5 RAB aspects

26.2023GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecInterface to Iu, Uu and NbRelease 17Speech codec speech processing functionsTS

During the RAB Assignment procedure initiated by the CN to establish the RAB for UMTS_AMR-WB, the RAB parameters are defined. The UMTS_AMR-WB RAB is established with one or more RAB co-ordinated sub-flows with predefined sizes and QoS parameters. In this way, each Transport Format Combination between sub-flows corresponds to one UMTS_AMR-WB frame type. On the Iu interface, these RAB parameters define the corresponding parameters regarding the transport of UMTS_AMR-WB frames.

Some of the QoS parameters in the RAB assignment procedure are determined from the Bearer Capability Information Element used at call set up. These QoS parameters as defined in [3], can be set as follows:

Table 5-1: Example of mapping of BC IE into QoS parameters for UMTS_AMR-WB

RAB service attribute

RAB service attribute value


Traffic Class


RAB Asymmetry Indicator

Symmetric, bidirectional

Symmetric RABs are used for uplink and downlink

Maximum bit rate

12,65 kbit/s in configurations 0 and 1
15,85 kbit/s in configurations 2 and 3
23,85 kbit/s in configurations 4 and 5

This value depends on the highest mode rate in the RFCS (note 2)

Guaranteed bit rate

6,60 kbit/s

One of the values is chosen, depending on the lowest rate controllable SDU format (note 2)

Delivery Order


(note 1)

Maximum SDU size

253 in configurations 0 and 1

317 in configurations 2 and 3

477 in configurations 4 and 5

Maximum size of payload field in Iu UP, according to the highest mode rate in the RFCS (note 2)

Traffic Handling Priority

Not applicable

Parameter not applicable for the conversational traffic class. (note 1)

Source statistics descriptor


(note 1)

SDU Parameters

RAB subflow 1 (Class A bits)

RAB subflow 2 (Class B bits)

The number of SDU, their number of RAB subflow is subject to operator tuning (note 3)

SDU error ratio

7 * 10-3

(note 3)

Residual bit error ratio



(note 3 – applicable for every subflow)

Delivery of erroneous SDUs


Class A bits are delivered with error indication;
Class B bits are delivered without any error indication.

SDU format information 1-5

(note 4)

sub-flow SDU size 1-5

(note 5)

(note 5)

NOTE 1: These parameters apply to all UMTS speech codec types.

NOTE 2: The guaranteed bit rate depends on the periodicity and the lowest rate controllable SDU size. All UMTS_AMR-WB configurations as defined in TS 26.103 contain the 6,60 kbps codec mode as lowest and therefore "guaranteed bit rate". The "maximum bit rate" and the "maximum SDU size" depend on the selected UMTS_AMR-WB configuration.

NOTE 3: These parameters are subject to operator tuning.

NOTE 4: SDU format information has to be specified for each AMR-WBcore frame type (i.e. with speech bits and comfort noise bits) included in the RFCS as defined in [2].

NOTE 5: The subflow SDU size corresponding to an AMR-WBcore frame type indicates the number of bits in the class A, class B fields.

The conversational traffic class shall be used for the speech service, which is identified by the ITC parameter of the bearer capability information element in the SETUP message. This shall apply for all UMTS speech codec types.
The parameters traffic class, transfer delay, traffic handling priority and source statistics descriptor shall be the same for all speech codec types applicable for UMTS.