Jim Zyren, director of strategic marketing for wireless networkingat Intersil explains the draft standard and explains how its adoption
is an important step for the introduction of dual band radios.
Intersil's OFDM-based proposal for the IEEE 802.11g draft standard
was adopted last November after a long and often heated debate. The
draft standard extends data rates for 2.4GHz WLAN systems to 54Mbps
and provides backward compatibility with existing Wi-Fi (802.11b)
There are both mandatory and optional aspects included. This
article explains the main aspects of the draft standard and clearly
distinguishes the mandatory and optional elements.
The draft standard mandates use of Orthogonal Frequency Division
Multiplexing (OFDM) for higher data rates (>20Mbps) and requires
support for Complementary Code Keying (CCK) to ensure backward
compatibility with existing 802.11b radios. The draft standard also
includes some optional elements. The optional elements included in
the 802.11g draft standard are two hybrid waveforms called CCK/OFDM
and PBCC. Developers may elect to include either optional element,
or omit both options entirely.
IEEE 802.11a and 802.11g now share a common high rate waveform
(OFDM) and offer complementary advantages to consumers. 802.11a
systems enjoy more spectrum at 5GHz, which allows for more channels
and, by extension, more users. On the other hand, 802.11g systems
provide backward compatibility with existing Wi-Fi devices and will
offer a range advantage relative to systems operating at 5GHz.
The use of OFDM in the 2.4GHz band will also facilitate
development of dual band radios. The reason is quite simple.
Developers of dual band radios will need to include OFDM capability
for 5GHz operations and CCK capability to support Wi-Fi at 2.4GHz.
By using OFDM at 2.4GHz, implementing 802.11g in a dual band device
will add no additional hardware complexity.
Packet structure: preambles & payloads
Every packet of transmitted data can be thought of as consisting
of two main parts:
The Preamble/Header alerts all radios sharing a common channel
that data transmission is beginning. The Preamble is a known
sequence of 1's and 0's and enables radios to get ready to receive
data (think of it as a wake-up call). The Header immediately follows
the Preamble and conveys several important pieces of information,
including the length (in ms) of the payload. Other radios will not
begin transmission during this period, thus preventing a network
collision. The Preamble/Header and the Payload are normally sent
using the same modulation format (CCK for example). However, there
are exceptions to this rule in the optional elements of the 802.11g
draft standard. Figure 1 shows the packet structures for all of the
elements included in the IEEE 802.11g draft standard.
Fig 1: Various elements of IEEE 802.11g draft standard differ
in packet format
Complementary Code Keying (CCK) is the modulation format for
current Wi-Fi (IEEE 802.11b) systems. Referring to figure 1, the
Preamble/Header and the Payload are both transmitted using CCK
modulation. CCK is a 'single carrier' waveform. Data is
transmitted by modulating a single radio frequency or carrier, as
shown in figure 2.
Fig 2: CCK is a 'single-carrier' modulation
Orthogonal Frequency Division Multiplexing is just now beginning
to reach the market in IEEE 802.11a devices operating at 5GHz. Until
very recently, FCC regulations prohibited the use of OFDM in the
2.4GHz band. Due to recent regulatory changes, a common modulation
format can now be used in both bands.
OFDM employs a much shorter Preamble length as shown in figure 2.
An OFDM Preamble is just 16ms in length as compared to 72ms for CCK.
A shorter preamble reduces network overhead. Note that OFDM
modulation is used for both the Preamble/Header and the Payload.
OFDM is a multi-carrier modulation scheme. The data is split up
among several closely spaced subcarriers (see figure 3). This helps
OFDM provide very reliable operation even in the presence of severe
signal distortion due to multipath. In addition, OFDM systems can
support higher data rates than single carrier systems without
incurring a huge penalty in terms of complexity. For data rates up
to 11Mbps, CCK is a good option. However, as data rates go higher,
OFDM becomes the clear choice.
