Just as PCs were the initial driving force behind wired Ethernet
innovations in the 80s, the wide acceptance of laptops, palmtops, PDAs,
and (of course) wireless telephony is creating an environment that is ripe
for innovation in the form of wireless Ethernet connectivity. To find
evidence of this trend, we need look no further than wireless LANs, which
are becoming more and more refined. Already, products exist that conform
to standards while offering high performance, security, and ease of use.
Who might use these products? Businesses considering an add-on network
strategy, whether they are large, multi-site corporations or small/medium
businesses. Such a strategy, once implemented, could help these businesses
meet the need for continuous connectivity to critical network resources
(such as financial and inventory databases) and for intranet and Internet
access.
More generally, wireless networking offers a new dimension in
productivity for PC users, enabling true portability through continuous,
non-stop network access. Continuous connectivity makes for continuous
productivity.
STANDARDIZATION LEADS TO BROADER ACCEPTANCE
Until a couple of years ago, the world of wireless data networking had
been dominated by proprietary niche products that targeted vertical
markets. But with most vendors adopting the IEEE 802.11 standards,
wireless LANs are reaching a new level of acceptability with mainstream
corporate users.
But what are the IEEE 802.11 standards? Let�s take a close look. Such
an examination will help us weigh the implications of adding telephony. In
addition, a review of the standards will put us in a better position to
relate these developments to events in the public wireless space.
(Wireless LANs are well positioned to fulfill the wireless Internet�s
inherent infrastructure requirements. These requirements include both the
unification of untethered data and telephony systems, as well as the
deployment of next-generation public wireless systems.)
The Original Wireless Ethernet Standard
Wireless Ethernet standards are the responsibility of the IEEE 802.11
committee. The original 802.11 standard delivered throughputs in the 1 to
2-Mbps range shared across a number of users. With technology advances,
the new standard supports throughputs of 11 Mbps.
Analogous to wired Ethernet, the IEEE 802.11 standard specifies the
operation of the Physical Layer and the Media Access Control or MAC Layer.
It uses the 83.5-MHz-wide, 2.4-GHz ISM (Industry, Science, and Medical)
band, which is available for unlicensed operation worldwide, using low
power spread spectrum radio frequency modulation technology to minimize
interference.
Physical Layer: Two Physical Layer modes of operation are
supported: namely, Direct Sequence and Frequency Hopping. A third mode is
Diffuse Infrared, which, while cost effective, has very limited reach.
Direct Sequence (DS) operation takes the original data stream and
multiplies it by a spreading factor or �chipping code.� Depending on
the coding method, this approach can result in 2 or 11-Mbit/s throughputs
to a particular user. Limited simultaneous transmissions are possible, but
they have to be substantially non-overlapping spectrum.
In contrast, Frequency Hopping (FH) consists of grabbing one of 78
separate 1-MHz channels for short intervals (usually on the order of 100
ms) and changing frequencies in some algorithmic fashion. Scalability is
greater with FH because it allows many multiple simultaneous
transmissions. Interference from other ISM sources (for example, microwave
ovens) is low because the frequency is changing 10 times per second. At
the same time, FH is limited to 1 Mbps throughputs per user. Either spread
spectrum technique is relatively immune to tapping.
MAC Layer: The MAC layer specified rules of access,
providing mechanisms to provide contention mechanisms independent of which
Physical Layer mode is used. Like the Ethernet IEEE 802.3 standard, IEEE
802.11 uses Carrier Sense Multiple Access (CSMA). However, unlike IEEE
802.3, it uses collision avoidance rather than collision detection.
Simply, rather than detecting collisions on the shared media LAN and then
backing off if collisions occur, it uses a request to send/clear-to-send
handshake to minimize possibilities of collisions. During this handshake,
the length of the data burst is also communicated. Collision avoidance is
used instead of carrier sensing as other transmitters may be �hidden�
and therefore not detected. This approach makes 802.11 more
bandwidth-efficient than traditional 802.3 systems.
Wireless Ethernet Refinements
While the first-generation wireless LAN standards and products operated in
the 1�2-Mbps range, two new standards have been developed which deliver
much higher bandwidths. These standards are called IEEE 802.11a and IEEE
802.11b.
IEEE 802.11a: Based on a variant of Frequency Division Multiplexing, IEEE
802.11a can accomplish data modulation, the main benefit of which is its
immunity to multipath echoes, which are typical to the indoor and mobile
environments. While other bit rates are allowed, all implementations are
required to support 6, 12, and 24 Mbps. The multi-rate mechanism of the
MAC protocol ensures that all devices communicate with each other at the
best data rate.
IEEE 802.11a is specified for use at 5 GHz. Currently, this band is
open to unlicensed devices only in the United States. Another downside is
that 802.11a�s low penetration power may limit its value in many office
environments.
