6 Iu Interface User Plane (RAN-CN)

26.4543GPPCodec for Enhanced Voice Services (EVS)Interface to Iu, Uu, Nb and MbRelease 17TS

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

6.1 Frame structure on the Iu UP transport protocol

6.1.1 General

The data structures exchanged on the Iu interface are symmetrical, i.e. the structure of the uplink data frames is identical to that of the downlink data frames.

6.1.2 Initialisation

At the initialisation of the SMpSDU mode of operation, several parameters are set by the CN on Control Plane. Some of these parameters are related to the MSC-selected EVS Configuration for Iu, corresponding to one of the EVS Configurations for UMTS_EVS, as defined in TS 26.103 [20].

The EVS implementation shall use the "Support mode for predefined SDU sizes, version 2", see TS 25.415 [4].

The initialisation procedure for the Iu User Plane is described in TS 25.415 [4].

– RFCS (RAB sub-flow Combination Set):
The RFCS on Iu corresponds to the EVS Configuration for Iu, as selected by the MSC.
Each RFCS Index (RFCI) corresponds to one bit rate of the selected EVS Configuration for Iu.

Note: The Traffic Format Combination Set (TFCS) on Uu, selected by the RNC, may belong to a smaller EVS Configuration. Example: Instead of EVS (Set 2), the RNC may select EVS (Set 1) or even EVS (Set 0).
The UE determines the EVS Configuration for UE based on the selected TFCS, see also clause 7.

The RFCS on Iu in uplink is the same as the RFCS on Iu in downlink.

The actual RFCI used in downlink in a speech frame may, however, be different to the RFCI used in uplink for another speech frame.

– SCR (Source Controlled Rate), also known as DTX (Discontinuous Transmission),
shall be supported in uplink and downlink.

– Delivery of erroneous SDUs:
This parameter shall be set to YES. Erroneous or lost frames may be used to assist the error concealment procedures. PDU Type 0, containing a Payload CRC, shall be used for the transport of EVS data on Iu.

6.1.3 Time Alignment Procedure

The Transcoder in the Iu-terminating MGW, if present, should adjust the timing of the speech data transmission in downlink direction according to the time alignment frames sent by the RNC in Iu PDU Type 14 messages, see TS 25.415 [4]. The Time alignment procedure shall be dismissed in case of TrFO.

6.2 Mapping of the bits

The mapping of the bits between the generic frame format for EVS and Iu PDU Type 0 is the same for both, uplink and downlink frames. The bits of the generic frame format for EVS, as defined in TS 26.453 [19], follow immediately after the Payload CRC of Iu PDU Type 0. Then follows the EVS-CMR, see also TS 26.453 [19]. All parameters are sent MSB first. Figure 6.2-1 gives the general example for EVS primary rates, Figure 6.2-2 for the CMR-Only frame and Figure 6.2-3 for the example EVS AMR-WB IO 6.60 kbps.

Figure 6.2-1: Iu PDU Type 0 for all EVS Primary rates

Figure 6.2-2: Iu PDU Type 0 for CMR-Only packets

Figure 6.2-3: Iu PDU Type 0 for the EVS AMR-WB IO rate 6.60 kbps

The following table 6.2-1gives the correspondence of the bit fields between the generic frame format for EVS at the transcoder interface and the Iu PDU Type 0, exchanged with the Iu transport layer.

Table 6.2-1: Mapping of generic frame format for EVS onto Iu PDU Type 0

PDU field	Corresponding field within the generic frame format	Comment
PDU Type	N/A	Type 0
Frame Number	N/A
FQC	FQC	The FQC of the Iu Frame is used
RFCI	payload size	the payload size identifies the EVS rate
Payload CRC	N/A	result included in the FQC
Header CRC	N/A	result included in the FQC

Payload Fields (N Sub-flows)	Class A: mandatory Class B: not applicable Class C: not applicable	equal error protection is applied on the Uu interface for UMTS_EVS, therefore only class A bits are defined.
SDU #1	Most important bits come first	mandatory
SDU #2	n/a
SDU #3	n/a

The following Table 6.2-2 shows examples of mapping between RFCIs (RAB sub-Flow Combination Indicators) and EVS rates. The RFCI definition is given in these examples in order of increasing SDU sizes. Note that the RFCIs for EVS (Set 3) are in this example not contiguous: RFCI values 3, 5, 6 are punctured out. In EVS (Set 3) the EVS Primary rates 2.8, 7.2 and 8.0 are not allowed.

