6 IMS eCall Conformance of In-band Modem

26.2693GPPConformance testingeCall data transferIn-band modem solutionTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.1 General

When supported by the IVS, mobile network, and PSAP, IMS eCall uses SIP signalling to send the MSD to the PSAP as this provides a compact and reliable means to transfer the information without interrupting the voice conversation between the motorist and the PSAP operator. During the deployment of IMS eCall, there may be temporary scenarios where the mobile network does not support a circuit-switch bearer and the network or PSAP do not support the IMS eCall protocol. In such scenarios, the IVS has no reliable means to send the MSD to the PSAP. As a solution of "last-resort" clause 7.7.2 of 3GPP TS 23.167 [7] and clause 5.1.6.11.2 of 3GPP TS 24.229 [8] require that the IVS attempt to transfer the MSD via the in-band modem sending the voice-band signals through the Voice-Over-IP bearer.

As ETSI and 3GPP determined that the operation of the in-band modem over the VoIP bearer cannot guarantee reliable transmission of the MSD, the deployment scenario that requires use of this "last-resort" solution must be avoided as much as possible. Furthermore, the testing of this mode of operation in the IVS does not require testing of reliability. The testing only focuses on conformance as follows:

1) The in-band modem conformance shall be tested separately from the VoIP bearer using the tests specified in clause 5.

2) To test that the in-band modem conforming to clause 5 is operating over the VoIP bearer in the IVS, the test described in clause 6.3 should be used.

To avoid the effects of jitter and time-warping interfering with the testing of the in-band modem, the test set-up should not introduce any time-warping when decoding the VoIP packets encoding the in-band modem signals. Therefore the test specified in clause 6.3 is recommended.

If the test set-up is not able to prevent time-warping in any adaptive de-jitter buffers at the IVS, PSAP, or intermediate nodes (e.g., media gateway) then the test in clause 6.3 can not be used to reliably test the in-band modem. The test in clause 6.4 shall only be used if the test in clause 6.3 can not be used.

6.2 Initial Settings

Following are the initial settings to be used before starting the test:

1) Place the PSAP simulator into PULL mode so that it sends START signaling ([1] and [2]) once the voice bearer is connected with the IVS under test

2) Negotiate use of the AMR 12.2 speech codec for encoding the in-band modem signal

3) Use an error free channel model in the channel simulator

4) Set-up the IMS eCall session so that it triggers the "last-resort" condition in the IVS that requires the in-band modem operation over the VoIP bearer.

6.3 No Time-Warping Conformance Test

If the test set-up can prevent time-warping of the decoded VoIP signal (i.e., it does not introduce jitter or the de-jitter buffers in the IVS and PSAP do not perform time-warping) then the following test procedures shall be used:

1) When the IVS detects the START messages sent by the PSAP (after the voice bearer is established), the IVS is expected to send the MSD.

2) If the PSAP simulator detects that the IVS has started sending the MSD then the IVS PASSES conformance. Detecting the start of MSD transmission does not require decoding the entire MSD correctly. The PSAP simulator could detect the sync ([1] and [2]) from the IVS that precedes the MSD information.

3) If the test fails, i.e., the PSAP simulator does not detect the sync or the MSD within 20s, the call is terminated.

a) If the IVS has failed five consecutive tests then it FAILS conformance.

b) Otherwise the test is re-run starting with the triggering event in step 4 of clause 6.2.

6.4 Time-Warping Conformance Test

If the test set-up can not prevent time-warping of the decoded VoIP signal (i.e., it could introduce jitter and the de-jitter buffers in the IVS and PSAP could perform time-warping) then the following test procedures shall be used:

1) When the IVS detects the START messages sent by the PSAP (after the voice bearer is established), the IVS is expected to send the MSD.

3) If the test fails, i.e., the PSAP simulator does not detect the sync or the MSD within 20s, the call is terminated.

a) If the IVS has failed 50 consecutive tests then it FAILS conformance.

b) Otherwise the jitter-model of the channel simulator is re-randomized to ensure that the same jitter profile is not re-used and the test is re-run starting with the triggering event in step 4 of clause 6.2.

Annex A (normative):
Test sequences and scripts for testing bit-exact implementation of eCall in-band modem

This Annex describes the test set-up, test sequences, and scripts designed to evaluate the bit-exact implementation of the eCall in-band modem [2].

For the purpose of conformance testing, the control sequences at the receiver-transmitter interfaces of the IVS and PSAP are defined as the content of certain state variables at the end of the receiver processing stage after each frame. The IVS and PSAP input/output data signals are standard PCM data signals with 16 bits per sample and 8 kHz sampling rate. Three sets of state variables and PCM data signals are provided to carry out the bit-exactness tests for the eCall core, the eCall push trigger, and the eCall HLACK transmission. The eCall core is defined as a transmission that is initiated by the PSAP and terminated by the transmission of lower layer ACK (LLACK) messages after the successful reception of the MSD by the PSAP. The push trigger test files are intended to test an IVS initiated eCall transmission, which starts with the transmission of SEND messages and is terminated by the transmission of lower layer ACK messages after the successful reception of the MSD by the PSAP. The HLACK transmission test files are intended to test the transmission of HLACK messages, which starts with an PSAP initated MSD transmission and terminates with the successful reception of the HLACK by the IVS. The three data sets are distinguished by their ‘basename’ which is to be replaced by ‘core’, ‘corepush’, or ‘corehlack’ in the tables below, respectively.

Note that the number of LLACK and HLACK messages to be transmitted is a configuration parameter of the in-band modem [2]. It is up to higher-layer protocol requirements whether link-layer and/or higher-layer ACK messages are transmitted. From a modem protocol perspective [1], at least five ACK messages of one type (either link-layer or higher-layer) shall be transmitted consecutively for security. For conformance testing, the modem shall be configured to send four LLACK and then five HLACK messages.

The attached test campaign file campaign_conformance_test.txt specifies the MSDs, delays, codecs and channel conditions to be used according to Tables A.1 to A.4.

The test scripts conftest.bat (for Windows) and conftest.sh (for Linux) are provided in order to perform all tests according to clauses A.1 to A.4. Note that these are exemplary test scripts for the case that the IVS and PSAP implementations exhibit the same output signal delays as the reference implementation [2]. For other signal output delays within the limits specified in clause 5.1, the scripts should be adapted.

The test setup included in [2] automatically records input/output PCM data, as well as the contents of the state variables at the transmitter/receiver interface. The test setup also checks for the correct reception of the MSD. It is required for the execution of the above test scripts.