CCK/OFDM is an optional part of the draft proposal. As the name
implies, it is a hybrid of CCK and OFDM. CCK and OFDM are used for
separate and distinct parts of the packet. CCK modulation is used
for the Header/Preamble while OFDM modulation is used for the
Payload. The CCK header alerts all legacy Wi-Fi devices that a
transmission is beginning and to inform those devices of the duration
(in ms) of that transmission. The Payload can then be transmitted at
a much higher rate using OFDM. Even though existing Wi-Fi devices
will not be capable of receiving the Payload, collisions are
prevented because the Preamble/Header is transmitted using CCK.
The mandatory OFDM waveform can also coexist and interoperate with
existing Wi-Fi devices. However, a different method referred to as
'RTS/CTS' is required. This method is described in greater detail
Packet Binary Convolutional Coding is a single carrier system, but
it is far different from CCK. It employs a more complex signal
constellation (8-PSK for PBCC vs. QPSK for CCK) and a different code
structure. PBCC can also be thought of as a hybrid waveform because
it uses a CCK Preamble/Header with a PBCC payload (see figure 2).
The maximum data rate for PBCC option 33Mbps.
OFDM interoperability with Wi-Fi devices
It is very likely that many IEEE 802.11g radios will implement
only the mandatory modes. This section describes how radios using
OFDM modulation (OFDM Preamble/Header and OFDM Payload) can
interoperate with existing Wi-Fi radios (CCK Preamble/Header and CCK
Fig 3: OFDM systems transmit data on multiple
Normally, all of the radios on a given channel share access to the
airwaves by means of a 'listen-before-talk' mechanism referred to as
Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA). In
simple terms, the radios listen to determine if another device is
transmitting. Each radio on a channel waits until there is no other
transmission in progress before beginning to transmit. There are
additional provisions to reduce the probability that more than one
radio will attempt to transmit at the same moment.
IEEE 802.11g radios will be able to receive either CCK or OFDM
transmissions. However, existing Wi-Fi devices can only receive CCK
transmissions. This presents a problem: If existing radios cannot
receive OFDM transmissions, how will they avoid colliding with those
The CSMA/CA mechanism will not be suitable when CCK radios and
OFDM radios operate on the same channel. Fortunately, another
mechanism already exists in the 802.11protocol that addresses this
problem very efficiently.
The hidden node problem and RTS/CTS
Normally, all radios sharing a given channel (including the Access
Point) can 'hear' one another. However, this is not always the case.
There are instances when all radios can hear and be heard by the
Access Point (AP), but they cannot hear each other (see figure 4).
Fig 4: Hidden node problem occurs when some radios cannot
'hear' each other
Under these conditions, the listen-before-talk mechanism would
break down because radios might detect a clear channel and begin
transmitting to the AP while the AP is already in the process of
receiving another transmission from a 'hidden' radio. This is
commonly referred to as the 'hidden node' problem.
For this reason, another mechanism called Request-To-Send /
Clear-To-Send (RTS/CTS) was included in the existing 802.11 Standard.
Under the RTS/CTS mechanism, each node must send an RTS message to
the AP and receive a CTS reply before transmission can begin. The
situation of CCK and OFDM radios operating on the same channel is
very analogous to the 'Hidden Node' problem because the CCK radios
cannot 'hear' the OFDM transmissions. However, via the use of the
RTS/CTS mechanism, OFDM radios will be able to operate on the same
channel as existing Wi-Fi radios without collision.
The RTS/CTS mechanism results in additional network overhead.
However, the penalty is fairly modest. The benefit is a migration
path to higher data rates for radios operating in the 2.4GHz band.
In the future, networks may make exclusive use of OFDM in the 2.4GHz
band, thus removing the need to use RTS/CTS at some point.
The emergence of IEEE 802.11g is extremely beneficial for the WLAN
market. OFDM is the MANDATORY high rate waveform in the 2.4GHz band.
Data rates of up to 54Mbps are now available in the 2.4GHz band. In
addition, backward compatibility with existing Wi-Fi devices is
Longer term, the IEEE 802.11g draft standard represents an
important step toward the realization of dual band (2.4GHz and 5GHz)
radios. Because OFDM is already required for operation in the 5GHz
band, implementing 802.11g in a dual band device adds no extra
hardware complexity to the resulting product. For dual band devices,
'G is free!'
Published in Embedded Systems (Europe) May