Controller-Based Access Points; Rf Modulation Standard; 802.11B/G Rf Modulation Standard - ASCOM VoWiFi System System Description

System Description
Ascom VoWiFi System
2.1.2

Controller-Based Access Points

Figure 3. Controller-based APs with centralized management.
The example in
can share the features that enhance wireless communications. Much of the configuration is
centralized in the Controller. The Controller handles switching functionality to the WLAN,
security functionality, RF management, QoS, and extended roaming features. It is physically
connected to the LAN and not to the APs.
It is also possible to use a Controller that incorporates a switch. The APs are then directly
connected to the Controller and are basically just a radio and an antenna. The Controller
handles most of the MAC layer all by itself.
The use of controller-based APs enhances operation of the VoWiFi system, makes
centralized administration over the WLAN possible, and makes it easy to expand the
network. If several Controllers are used in a system, a centralized intelligent server
(Centralized Wireless Management) can be used for administration and management.
2.2

RF Modulation Standard

2.2.1

802.11b/g RF Modulation Standard

WLAN products based on the 802.11b/g standards operate in the 2.400 to 2.497 GHz
frequency range, which is one of the Industrial Scientific Medical (ISM) bands.This is an un-
licensed frequency band also used by Bluetooth devices, medical equipment, and many
other devices that do not require a license.
The frequency band is divided into channels on which data packets are sent in parallel. It is
important that the channels do not overlap each other as this would decrease the
performance of the network.
APs supporting the 802.11b/g standard have up to 14 channels with centre frequencies
that are 5 MHz apart (applies to channel 1 to 13)
either 22 MHz (802.11b) or 20 MHz (802.11g), the centre frequencies for non-overlapping
channels must be at least 25 MHz apart. This gives us three non-overlapping channels at the
most, see the example with 11 channels in
to APs in different regions, one channel per AP.
Use of other channels than 1, 6 and 11 will have a major negative impact on performance in
the system since those channels will interfere with each other. Not only due to RF
interference, but also due to the 802.11 protocol specification, a selection of channels other
than 1, 6 and 11 will cause a performance reduction.
1. The centre frequency in channel 14 differs from the other (12 MHz apart). It was previously used only in Japan.
7 September 2011 / Ver. G
Centralized Wireless Management
Controller
AP AP
3. Figure
figure 3 shows a system with controller-based APs. With this approach, APs
Controller
LAN
AP
1
. Since the bandwidth of each channel is
figure 4. These channels are reused and allocated
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