7 Transmission protocol and error handling

26.2673GPPeCall data transferGeneral descriptionIn-band modem solutionRelease 17TS

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

This clause describes the employed eCall transmission protocol in normal operation, and its handling of transmission errors.

7.1 Normal operation

The operation of the eCall data transmission in the "normal" non-erroneous case works as outlined on a high level in the previous clauses.

Upon request by the operator or by the IVS push message, the PSAP transmitter starts sending START messages. The IVS receiver shall detect the synchronization preambles that are transmitted along with the START messages and obtain synchronization. This enables the IVS receiver to demodulate and detect the START messages. The PSAP transmitter continues sending START messages to the IVS at this stage. The maximum allowed number of START messages to be transmitted by the PSAP modem is determined by the higher-layer protocols and timers (which are out of scope for this specification).

Upon detection of the START message, the IVS starts the transmission of the first MSD message with incremental redundancy version rv0 which is preceded by a synchronization frame. The PSAP receiver shall detect the synchronization frame and obtain exact synchronization on the synchronization preamble. The PSAP receiver is then enabled to demodulate the MSD and to decode it.

As soon as the PSAP receiver has obtained synchronization, it changes the PSAP to NACK message transmission and continues sending this message repeatedly. The IVS transmitter then should detect the NACK messages. The IVS continues sending MSD data. When transmission of the MSD message with rv0 has been completed, the IVS transmitter continues sending the next redundancy version rv1 of the same MSD, and so on.

The PSAP receiver, after demodulation of the full MSD with rv0, performs a CRC check. If the CRC check fails, the PSAP receiver continues sending NACK messages. If the CRC check succeeds, the PSAP transmitter changes the message to the link-layer or higher-layer ACK message. It is up to higher-layer protocol requirements whether link-layer and/or higher-layer ACK messages are transmitted. From a modem protocol perspective, at least five ACK messages of one type (either link-layer or higher-layer) shall be transmitted consecutively for security. No higher-layer ACK message shall preceed a link-layer ACK message, and no link-layer ACK message shall succeed a higher-layer ACK message. For this purpose, the modem reference implementation transmits up to five link-layer ACK messages until a trigger from the higher-layer protocol is received. After the trigger, five higher-layer ACK messages are transmitted. The IVS receiver should detect an ACK of one particular type (link-layer or higher-layer) and then stop the IVS transmitter sending the MSD.

7.2 Abnormal operation

This clause describes some abnormal scenarios which may occur due to severe signal distortion on the transmission channels and which need to be handled by the overall transmission protocol to avoid any deadlock situations. This description of abnormal scenarios is not necessarily exhaustive.

Table 5 lists potential abnormal scenarios together with the measures implemented against it in the eCall solution. In the table, the reference numbers refer to the following case categorization:

1. IVS error cases

1.1 Sync error

1.1.1 No successful synchronization

1.1.2 Successful synchronization although no sync frames were sent

1.1.3 Successful synchronization, but wrong timing

1.1.4 Lost synchronization

1.2 Sync detected, correct timing, false detection of PSAP messages

1.2.1 Errors at transmission of START messages

1.2.1.1 START message sent, no downlink message detected

1.2.1.2 START message sent, NACK detected

1.2.1.3 START message sent, ACK detected

1.2.2 Errors at transmission of NACK messages

1.2.2.1 NACK message sent, no downlink message detected

1.2.2.2 NACK message sent, START detected

1.2.2.3 NACK message sent, ACK detected

1.2.3 Errors at transmission of ACK messages

1.2.3.1 ACK message sent, no downlink message detected

1.2.3.2 ACK message sent, START detected

1.2.3.3 ACK message sent, NACK detected

2. PSAP error cases

2.1 Sync error

2.1.1 No sync preamble detected

2.1.2 Sync preamble detected, but wrong timing or false detection

2.1.3 False evaluation of sync tone

2.2 Sync detected, correct timing, false detection of MSD messages

Table 5: List of potential abnormal cases and protocol solutions

Ref #	Scenario	Error description	Error handling	Comments
1.1.1	IVS receiver does not detect the sync frames or gets different delays from subsequent preamble detections.	MSD message will never be sent	Start message is sent a defined number times	If the start message is not detected within a defined time, the PSAP goes back to idle state. In this case another attempt could be started manually by the PSAP operator.
1.1.2	IVS receiver detects equal sync frames while none were sent.	Sync false alarm	Sync Check verifies the validity of the sync continuously and resets the IVS if necessary.	Synchronization at the IVS has been designed such that the probability of this error is negligibly small (virtually zero).
1.1.3	IVS receiver detects sync frames incorrectly and triggers nevertheless.	START message is usually not detected correctly and MSD message will never be sent	Same as #1.1.2	Same as #1.1.2
1.1.4	Synchronization gets lost due to some abnormal channel conditions	Feedback messages are skipped due to Sync Check and unsuccessful Sync Tracker	Same as #1.1.2	The Sync Tracker usually avoids this situation (e.g., due to adaptive jitter buffers or in the unlikely case of a handover)
1.2.1.1	In-sync, but detection of START messages fails	If START message is never detected, same as 1.1.1.	START message is repeated a defined number of times. This decreases the likelihood of this case to almost zero
1.2.1.2	In-sync, instead of a START message a NACK is detected	MSD message is not sent if transmission has not yet started. If a transmission is ongoing, it will not be restarted although the PSAP would want the IVS to restart.	PSAP always transmits more than just one START message. A NACK before the first START message is ignored
1.2.1.3	In-sync, instead of a START message an ACK is detected	MSD message is not sent if transmission has not yet started. If a transmission is ongoing, the IVS could terminate the transmission erroneously	PSAP always transmits more than just one START message. An ACK before the first START message is ignored. If the transmission is terminated, the PSAP operator could retrigger the transmission of the MSD.
1.2.2.1	In-sync, NACK message is not detected		NACK messages are repeated until the correct MSD is received	IVS behaviour does not change in this case
1.2.2.2	In-sync, instead of a NACK message a START message is detected	MSD transmission may be interrupted and restarted	Only after a successive reception of three START messages the MSD transmission is restarted	The probability of subsequent erroneously detected START messages is very low.
1.2.2.3	In-sync, instead of a NACK message an ACK is detected	IVS stops MSD transmission before PSAP has detected it.	At least two ACK messages need to be detected subsequently in order to stop transmission at the IVS. PSAP operator could retrigger the transmission of the MSD.	The probability of the subsequent erroneously detected ACK messages is very low.
1.2.3.1	In-sync, ACK message is not detected	Results in prolonged MSD transmission	ACK messages are sent several times	Not a problem as long as only few ACKs are missed.
1.2.3.2	In-sync, instead of a ACK message a START message is detected	Same as 1.2.2.2	Same as 1.2.2.2
1.2.3.3	In-sync, instead of a ACK message a NACK is detected	Same as 1.2.3.1	Same as 1.2.3.1	Same as 1.2.3.1
2.1.1	No sync frame detection	PSAP misses the MSD	The PSAP continues sending START messages to the IVS until it detects a sync preamble. The IVS restarts transmission of the MSD with sync frame when it continuously receives START messages from the PSAP.
2.1.2	PSAP receiver detects a sync frame while none was sent	Sync frame false alarm, PSAP tries to decode data, but fails.	After having failed the detection of a valid sync delay in subsequent sync frames, of after an unsuccessful reception of the MSD, the PSAP will ask the IVS to resend the MSD by transmitting START messages again.	Introduces a delay in MSD transmission
2.1.3	Sync tone evaluated incorrectly	Wrong modulator mode used to demodulate the MSD	If there are doubts about the reliability of the tone detection, the PSAP uses the fast modulator mode for the first trial of demodulating the MSD (first set of 8 RVs) and the robust modulator mode otherwise.	This assumption about the modulator mode will be incorrect only if the IVS completely fails to evaluate many feedback messages, which is very unlikely.
2.2	Sync detected, correct timing, false positive detection of MSD messages	CRC gives incorrect result		Very unlikely

7.3 PSAP and IVS protocol state models

The state model of the PSAP is shown in Figure 19.

Figure 19: State model of the PSAP

The state model of the IVS is shown in Figure 20.

Figure 20: State model of the IVS

Annex A (informative):
eCall Performance Requirements/Objectives and Design Constraints

Minimum performance requirements for non-bitexact implementations of the eCall modems, as well as exact performance figures of the bit-exact implementation, are given in a separate Conformance Testing document.

The following is reproduced for information form the permanent document "eCall Performance Requirements / Objectives and Design Constraints", for eCall Phase 2.