IEEE802.11b: This variant uses the 2.4 GHz band, but achieves 11 Mbps
rates (with fallback to 5.5 Mbps) through a coding scheme derived from the
Direct Sequence technology. The industry is focusing on 802.11b as the
volume market wireless LAN technology. The first PC product supporting 11
Mbps operation is the Apple iMac.
More IEEE 802.11 Features
Two other features of IEEE 802.11 should be noted: ad hoc mode and low
latency mode. The ad hoc mode allows two laptops or PDAs to communicate
directly with each other without needing to go through a base station (or
Access Point) � a useful feature for exchanging data between users on an
as-required basis, even in the absence of a wireless LAN infrastructure
deployment. The advantage over generally available infrared links is range
and the fact that IEEE 802.11 is an omni-directional technology
(eliminating the need to align the communicating devices).
Latency mode introduces virtual Class of Service support over the radio
spectrum and opens up wireless telephony opportunities, not just in the
context of data devices but also as the next generation wireless telephony
systems.
THE CONVERGENCE OF WIRELESS LANs AND IP TELEPHONY
Wireless LANs have evolved driven by the needs for data connectivity,
initially based on proprietary standards and more recently on IEEE 802.11.
The penetration has been compatible with technologies in the early
adoption stage and limited by the perception of low bandwidth (that is, 2
Mbps) and by the per user costs (that is, in the $500-plus range).
At the same time, in-building wireless telephony systems have been
developed using traditional circuit establishment and switching
mechanisms. Early adopters such as the healthcare and retail industries
have been faced with deploying separate wireless infrastructures to
support voice and data services.
Three major factors are changing this landscape significantly:
- The introduction of 11-Mbps wireless LANs and the anticipated
lowering in per user equipment costs towards the $100 range.
- The explosion in laptop, palmtop, and PDA devices.
- More connectivity options and new applications enabled through IP
telephony.
These three factors have created the opportunity for the convergence of
wireless telephony and wireless LAN systems onto IEEE 802.11b systems.
Therefore, in 2000, we can expect the introduction of 802.11b devices in
the form of integrated and PCMCIA solutions in lap, palm, and handheld
data devices; screen-based multifunction wireless �telephones�; and
specialized devices targeted at specific vertical markets such as
healthcare, retail, and hospitality.
A significant technology enabler for these devices, given their limited
display and storage capacity is the Wireless Application Protocol (WAP),
which has been developed specifically for wireless mobility.
BUT WHAT ABOUT PUBLIC WIRELESS ALTERNATIVES?
IEEE 802.11b systems are the preferred solution for
in-building-centered wireless LAN applications, delivering 11 Mbps of
bandwidth � a significant enhancement over today�s systems. However,
when wide area broadband mobility is a requirement from a single device,
some new opportunities emerge, opportunities enabled by new 2.5G and 3G
(third generation) public wireless systems.
For example, 2.5G GSM systems allocate multiple 25 KHz channels (up to
eight per carrier for approximately 100 Kbps of effective bandwidth) for
IP, avoiding contention between voice and data. This contrasts with 3G
systems that intermix IP and telephony across a 5 MHz band using
spread-spectrum techniques for an effective peak burst data rate of 2 Mbps
(when stationary), 384 Kbps (when walking), and 144 Kbps (when in a moving
car). In any case, these public systems will support the same types of
applications (albeit at a lower rate), as discussed above, using the WAP
standard.
Customers desiring in-building and wide area broadband mobility will
have several choices:
- Use the public wireless standard in both the WAN and the building
environments on a pay-as-you-go basis, and accept the lower bandwidth
capacity.
- Rely on microcellular technology. This option is similar to the
option just mentioned; however, in this case, low-power 2.5G or 3G
transmitters are distributed as microcells around the building
(contrasted with standard multi-kilometer wide cells in the WAN). The
advantage is that this approach avoids usage-based airtime charges,
though there is a charge for sharing of the spectrum with the carrier.
- Use dual-mode devices that can operate using 802.11b in the building
and one of the public standards outside of the building.
- Create �islands� of wireless LAN coverage in public areas such
as airport lounges that can be connected seamlessly to the corporate
LAN. Such a service is available today from Mobilestar.
THE NEXT WAVE OF WIRELESS LANs
The development of 802.11 wireless LAN products should make the idea of
actually deploying wireless LANs more attractive. In fact, the Cahners
In-Stat Group predicts dramatic growth in the number of wireless LAN users
in the United States, from 2.3 million today to 23 million in the year
2003. Going forward, the introduction of IEEE 802.11b high-capacity
wireless LANs will enable new telephony/data mobile applications and
devices and bring us closer to Star Trek-style communications.
Tony Rybczynski is director of strategic marketing and technologies
for Nortel Networks� Enterprise Solutions unit. This business unit offers
a full range of enterprise terminal, workgroup, campus, and wide-area
unified networks and applications, through direct and indirect channels.
For more information, visit the company�s Web site at www.nortelnetworks.com.
E-mail questions or comments to [email protected].
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