In June 1997, the Institute of Electrical and Electronic Engineers (IEEE)
finalized the initial standard for wireless LANs, IEEE 802.11. This standard
specified a 2.4GHz operating frequency with data rates of 1 and 2Mbps. When
deploying a wireless LAN using the initial version of 802.11, you could opt
for using frequency hopping spread spectrum (FHSS) or direct sequence spread
spectrum (DSSS). Since the ratification of the initial 802.11 standard, the
IEEE 802.11 Working Group (WG) has
made several revisions through various task groups.
What do the letters mean?
Task groups within the 802.11 WG enhance portions of the 802.11 standard. A
particular letter corresponding to each standard/revision, such as 802.11a,
802.11b, and so on, represents the different task groups. For example, Task
Group B (i.e., 802.11b) was responsible for upgrading the initial 802.11 standard
to include higher data rate operation using DSSS in the 2.4GHz band.
Let’s take a closer look at each of the 802.11 task groups and how they impact
your WLAN deployments.
802.11a — OFDM in the 5GHz Band
802.11a is a Physical Layer (PHY) standard (IEEE Std. 802.11a-1999) that specifies
operating in the 5GHz UNII band using orthogonal frequency division multiplexing
(OFDM). 802.11a supports data rates ranging from 6 to 54Mbps. 802.11a-based
products became available in late 2001.
Because of operation in the 5GHz bands, 802.11a offers much less potential
for radio frequency (RF) interference than other PHYs (e.g., 802.11b and 802.11g)
that utilize 2.4GHz frequencies. With high data rates and relatively little
interference, 802.11a does a great job of supporting multimedia applications
and densely populated user environments. This makes 802.11a
an excellent long-term solution for satisfying current and future requirements.
Strongly consider the deployment of 802.11a unless extenuating circumstances
point you toward a different PHY, such as 802.11b.
802.11b — High Rate DSSS in the 2.4GHz band
The task group for 802.11b was responsible for enhancing the initial 802.11
DSSS PHY to include 5.5Mbps and 11Mbps data rates in addition to the 1Mbps and
2Mbps data rates of the initial standard. 802.11 finalized this standard (IEEE
Std. 802.11b-1999) in late 1999. To provide the higher data rates, 802.11b uses
CCK (Complementary Code Keying), a modulation technique that makes efficient
use of the radio spectrum.
Most wireless LAN installations today comply with 802.11b, which is also the
basis for Wi-Fi certification from the Wireless Ethernet Compatibility Alliance
(WECA). These products have been available
for the past two years. In some cases, you should deploy 802.11b networks today
to take advantage of the installed base of 802.11b-equipped users. For example,
utilize 802.11b as the basis for public wireless LANs to maximize the number
of subscribers.
802.11c — Bridge Operation Procedures
802.11c provides required information to ensure proper bridge operations. This
project is completed, and related procedures are part of the IEEE 802.11c standard.
Product developers utilize this standard when developing access points. There’s
really not much in this standard relevant to wireless LAN installers.
802.11d — Global Harmonization
When 802.11 first became available, only a handful of regulatory domains (e.g.,
U.S., Europe, and Japan) had rules in place for the operation of 802.11 wireless
LANs. In order to support a widespread adoption of 802.11, the 802.11d task
group has an ongoing charter to define PHY requirements that satisfy regulatory
within additional countries. This is especially important for operation in the
5GHz bands because the use of these frequencies differ widely from one country
to another. As with 802.11c, the 802.11d standard mostly applies to companies
developing 802.11 products.
802.11e – MAC Enhancements for QoS
Without strong quality of service (QoS), the existing version of the 802.11
standard doesn’t optimize the transmission of voice and video. There’s currently
no effective mechanism to prioritize traffic within 802.11. As a result, the
802.11e task group is currently refining the 802.11 MAC
(Medium Access Layer) to improve QoS for better support of audio and video (such
as MPEG-2) applications. The 802.11e group should finalize the standard by the
end of 2002, with products probably available by mid-2003.
Because 802.11e falls within the MAC Layer, it will be common to all 802.11
PHYs and be backward compatible with existing 802.11 wireless LANs. As a result,
the lack of 802.11e being in place today doesn’t impact your decision on which
PHY to use. In addition, you should be able to upgrade your existing 802.11
access points to comply with 802.11e through relatively simple firmware upgrades
once they are available.
802.11f – Inter Access Point Protocol
The existing 802.11 standard doesn’t specify the communications between access
points in order to support users roaming from one access point to another. The
802.11 WG purposely didn’t define this element in order to provide flexibility
in working with different distribution systems (i.e., wired backbones that interconnect
access points).