Table 6.2-2: Examples for EVS with SCR (DTX) and equal error protection

UMTS_EVS	UMTS_EVS	UMTS_EVS	UMTS_EVS	RAB sub-flows			Source rate (kbit/s)	Comment
RFCIs for EVS (Set 0)	RFCIs for EVS (Set 1)	RFCIs for EVS (Set 2)	RFCIs for EVS (Set 3)	sub- flow 1	sub- flow 2 (N/A)	sub- flow 3 (N/A)	Source rate (kbit/s)
0	0	0	0	7	–	–	(rare)	CMR-Only
1	1	1	1	40	–	–	–	EVS AMR-WB IO SID
2	2	2	2	55	–	–	–	EVS Primary SID
3	3	3	–	63	–	–	2.8	EVS Primary 2.8
4	4	4	4	139	–	–	6.6	EVS AMR-WB IO 6.6
5	5	5	–	151	–	–	7.2	EVS Primary 7.2
6	6	6	–	167	–	–	8.0	EVS Primary 8.0
–	7	7	7	184	–	–	8.85	EVS AMR-WB IO 8.85
–	8	8	8	199	–	–	9.6	EVS Primary 9.6
–	9	9	9	260	–	–	12.65	EVS AMR-WB IO 12.65
–	10	10	10	271	–	–	13.2	EVS Primary 13.2
–	–	11	–	335	–	–	16.4	EVS Primary 16.4
–	–	12	–	495	–	–	24.4	EVS Primary 24.4

6.3 Frame handlers

6.3.0 General

The Iu PDU Frame handling functions are described in 3GPP TS 25.415 [4].

The following clauses describe the mandatory frame handling functions at the EVS generic frame interface.

6.3.1 Handling of frames from TC to Iu interface (downlink)

6.3.1.0 General

The frames from the EVS Transcoder (TC) in generic EVS frame format are mapped onto the Iu PDU Type 0 as follows.

6.3.1.1 Frame Quality Classification

The Frame Quality Classification (FQC EVS) from the EVS Transcoder is directly mapped to the Frame Quality Classification (FQC PDU) of the Iu frame according to Table 6.3-1.

Table 6.3-1: FQC EVS to FQC IuPDU mapping in downlink

FQC EVS	FQC value (2 bit)	FQC PDU	FQC value (2 bit)
GOOD	00	GOOD	00
BAD	01	BAD	01
reserved	10	BAD Radio	10
reserved	11	reserved	11

Note: The EVS Transcoder in the Iu-terminating MGW sends always FQC=00 (GOOD).

6.3.1.2 Frame size

The received generic EVS frame with a given frame size is mapped onto the corresponding RFCI in the Iu PDU Type 0 Header thanks to the assigned RFCS table, defined at RAB Assignment.

6.3.1.3 Codec Mode Indication

void

6.3.1.4 EVS Codec Mode Request and Rate Control Command in downlink

The EVS Codec Mode Request (EVS-CMR) in downlink direction (DL EVS CMR) is part of the Iu PDU Type 0 payload. The EVS-CMR is defined in TS 26.453 [19].

The EVS-CMR follows in the Iu PDU Type 0 Frame immediately after the Speech or SID payload bits.

The DL EVS CMR shall be mapped into the selected EVS Configuration for Iu, see clause 6.1.1.

The PDU Type 14 Rate Control Procedure, downlink from the MGW to the RNC (DL RC Proc), is not applied for EVS. See also clause 6.3.2.4. Only DL RC Ack is sent from the Iu-terminating MGW to the RNC.

6.3.1.5 Optional internal 8 bits CRC

void

6.3.1.6 Mapping of Speech or SID bits

As defined in TS 26.453 [19], see also clause 6.2.

6.3.2 Handling of frames from Iu interface to TC (uplink)

6.3.2.0 General

If the EVS Transcoder (TC) is within the Iu-terminating MGW, then the uplink Iu frames are mapped onto the EVS generic frame format as follows.

6.3.2.1 Frame Quality Classification

At reception of UL Iu PDU Type 0 the Iu frame handler function set the EVS Frame Quality Classification (EVS FQC) according to the received PDU FQC, the Header-CRC check, and the Payload-CRC check (see 25.415 [4]). The EVS payload size (EVS bit rate) and EVS Frame Quality Classification are determined according to the following table, if the Header-CRC and the Payload-CRC indicate a correctly received PDU:

Table 6.3-2: PDU FQC to EVS FQC and EVS payload size mapping

PDU FQC	PDU FQC value (2 bits)	EVS FQC value (2 bits)	EVS FQC	resulting payload size
GOOD	00	00	GOOD	from RFCI
BAD	01	01	BAD	from RFCI
BAD Radio	10	10	BAD Radio	from RFCI
Reserved	11	11	Reserved	Reserved

Note: A Speech or SID Frame received on Iu with FQC "BAD Radio" is an indication that the UE is still sending and at least the payload is partly received, i.e. the connection is not lost. In case of BAD SID frames Comfort Noise Generation is continued, based on the previous SID parameter set.

