WGB Roaming time is the time taken by a WGB radio role to disassociate from one AP and reassociate to another AP. During this interval there is no data transfer & therefore roaming time is significant to maintain sessions.

Roaming involves two main processes

1. Scanning
2. Reassociation

Scanning:
WGB support two main modes of roaming operation

1. Static mode (default) – Roaming is based on two main vairiables: “Packet retransmissions” or “loss of 8 consecutive beacons”
2. Mobile Station mode – On top of previous variables, the AP can do periodic analysis of signal level drops and data rate shifts.

When any of the above criteria is met, WGB will trigger roaming process, scanning approximately 10-20ms/channel. You can also limit the channels to be scanned through configuration. For example you can set only CH1, CH6 & CH11 to be scan in 2.4GHz radio deployment.

Scanning methodology followed is “Active Scanning“. Instead of listening to beacons from APs, WGB will actively send out  “probe request” packets and waits for 20ms to get a response in every channel. The AP will stop scanning after it receives the first response with satisfying signal. So scanning time may last approximately 40ms & may be shorter depending on radio hardware type.

There are two forms of configuring WGB roaming parameters

1. Use “packet retries” command : This will allow more conservative approach where WGB will not start a roaming process until data loss is detected or 8 consecutive beacons are missed.

int d0 or d1
packet retries <1-128> {drop-packet}

2. Use “mobile station” command : This will start a regular process on WGB to do preemptive roaming, which monitor the signal levels & rate speed changes and force a new roaming before the current AP signal is too low. This scan process will trigger small gaps in radio transmission when the radio is performing the channel scan. Starting from 12.4(25d)JA, minimum data rate can be configured to trigger a roaming event in case of data rate change. If you want to limit the number of channel to scan you can use the “mobile station scan” command.

int d0 or d1
mobile station
mobile station period <1-1000s> threshold <1-100>
mobile station minimum-rate <min-data-rate>
mobile station scan <scanning-channels>

If the WGB starts scanning because of a loss of eight consecutive beacons, the message “Too many missed beacons” is displayed on the console. In this case, the WGB is acting as a Universal Bridge Client, much like any other wireless client in its behavior.

In some situations, it is interesting to use the optional “drop” option in the packet retries, to preserve the association, even on the failure to transmit a data packet. This is useful for challenging RF environments, where the roaming can be also triggered by mobile scan command.

The mobile station algorithm evaluates two variables: data rate shift and signal strength and responds as:
1. If the driver does a long-term down shift in the transmit rate for packets to the parent, the WGB initiates a scan for a new parent (no more than once every configured period).
2. If the driver detect the RSSI from its parent is below the configured threshold, WGB initiates a scan for a new parent (no more than once every configured period).

The data-rate shift can be displayed using this command:

debug dot11 dot11Radio 0 trace print rates
!
*Mar  1 00:33:20.371: 436472AA-0 BBF420 - Set rate: m15.-2s 144 Mbps (20F), Rssi 29 dBm
*Mar  1 00:33:44.379: 44D32696-0 BBF420 - Set rate:    m15. 130 Mbps (10F), Rssi 29 dBm
*Mar  1 00:33:47.380: 4500FD0C-0 BBF420 - Set rate: m15.-2s 144 Mbps (20F), Rssi 29 dBm
*Mar  1 00:34:04.386: 4604BC7B-0 BBF420 - Set rate:    m15. 130 Mbps (10F), Rssi 29 dBm
*Mar  1 00:34:05.386: 461403FF-0 BBF420 - Set rate: m15.-2s 144 Mbps (20F), Rssi 30 dBm

However, this will not show the actual data rate shift algorithm in action, but only the changes in data rate. This determines the time period to scan, depending on how much the data rate was decreased.

The mobile station period should be set depending on the application. The default is 20 seconds. This delay period prevents the WGB from constantly scanning for a better parent if, for example, the threshold is below the configured value.

Some situations may require a faster timer; for example, on high speed trains. The period should not be lower than the time that is required by the AP to complete the authentication process. For example, for 802.1x + CCKM networks, it should not be set below 2 seconds. PSK networks may use one second. The actual period will always have one second added to the timer, product of the AP scheduler resolution for this task.

The threshold sets the level at which the algorithm is triggered to scan for a better parent. This threshold should be set to noise+20dBm but not more than -70dBm (+70 because input for threshold is positive). The default is -70 dBm. The correct threshold will depend on the intended data rate, versus the coverage level offered in the environment where the WGB will operate. Assuming a proper coverage, we should set this threshold to be a little less than then “breaking point” for the needed data rate for the applications in use. Here is the RSSI sensitivity values for a 3502I AP (you can find the full table from AP’s data sheet)

When you enable these settings, the WGB scans for a new parent association when it encounters a poor Received Signal Strength Indicator (RSSI), excessive radio interference, or a high frame-loss percentage. Using this criteria, a WGB configured as a mobile station searches for a new parent association and roams to a new parent before it loses its current association. When the mobile station setting is disabled (the default setting) the WGB does not search for a new association until it loses its current association.

There are 3 types of scans implemented for the WGB
1. Normal Scan
2. Fast Scan
3. Very fast Scan

A normal scan begins on the associated channel & continues to cycle through the rest of the channels. For example if WGB is associated to CH6, then it will start  its scan on CH6, CH7 … CH13, CH1…. CH5. Upon scanning all channels & receiving more than one probe response WGB will do compare function that compares (RSSI, Load, Hops, etc) all responding APs to the one it was previously associated.

WGB perform a fast scan when traffic is between 10-20 packets per second. The WGB scans and associate to first responding AP during fast scan.

During a very fast scan, the WGB does not scan at all and try to associate to the best AP in the adjacent list that is build up with IAPP and CCX.

In certain situations depending the application parent list may have “directionality”. For example a train is traveling to given directions would not have any benefits of the neighbor-list since train is moving away from them. You can configure the following command to ignore neighbor-list.

int d0 or d1
mobile station ignore neighbor-list

We will look at the Reassociation in the next post.

Reference
1. Outdoor Mobility Design Guide
2. https://supportforums.cisco.com/docs/DOC-14944

Related Posts

1. WGB Configuration
2. WGB with EAP-FAST
3. WGB with CAPWAP
4. WGB with PSK
5. IOS AP-WGB with Multiple VLAN
6. Unified AP-WGB with Multiple VLAN
7. Packet Retries & Max-Retries
8. WGB Roaming-Part 2
9.