Maintaining Connectivity with Link Policies and Settings

Jul 08, 2015 - Author: Sierra Wireless - 1219 Views

Introduction and Pre-Requisites

This article provides supplemental information on using oMG policies to maintain connectivity, and describes some of the similar/related WAN and cellular link properties.

Note: this article requires an understanding of policies and WAN link settings. For information on these settings, see the oMG Operation and Configuration Guide.

Goals of Policies Versus Settings

The primary goal behind all of the policies is to proactively switch between WAN links to ensure that WAN connectivity is never lost, while a secondary and closely‑related goal is to also ensure that a switch between links does not occur unnecessarily.

Most WAN settings on the other hand are more concerned with identifying links as being eligible for selection, as well as maintaining and recovering from low‑level connection failures.

When tuning WiFi and cellular link settings as well as policy settings, it’s important to determine the expected usage of the vehicle in which a given oMG is installed. In particular, be sure to identify how “mobile” or “stationary” the vehicle is expected to be and whether WiFi or cellular is the preferred type of link in these situations. Identifying these factors will help you to select the appropriate WAN link settings and determine which policies, if any, should be applied.

Priority Score Explained

One of the key settings is the Priority Score which allows links to be given an order of preference. Although the Priority Score setting appears in the LCI on the Dynamic Priority Policy screen and includes an “Enable Policy” field, it is not related to the Dynamic Priority Policy specifically, and should not be thought of as a policy in and of itself or related to the configuration of another policy. However, other policies can and will assign an overall score to a link by adding this Priority Score to a base score of 1000.

Commonly Used Policies

Of the five policies available (Dynamic Priority, Geographical Region, Time Period, Velocity, and Signal Strength), the following are the most commonly used in practice:

Dynamic Priority Policy –the most widely used policy because it handles connectivity issues, and forces a failed link to prove itself before it can be re‑enabled as the active link.

Velocity Policy– frequently used when an oMG has both WiFi and cellular links where WiFi is the preferred link. This policy is generally applied only to a WiFi link to make it preferred over cellular (e.g. when operating near a depot which provides WiFi).

When choosing the speed threshold at which the penalty is to be applied be sure to take the distance away from the WiFi hot spot into consideration as well as the vehicle’s expected speed at that distance. The speed threshold should occur long before the WiFi coverage degrades too low so that a proactive switch to a cellular WAN link can take place before WAN performance is affected.

Signal Strength Policy –frequently used when both WiFi and cellular links are available and WiFi is the preferred link.

This policy can also be used when multiple cellular links are available such that the oMG can switch away from a degraded, but otherwise preferred (e.g. higher Priority Score) cellular network. In order to choose a threshold, it is best to monitor the signal levels across the operating region over a period of time to ensure that an undesirable level is not frequently encountered for a preferred carrier.

Note too that wide discrepancies in signal strengths amongst carriers are becoming less common in most areas today, thus reducing the need to use this policy. Therefore, be sure to assess the expected signal strengths amongst your carriers over your expected operational regions, to determine if it’s even worth setting up the Signal Strength Policy for selecting cellular links.

Link Settings

When setting up a link’s policies, it’s important to be aware of the link’s underlying connectivity settings, some of which can affect how the policies perform. However, most can be left with their default values.

WiFi Link Settings

The Association Settling Period and Disassociation Settling Period settings are intended to control the eligibility of a WiFi link for selection/de‑selection as the active link, respectively. The Association Settling period is similar in concept to the Dynamic Priority policy which forces a link to prove itself before being selected as the active link. The key difference is that the Association Settling period forces a WiFi link to be stably connected to a single access point for a period of time before declaring it eligible for selection, where as the Dynamic Priority policy performs the analysis and process of selecting an active link from a collection of eligible links.

Thus the goal of the Dynamic Priority policy is to prevent an unnecessary switch by having a link prove itself, where as the goal of the Association Settling period is to avoid the hardware‑level overhead associated with activating a link (e.g. authentication, IP address assignment. etc.), by preventing an unstable link from becoming eligible for selection.

