 'hidden computing' applications. These will allow synchronization of laptops, PDAs and mobile phones to automatically update calendars, appointments and e-mail when within range. Envisaged industrial applications include the wireless monitoring of transported goods and chemical processes.
However, most of the early applications are essentially cable replacement or connection substitutes primarily aimed at the cell phone and PDA markets. The industry needs to walk before it can run so it should be, and to a great extent is, concentrating on steady development and addressing ways of ensuring interoperability, standardization and coexistence issues. Bluetooth has its origins in Europe with its initial development concentrated in Scandinavia, and although it is truly a global technology, that is where its early deployment will be greatest. Bluetooth has attracted all the key players, investment is considerable and perhaps some of the hype is justified.
On the other side of the coin and the Atlantic, but in the same 2.4 GHz unlicensed frequency band, the IEEE 802.11b (WiFi) WLAN standard has been developed steadily without any razzmatazz. Its high data rate, together with the falling costs of PC cards, allied to the mobility and flexibility it offers has seen significant market growth. It is well placed to benefit from the rise in the use of laptops and growth in home broadband access. Globally, 802.11b networks are making inroads in 'hot spot' applications at airports, conference centers and hotels, and WiFi products are hitting the market. Again, issues of interoperability, coexistence and standardization are being addressed. However, although the establishment of a registered test house in Europe will aid acceptance, certification needs to be more widespread.
With the inevitability that the unlicensed 2.4 GHz band will become congested, the development of the 5 GHz band for next generation high speed WLANs is vital. However, the possibility of fragmentation, with separate standards being adopted in the US and Europe is a real threat to global development and could delay deployment significantly. A standards war will benefit nobody, possibly undermining confidence and making manufacturers wary of significant investment.
Going wireless has come with some strings attached but short range wireless systems have a long term future. Its ability to satisfy the industry's desire for seamless connectivity will ensure continued market growth and development.
The author would like to thank the following individuals and companies for their help in compiling this supplement:
· Mobilian Corporation, www.mobilian.com
· Vincent Vermeer, business development manager Wireless Connectivity Division, 3COM (Europe), www.3com.com
· Dr Jamshid Khun Jush, chairman of ETSI BRAN and senior specialist Wireless LANs at Ericsson, www.ericsson.com
· Martin Johnsson, chairman HIPERLAN2 Global Forum and WLAN product manager at Ericsson, www.ericsson.com/wlan
· Peter Bates, VP business development, www.bluesocket.com
· Andy Craigen, senior manager, Wireless Terminals Applications, Agere Systems
· Bob Heile, chairman IEEE 802.15 Working Group
· The organizers and speakers at the Wireless LAN conference in London in April 2001. Organized by EF-Telecoms, www.ef-international.co.uk
· Frost & Sullivan, www.frost.com
· Figure 2 and Figure 3 are taken with permission from presentations available on www.ieee802.org/15/