8 IEEE 802.11 Amendments

The most widely accepted amendments of the IEEE 802.11 family is currently b, a and g. All of them have reached the mass markets with cost efficient products. Other amendments in the family are [c-f], [h-j] and n which are enhancements and extensions or corrections to previous specifications in the family. We will take a closer look at b, a, g and n in this section.

IEEE 802.11b includes enhancements of the original 802.11 standard to support higher data rates (5.5 and 11 Mbit/s). IEEE 802.11b uses the same access method as defined the original standard IEEE 802.11.

IEEE 802.11b uses the DSSS modulation technique which is also defined in the original standard.

An IEEE 802.11b card can theoretically operate at 11 Mbit/s, but will, due to Adaptive Rate Selection scale, fall back to 5.5, then 2 and then 1 Mbit/s when packet loss takes place. The lower data rates are less sensitive to interference and attenuation since they are using a more redundant method to encode the data (i.e. the relation of signal and noise is better at lower data rates).

Just like IEEE 802.11b, this amendment uses the same core protocol as the original standard. IEEE 802.11a operates in the 5 GHz band and uses OFDM as modulation technique which gives it a maximum raw data rate of 54 Mbit/s. By using adaptive rate selection, the data rate is reduced to 48, 36, 24, 18, 12, 9 and, 6 Mbit/s if required.

802.11a has 12 non-overlapping channels whereas 8 of them are dedicated for indoor use and the remaining 4 are used for point to point links. 802.11a is NOT interoperable with 802.11b, except for equipment that specifically implements both standards (two radios).

As today, 802.11a has not reached the hype that 802.11b has. It has not been widely adopted due to the presence of 802.11b, poor initial product implementations and more restrictive regulations in the 5 Ghz band.

In June 2003, a third amendment to the 802.11 standard was ratified. It was given the name IEEE 802.11g. and just like 802.11b, it operates in the 2.4 GHz band,

802.11g uses the same modulation technique as 802.11a (OFDM) in high bit rates and can hence operate at a maximum raw data rate of 54 Mbit/s. To ensure interoperability with b products, at data rates of 5.5 and 11 Mbps it reverts back to CCK+DSSS (like 802.11b) and uses DBPSK/DQPSK+DSSS for data rates of 1 and 2 Mbit/s.

It is the 802.11g interoperability with 802.11b hardware that is one of the main reasons behind its major acceptance. However, it suffers the same problem at 802.11b regarding interference (crowded urban spots) since they operate in the same frequency band.

The latest amendment of 802.11 is IEEE 802.11n¹⁾ which “aims” to reach a maximum theoretical bit rate of 540 Mbit/s which would make it up to 40 times faster than 802.11b and 10 times faster than 802.11a or 802.11g. 802.11n is based on previous 802.11 amendments with the greatest difference of introducing MIMO, multiple-input multiple-output. MIMO implies that multiple transmitter and receivers are used to increase the data throughput and the transmitting range.

Several experts claims that MIMO is the future of the wireless LAN²⁾.

MIMO takes advantage of multipath propagation to increase the throughput (or to reduce bit error rate) instead of trying to eliminate the effects of the unavoidable multipath phenomena that other standards do. In simple words, MIMO takes advantage of what other standards sees as a hurdle: multipath.

When a radio signal is sent out though the air it is spread out as a beam. The receiver receives first the main line-of-sight signal and some time later echoes and fragments of the signal that has been reflected in buildings or in other obstacles. Normally, these echoes and fragments are seen as noise to the real signal but MIMO is able to use the information of this “non direct signals” to improve the main signal. This results in clearer signals (less noise) and longer signal ranges.

Another feature that MIMO includes is the use of many transmitters for the same data stream, so called Spatial Division Multiplexing (SDM). A set of independent data streams are sent out within a single channel of bandwidth. This increases the throughput as the number of data streams is increased. Since a MIMO antenna need a dedicated processing hardware, the cost of it is unavoidable higher than any standard WLAN antenna.

The IEEE 802.11n amendment is expected to be finalized in mid 2006.

Below follows a short summary and comparison of the 4 most important IEEE 802.11 amendments.

Table 1: Summary of IEEE 802.11b/a/g/n characteristics