MCS values for 802.11n-only mode in Comware 5 routers & access points

[lorn10]

Okay folks,

This topic was first posted under "Switching and Routing", - I didn’t know that there exists also a "Comware Wireless section". ;-)

As everybody knows there exists some MSR based (formerly 3Com and H3C branded) routers with Comware 5 and 802.11n WiFi functionality. Furthermore there exist also some old 3Com access-points like the AP9552 series which contain near the same Comware 5 OS.

Because there isn’t anymore any legacy WiFi hardware present I want to set the WiFi to 802.11n-only setting. This is sometimes also called "Greenfield Mode".

Well, to do this, I had first to set at least three MCS values manually. Here starts my problem; unfortunately I have no idea which numbers are needed.

With the exception of “Supported Maximum MCS”, - there exist no recommendation.  I was not able to find regarding “Mandatory Maximum MCS” and the “Multicast MCS” any examples at the web. The default “Supported Maximum MCS” value is defined with 76.

To make it now even more confused, this value makes for 2x2 MIMO “spatial stream” devices absolutely no sense. According to the IEEE_802.11n-2009 Wikipedia article the maximum possible MCS value for 2x2 802.11n hardware is 15 and not 76. This will give a total theoretical performance of 300 Mbit which corresponds perfectly to the hardware I use. (Source: https://en.wikipedia.org/wiki/IEEE_802.11n-2009)

So I am now in the situation that I have two MCS functions which require nebulous values, and the default MCS value for “Supported Maximum MCS” seems totally absurd.

Yes, I have tried several configurations with different numbers but until now without any success. The WiFi network get mostly immediately instable; - the WiFi devices cannot connect any longer or they have very long to connect. And if they sometimes connect they are crashing out quite fast.

In the attachment I have uploaded several screenshots of the Comware 5 Web UI.

Any hint is welcome. J

Kind regards
lorn10

[lorn10 · Accepted solution]

Well, this question will be now answered by myself. J

In short, as I suspected, the “Supported Maximum MCS” value cannot be higher than 15! This is the absolute limit on 2x2 (or 2x3) 802.11n WiFi hardware. It is really strange why 3Com has defined at the 9552 AP series a standard value of 76.

The other two functions, “Mandatory Maximum MCS” and “Multicast MCS” should be zero 0. It is possible to set a higher mandatory value but this will reduce the stability in a 802.11n-only network, - I think.

A big help to resolve that whole MCS issue was the “HP A-MSR Router Series WLAN Configuration Guide”. Under the section “Configuring 802.11n rates” the MCS index values are described in full detail and with a nice MCS index table (see attachment).

802.11n (MCS) rates fall into the following types:

• Mandatory rates: Mandatory rates must be supported by the AP. Clients can associate with the AP only when they support the mandatory rates.
  
  
• Supported rates: Higher rates supported by the AP besides the mandatory rates. Supported rates allow some clients that support both mandatory and supported rates to choose higher rates when communicating with the AP.
  
  
• Multicast rates: Multicast rates supported by the AP besides the mandatory rates. Multicast rates allow clients to send multicast traffic at the multicast rates.

When you specify the maximum MCS index, you actually specify a range. For example, if you specify the maximum MCS index as 5 for mandatory rates, rates corresponding to MCS indexes 0 through 5 are configured as 802.11n mandatory rates.

3Com had a somewhat different naming, “Mandatory Maximum MCS”, “Supported Maximum MCS” and “Multicast MCS”. In later HP Comware 5 (MSR router) releases it also seems to be possible to define a Multicast value which is higher than the Mandatory MCS rate. On 3Com Comware devices like the AP 9552 this is NOT possible. The Multicast MCS value cannot be higher than the Mandatory MCS rate. ;-)