Compatibility With Other Standards; The Physical Layer - Lancom 1821n Wireless Manual

1.2.2

Compatibility with other standards

The 802.11n standard is backwardly compatible to previous standards
(IEEE 802.11a/b/g). However, some of the advantages of the new technology
are only available when, in addition to the access points, the wireless LAN cli-
ents are also compatible with 802.11n.
In order to allow the co-existence of wireless LAN clients based on 802.11a/
b/g (called "legacy clients") 802.11n access points offer special mechanisms
for mixed operation, where performance increases over 802.11a/b/g are not
as high. Only in all-802.11n environments is the "greenfield mode" used,
which can exploit all the advantages of the new technology. In greenfield
mode both access points and wireless LAN clients support the 802.11n stan-
dard, and access points reject connections with legacy clients.
1.2.3

The physical layer

The physical layers describes how data must be transformed in order for them
to be transmitted as individual bits over the physical medium. In this process
the following steps are performed in a wireless LAN device:
Modulation of digital data into analog carrier signals
Modulation of the carrier signal into a radio signal in the selected fre-
quency band, which for a wireless LAN is either 2.4 or 5 GHz.
The second modulation step in IEEE 802.11n occurs in the same way as in
conventional wireless LAN standards and is therefore not covered here.
However, there are a number of changes in the way digital data are modula-
ted into analog signals in 802.11n.
Improved OFDM modulation (MIMO-OFDM)
Like 802.11a/g, 802.11n uses the OFDM scheme (Orthogonal Frequency Divi-
sion Multiplex) as its method of modulation. This modulates the data signal
not on just one carrier signal but in parallel over several. The data throughput
that can be achieved with OFDM modulation depends on the following para-
meters, among other things:
Number of carrier signals: Whereas 802.11a/g uses 48 carrier signals,
802.11n can use a maximum of 52.
LANCOM 1811n Wireless – LANCOM 1821n Wireless
Chapter 1: Introduction
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