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    Configuring Advanced Wireless Settings

    Use the advanced wireless settings to control low-level wireless networking settings, such as aging values and collision protection. When you first deploy the NetScreen-5GT Wireless device on your network, the network settings are already configured with default settings designed to work in most networking environments. However, you might want to edit these settings to meet your specific wireless networking needs.

    You can edit the default values for the following wireless networking settings.

    • aging
    • beacons
    • burst and fragment size
    • control frame protection
    • short slots
    • preambles

    Configuring Aging

    The aging interval is the amount of time (in seconds) that a wireless client or bridge remembers an access point after communication with the WAP is lost. To configure the aging setting:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and then edit the default aging value.

    The default is 300 seconds; acceptable range is 60 to 1,000,000 seconds. To disable aging, set the value to 0 (zero).

    Configuring Beacons

    A WAP broadcasts beacon packets to keep the wireless network synchronized and to inform wireless clients of waiting data. A beacon packet includes data such as the wireless LAN service area, the WAP address, and delivery traffic indicator maps (DTIMs).

    To configure the beacon settings:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and then edit the default values for the following settings:
    • Beacon Interval—The beacon interval is the amount of time between beacons sent by the NetScreen-5GT Wireless to wireless clients. A beacon transmission includes the beacon interval; the interval informs receiving devices how long they can wait in low-power mode before waking up to handle beacons. Increasing the beacon interval lessens the number of beacon responses required by a wireless client, enabling clients to reduce battery power. The default value is 100 time units; acceptable range is 20 to 1,000 time units (1 time unit equals 1024 µs).
    • Beacon Interval Between DTIMs—This interval is the amount of beacon intervals between DTIM messages, which inform wireless clients of waiting data. A lower value enables wireless clients to download waiting data more often; a higher value enables wireless clients to wait in low-power mode longer between DTIMs. When using a high DTIM value, however, the client must stay active longer to collect waiting data, and clients might miss broadcast and multicast traffic messages. The default value is 1 beacon interval; acceptable range is 1 to 255.

    Configuring Burst and Fragment Size

    Use the burst and fragment setting to configure how the device transmits wireless packets over the network. To configure the burst and fragment settings:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and then edit the default values for the following settings:
    • Maximum Number of Frames in a Burst—The burst threshold defines the average maximum number of frames a WAP can use to handle wireless traffic before the device begins sending traffic in bursts. When wireless traffic exceeds the specified threshold, the device sends wireless packets in bursts to clients, who can switch to a low-power sleep state between bursts. The default value is 3 frames; acceptable range is 2 to 255 frames.
    • Fragmentation Threshold—The fragmentation threshold defines the maximum size of a packet that can be transmitted without fragmentation. If the packet size exceeds the specified threshold, the sender (client or WAP) must fragment the packet before transmitting.

      Using a high fragmentation threshold reduces the number of fragments on the wireless network, which can increase efficiency. However, large, unfragmented packets can be corrupted during transmission, requiring resend attempts that can decrease efficiency. The default value is 2346; acceptable range is even numbers between 256 and 2346.

    Configuring Control Frame Protection

    Control frame protection is designed to help avoid collisions on the wireless network. Transmission collision usually occurs when two wireless devices are within range of the same WAP, but are not within range of each other (they are hidden nodes). If two wireless transmissions collide at the WAP, the data in each transmission is lost.

    To avoid collisions, you can require wireless clients to first request permission to send data (clients must send a request-to-send (RTS) frame) and/or receive approval of that request (client must receive a clear-to-send (CTS) frame) before transmitting data.

    Because 802.11b stations cannot hear 802.11g stations using orthogonal frequency-division multiplexing (OFDM), a method for wireless transmission that divides a signal and transmits the pieces at different frequencies simultaneously, traffic from these stations can collide on the network, reducing network efficiency. We recommend you enable protection to avoid collisions when supporting 802.11b and 802.11g operation modes.

    Note: CTS protection is not supported when using 802.11b only.

    To configure the control frame protection settings:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and then edit the default values for the following settings:
    • Threshold for RTS to Transmit—The request-to-send (RTS) threshold defines the maximum size of a packet that a wireless client can send without obtaining permission from the WAP. If a packet exceeds this threshold, the client must send an RTS message to the WAP requesting permission to send the packet. You might want to adjust this setting to control traffic flow through an access point that services a large number of clients. The default is 2346; accepted range is 256 to 2346.
    • CTS Protection Mode—Enables clear-to-send (CTS) control frame protection, which requires wireless client to first receive a CTS frame from the WAP before sending data. Select one of the following protection modes:
      • On—When selected, wireless clients must first receive a CTS frame from the device before sending data.
      • Off—When selected, wireless clients do not send CTS control frames.
      • Auto—When selected, the device automatically detects the CTS mode used by the wireless client. This is the default setting.
    • CTS Protection Type—The protection type defines the level of control frame protection enforced by the device. Select one of the following protection types:
      • CTS Only—When selected, wireless clients must first receive a single, self-directed CTS frame from the device before sending data. This is the default setting.
      • CTS-RTS—When selected, wireless clients must first send an RTS frame and receive a CTS frame from the device before sending data (a two-frame exchange occurs prior to the actual network transmission).
    • CTS Rate—The CTS rate defines the data rate (in Mbps) at which CTS frames are sent. The default rate is 11 Mbps; acceptable values are 1, 2, 5.5, and 11.

    Configuring Short Slots

    Short slots, an 802.11g-only feature, can increase efficiency and throughput for wireless traffic. By default, the device supports 802.11g traffic that uses short slots. However, because 802.11b does not support short slots, you might want to disable short slots for all protocols when your wireless network is handling 802.11b traffic.

    To disable short slot for 802.11g packets:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and then select Long in the Set Slot Time option.

    Configuring Preambles

    A preamble is the sequence of bits within a transmission that, when recognized and received by a wireless client, enables the client to locate the remaining packets in the transmission. The preamble length is defined in the Synchronization field of a wireless packet, and can be long or short:

    • A long preamble (128 bits) provides the wireless client more time to process the preamble, which can provide greater interoperability with older wireless protocols and non-short-preamble equipment. All 802.11 devices support a long preamble.
    • A short preamble (56 bits) can improve efficiency because the client does not spend time processing the preamble. However, older wireless protocols do not support short preambles.

    By default, the device does not support long preambles. To enable long preambles for 802.11b packets only:

    1. In the NSM navigation tree, select Device Manager > Devices. Double-click the device object to open the device configuration.
    2. In the device navigation tree, select Wireless Settings > Advanced, and, then select Long Transmit Preamble.

    Published: 2013-01-02