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Criss_Hyde Escribi?3:

Hi:

The IEEE 802.11 standard leaves the choice of transmission rate for multicast frames by a multirate capable station largely up to the vendor.

"9.6 Multirate support
Some PHYs have multiple data transfer rate capabilities that allow implementations to perform dynamic rate switching with the objective of improving performance. The algorithm for performing rate switching is beyond the scope of this standard, but in order to ensure coexistence and interoperability on multirate-capable PHYs, this standard defines a set of rules to be followed by all STAs. ... All frames with multicast and broadcast in the addresses1 field shall be transmitted at one of the rates included in the BSS basic rate set, regardless of their type or subtype. ..."

Multirate decisions are made by each transmitting station. Multirate decisions affect but do not determine the multirate decisions made by other stations in the service set.

The only stations that transmit frames with multicast/broadcast in the address1 field are IBSS clients and infrastructure BSS access points; infrastructure BSS clients only transmit with unicast in the address1 field. So in most multicast/broadcast cases we are concerned with the multirate choice of the access point.

Multicast/broadcast frames are never acknowledged nor retransmitted, so the onus is on getting it right the first time.

An access point must record, 1) the basic rates of the service set, 2) its own operational or supported rates, a super set of the basic rates, and 3) the operational rates of each associated client. An access point might record, 4) the rates used when recently receiving from each client, and 5) the rates used when recently transmitting and retransmitting to each client.

Based on all this information a wise access point vendor, such as Cisco, might transmit a multicast frame at the highest rate that is both shared by all the operational rate sets that make up the BSS, and yet no higher than recent experience shows all its clients are likely to successfully receive.

However some access points in my experience simply set the multicast rate at the lowest or nearly lowest basic rate and leave it at that. These access points may provide more reliable but lower speed multicast performance.

I hope this helps. Thanks. /criss

Criss, thanks. It really helps. Just want to be sure that I understand correctly, then can I conclude as follows :
For an Cisco AP with only 802.11b clients, then settings all speeds ( 1,2,5.5 and 11 ) to basic rates cannot have poorer performance than the case that basic rates = 1,2 and enable rates = 5.5 and 11 ?

Thanks,

Vu

Hi Vu:

The IEEE 802.11 standard restricts multicast to one of the basic rates. Thus your second case is more limiting than your first.

The actual result for any given make and model of access point is better determined on the lab bench with a protocol analyzer.

I hope this helps. Thanks. /criss

Criss_Hyde Escribi?3:

Hi Vu:

The IEEE 802.11 standard restricts multicast to one of the basic rates. Thus your second case is more limiting than your first.

Thanks. That's what I think, too, but was not sure :)

Criss_Hyde Escribi?3:

The actual result for any given make and model of access point is better determined on the lab bench with a protocol analyzer.

I hope this helps. Thanks. /criss

Will do. Got the Observer and YellowJacket. I am going to spend this whole week with them.

Thanks again,

Vu