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HiperLAN/2: An Efficient High Speed WLAN

HiperLAN/2, which stands for High Performance Radio Local Area Network, is a wireless LAN standard developed by the Broadband Radio Access Networks (BRAN) division of the European Telecommunications Standards Institute (ETSI). HiperLAN/2 defines a very efficient, high-speed wireless LAN technology that fully meets the requirements of Europe's spectrum regulatory.

Similar to IEEE 802.11a, HiperLAN/2 operates in the 5GHz frequency band using orthogonal frequency division multiplexing (OFDM) and offers data rates of up to 54Mbps. In fact, the physical layer of HiperLAN/2 is very similar to the one that 802.11a defines.

Basic Differences

The similarities between 802.11a and HiperLAN/2, however, stop at the medium access control (MAC) layer. While 802.11a uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to transmit packets, HiperLAN/2 uses Time Division Multiple Access (TDMA).

With CSMA/CA, all stations share the same radio channel and contend for access. For example when an 802.11 station (client) needs to send a packet, the station first listens for other transmissions and then attempts to send frames when no other station is transmitting. If another station happens to be transmitting, all other stations will wait until the channel is free.

A problem is that CSMA/CA causes stations to wait for an indefinite period of time, which in communications terms is referred to as asynchronous access. With this mode of operation, there are no regular time relationships associated with medium access. As a result, there's no guarantee of when a particular station will be able to send a packet. The lack of regular access to the medium draws down the efficiency of the system, which is not good when supporting voice and video information.

The use of TDMA in HiperLAN/2, however, offers a regular time relationship for network access. TDMA systems dynamically assign each station a time slot based on the station's need for throughput. The stations then transmit at regular intervals during their respective time slots, making more efficient use of the medium and improving support of voice and video applications.

HiperLAN/2 Features

HiperLAN/2 has a number of attractive features as compared to 802.11. The first, and probably most important, is higher throughput. Both 802.11a and HiperLAN/2 boast maximum data rates of 54Mbps, but this doesn't represent the actual rate that information flows between the station and the access point.

The true usable maximum throughput of HiperLAN/2, however, is 42Mbps, while the maximum usable throughput of 802.11a is only around 18Mbps (based on Ethernet packets with an average size of 512 bytes). This puts HiperLAN/2 well ahead of 802.11a in terms of throughput capacity of each access point.

Unlike 802.11, HiperLAN/2 implements quality of service (QoS) protocols for different sorts of connections. This allows HiperLAN/2 to support the transmission of a variety of information types, such as data, video and voice. The 802.11e Task Group is developing QoS mechanisms, but they won't likely become part of 802.11 products until the end of 2003.

HiperLAN/2 is presumably more cost effective than 802.11a. While the initial HiperLAN/2 products will probably cost more than 802.11a counterparts, supporters say that the better throughput will outweigh the slight price difference.

The rationale is that HiperLAN/2 provides lower cost per unit of throughput as compared to 802.11a. Of course this isn't really a benefit unless you really need high throughput. Lower bandwidth applications will benefit from the somewhat lesser throughput and lower prices of 802.11 technologies.

A unique feature of HiperLAN/2 technology is the ability to interface with other high-speed networks, including 3G cellular, asynchronous transfer mode (ATM), and other Internet protocol based networks. This can be a real advantage when integrating wireless LANs with cellular systems and wide area networks.

Is HiperLAN/2 a threat to 802.11?

Despite bold predictions of mass production and deployment of HiperLAN/2 products during the second half of 2002, not many, if any HiperLAN/2 products are currently available for consumer purchase. In fact, exhaustive searches on the Internet reveal no HiperLAN/2 products available to consumers. HiperLAN/2 doesn't seem to be moving forward at any discernable pace.

Much of this probably has to do with regulatory issues and big supporters pulling out of the HiperLAN/2 movement. For example, Ericsson, a founder and major supporter of HiperLAN/2 technology, has begun to focus more on 802.11a. In addition, the 802.11h Task Group has been working on revisions to 802.11 that make it more suitable for deployment in Europe, which is where HiperLAN/2 could dominate if anywhere.

Essentially 802.11h is 802.11a with two additional European features. The first of these is Transmit Power Control (TPC), which enables automatic controls for keeping transmissions from interfering with other nearby systems. The second feature is Dynamic Frequency Selection (DFS), which lets the station listen to the airspace before picking a channel. This is also an interference avoidance mechanism that the ETSI requires for operation within Europe.

802.11 currently has a definite lead in the worldwide market as the top choice for wireless LAN deployments. This makes 802.11 the only alternative for wireless LAN deployments today. Combined with the absence of HiperLAN/2 products, it's doubtful that HiperLAN/2 will catch up and become the dominate player in the wireless LAN market.

As a result, don't abandon 802.11 and go with HiperLAN/2. You should, nevertheless, plan the deployment of a wireless LAN with migration paths to other technologies in mind. Certainly the use of dual radio access points with the ability to upgrade applicable software is a smart way of moving forward. Definitely keep your alternatives open and position your network to include new technologies, such as HiperLAN/2, when they become available and feasible to implement.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers workshops on deploying wireless LANs.

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