Radio settings

LANconfig: Wireless LAN / General / Physical WLAN settings / Radio

WEBconfig: LCOS menu tree / Setup / Interfaces / WLAN / Radio settings

• Frequency band, Subband When selecting the frequency band on the 'Radio' tab under the physical interface settings, you decide whether the WLAN module operates in the 2.4 GHz or in the 5 GHz band (also see ), and thus the available radio channels. In the 5 GHz band, a subband can also be selected which is linked to certain radio channels and maximum transmission powers.
  Note: In some countries, the use of the DFS method for automatic channel selection is a legal requirement. Selecting the subband also defines the radio channels that can be used for the automatic channel selection.
  With the DFS method (Dynamic Frequency Selection) an unused frequency is automatically selected, for example, to avoid interference from radar systems or to distribute WLAN devices as evenly as possible over the entire frequency band. After switching on or booting, the device randomly selects one of the available channels (e.g. based on the country settings). It checks whether radar signals exist on this channel, and whether it is already in use by another WLAN. This scan procedure repeats until a channel is found that is free of radar signals and which has the lowest possible number of other networks. The selected channel is then monitored for radar signals for a further 60 seconds. For this reason, data traffic may be interrupted for a period of 60 seconds while the frequencies are scanned for a free channel. To avoid these pauses in data transmission every time the channel is changed, LANCOM devices carry out the scan before a channel is chosen. Information about scanned channels is stored to an internal database.
  • Was a radar signal detected on the channel?
  • How many other networks were found on the channel?
  This database helps the WLAN device to select a channel from the list that is free of radar signals and that has the lowest number of other networks (the operating channel). After the channel has been selected, data transmission can continue immediately without any waiting.
  • The "blacklist" in the database stores the channels to be blocked due to the detection of radar signals. This entries are removed from the list every 30 minutes in order to keep the information up to date.
  • The "whitelist" in the database stores the channels where no radar was detected. These entries remain valid for 24 hours, although if radar signals be detected in the meantime, an entry is made to the blacklist.
  The access point generally uses the operating channel selected after the first scan permanently. Connections can now be operated for any length of time on the channel selected by the DFS algorithm until either a radar signal is detected or the radio cell is restarted (e.g. by changing the device configuration, firmware upload, or restart). When is it necessary to carry out a new 60-second scan?
  • The device is switched on or cold-started. Under these circumstances the database is empty. The device cannot select a channel from the whitelist, and so a scan has to be carried out.
  • Within the first 24 hours of scanning, it becomes necessary to switch channels because a radar signal is detected within range of the access point. In this case, the access point can refer to alternatives in the whitelist. It then informs associated WLAN clients and/or P2P partners of the new operating channel and switches to this channel. This process takes place within about a second, so the switch can be considered to be uninterrupted.
  • The device is in operation for 24 hours already, and then a channel switch becomes necessary. Entries in the whitelist are out of date and thus discarded. The access point has no alternative channel to which it can switch directly. In this case the database requires new information from a scan and WLAN operation is interrupted for one minute.
    Note: To avoid having the 60 second pause at an inconvenient time, you can set the time of the scan and thus the database update. Do this with WEBconfig or telnet in the menu Setup/Interfaces/WLAN/Radio settings. To define the time you can use the options provided by cron commands, e.g. '1,6,13' to force a DFS scan at 01:00h, 06:00h or 13:00h, or '0-23/4' for a DFS scan between 0:00h and 23:00h every 4 hours. Forced DFS scans require that the device is set with the correct system time.
  The ETSI standard 301 893 version 1.4.1 is the latest set of regulations concerning the operation of 5-GHz wireless LANs. In the context of the wireless LAN modules used in the LANCOM Wireless Routers and LANCOM Access Points, this standard is also referred to as DFS 2. This standard makes tougher demands on the radar detection patterns used when operating 5-GHz WLANs. The standard applies to all devices brought into circulation after April 01, 2008. Devices brought into circulation before this date do not have to meet this standard. In particular devices with older WLAN chips (two- or three-chip modules) do not have to meet this standard and, as such, do not have to be upgraded. LANCOM Systems supplies LCOS firmware of the versions 7.30 (for the current Wireless Routers and Access Points) and 7.52 (for LANCOM Wireless L-310agn and LANCOM Wireless L-305agn) with DFS 2 support. These firmware versions have different threshold values for radar-pattern recognition than with the former DFS.
  Note: In principle the operator of the WLAN is responsible for maintaining the new ETSI standards. For this reason LANCOM Systems recommends that you perform an update to a firmware version with DFS 2 support.
• Channel number The radio channel selects a portion of the conceivable frequency band for data transfer.
  Note: In the 2.4-GHz band, two separate wireless networks must be at least three channels apart to avoid interference.
  
