HiperLAN2 wireless LANs and the Bluetooth specification promise to revolutionize connectivity. By Peter Rysavy.
A lot has happened recently in the local wireless market, so this article will briefly survey these developments. Also, because HiperLAN and Bluetooth are not the only games in town, this article will stack them up against some of the competing technologies before examining them on their own merits.
In addition, home use for sharing peripherals and broadband Internet connections is shaping up to be the killer application. Increasing numbers of users are standardizing on laptops as their only computer, making mobile connectivity highly desirable. The market doesn’t stop there, however: Hundreds of millions of cell phones are about to become Internet-enabled, and users will want to link up to laptops, headsets, hands-free kits in cars, and LAN access points.
Clearly a need exists for low-cost, high-speed, local-area wireless connections, and numerous technologies are becoming available for these connections. The wireless LAN space has a variety of proprietary approaches, industry consortium standards (including OpenAir and HomeRF), IEEE 802.11 standards, and HiperLAN standards. The table contrasts the major technologies.
The HiperLAN1 standard is complete, and leading Wireless LAN vendor Proxim (www.proxim.com) is now delivering products based on it. HiperLAN1 offers the fastest route to market for a high-speed wireless LAN technology while minimizing the complexity of the radio technology.
HiperLAN1 uses Gaussian Minimum Shift Keying (GMSK) modulation, which is well understood and broadly used in Global System for Mobile Communications (GSM) cellular networks and CDPD. In contrast, HiperLAN2 uses a new type of radio technology called Orthogonal Frequency Division Multiplexing (OFDM), which imposes significant technical challenges.
Spectrum plays a crucial role in the deployment of next-generation wireless LANs. Currently, most local area wireless products operate in the unlicensed 2.4GHz band, which has several limitations: The band is only 80MHz wide, it mandates the use of spread spectrum technology, and wireless LAN users must not interfere with primary license holders.
Recognizing the limitations of the 2.4GHz spectrum, licensing authorities around the world have allocated large blocks of spectrum in the 5GHz band. In the United States, 300MHz is available from 5.15GHz to 5.35GHz and 5.725GHz to 5.825GHz. In Europe, 300MHz is available from 5.15GHz to 5.35GHz and from 5.470GHz to 5.725GHz. Japan is considering a similar allocation.
These broad blocks, combined with more lenient rules of operation, enable high-speed operation by large numbers of users. Both HiperLAN standards enjoy one key advantage over IEEE 802.11a in that they are approved standards for the European spectrum. IEEE 802.11a products may not be usable in Europe.
HiperLAN2 is being promoted by an industry group called the HiperLAN2 Global Forum (www.hiperlan2.com), which features such heavyweights as Bosch, Dell Computer, Ericsson, Nokia, Telia, TI, and Xircom. The HiperLAN2 standard will be completed in 2000, but the first HiperLAN2 products will not appear until 2001 and will not be widely available until 2002.
The most compelling feature of HiperLAN2 is its high speed, which is sometimes misleadingly quoted at 54Mbits/sec. The raw over-the-air rate is indeed 54Mbits/sec, but sustained throughput for applications is closer to 20Mbits/sec. Another key feature is support for QoS, which will be important for applications like video and voice.
The HiperLAN2 architecture provides for connections to multiple types of network infrastructures, including Ethernet (which will be the first one supported), IP, ATM, and PPP. Security features include authentication and encryption. An especially innovative feature is automatic frequency management, which significantly simplifies deployment. Each of these features is examined in the next section of this article.
With its combination of high speed and QoS, HiperLAN will open up entire new classes of applications, such as video signal distribution into homes.
As Figure 1 shows, HiperLAN2 defines a Physical layer and a Data-link layer. Above these is a Convergence layer that accepts packets or cells from existing networking systems and formats them for delivery over the wireless medium.
The first unique aspect of HiperLAN2 is OFDM. Though OFDM has been used before—in the European Digital Audio Broadcast (DAB) standard and in Asymmetric Digital Subscriber Lines (ADSLs)—it has never before appeared in a wireless LAN standard.
