IEEE 802.11g Wi-Fi Tutorial
- the IEEE 802.11g Wi-Fi / WLAN standard provides data transfer speeds of 54 Mbps data transfer rates using the 2.4 GHz ISM band.
After the introduction of Wi-Fi with the 802.11a and 802.11b standards, the 802.11b standard became the most popular operating in the 2.4 GHz ISM band. This standard proved to be the most popular despite the faster operating speed of the a variant of the standard because the cost of producing chips to operate at 2.4 GHz were much less than ones to run at 5 GHz.
In order to provide the higher speeds of 802.11a while operating on the 2.4 GHz ISM band, a new standard was introduced. Known as 802.11g, it soon took over from the b standard. Even before the standard was ratified, 802.11g products were available on the market, and before long it became the dominant Wi-Fi technology.
The 802.11g standard provided a number of improvements over the 802.11b standard which was its predecessor. The highlights of its performance are given in the table below.
|IEEE 802.11g Wi-Fi Features|
|Date of standard approval||June 2003|
|Maximum data rate (Mbps)||54|
|Modulation||CCK, DSSS, or OFDM|
|RF Band (GHz)||2.4|
|Channel width (MHz)||20|
802.11g physical layer
Like 802.11b, its predecessor, 802.11g operates in the 2.4 GHz ISM band. It provides a maximum raw data throughput of 54 Mbps, although this translates to a real maximum throughput of just over 24 Mbps.
Although the system is compatible with 802.11b, the presence of an 802.11b participant in a network significantly reduces the speed of a net. In fact it was compatibility issues that took up much of the working time of the IEEE 802.11g committee.
In order to provide resilience against multipath effects while also being able to carry the high data rates, the main modulation method chosen for 802.11g was that of OFDM - orthogonal frequency division multiplex, although other schemes are used to maintain compatibility, etc..
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
Click on the link for an OFDM tutorial
In addition to the use of OFDM, DSSS - direct sequence spread spectrum is also used.
To provide the maximum capability while maintaining backward compatibility, four different physical layers are used - three of which are defined as Extended Rate Physicals, ERPs.These coexist during the frame exchange so that the sender can use any one of the four, provided they are supported at each end of the link.
The four layer options defined in the 802.11g specification are:
- ERP-DSSS-CCK: This layer is that used with 11b. Direct sequence spread spectrum is used along with CCK - complementary code keying. The performance is that of the legacy 802.11b systems.
- ERP-OFDM: This physical layer is a new one introduced for 802.11g where OFDM is used to enable the provision of the data rates at 2.4 GHz that were achieved by 11a at 5.8 GHz.
- ERP-DSSS/PBCC: This physical layer was introduced for use with 802.11b and initially provided the same data rates as the DSS/CCK layer, but with 802.11g, the data rates have been extended to provide 22 and 33 Mbps. As indicated by the title, it uses DSSS technology for the modulation combined with PBCC coding for the data.
- DSSS-OFDM: This layer is new to 11g and uses a combination of DSSS and OFDM - the packet header is transmitted using DSSS while the payload is transmitted using OFDM
802.11g occupies a nominal 22 MHz channel bandwidth, making it possible to accommodate up to three non-overlapping signals within the 2.4 GHz band. Despite this, the separation between different Wi-Fi access points means that interference is not normally too much of an issue.
|IEEE 802.11g Wi-Fi Physical Layer Summary|
|Physical Layer||Use||Data Rates (Mbps)|
|ERP-DSSS||Mandatory||1, 2, 5.5, 11|
|ERP-OFDM||Mandatory||6, 9, 12, 18, 24, 36, 48, 54|
|ERP-PBCC||Optional||1, 2, 5.5, 11, 22, 33|
|DSSS-OFDM||Optional||6, 9, 12, 18, 24, 36, 48, 54|
802.11g packet structure
It is customary for data packets to be split into different elements. For Wi-Fi systems the data packets sent over the radio interface can be thought of as consisting of two main parts:
- Preamble / Header: As with any other preamble / header, it serves to alert receivers, in this case radios, that a transmission is to start, and then it enables them to synchronise. The preamble consists of a known series of '1's and '0's that enable the receivers to synchronise with the incoming transmission. The Header element immediately follows the pre-amble and contains information about the data to follow including the length of the payload.
- Payload: This is the actual data that is sent across the radio network and can range from 64 bytes up to 1500 bytes. In most cases the preamble/header are sent using the same modulation format as the payload, but this is not always the case. When using the DSSS-OFDM format, the header is sent using DSSS, while the payload uses OFDM.
The initial 802.11 standard defined a long preamble PLCP frame set. In the later 802.11b standard, an optional short preamble was defined. Then for 802.11g the short preamble PPDU was defined as mandatory.
802.11g ERP-DSSS/CCK PPDU frame
PPDU: This is the format into which data is converted by the PLCP for transmission.
PLCP: This is the PHY Layer Convergence Procedure and it transforms each 802.11 frame that a station wishes to send into a PLCP protocol data unit, PPDU.
PDSU: This is the Physical Layer Service Data Unit, it represents the contents of the PPDU, i.e., the actual data to be sent.
Service: This field is always set to 00000000. The802.11 standard reserves its data and format for future use.
For the ERP-OFDM PHY option an ERP packet must be followed by a 6 µs period of no transmission called the signal extension period. The reason for this that for a 16 µs period was allowed in 802.11a to enable convolutional decode processing to finish before the next packet arrived.
Within 802.11g, the ERP-OFDM modulation scheme still requires 16 µs to ensure that the convolutional decoding process is able to be completed within the overall process timing. To enable this to occur, a signal extension of 6 µs is included. This enables the transmitting station to compute the Duration field in the MAC header. In turn this ensures that the NAV value of 802.11b stations is set correctly and compatibility is maintained.
By Ian Poole
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