What information do you get from the signal recorder of the SCALANCE W components?
The SCALANCE W components have an integrated signal recorder. The signal recorder records the signal strength of the access points and other connection data on the IWLAN client side. It is a useful tool for obtaining initial information about the connection quality and transmitting aperture illumination of a plant with a view to possible optimization.
Since firmware version V6.1 there is an extended version of the signal recorder that records the data of the access point at the same time. This makes it possible to obtain even more information from the signal recorder.
Operation of the signal recorder
You must plan a signal recorder recording in advance in order to obtain useful information from the signal. The following questions among others are to be dealt with:
- Is the application static or mobile?
- Where can the IWLAN clients be located in the plant?
- What critical points are there in the plant (shielding, sources of interference, ...)?
- Is the plant in full operation for the trial run (with regard to radio transmission)?
The list below shows how to set the signal recorder on an IWLAN client via the WBM (Web Based Management), start and stop the signal recorder as well as how to download the data from the SCALANCE W device upon completion of the signal recorder recording.
- Open the WBM and log on.
- In the navigation bar you navigate to "Interfaces > WLAN" and select the "Signal Recorder" tab.
The signal recorder is only available in the Client mode.
- Select the time interval required between two measurements. When the time interval is ≥ 100 you can enable the "Endless" option to perform an endless measurement. In this case the signal recorder runs until it is stopped manually or the device is reconfigured.
With the "Endless" setting, after 10,000 samples the recording runs in a cyclic buffer and overwrites the last values.
- Then select how many measurements you want made. The combination of these values determines the maximum duration and resolution of the recording.
This field is grayed out in the case of an endless measurement.
A value of about 100ms as sampling interval permits a recording of about 17 minutes. This gives you conclusive values about the signal strength. You can then also obtain limited information about the roaming speed.
With a recording of about 20 minutes, for example, the evaluation includes the last 17 minutes of the recording or always the last 10,000 samples.
You can also operate the signal recorder via the Command Line Interface (CLI). Refer here to the manual in Entry ID 109745196, Section 5.9.2.
- Click the "Start" button to start the recording. To record with the IWLAN client in the area you require, you move the client there. It is useful here to create a movement profile to be able later to pin the measured values to specific locations in the plant.
- Click the "Stop" button to finish the recording or wait until the recording terminates automatically.
- In the navigation bar you navigate to "System > Load&Save" and select the "HTTP" tab.
- Click "Save" to download the data after a completed signal recorder recording.
Save the .zip file in a storage location of your choice and unpack the file.
The file contains a PDF file and a .csv file with the single measuring points.
Results of the signal recording
The PDF file includes a graphical display of the effective useful signal in dBm and the data rate in Mbps. The colors of the graph correspond to the display in the Web Based Management. If the client changes the Access Point (roaming) during the measurement, vertical black bars with a black square at the tip indicate the event.
If the client has an iPCF-M connection, the useful signal of the management channel is also displayed with a black line.
The PDF file of the signal recorder is split into four areas. A sample recording is available in this entry under "Download".
- Graphical area (signal strength, noise floor and roaming)
- Graphical area (retries in percent, data rate and roaming)
- Information about the IWLAN client
- List of measured values in table form
On the first page is the data for the client, on the second page the data for the connected access point. The data for the access point is only complete with a bidirectional recording.
The graphical overview of the signal recorder is very good to get a first general idea. Other pages of the PDF file include the list of individual measured values in the recording. The complete list of measured values is also available in the .csv file.
The signal recorder run in the download shows a recording of a simulated RCoax segment. The client moves along the RCoax cable and roams from AP to AP (access point).
The following observations can be read very easily from the graphical area (the conclusions apply for the application iPCF with RCoax and depend on the real plant).
- Observation: Is the signal strength in the required range (between -35dBm and -65dBm for iPCF)?
Measure: If not, you can make changes to the transmission power of the client and AP.
The clients do not necessarily roam when connected to an AP with low signal strength. Nevertheless, even at low signal strength there should be a "better" AP available to which the client can roam as soon as the connection breaks down. You can tell this by the fact that after roaming a connection is established immediately to an access point with a good signal strength.
- Observation: Are the signal runs similar at constant speed or are there sometimes very sharp drops in signal strength?
Measure: One reason for unusual drops in signal strength might be incorrect assembly. For example, incorrectly connected connectors, wrongly connected RCoax cable or damaged RCoax cable.
- Observation: When and how often does the client roam to another AP?
Measure: If the client roams very frequently between two APs, this indicates that the setting is not optimized. There are two possible reasons for the "jumping" between two APs. One reason might be bad transmitting aperture illumination (both APs have only a very weak signal strength), another might be the setting of the "Roaming Threshold" parameter. A client roams sooner or later to the next AP depending on the setting of the roaming threshold.
- Observation: Is the data rate (black line in the second graphical area) in the expected frame with good signal strength or are there strong fluctuations in the data rate despite good signal strength?
Measure: If the data rate is too low, this might indicate bad signal quality caused, for example, by the proximity of a source of interference. It is therefore recommended to create and maintain a channel plan if you want to operate various radio applications in one plant.
Observation: The noise floor is relatively high: There are other interferences on the channel or neighboring WLAN APs are impairing communication.
Measure: If the signal is significantly above the noise floor (SNR), there are no measures to be taken. If the ratio is too low you can, for example, increase the transmission power of the AP and clients or use different antennas. Alternatively you can consider switching to a noise-free channel if available.
Note: Time-critical applications should always be run on free channels. This requires channel planning.
Observation: Is there a significant difference in the signal strength at the AP to the signal strength at the client at the same time?
Measure: You should monitor this behavior if the transmission strength at the AP is different to that at the client. In addition, this behavior can occur if either the transceiver or the receiver is defective. You can run a test with a second reference AP or client to determine whether the transceiver or receiver is defective.
Observation: Are retries shown during the process?
Measure: Retries are always normal with WLAN. Retries might occur due to the environment and reflections, other WLAN devices or sources of interference. Furthermore, you should monitor whether the retries are displayed in percent per time unit. If, for example, only one package is sent in 100ms, but this has to be repeated once, then you have a retry rate of 100%. No measures are necessary here.
Observation: Are frequently high retry rates shown?
Measure: Check the configuration and your antenna connections. Check your channel planning and if necessary, remove other sources of interference from the channel. Check the location where the interference occurs frequently: You might have to optimize the antenna alignment or metal might be causing reflections that lead to interference. Generally a high retry rate does not pose a problem. However, if you have an application problem and a high retry rate, then you should take a closer look at it.
The following conclusions may be drawn from the measured values of the signal recorder:
- In what order does the client log on to each separate AP (detectable through the MAC addresses and SSIDs)?
- Is the client temporarily not connected to an AP and for how long?
From these observations you can conclude at which point there is need for optimization of the segment and/or the transmission aperture illumination.
The signal recorder records the signal strength of the currently connected AP. It is therefore not possible to analyze the radio environment conditions (for example, Bluetooth applications, radar, other WLAN applications, ...)! Use a spectrum analyzer for this.
If the client is not connected to an access point, the MAC address 00:00:00:00:00:00 and the signal strength -95dBm are specified in the signal recorder file.
The download includes the signal recorder run with DCF in a simulated environment.
The pictures and downloads in this FAQ response were created with the firmware V6.1 of the SCALANCE W components.
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