Page 431 - From GMS to LTE

Wireless Local Area Network (WLAN) 417

Default SIFS
data transmission

Data Data Data
Device 1
Device 2 t

Ack Ack Ack
Access delay
Frame SIFS
aggregation

Data Data Data
Device 1
Device 2 t

Subframe header
Ack
Access delay

Figure 6.16 Default frame transmission compared to frame aggregation.

decreases the transmission time of an OFDM symbol from 4 to 3.6 microseconds, and
hence, more symbols can be transmitted in a certain time frame.
To further increase transmission speeds, a new coding scheme was introduced with a
reduced number of error detection and correction bits. In 802.11g, the least conservative
coding scheme defined was 3/4, that is, three user data bits are encoded into 4 bits
transferred over the air interface. Under very good signal conditions, 801.11n devices
can now use a 5/6 coding rate, that is, 5 user data bits are encoded into 6 bits, which are
then transferred over the air interface.
Use of all of these methods simultaneously increases the maximum datarate by about
2.5 times compared to 802.11g. This results in a maximum speed on the air interface of
150 Mbit/s. As in previous WLAN systems, application layer speeds are around half of
this value owing to acknowledgement frames and other air interface properties.
As discussed earlier in this chapter, only three independent 20 MHz networks can be
operated in the 2.4 GHz band. Especially in cities, many networks overlap each other.
This significantly reduces the throughput of each network if a high amount of data is
transferred on several networks that share the same channel. If an AP detects 20 MHz
channels, the standard mandates that a network using a 40 MHz channel has to imme-
diately switch to a 20 MHz channel and remain in this mode for at least 30 minutes after
it has received the last frame from an AP of another network. A 40 MHz channel does
not, therefore, result in a reliable and significant speed improvement in the 2.4 GHz
band. In theory, the AP could change to another frequency and inform devices of the
new channel number via a channel switch announcement message, but this is unlikely
to improve the situation in the overcrowded 2.4 GHz band. In practice, some manufac-
turers have therefore decided to ignore the 20 MHz fallback requirement, and configur-
ing a 40 MHz channel in the 2.4 GHz band results in a 40 MHz channel independent of