Questions on Channel Agility bit
Last Post: March 3, 2005:
There does not seem to be a lot of details on the Channel Agility bit in the capability field of an MMPDU. Should I assume since not much of 802.11 using FHSS is around any more that this is no longer useful? Was it ever useful? Can I also assume that this would only be useful with 802.11 FHSS and no other technologies using FHSS since I also assume that the hopping sequence and dwell times may be different than 802.11 FHSS? Any info would be welcome.
You seem to be on the right track. Almost nothing is written about this HR/DSSS feature and I am not aware of it ever having been built (or knowledge of it ever having been tested by The CWNP Program). Here are the pertinent paragraphs from the 802.11b amendment.
I hope this helps. Thanks. /criss
18.104.22.168 Channel Agility (optional) This Channel Agility option allows an implementation to overcome some inherent difficulty with static channel assignments (a tone jammer), without burdening all implementations with the added cost of this capability. When the Channel Agility option is enabled, the PHY shall meet the requirements on channel switching and settling time, as described in 22.214.171.124, and the hop sequences described below. This option can also be used to implement IEEE 802.11-compliant systems that are interoperable between both FH and DS modulations. Annex F contains a description of the expected behavior when such networks are employed.
Annex F High Rate PHY/frequency-hopping interoperability:
The Channel Agility option described in 126.96.36.199 provides for IEEE 802.11 FH PHY interoperability with the High Rate PHY. The frequency-hopping patterns, as defined within this annex, enable synchronization with an FH PHY compliant BSS in North America and most of Europe. In addition, CCA requirements on a High Rate station using this mode provide for CCA detection of 1 MHz wide FH signals within the wideband DS channel selected. FH PHY stations operating in mixed mode FH/DS environments are advised to use similar cross PHY CCA mechanisms. The frequency-hopping (Channel Agility) and cross CCA mechanisms provide the basic mechanisms to enable coexistence and interoperability.
The MAC elements include both DS and FH elements in beacons and probe responses when the Channel Agility option is turned on. Added capability fields indicate the ability to support the Channel Agility option and to indicate whether the option is turned on. These fields allow synchronization to the hopping sequence and timing, identification of what modes are being used within a BSS when joining on either High Rate or FHSS sides, and rejection of an association request in some cases.
Interoperability within an infrastructure BSS can be achieved, as an example, using a virtual dual access point (AP). A virtual dual AP is defined, for purposes of discussion, as two logically separate APs that exist within a single physical AP with a single radio (one transmit and one receive path). Both FHSS and High Rate logical APs send out their own beacons, DTIMs, and other nondirected packets. The two sides interact in the sharing of the medium and the APÃ¢Â€Â™s processor and radio. Addressing and association issues may be handled in one of several ways and are left as an implementation choice.
Minimal interoperability with a non-hopping High Rate or legacy DSSS is provided by the use of a channel at least 1/7 or more of the time. While throughput would be significantly reduced by having a channel only 1/7 of the time, connection and minimal throughput can be provided.
When the frequency-hopping option is utilized, the HR/DSSS PHY should provide the CCA capability to detect 1 MHz wide FH PHY signals operating within the wideband DS channel at levels 10 dB higher than that specified in 188.8.131.52 for wideband HR/DSSS signals. This is in addition to the primary CCA requirements in 184.108.40.206. A timeout mechanism to avoid excessive deferral to constant CW or other non-IEEE 802.11 type signals is allowed.
FH PHY stations operating in mixed environments should provide similar CCA mechanisms to detect wideband DSSS signals at levels specified in 220.127.116.11, but measured within a 1 MHz bandwidth. Signal levels measured in a full DSSS channel will be generally 10 dB or higher.