OFDM is extremely effective in a time-dispersive environment where signals can take many paths to reach their destinations, resulting in variable time delays. At high data rates, these time delays can reach a significant proportion of the transmitted symbol (a modulated waveform), resulting in one symbol interfering with the next in what is called “intersymbol interference.”
OFDM combats this by dividing a radio channel into multiple subcarriers and transmitting data in parallel on them (see Figure 2). The aggregate throughput ends up being the same, but the data rate of each subcarrier is much lower, making each symbol longer—thus practically eliminating the effect of the variable time delays. However, OFDM demands extremely linear power amplifiers, which increase the cost of the radio. Consequently, HiperLAN2 products will likely cost more than lower-speed alternatives.
In the spectrum allocation for Europe, HiperLAN2 channels will be spaced 20MHz apart—for a total of 19 channels. Each channel will be divided into 52 subcarriers, with 48 for data and four as pilots that provide synchronization. Synchronization enables coherent (in-phase) demodulation. Through digital signal processing, subchannels are divided through mathematical processing, rather than in the analog domain.
OFDM by itself does not fully describe the Physical layer. There is also the question of how data is encoded and the type of modulation used in each subchannel. Encoding involves the serial sequencing of data, as well as Forward Error Correction (FEC). Most lower-speed wireless LANs do not employ FEC, but HiperLAN2 provides multiple levels, each capable of protecting against a certain percentage of bit errors.
HiperLAN2 also employs multiple types of modulation. By dynamically adapting the FEC and modulation according to varying conditions, HiperLAN2 can transmit at higher data rates with a strong signal relative to noise; it can also transmit data at lower throughputs under adverse conditions.
The next layer is the Data-link layer. In HiperLAN2, the Data-link layer is connection-oriented, which differentiates it from other wireless LAN technologies. Before a mobile terminal transmits data, the Data-link layer communicates with the access point in what is called the signaling plane to set up a temporary connection. This connection approach permits the negotiation of QoS parameters like bandwidth and delay requirements. It also assures that other terminals will not interfere with the subsequent transmission.
By contrast, a mobile terminal that conforms to the IEEE 802.11 standards will communicate when the radio channel becomes available, and it may experience packet collisions from other terminals. It should be mentioned, however, that IEEE 802.11 does provide a separate mechanism for synchronous applications like voice.
HiperLAN2 implements QoS through time slots. 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. Collisions from other mobile terminals can occur in this random-access channel, but since these messages are brief, this is not a problem.
The access point grants access by allocating specific time slots for a specific duration in what are called 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 needs to be retransmitted.
Above the Data-link layer is the Convergence layer, which responds to service requests from higher layers and formats data as required. This layer supports both packet-based (Ethernet) or cell-based (ATM) communications. When implemented for Ethernet, the Convergence layer preserves Ethernet frames and uses either conventional best-effort communications or the IEEE 802.1p priority scheme.
HiperLAN2 also comes with Automatic Frequency Allocation (AFA). To provide continuous coverage, access points need to have overlapping coverage areas. Coverage typically extends 30 meters indoors and 150 meters in unobstructed environments. Access points monitor the HiperLAN radio channels around them and automatically select an unused channel. This eliminates the need for frequency planning and makes deployment relatively straightforward.
When a mobile terminal roams from the coverage area of one access point to another, it initiates a handoff to the new access point after detecting a better signal on another radio channel. The new access point obtains details of the mobile terminal’s connection from the old access point, and communications continue smoothly.
HiperLAN secures communications for a mobile terminal, creating a session (called an association) with an access point by first using a Diffie Hellman key exchange to negotiate a secret session key, then a mutual authentication process via either a secret key or a public key, if a PKI is available. Data traffic is encrypted using DES or Triple DES.
With these security mechanisms, communication over HiperLAN should be as secure—if not more so—as over a wired LAN.