The Background Scanning Interval is a WiFi setting that controls the interval at which the oMG looks for eligible WiFi access points when already connected on a link. The default value is 300 seconds and can be left as is. However since WiFi access points remain stationary, the value can be set to an even more conservative value (e.g. up to 15 minutes) for a vehicle that doesn’t move around a lot.

For vehicles that do move around a lot, the value should be adjusted to between 5 and 10 seconds so that upcoming WiFi access points can be detected early, their stability proven, and selection as an active link completed before connectivity on the current link breaks. Note that the interval should not be set too short because scans affect data throughput. Therefore the range of 5 to 10 seconds should be considered the optimal and shortest interval values for highly mobile vehicles.

The Signal Strength Average Length setting specifies the number of signal strength samples to be obtained at each Background Scanning Interval. This can be thought of as a filter which averages out the samples obtained and reports the average value (i.e. moving average).

When a vehicle is stationary, signal strength is affected by environmental factors such as moving objects which interfere with the signal, and therefore the default of 10 samples can be left as is. For vehicle’s that move around a lot, the vehicle’s movement is the primary cause for signal strength fluctuations and therefore this setting should be dropped down to between 1 and 3.

It’s important to be aware of this setting, because the averaged signal strength value will be observed by the Signal Strength Policy when it evaluates a WiFi link. Therefore, the recommended settings above will ensure that a WiFi link will be eligible for selection quickly, thus allowing the policy to perform optimally and react fast enough for WiFi link selection when mobile.

The Roaming Squelch setting is typically enabled for oMG’s that remain primarily stationary (e.g. spend a lot of time at depots) and ensures that a WiFi connection remains on the current access point unless the signal‑to‑noise ratio settings (Minimum Quality of Signal, Satisfactory Quality of Signal, and Minimum Quality of Signal Differential) are not being satisfied.

The settings operate as follows:

Minimum Quality of Signal: prevents switching to an access point that has a poor signal‑to‑noise ratio. Note: it’s recommended that the default value of 8dB be increased to 20dB since most access points should be accessible at this value. 8dB is fine for rudimentary low‑bandwidth operation, but 20dB is recommended for optimal performance on 802.11a/g/n networks.

Satisfactory Quality of Signal: prevents switching to another access point that has a better signal‑to‑noise ratio, when the current access point has a sufficiently good signal‑to‑noise ratio.

Minimum Quality of Signal Differential: specifies the minimal difference in signal‑to‑noise ratio between the current access point and a potential access point that must be satisfied in order for the latter to be considered. This prevents an unnecessary switch to an access point which may only have a marginally better ratio.

Roaming Squelch and its associated settings, cause the oMG to judge WiFi access points based on their signal‑to‑noise ratios where as the Signal Strength Policy is typically used in the assessment of cellular signal strengths in outlying areas. For vehicle’s that are primarily mobile, Roaming Squelch should be disabled.

Cellular Link Settings

The Signal Strength Filter Length for a cellular link setting operates the same as the Signal Strength Average Length setting for WiFi, by specifying the number of signal strength samples to obtain to calculate the moving average. This average is then used by the Signal Strength Policy when assessing the viability of a cellular link.

The polling interval is specified via the Signal Polling Interval field (default is 2000ms). One point worth noting is that as of oMG 3.11, this field is currently located at the bottom of the settings screen in the LCI even though it relates directly to the Signal Strength Filter Length field.[1]

The values of both fields can generally be left as they are (10 for Signal Strength Filter Length and 5 for Signal Strength Change Threshold) as they provide adequate agility and reactiveness for policies to select a cellular link.

Common Settings

Both WiFi and cellular links contain a Split Access field that, when enabled, allows sessions to be initiated on a link that has not been selected as active. This can be used by Support to SSH into an oMG when diagnosing issues, or for features like live video look-in.

It’s important to be aware of this setting, since a policy may have selected a different active link, yet data traffic such as that described in the examples above may be occurring on another link with Split Access enabled.
[1] The field’s location may change in future releases.
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