User Guide
Advanced Configuration
SYSTIMAX
AirSPEED
AP541
User Guide
46
Dynamic Frequency Selection (DFS)
AirSPEED AP541 devices sold in Europe use a technique called Dynamic Frequency Selection (DFS) to automatically
select an operating channel. During boot-up, the AP scans the available frequency and selects a channel that is free of
interference. If the AP subsequently detects interference on its channel, it automatically reboots and selects another
channel that is free of interference.
DFS only applies to AirSPEED AP541 devices used in Europe (i.e., units whose regulatory domain is set to ETSI). The
European Telecommunications Standard Institute (ETSI) requires that 802.11a devices use DFS to prevent
interference with radar systems and other devices that already occupy the 5 GHz band.
If you are using an 802.11a AP in Europe, keep in mind the following:
•
DFS is not a configurable parameter. It is always enabled and cannot be disabled.
•
You cannot manually select the device’s operating channel; you must let DFS select the channel.
•
You cannot configure the Auto Channel Select option. Within the HTTP or CLI interface, this option always
appears enabled.
RTS/CTS Medium Reservation
The 802.11 standard supports optional RTS/CTS communication based on packet size. Without RTS/CTS, a sending
radio listens to see if another radio is already using the medium before transmitting a data packet. If the medium is
free, the sending radio transmits its packet. However, there is no guarantee that another radio is not transmitting a
packet at the same time, causing a collision. This typically occurs when there are hidden nodes (clients that can
communicate with the Access Point but are out of range of each other) in very large cells.
When RTS/CTS occurs, the sending radio first transmits a Request to Send (RTS) packet to confirm that the medium
is clear. When the receiving radio successfully receives the RTS packet, it transmits back a Clear to Send (CTS)
packet to the sending radio. When the sending radio receives the CTS packet, it sends the data packet to the receiving
radio. The RTS and CTS packets contain a reservation time to notify other radios (including hidden nodes) that the
medium is in use for a specified period. This helps to minimize collisions. While RTS/CTS adds overhead to the radio
network, it is particularly useful for large packets that take longer to resend after a collision occurs.
RTS/CTS Medium Reservation is an advanced parameter and supports a range between 0 and 2347 bytes. When set
to 2347 (the default setting), the RTS/CTS mechanism is disabled. When set to 0, the RTS/CTS mechanism is used for
all packets. When set to a value between 0 and 2347, the Access Point uses the RTS/CTS mechanism for packets that
are the specified size or greater. You should not need to enable this parameter for most networks unless you suspect
that the wireless cell contains hidden nodes.
Wireless Service Status
The user can shutdown (or resume) the wireless service on the wireless interface of the AP through the CLI, HTTP, or
SNMP interface. When the wireless service on a wireless interface is shutdown, the AP will:
•
Stop the AP services to wireless clients connected on that wireless interface by disassociating them
•
Disable the associated BSS and WDS ports on that interface
•
Disable the transmission and reception of frames on that interface
•
Indicate the wireless service shutdown status of the wireless interface through LED and traps
•
Enable Ethernet interface so that it can receive a wireless service resume command through CLI/HTTP/SNMP
interface
NOTE
The wireless service cannot be shutdown on an interface where Rogue Scan is enabled.
In shutdown state, AP will not transmit and receive frames from the wireless interface and will stop transmitting
periodic beacons. Moreover, none of the frames received from the Ethernet interface will be forwarded to that wireless
interface.
Wireless service on a wireless interface of the AP can be resumed through CLI/HTTP/SNMP management interface.
When wireless service on a wireless interface is resumed, the AP will:
•
Enable the transmission and reception of frames on that wireless interface
•
Enable the associated BSS port and WDS ports on that interface
•
Start the AP services to wireless clients
•
Indicate the wireless service resume status of the wireless interface through LED and traps