In addition, Bluetooth enjoys wide industry support from approximately 1,400 companies that now belong to the Bluetooth Special Interest Group (SIG). And unlike most other wireless technologies, Bluetooth does not have direct competition (other than cables). The Bluetooth specification could become an official standard if adopted by IEEE 802.15, which seeks to develop a standard for Personal Area Networks (PANs).
The connections enabled by Bluetooth will be quite novel: headsets to cellular telephones; cellular telephones to portable computers; electronic wallets to point-of-sale systems; portable computers to Internet connections in airports or hotels, and on and on.
In a world of computerized electronic gadgets, Bluetooth will not only be indispensable but will also make other technologies, such as cellular data, more attractive by making them easy to use.
Bluetooth was designed to be inexpensive and to fit into a single 9-millimeter-by-9-millimeter microchip, and could eventually be designed into a variety of devices, whether mobile or stationary. However, Bluetooth only permits a small number of devices to engage primarily in point-to-point communications at medium speeds.
The key architectural element of Bluetooth, the piconet, consists of up to eight devices. One device is designated the master, while the rest are slaves. The master is the device that initiates communication. Most devices will be capable of being either masters or slaves.
Bluetooth uses Frequency Hopping Spread Spectrum (FHSS) at the Physical layer, which means that by using different hopping sequences, multiple piconets can operate in one area and not interfere with each other. The master chooses the hopping sequence, which operates at 1,600 hops per second. This is significantly faster than in most frequency-hopping wireless LANs.
The master controls communication through a polling protocol. All communication is between a master and a slave, and the only way one slave device can communicate with another is via the master. Within one piconet, asynchronous data can be transmitted at a rate of up to 721Kbits/sec, with 57.6Kbits/sec available in the return direction, or up to 432.6Kbits/sec in symmetric operation. One piconet also supports up to three voice channels at 64Kbits/sec each, though use of all three voice channels fills the capacity of the piconet.
Rather than using separate send and receive frequencies, Bluetooth uses the same frequency for sending and receiving via Time Division Duplexing (TDD). TDD enables the same device to easily be a master or a slave. In fact, a device can be a master of one piconet and a slave to another.
Time Division Multiple Access (TDMA) enables both asynchronous and synchronous communication. The master allocates slots according to the type of communication. For instance, synchronous voice channels require time slots at regular intervals, whereas data packets can be dispatched as slots become available. FEC and automatic repeat requests help protect against errors.
The radio has a range of 10 meters, which is far enough for both personal devices and for other people’s devices. To restrict communications between desired devices, each Bluetooth machine has a Personal Identification Number (PIN). Users can specify what other PINs their devices can communicate with.
A maximum of eight devices in a piconet sounds restrictive, but some additional mechanisms extend the network. Though a piconet can only have seven active slave devices, an additional 254 can be in a “parked” state, monitoring the network and ready to enter it. For example, a LAN or Internet POP could grant access to devices that have packets to send and receive, while keeping the rest in a parked state.
It will also be possible to interconnect piconets into a “scatternet” that can include up to eight piconet masters and can support 20 voice channels. However, scatternets (along with proposed LAN emulation schemes) may be pushing the technology just a bit too far. The present specification just wasn’t designed for the complexity of LAN emulation or tying together a bunch of so-called scatternets.
Bluetooth chipsets will be available in 2000 from a variety of chipmakers. The first products will roll out later in 2000, as well, but widespread use probably won’t occur until 2002. Once Bluetooth gains hold, you can expect it to become as ubiquitous as cordless phones and TV remote controls.
Both HiperLAN and Bluetooth promise to have a tremendous impact on network connectivity, and users can look forward to a host of new products and services that will free them from the restrictions of a wired infrastructure.
The HiperLAN2 Global Forum features information about the forum and technical information about HiperLAN2. Go to www.hiperlan2.com.
Proxim is a market leader in wireless LAN products and the first company to announce a HiperLAN product. See www.proxim.com.
The official Bluetooth Web site, www.bluetooth.com, contains information about the Bluetooth Special Interest Group (SIG) and the Bluetooth specification.
Copyright 2000 CMP Media Inc.