16 - Understanding Wireless Roaming and Location Services Flashcards
Moving from one access point to another is called ____.
roaming
What are common situations that typically cause roaming?
- Maximum data retry count is exceeded – excessive numbers of data retries
- Low RSSI – when the received signal strength drops below a threshold
- Low SNR – when the difference between the received signal strength and noise floor drops below a threshold
- Proprietary load-balancing schemes – schemes in which clients roam to more evenly balanced client traffic across multiple APs
This occurs when a client changes it’s 802.11 radio to the channel that is being scanned, broadcasts a probe request, then waits to hear any probe responses from AP’s on that channel.
active scan
A client sends probe request to specific destination SSID, only APs with a matching SSID reply with a probe response.
directed probe
A client sends a broadcast SSID (actually a null SSID) in the probe request, all APs receiving the probe request respond with a response
broadcast probe
This is performed by changing to 802.11 radio of the client to the channel that is being scanned and waiting for a periodic beacon from any ap’s on that channel. By default, APs send beacons approx. every 100 ms
passive scan
During a channel scan, the client is unable to transmit or receive client data traffic. Clients take various approaches to minimise this impact to their data traffic. What are these approaches?
- Background scanning
* On-roam scanning
Clients can scan available channels before they need to roam. This scan builds on the knowledge of the RF environment and available APs so clients can roam faster if necessary
background scanning
In contract to background scanning, ___ occurs when a roam is necessary. Each vendor/ device can implement its own algorithms to minimise the roam latency and the impact on data traffic.
on-roam scanning
A ___ is a collection of mobility controllers (MCs) across which roaming needs to be supported. A ___ can consist of up to 24 WLCs that share the context and state of client devices and WLC loading information. The WLCs of a ___ forward data traffic among the group, which enables intercontroller WLAN roaming and WLC redundancy.
mobility group
A wireless client can roam between any AP in a mobility group without being required to ___ to the network.
re-authenticate
A controller can recognise controllers that belong to another mobility group. When this occurs, the controllers are said to be in the same ___.
mobility domain
For two controllers to be in the same mobility domain, they must know each other, which means that the built in ______of each controller must be entered in to the other controller.
mac address and management IP address
If a client moves from one controller to another in a different mobility domain (that is, to a controller that is not known by the controller that the client leaves), the client will have to ___, ___ and ___.
re-authenticate, re-associate and get new IP information
To ensure fast secure roaming, all controllers between which roaming should occur should be in the same ___.
mobility group
Controllers in a MG must all have these characteristics in common:
- Mobility domain name
- Version of controller code
- CAPWAP mode
- ACLs
- WLANs (SSIDs)
There are two types of roaming…
- Layer 2 roaming
* Layer 3 roaming
__ roaming occurs when the client moves from one AP to another and is maintained in the same client subnet/VLAN.
L2
L2 roaming occurs in two ways:
- Intracontroller roaming
* Intercontroller roaming (layer 2)
• ___ roaming
o when a client moves from location t1 to t2, it asks for reauthentication on a new AP.
o this authentication is done by a query that is sent to the controller to which the AP is connected.
o If the controller is the same one to which the AP that the client is leaving was associated, the controller simply updates the client database with the newly associated AP.
intracontroller roaming
• ___ roaming
o In this process, a client roams from an AP that is joined to one controller and roams to an AP that is joined to a different controller
o When a client at location t2 moves to t3, it associates to an AP that is joined to the new controller, the new controller exchanges mobility messages with the original controller and the client database entry is copied to the new controller.
intercontroller roaming (L2)
Intercontroller L2 roaming remains transparent to the user unless:
- Client sends a DHCP discover request
* The session timeout is exceeded
__ roaming occurs when the client moves from an SSID on one AP (that is associated with one VLAN and respective IP subnet) to the same SSID on a different AP (that is associated with a different VLAN and IP subnet).
L3
L3 roaming can occurring the following way:
Intercontroller Roaming (layer 3)
