802.11a Physical Layer Revealed

In a previous tutorial,
we discussed the inner workings of the 802.11b Physical (PHY) Layer. Similar
to 802.11b, the 802.11a PHY Layer specifies the transmission and reception of
802.11 frames.
Now we’ll take a look inside the 802.11a PHY Layer, which uses orthogonal frequency division multiplexing (OFDM) technology
to support operation of up to 54Mbps data rates in the 5GHz band.

802.11a PLCP Frame Fields

The 802.11a PHY Layer Convergence Procedure
(PLCP) transforms each 802.11 frame that a station wishes to send into a PLCP
protocol data unit (PPDU). The PPDU includes the following fields in addition
to the frame fields imposed by the Medium Access
Control (MAC) Layer:

• PLCP Preamble. This field consists of 12 symbols and enables the
  receiver to acquire an incoming OFDM signal.
• Rate. This field identifies the data rate of the 802.11 frame. As
  with 802.11b, the 802.11a PLCP fields, however, are always sent at the lowest
  rate, which is 6Mbps. The following represents the data rates represented
  by specific field values:

• Reserved. This field is set to a logic zero.
• ength. This field represents the number of octets contained with
  the frame.
• Parity. Based on values of the Rate, Reserved, and Length fields,
  this field contains a single-bit value that provides positive (even) parity.
• Tail. This field is always set to logic zeros.
• Service. This field consists of seven bits as logic zeros to synchronize
  the descrambler in the receiver and another nine bits (currently all logic
  zeros) reserved for future use.
• PSDU. The PSDU, which stands for Physical Layer Service Data Unit,
  represents the contents of the PPDU (i.e., the actual 802.11 frame being sent).
• Tail. This field consists of six bits (all zeros) for receiver processing
  functions.
• Pad Bits. This field contains a number of bits in order to modify
  the frame size to equal a specific multiple of bits coded in an OFDM symbol.

As with 802.11b,
802.11 analyzers don’t display the 802.11a PHY Layer fields. The 802.11 radio
card removes the fields before processing occurs by the MAC Layer.

OFDM in Operation

OFDM is not a form of spread spectrum.
Instead, OFDM divides a data signal across 48 separate sub-carriers within a
20MHz channel to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or 54Mbps.
Data rates of 6Mbps, 12Mbps, and 24Mbps are mandatory for all 802.11-compliant
products. OFDM is extremely efficient, which enables it to provide the higher
data rates and minimize multi-path
propagation problems.

An 802.11a modulator converts the binary
signal into an analog waveform through the use of different modulation types,
depending on which data rate is chosen. For example with 6Mbps operation, the
PMD uses binary phase shift keying (BPSK), which shifts the phase of the transmit
center frequency to represent different data bit patterns. The higher data rates,
such as 54Mbps, employ quadrature amplitude modulation (QAM) to represent data
bits by varying the transmit center frequency with different amplitude levels
in addition to phase shifts.

Transmit Frequencies

The 802.11a PMD translates the signal into
an analog form with a transmit center frequency corresponding to the radio channel
chosen by the user. The corresponding operating frequencies in the U.S. fall
into the national information structure (U-NII) bands: 5.15-5.25GHz, 5.25-5.35GHz,
and 5.725-5.825GHz. Within this spectrum, there are twelve, 20MHz channels,
and each band has different output power limits.

The following identifies the center frequency and maximum output power of each
of the U-NII bands:

OFDM is becoming very popular for high-speed transmission. In addition to being
selected for use within the 802.11g PHY
Layer, OFDM is the basis for the European-based HiperLAN/2 wireless LAN standards.
In fact the 802.11a PHY Layer is very similar to the HiperLAN/2 PHY. In addition,
OFDM has also been around for a while supporting the global standard for asymmetric
digital subscriber line (ADSL).

Stay tuned: In future tutorials, we’ll
take a closer look at the 802.11 infrared and 802.11g physical layers as well.

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.