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The oMG’s Dynamic Priority policy is used to proactively select the most preferred WAN link. This article provides an example of how to use this policy to select between two carrier links and a WiFi link.
Before continuing with this article, be sure to read the oMG Operation and Configuration Guide which details the various policy settings.
A common scenario is to configure an oMG with two carrier cards as well as WiFi access, where one carrier card is the primary, the other is a backup, and WiFi is the most preferred WAN link when available (e.g. when in or around a depot). Since each link is assigned a base score of 1000 by the oMG, priority is specified by assigning a “Priority Score” to each link which is then added to the base score.
The following example shows how this could be configured using the dynamic priority policy where a Verizon E362 is set as the primary carrier and a Sprint CC208 as the secondary:
Verizon E362 (primary link):
• Priority: 100
• Penalty: 150
• Recovery time is 60s
• Default score: (1000 + 100=1100)
Sprint CC208 (secondary link):
• Priority: 0
• Default score: 1000
WiFi (most preferred):
• Priority: 200
• Default Score: 1200 (1000+200)
If a vehicle is utilizing its Verizon link while travelling but the link drops, then the link’s score becomes 950 and the Sprint link will take over since the Sprint link now has a higher score. When Verizon connectivity becomes available again, the score for the Verizon link will increase by 2.5 points for every second over the recover period. Note that the increment is calculated using the penalty divided by the recovery time: 150/60s= 2.5points/second.
At 20 seconds, the score for the Verizon link will be 1000 which is equivalent to that of the Sprint link, but Sprint will remain the active link. However, at 21 seconds, the score for the Verizon link will be 1002.5 at which point it will become the active link again.
If the Verizon link goes down again before reaching its default score, it will subtract the penalty from the score that it currently has (e.g. if the Verizon link is 980 and climbing when the Verizon link goes down again, the new score would be 980-150=830). This ensures that if the oMG is located in an area of poor Verizon coverage, the link has a chance to stabilize before being set to the active link.
When WiFi connectivity is available (e.g. the depot’s WiFi), the oMG will utilize the WiFi link since it has the highest score (1200) even if the cellular links are available.
One common issue that can occur when WiFi is the most preferred link, is a “flip flop” between the WiFi and preferred cellular links as the oMG enters the cellular coverage area. To rectify this, it’s recommended that the Signal Strength Policy be applied to the WiFi link to drop its score below that of both cellular links as the oMG travels away from the hotspot, thus forcing a switch to a cellular link even when a weak WiFi signal is available. A Signal Strength Threshold to -75dBm is typically used in this configuration.