  
  
  
  
• 2.4-GHz mode Three different wireless standards are based on the 2.4-GHz band: IEEE 802.11b, IEEE 802.11g and IEEE 802.11n. If 2.4 GHz is selected as the operating frequency, the compatibility mode can be selected in addition.
  Note: Please observe that clients supporting only the slower standards may not be able to register with the WLAN if compatibility mode is set to a high value.
  The 802.11gbn compatibility mode offers the highest possible speeds and yet also offers the 802.11b standard so that slower clients are not excluded. In this mode, the WLAN module in the access point principally works with the faster standard and falls back on the slower mode should a client of this type log into the WLAN. In principle 802.11n is backwardly compatible to the previous IEEE 802.11b/g wireless LAN standards, although not all 802.11n functions are supported in this mode. In the 2.4 GHz band you can allow operation in accordance with 802.1b/g/n either exclusively or in various mixed modes. When 802.11b is supported you can also select whether only 11 Mbps mode or the older 2 Mbps should be supported.
  Note: Compatibility is always achieved at the expense of performance. It is therefore recommended to allow only those modes of operation that are absolutely necessary for the wireless LAN clients in use.
• 5-GHz mode Using two neighboring, vacant channels for wireless transmissions can increase the transfer speeds up to 108 Mbps. With the base station in the 108Mbit/s Turbo mode, only those WLAN clients that also support the 108Mbit/s Turbo mode can connect to this base station. In the 5 GHz band you can choose to allow either greenfield mode (802.11n only) or mixed operation with 802.11a. Greenfield mode should be chosen if there are only 802.11n devices in operation in a network, as these guarantee the highest possible throughput rates.
• Double bandwidth (20/40 MHz) A wireless LAN module normally uses a frequency range of 20 MHz in which data to be transmitted is modulated to the carrier signals. 802.11a/b/g use 48 carrier signals in a 20 MHz channel. The use of double the frequency range of 40 MHz means that 96 carrier signals can be used, resulting in a doubling of the data throughput. 802.11n can use 52 carrier signals in one 20 MHz channel for modulation and up to 108 in a 40 MHz channel. The use of the 40 MHz option for 802.11n therefore means a performance gain of more than double.
• Antenna grouping LANCOM Access Points with 802.11 support can use up to three antennas for transmitting and receiving data. Using several antennas with 802.11n can have different purposes:
  • Improved data throughput: Using "spatial multiplexing" allows parallel data streams to be implemented to transmit double the amount of data.
  • Improving wireless coverage: Cyclic shift diversity (CSD) can be used to transmit a radio signal in different phases. This reduces the risk of the signal being erased at certain points in the radio cell.
  Depending on the application the use of the antennas can be set:
  • When using the device in access point mode to connect wireless LAN clients it is generally recommended to use all three antennas in parallel in order to achieve good network coverage.
  • To work with 2 parallel data streams; for example for point-to-point links with an appropriate dual slant antenna, the antenna ports 1 + 2 or 1 + 3 are used. The unused antenna port is deactivated.
  • For applications with only one antenna (for example an outdoor application with just one antenna) the antenna is connected to port 1 and ports 2 and 3 are deactivated
  • The ''Auto' setting means that all available antennas are used
  Please note the following when connecting antennas: Antenna connector 1 must always be used. Depending on the mounting and cabling, the second antenna may be connected either to connector 2 or connector 3.

