Testing analog lines

Why this white paper ?

In a world of Voice over IP (VoIP), we could think analog is abandoned but it is not mostly the case.

Analog lines are still used for lots of services. The main reason is the cost of that technology.

Working with analog line is very cheap, does not require lots of components, grant access even in case of power outage and provides acceptable speech quality.

Analog connection offers multiple services:

  • For Internet Service Providers: the boxes (fiber or xDSL connections) include a phone line port which is analog.
  • For Emergency services: fire alarms, security systems, elevator emergency phones
  • For telecom backup systems: in case of digital system failure
  • For transportation: train stations, air bases in outdoor conditions

Typical analog phone behavior



a. When a phone wants to place a call it will first off-hook.

b. The PBX will send a tone. This is called a dialing tone. This is where we calculate the initial response delay.

c. Then the phone sends DTMFs corresponding to phone number to call. In the meantime , as the DTMF are sent, the PBX is stopping transmitting the dial tone.

d. The PBX will transmit the call request over the phone network.

In the meantime, and depending on network performances and services:

    • If the destination number is valid, the phone receives a calling ringback tone.
    • Otherwise, several tones/vocal messages can be sent from the PBX including “unavailable”,“busy”:
        • If the destination number is not valid, the phone receives an unavailability ringback tone.
        • If the destination number is already engaged, then, the phone gets a busy tone
        • If the phone had subscribed some services that do not permit to place a call, then the phone will receive a vocal message from the PBX.
        • If the destination phone number does not accept incoming calls, then a vocal message or a tone should be played.

Introducing MultiDSLA

MultiDSLA application server

MultiDSLA is the solution able to perform signal, speech and music generation/measurement.


DSLA  stands for Digital Speech Level Analyzer. It is a hardware box containing what we call channels.

A channel can be configured in phoneline mode or hanset mode.

In phone line mode, DSLA line interface behaves as an analog phone that connects to a PBX with 2 wires.

In handset mode, DSLA uses a 4-wires connection (balanced send and receive).

Troubleshoot call setup issues

Call setup recording

To record the call setup, the MultiDSLA node must have call progress option enabled:

Viewing a result

By viewing a result within MultiDSLA GUI or through SPV, you can listen to the recorded call setup:

Issue resolution

No dial tone in call setup record

That means that PBX is not sending the dial tone.

Please verify the line connection between DSLA line port and the PBX.

Try to force the on-hook and perform a new test.

Dial tone available but ringback tone is not a call ringback tone

When a call is not established, you can face those situations:

  • Line unavailability ringing. When it occurs, that means that the destination phone number is in unavailability state. The tones are countries specific. The only way to characterize that tone is to listen to it.
  • Listen vocal message saying that the destination does not exists or is not available. You must look on both PBX and MultiDSLA to be sure that destination phone number is an allowed one.
  • Nothing after the dial tone and DTMF sending. The issue is on network side. You must troubleshoot why, when placing a call, nothing happens.


Is DSLA line port a FXO or FXS ?

DSLA line port is only FXO. DSLA is working like a phone connected to a PBX.

Can we perform a loopback between two lines channels on a DSLA with a simple cable?

The DSLA line port is acting as a phone. It is FXO interface. That means loopback is not possible.

What are the output and input level limits on the DSLA line port?

Input and output level limits are +10 dBm.

How can I force on-hook on a phone line in a DSLA ?

You can force on-hook with a dedicated tasklist.

Tasklist properties:

Put rotate speaker field to false

Put auto call setup field to false

Then add a control event on Node A

Then you can execute the tasklist with a phantom node.

Is there a way to identify the phone line state within a DSLA ?

This is not possible in MultiDSLA.

There is a way through the DSLA configurator to see if the phone line port on the DSLA is on-hook or off-hook

If you want to know more about that point, please contact Opale Systems support team.

Is the DSLA a voltmeter ?

Yes and no.

It measures signal in tones or speech. On analogue side it’s an AC voltmeter providing dBm or mV measurement.

How do I get mV measurement ?

First you change your node configuration through the node manager:

  • Default value is dBm
  • mV can be selected
  • Custom permits to switch to dBPa and to modify the sensitivity for dBPa calculation but it is not the subject of this document.

Then you can create/modify a tasklist  and add some level measurement events:

  • Speech Level: Measures the P.56 speech level, speech activity, noise level and long-term level on a particular channel. If signal is too low (< -65 dBm, or 0.44 mV) the speech level measurement cannot be performed. Please prefer the usage of other level measurements
  • Tone level: Determines the peak frequency between start and end frequencies and measures its level. If no value is specified for End Frequency, the measurement is taken at the Start Frequency only. The level measurement is a true RMS level and includes the influence of silence within the measurement window.
  • Tone Burst: Measures the P.56 active level, activity, noise level and long-term level on a particular channel with hangover processing disabled. This can be useful in determining the level of call progress tones that contain periods of silence.
  • Band Level: measure the energy on a specific frequency band.
  • RMS Level: measures the Root Mean Square level of the incoming signal

Are all countries using the same tones in analog signaling world?

Unfortunately, not. That is why DSLA conception was done to be agnostic on incoming signals.

Is DSLA able to act like an analog fax ?

It needs firmware development that is not done today. If you request one, please contact sales@opalesystems.com


FXO - Foreign Exchange Office

The FXO interface is typically found on the device that connects to the telephone company's central office (CO) or PBX (Private Branch Exchange).

An FXO interface allows the device to receive calls from and place calls to the telephone network.

In other words, it provides the connection to the telephone line from the perspective of the telephone network.

Example: An FXO interface is commonly used on devices like analog telephone adapters (ATAs) or PBX systems to connect to the PSTN (Public Switched Telephone Network).

​FXS - Foreign Exchange Station

The FXS interface is typically found on the device that connects to analog telephones, fax machines, or other analog devices.

An FXS interface provides the necessary voltage and signaling to power and control the connected analog device.

In other words, it provides the connection to the analog device from the perspective of the telephone network.

Example: An FXS interface is commonly found on analog telephone adapters (ATAs), analog telephone ports on PBX systems, or analog telephony cards in computers.

POTS - Plain Old Telephone Service

It is the traditional analog voice transmission service that has been in use since the early days of telecommunication.

PSTN - Public Switched Telephone Network

It is the global network of interconnected telephone lines and systems. PSTN encompasses both analog and digital components, including the infrastructure and switching equipment that route calls.

Level calculation – relation between dBm and mV

Line ports is using an impedance of 600 ohm in most of the cases.

From the voltage in V:

P1 in W = (voltage²/impedance)

Level in dBm = 10 * log (P1/P0) with P0 = 0.001. P0 is the power reference in dBm calculation (1 milliwatt)

From Level in dBm:

Power in mW = 10 (level/10)

Voltage in V = square root (Power*impedance/1000)

Contact us

For any request do not hesitate to send a mail to sales@opalesystems.com

Speech To Text and IVR testing
MultiDSLA tech note