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Network Parameters
Dynamic Frequency Selection (DFS)
802.11a 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 802.11a 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 AP with a 5 GHz upgrade kit 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 Web browser 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 (802.11b)
You can configure and view the following parameters within the Wireless Interface Configuration screen for an
802.11b radio:
NOTE
You must reboot the Access Point before any changes to these parameters take effect.
• Physical Interface Type: This field reports: “802.11b (DSSS 2.4 GHz).” DSSS stands for Direct Sequence Spread
Spectrum; this is the name for the radio technology used by 802.11b devices.
• MAC Address: This is a read-only field that displays the unique MAC (Media Access Control) address for the
Access Point’s wireless interface. The MAC address is assigned at the factory.
• Network Name (SSID): Enter a Network Name (between 1 and 31 characters long) for the wireless network. You
must configure each wireless client to use this name as well. See Configure Network Names for the Wireless
Interfaces for more information.
• Auto Channel Select: The AP-2500 scans the area for other Access Points and selects a free or relatively unused
communication channel. This helps prevent interference problems and increases network performance. By default
this feature is enabled. However, if you are setting up a Wireless Distribution System (WDS), it must be disabled.
See Wireless Distribution System (WDS) for more information.
• Frequency Channel: When Auto Channel Select is enabled, this field is read-only and displays the Access Point’s
current operating Channel. When Auto Channel Select is disabled, you can specify the Access Point’s operating
Channel. If you decide to manually set the unit’s Channel, ensure that nearby devices do not use the same
frequency (unless you are setting up a WDS). Available Channels vary based on regulatory domain. See 802.11b
Channel Frequencies.