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.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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