Wireless Access

Hey Folks,

we're using a HP MSM 765zl Premium Mobility Controller Team at our university.

Thus, there is no detailed explanation 'bout how the roaming is handled, our question is, how the wlan-roaming is realized.

Is it only a simple, "client-based"-roaming, handled by clients/operating systems, which decides, when the system switches to the next accesspoint, reauthenticates, thats it...?

Or is it a real "enterprise"-roaming where the controller makes a pre-authentication and roames the clients, pro-actively, through the accesspoints, so that there is no interruption and (e.g.) voice-over-wlan would work without any interruption?

Thanks in advance for your answers!

Best regards,

Matthias Roeske

Hi

Roaming in a controlled AP environment (involving MSM controller and AP deployment), is based on client decision.  However there are few things you could tweak on the controller configuration that could influence "clients roaming behavior", such as:

- remove some of the lower data rates on the VSC profile, which basically reduces the RF cell size eliminating lower data rates.  For example on the VSC profile page --> Allowed wireless rates --> if you remove some lower data rates 1,2,5.5,6,9,12..etc, this would mean as soon as the client start to move away from an AP, then they wont be able to support lower data rates (with poor RSSI) to sustain the connection.  Eventually client wlan card will start thinking about moving to another access point which provides connection at a rate that a client can hear.

- reduce transmit power on the Radio page for respective band (5Ghz, 2.4Ghz).  This another method of reducing the RF cell size.  Smaller the cell better the data rate and less of co-channel/adjacent channel interferences.  When there are smaller donouts of RF coverage is deployed, its easy for the client to make the decision of switching swiftly between the nearest APs

- Again on the Radio page - "distance between AP" - default setting is LONG.  If you set it to Small, then Radio will not accept client connection with poor RSSI (more could be read on the online help page of the webgui - click on ? icon)

At the end of the day, its the CLIENT who makes the decision on when to roam.  WLAN configuration can only help in a way that the client can make the decision more effectively.

Hope this helps.

Adding a bit...

As Ram says, roaming decision is always, without exceptions, made by the client. It can't be forced by the infrastructure.

A client NIC's purpose is to maintain network connectivity. It will not roam when it detects an AP with a stronger signal, it will roam when the RSSI of the first association falls below a certain threshold. This threshold is determined by the NIC manufacturer. For example Intel Wless NICs have a setting "Roaming aggressiveness" which is by default set to Medium - this setting changes the threshold. The methods Ram described are a way of making the RSSI fall below the threshold faster, thus forcing the client to search for a stronger signal.

What you are looking for is called "Fast Roaming", and there are several methods to achieve that. With MSM products we use Opportunistic Key Caching.

When the client first associates with an AP, during EAPOL exchange a Pairwise Master Key is created. The client stores the key identifier (PMKID). When the client roams to another AP, it does not have to re-authenticate, it just provides the PMKID to the next AP and the AP will allow the client through. 

This feature is available for non-Access-Controlled VSCs that use WPA2. When you look at the VSC settings, you'll find "Fast Wireless Roaming". The APs also support Spectralink Voice Interoperability for Enterprise Wireless (Spectralink VIEW), you'll find this in the AP Radio settings.