The problem, however, is that access points from different vendors may not
interoperate when supporting roaming. 802.11f is currently working on specifying
an inter access point protocol that provides the necessary information that
access points need to exchange to support the 802.11 distribution system functions
(e.g., roaming). The 802.11f group expects to complete the standard by the end
of 2002, with products supporting the standard by mid-2003.
In the absence of 802.11f, you should utilize the same vendor for access points
to ensure interoperability for roaming users. In some cases a mix of access
point vendors will still work, especially if the access points are Wi-Fi-certified.
The inclusion of 802.11f in access point design will eventually open up your
options and add some interoperability assurance when selecting access point
vendors.
802.11g – Higher Rate Extensions in the 2.4GHz Band
The charter of the 802.11g task group is to develop a higher speed extension
(up to 54Mbps) to the 802.11b PHY, while operating in the 2.4GHz band. 802.11g
will implement all mandatory elements of the IEEE 802.11b PHY standard. For
example, an 802.11b user will be able to associate with an 802.11b access point
and operate at data rates up to 11Mbps. In early 2002, 802.11g decided to use
OFDM instead of DSSS as the basis for providing the higher data rate extensions.
An issue is that the presence of an 802.11b user on an 802.11g network will
require the use of RTS / CTS (request-to-send / clear-to-send), which generates
substantial overhead and lowers throughput significantly for all 802.11b and
802.11g users. RTS / CTS ensures that the sending station first transmit a RTS
frame and receive a CTS frame from the access point before sending data. A mixture
of 802.11b and 802.11g requires RTS / CTS to avoid collisions because 802.11b
stations can’t hear 802.11g stations using OFDM.
It’s unclear at this date when 802.11g will ratify the standard. In addition,
the FCC (Federal Communications Commission)
still needs to approve the use of OFDM in the 2.4GHz band, a generally necessary
action when messing with the PHY. As a result, it will likely take a relatively
long period of time before 802.11g products appear on the market.
There’s been much debate over the use of 802.11g
vs. 802.11a for satisfying needs for higher performance WLAN applications.
For the foreseeable future, your only selection for data rates beyond 802.11b’s
11Mbps is to use 802.11a. Because of the earlier time to market and superior
performance capacity, 802.11a will likely dominate the high performance WLAN
market in the near-term and distant future.
802.11h – Spectrum Managed 802.11a
802.11h addresses the requirements of the European regulatory bodies. It provide
dynamic channel selection (DCS) and transmit power control (TPC) for devices
operating in the 5GHz band (802.11a). In Europe, there’s a strong potential
for 802.11a interfering with satellite communications, which have "primary
use" designations. Most countries authorize WLANs for "secondary use"
only. Through the use of DCS and TPC, 802.11h will avoid interference in a way
similar to HiperLAN/2,
the European-based competitor to 802.11a. 802.11h hopes to have their standard
finalized sometime before the end of 2003.
To implement DCS and TPC, 802.11h is developing associated practices that affect
both the MAC and PHY Layers. The inclusion of DCS and TPC will likely enable
802.11h to become the successor to 802.11a. Fortunately, there shouldn’t be
any issues of non-interoperability between existing 802.11a and 802.11h users
and access points. The good news is that 802.11h is enabling sales
of 802.11a networks in Europe, which will eventually result in higher sales
volumes and lower prices.
802.11i – MAC Enhancements for Enhanced Security
802.11i is actively defining enhancements to the MAC Layer to counter the issues
related to wired equivalent privacy (WEP). The existing 802.11 standard
specifies the use of relatively weak, static encryption keys without any form
of key distribution management. This makes it possible for hackers to access
and decipher WEP-encrypted data on your WLAN. 802.11i will incorporate 802.1x
and stronger encryption techniques, such as AES (Advanced Encryption Standard).
In a previous
tutorial, I discuss more details of how 802.11i is beefing up security.
Don’t expect 802.11i to be available in the near future. The standard will
likely not have IEEE ratification before mid-2003. 802.11i updates the MAC Layer,
so you should be able to upgrade existing access points with firmware upgrades.
The implementation of AES, however, may require new hardware.
For now, you can obtain stronger forms of security that go well beyond WEP
by implementing proprietary security mechanisms available from access points
vendors. The problem is that you’ll probably need to deploy network cards and
access points from the same vendor. As a minimum, utilize WEP.
802.11 Next Generation
In addition to the above task groups, the 802.11 WG is studying new methods
to increase performance and make better use of the radio spectrum. For example,
the group is considering the use of ultrawideband modulation as a new mechanism
for supporting higher speed applications and reducing the potential for RF interference.
You won’t see these newer, faster standards for a number of years, though.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
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