In this tutorial, 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:

 
    Field Value     Data Rate       
    1101               6Mbps       
    1111               9Mbps       
    0101              12Mbps       
    0111              18Mbps       
    1001              24Mbps       
    1011              36Mbps       
    0001              48Mbps       
    0011              54Mbps

     
Reserved. This field is set to a logic zero.

Length. 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.
         

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).

802.11a: An Excellent Long Term Solution

As 802.11a products began shipping months ago, more and more companies have been taking advantage of 802.11a's superior performance. 802.11a radios transmit at 5GHz and send data up to 54Mbps using OFDM (orthogonal frequency division multiplexing). The results have been very good. 802.11a products deliver excellent performance.

Inside 802.11a

Before discussing benefits and implications of 802.11a, let's take a look at how 802.11a devices operate.

802.11a defines one of several different 802.11 Physical Layers (PHYs). The actual name of 802.11a is the "High Speed Physical Layer in the 5GHz band," commonly referred to as the "OFDM PHY." Another popular PHY of course is 802.11b, which most companies have been installing for the past couple years. Others include 802.11 FHSS (frequency hopping spread spectrum) and 802.11 IR (infrared).

No matter which 802.11 PHY you deploy, the MAC (medium access control) Layer is the same. The MAC Layer manages and maintains communications between 802.11 radio NICs and access points by coordinating access to a shared radio channel. The MAC Layer is actually a program that runs on a processor; whereas, the PHY involves digital communications circuitry and an RF (radio frequency) modulator to prepare data for transmission over the air medium.
The 802.11a PHY is quite different than 802.11b, which uses direct sequence spread spectrum (DSSS). 802.11a specifies the use of OFDM to support higher data rates.

OFDM divides the data signal across 48 separate sub-carriers to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or 54Mbps of which 6, 12, and 24Mbps are mandatory for all products. For each of the sub-carriers, OFDM uses PSK (phase shift keying) or QAM (quadrature amplitude modulation) to modulate the digital signal depending on the selected data rate of transmission. In addition, four pilot sub-carriers provide a reference to minimize frequency and phase shifts of the signal during transmission. This form of transmission enables OFDM to operate extremely efficiently, which leads to the higher data rates, and minimize the affects of multi-path propagation.

The operating frequencies of 802.11a 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 Code of Federal Regulations, Title 47, Section 15.407, regulates these frequencies in the U.S.
OFDM is becoming very popular for high speed transmission. In addition to being selected as the basis for the 802.11g PHY, OFDM is the basis for the European-based HiperLAN/2 wireless LAN standards. In fact the 802.11a PHY 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).

Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports, and equipment providers with the development and deployment of wireless networks.