6 posts by 2 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: August 15, 2006:
  • i have some problem regarding these questions:

    1. All STAs accessing a single AP may use different authentication mechanism? how
    2. Can a Bluetooth device cause attenuation to 802.11g device?
    3. The power decreases directly with the decrease in wavelength?
    4. DOes the RF waves move away from the antenna (source) in straight lines or in spherical concentric circles?

    5. when visual LOS is present then RF LOS will be present. this statement may be true or is it always false?

    6. one of the practise tests mention that if a WLAN frame contains more 2000byte payload to be carried to wired segment through ethernet port, the AP will drop the frame....!!!wont AP fragment frame into smaller manageable frames (1500bytes)???the qeustion also mentiones about the jumbo frame...what is the accurate size of the jumbo frame?

  • Hi Kapil:

    6. Jumbo Frames

    Yes, the AP cannot translate an 802.11 frame into an 802.3 frame unless it can fit the 802.3 maximum frame size limitations. Fragmenting/Defragmenting is unavailable in this context.

    Confession: I have administered Ethernets for twenty years and I have never read the IEEE 802.3 standard. I too have wondered what the IEEE says about jumbo frames. So today I searched -- and found nothing.

    "IEEE 802.3-2005 Section 4.4.2 Allowable implementations
    The following parameter values shall be used for their corresponding implementations: ...
    10 Mb/s maxUntaggedFrameSize 1518 octets
    100 Mb/s maxUntaggedFrameSize 1518 octets
    1 Gb/s maxUntaggedFrameSize 1518 octets
    10 Gb/s maxUntaggedFrameSize 1518 octets
    "WARNING Any deviation from the above specified values may affect proper operation of the network."

    It appears that jumbo Ethernet frames are vendor proprietary. If someone knows otherwise please let me know where the IEEE speaks to the matter.

    I hope this helps. Thanks. /criss

  • the study guide mentions that one of the PHYs of clause 19(802.11g) is interoperable with clause 17(802.11a)...both are od differnt frequency bands..then how do they become interoperable??

  • Hi Kapil:

    (Please provide page numbers with such questions.)

    Clause 19 ERP is one PHY with multiple modulations, some mandatory and some optional. Several of its modulations borrow heavily from Clause 17 OFDM techniques, but in a different frequency range.

    As you observed, ERP is limited to 2.4 GHz and OFDM is limited to 5 GHz. Stations based on these two PHYs do not interoperate regardless of what modulations they use.

    I hope this helps. Thanks. /criss

  • Hi Kapil:

    5. LOS

    The field guys are better at this than I am. Visual line of sight to me means that an eyeball at one antenna location can "see" the other antenna. RF line of sight to me means that an RF signal from one antenna reaches the other with sufficient signal strength and integrity as to be useful.

    I may see the other antenna but the Fresnel zone may be blocked. I may not see the other antenna but the RF signal arrives there by bouncing neatly off a building along the way.

    I hope this helps. Thanks. /criss

  • Hi Kapil:

    1. Authentication

    There is IEEE 802.11 authentication, Robust Security Network (RSN) pre-shared key (PSK) authentication, and RSN 802.1X/EAP authentication. Multiple EAP methods are available.

    RSN clients and access points (AP) negotiate authentication methods from a list managed on the AP. The list may include one or more than one authentication method.

    A vendor proprietary AP may operate as though it is multiple APs each servicing its own BSS. Typically a vendor bridges the traffic of each BSS to one Vlan on the AP's Ethernet port. Each BSS may support only one authentication method but the AP supports all, including pre-RSN methods.

    I hope this helps. Thanks. /criss

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