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I am reading the clause 9.6 of 802.11-1999, and get confused with some concepts.

1. What is the difference between the BasicRateSet and the PHY Mandatory rate ? I cannot find the defintion of "PHY Mandatory rate" in the standard.

2. Assuming the later matches the former, and the basic rates are 1,2,5.5 and 11, then how the sentence "all frames with multicast and broadcast RA shall be transmitted at one of the rates included in the BSSBasicRateSet" be understood ? Which rate will be used for transmit ?

Thanks for the explanation.

Vu

There's a good explanation of this in the Software Config Guide for Cisco Aironet APs:

You use the data rate settings to choose the data rates the wireless device uses for data transmission. The rates are expressed in megabits per second. The wireless device always attempts to transmit at the highest data rate set to Basic, also called Require on the browser-based interface. If there are obstacles or interference, the wireless device steps down to the highest rate that allows data transmission. You can set each data rate to one of three states:
- Basic (the GUI labels Basic rates as Required)?¡é?€?¡±Allows transmission at this rate for all packets, both unicast and multicast. At least one of the wireless device's data rates must be set to Basic.
- Enabled?¡é?€?¡±The wireless device transmits only unicast packets at this rate; multicast packets are sent at one of the data rates set to Basic.
- Disabled?¡é?€?¡±The wireless device does not transmit data at this rate.

Note At least one data rate must be set to basic.

Joel

I found the answer for the first question. The IEEE 802.11b -1999 explicitly delete the phrase "PHY mandatory rate" in part 9.6 Multirate support.

I still cannot find the answer for question 2. I *guess* the answer should be the lowest basic rate. Because 802.11b allows the STA to shift to lower rates when there is so much noise at some higher rate, then there may be the case that some STAs may have to use the lowest basic rates in order to be able to communicate with the APs ( or other STAs ). If the multicast/broadcasts are sent with a non-lowest basic rate, then those STAs may not be able to receive. Does it make sense ?

But if so, then why the standard says "one of the rates" instead of the lowest one ?

I am looking forward ot having your explanation.

Thanks,

Vu

joelb Escribi?3:

There's a good explanation of this in the Software Config Guide for Cisco Aironet APs:

[...]
Joel

Hi Joel,

Thanks for your reply. Yes, I read the paragraph many times (all of my APs at work and home are from Cisco :) ). Here is what I do not understand.
If I set the basic rates to 1,2,5.5, and 11, then which rate will be selected for the nulticast/broadcast ?

Another relating question is : if I set basic rate to those 4 rates on an AP, can a wireless device 802.11b shift its rate down, say from 11 to 2, when it experiences a lot of noise ? Or do I have set the basic rate to 1,2 and option rates to 5.5 and 11 ?

Thanks,

Vu

Broadcast and multicasts will be sent at the highest selected Basic rate. Regular unicast traffic will be sent at the highest enabled rate for that spectrum. For example, if you take the defaults for 802.11b, 1 and 2mbps are set to Basic, the rest (5.5 and 11mbps) are set to enabled. Broadcast/multicast will transit the medium at 2mbps and unicast traffic will go at 11mps (if you're connected at that rate).

Joel

Sorry, didn't answer the rest of your question. If you set 1, 2, 5.5, and 11 to basic, then all traffic will transit the medium at 11mbps, if that connection rate can be attained. Your connection will adjust datarates down as necessary until it can no longer maintain the connection. At that point, if there are no neighboring APs with the same SSID and authentication settings, the client will drop off the WLAN. When connection is regained, the client will perform the authentication and association process from the start.

Joel

joelb Escribi?3:

Broadcast and multicasts will be sent at the highest selected Basic rate. Regular unicast traffic will be sent at the highest enabled rate for that spectrum. For example, if you take the defaults for 802.11b, 1 and 2mbps are set to Basic, the rest (5.5 and 11mbps) are set to enabled. Broadcast/multicast will transit the medium at 2mbps and unicast traffic will go at 11mps (if you're connected at that rate).

Joel

Joel, thanks very much for bearing with me and for the explanation. I understand the part that the clients will adjust datarates down when necessary ( in your other reply ) , but then it raises a question in my previous post. Let's say the basic rates are 1,2,5.5 and 11 and a client gets some interference/weka signal and it decides to use the rate of 2 Mbps. If the AP sends a broadcast/multicast at a highest basic rate - which is 11 in this case - , then this device cannot receive that broadcast ?

Thanks again,

Vu

No it would send it at the highest connected rate set for Basic -- 2mbps in that situation.

Joel

joelb Escribi?3:

No it would send it at the highest connected rate set for Basic -- 2mbps in that situation.

Joel

"Connected". THANKS, Joe !!!

I can listen some music now :-)

Vu

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