The configuration of the device software must agree with the actual antenna connections.

• Diversity settings The diversity settings specify which antennas should be used for transmission and for reception:
  • 'Transmit via the primary antenna only' (RX diversity): In this default setting, the antenna connected to the access point's main connector is used for data transmission. For reception (RX), the antenna with the best signal is selected (at Main or AUX).
  • 'Automatically select the best antenna for transmission' (TX and RX diversity): If the diversity function is used for transmission (TX) as well, the antenna with the strongest signal is taken.
  • 'Send via the primary antenna and receive via the secondary antenna' (no diversity): The main antenna only is used for transmission, and for reception the antenna at the AUX connector is preferred. Using this alternative, high-performance antennas that are legally prohibited from transmitting can be used for reception.
• Antenna gain, transmission power reduction Where the transmission power of an antennae exceeds the levels permitted in the country of operation, the power must be attenuated accordingly.
  • The field 'Antenna gain' is for the gain of the antenna minus the actual cable loss. For an AirLancer Extender O-18a antenna with a gain of 18dBi and a 4m cable with a loss of 1dB/m, the 'Antenna gain' would be entered as 18 - 4 = 14. This value for true antenna gain is dynamically used to calculate and emit the maximum permissible power with regards to other parameters such as country, data rate and frequency band.
  • In contrast to this, the entry in the field 'Tx power reduction' causes a static reduction in the power by the value entered, and ignores the other parameters. Also see .
  Note: The transmission power reduction simply reduces the emitted power. The reception sensitivity (reception antenna gain) remains unaffected. This option is useful, for example, where large distances have to be bridged by radio when using shorter cables. The reception antenna gain can be increased without exceeding the legal limits on transmission power. This leads to an improvement in the maximum possible range and, in particular, the highest possible data transfer rates.
• Access point density The more access points there are in a given area, the more the reception areas of the antennae intersect. Information on 'access point density' is sent with the beacons for processing by older Agere clients.
• Maximum distance Large distances between transmitter and receiver give rise to increasing delays for the data packets. If a certain limit is exceeded, the responses to transmitted packets no longer arrive within an acceptable time limit. The entry for maximum distance increases the wait time for the responses. This distance is converted into a delay which is acceptable for wireless communications.
• Background scan interval If a value is entered here, the LANCOM Wireless Router searches the frequencies in the active band that are currently not in use in cycles within this interval in order to find available access points.
  • The background scan function is usually deployed for rogue AP detection for the LANCOM Wireless Router in access point mode. This scan interval should correspond to the time span within which unauthorized access points should be recognized, e.g. 1 hour.
  • Conversely, for the LANCOM Wireless Router in client mode, the background scan function is generally used for improved mobile WLAN client roaming. In order to achieve fast roaming, the scan time is limited here, for example, to 260 seconds.
  • When the background scan time is '0' the background scanning function is deactivated.
• Time unit for background scanning The background scan interval sets the time period between searches by a Wireless Router or Access Point for third-party WLAN networks within range. The time interval allows the entered value to be defined in milliseconds, seconds, minutes, hours or days.
  Note: To avoid adverse effects on data transfer rates, the interval between channel scans in access-point mode should be at least 20 seconds. Lesser values will be corrected to this minimum value automatically. For example, with 13 channels to scan in the 2.4GHz band, one scan of the full spectrum takes at least 13 x 20s = 260 seconds.
  Note: Background scanning can be limited to a lower number of channels when indoor mode is activated. This allows roaming for the mobile LANCOM Wireless Router in client mode to be improved even further.