6.3.2.2 Frame Type

void

6.3.2.3 Codec Mode Indication

void

6.3.2.4 EVS Codec Mode Request and Rate Control Procedure

The EVS codec rate in downlink direction is controlled by two complementing procedures, see Figure 6.3.2.4-1, which shows the example, where the Iu-terminating MGW hosts the Transcoder (TC), consisting of the EVS Encoder and the EVS Decoder, e.g. to/from a PCM interface.

Figure 6.3.2.4-1: EVS Rate and Mode Control for downlink speech

Each UL PDU Type 0, received by the MGW via the Iu-Interface in uplink, contains the UL EVS CMR (Iu), as sent by the UE in uplink.

UL PDU Type 14, received by the MGW via the Iu-Interface in uplink, may contain an UL RC Proc, specifying the maximum bit rate the RNC allows in downlink for all subsequent speech frames. This UL RC Proc contains all initialized RFCIs. The RFCIs with bit rates exceeding the maximum bit rate allowed in downlink are banned by the RNC by setting their RFCI flags to value 1, see TS 25.415 [4].

The DL RC Ack, to be sent in downlink, shall contain all initialized RFCIs. The RFCIs with bit rates exceeding the maximum bit rate allowed by the DL EVS CMR (Iu) shall be banned by setting their RFCI flags to value 1, see TS 25.415 [4].

The Iu-terminating MGW shall combine both, UL EVS CMR (Iu) and the maximum rate, as allowed by the UL RC Proc (see the RC combiner in Figure 6.3.2.4-1) and shall restrict the received UL EVS CMR (Iu) to the maximum rate the RNC allows for downlink, resulting in UL EVS CMR i ("i" like "internal").

If necessary, the RC combiner modifies the audio bandwidth contained in the UL EVS CMR (Iu) to the next smaller audio bandwidth that fulfils the maximum rate requirement set by the RNC for downlink with UL RC Proc or UL Iu_Init.

Note: The RNC parameters, also the RNC max rate in DL, are subject to operator policy.

The major operation mode contained in the UL EVS CMR (Iu) shall, however, not be modified by this RC combiner. If necessary, the RC combiner selects the next lower rate that corresponds to the received major operation mode.

In addition, subject to operator policy, the MGW may filter and modify the resulting UL EVS CMR i further. The MGW shall remember the resulting UL EVS CMR i, until it is modified again, see also clause 11.

In each new frame that is sent to the local EVS Encoder, if present, the remembered UL EVS CMR i is resent, in order to control the EVS Encoder-mode for the downlink direction.

The EVS Decoder within the Iu-terminating MGW, if present, shall accept and decode every (good) Speech and SID frame received in uplink direction. This decoder generates in addition the DL EVS CMR i, defining the maximum EVS mode this decoder wants to receive in uplink. Typically this "DL EVS CMR i" is identical with the highest EVS mode of the EVS Configuration for Iu. The decoder may reduce it to a lower mode, e.g. if the UL Speech frames have a high frame loss rate.

If the Iu-terminating MGW does not contain the Transcoder (i.e. the MGW is in TrFO mode), then the remembered UL EVS CMR i is sent on the other, outgoing interface, potentially mapped to the outgoing EVS Configuration, see also clause 11. Figure 6.3.2.4-2 shows this principle for the case that the Iu-terminating MGW hosts no transcoder and has an Nb-interface on the opposite outgoing termination in UL.

Figure 6.3.2.4-2: EVS Rate and Mode Control for downlink speech in TrFO

It is possible, that the EVS Configuration for Iu and the outgoing EVS Configuration (here on Nb) are different, although TrFO compatible. In such a case, the MGW shall forward all Speech and SID frames unmodified with respect to the Speech and SID payload. The EVS-CMR, however, may be filtered and modified by the MGW in both directions:

The MGW shall map each received DL EVS CMR (Nb), after potential filtering and modifying, into the EVS Configuration for Iu, before it send this as DL EVS CMR (Iu) towards the RNC. If a DL RC Ack has to be sent, then the latest DL EVS CMR (Iu) shall be considered to populate this DL RC Ack.

The MGW shall map each received UL EVS CMR (Iu), after combining with the latest UL RC Proc and potential filtering and modifying, into the EVS Configuration for Nb, before it send this as UL EVS CMR (Nb) towards the next MGW.

The details, how this filtering, modifying and mapping is performed, are specified in clause 11.

6.3.2.5 Optional internal 8 bits CRC

void

6.3.2.6 Speech and SID bits

The Speech and SID bits are mapped from the received PDU Type 0 to the payload of the generic EVS frames, as defined in TS 26.453 [19], with the reverse function of clause 6.3.1.6.