6.2 Radio Bearer Control Procedures

25.3033GPPInterlayer procedures in Connected ModeRelease 17TS

6.2.1 Radio Bearer Configuration

6.2.1.1 Radio Bearer Establishment

The procedures for establishing radio bearers may vary according to the relation between the radio bearer and a dedicated transport channel. Depending on the QoS parameters, there may or may not be a permanently allocated dedicated channel associated with the RB. Circuit-switched bearers, or bearers classified as real-time services typically need a permanent association to a DCH to meet the delay requirements. Packet-switched bearers, or bearers classified as non-real-time services can in many cases be served as best-effort, requesting capacity from an associated DCH based on need.

When establishing an RB together with a DCH, the DCH may be attached to either a newly activated physical channel or it may be accommodated by modifying an existing physical channel. The modification is further broken down into two different options: synchronised and unsynchronised. If the old and new physical channel settings are compatible (TFCI etc.) in the sense that executing the modification in the NW and the UE with arbitrary timing does not introduce transmission errors, the unsynchronised procedure can be applied. If the old and new settings are incompatible, due to e.g. assignment of the same TFCI value to a new set of physical layer configuration, the synchronised procedure must be used.

6.2.1.1.1 Radio Bearer Establishment with Dedicated Physical Channel Activation

The procedure in figure 5 is applied when a new physical channel needs to be created for the radio bearer. A Radio Bearer Establishment is initiated when an RB Establish Request primitive is received from the DC-SAP on the network side of the RRC layer. This primitive contains a bearer reference and QoS parameters. Based on these QoS parameters, L1 and L2 parameters are chosen by the RRC entity on the network side.

The physical layer processing on the network side is started with the CPHY-RL-Setup request primitive issued to all applicable Node Bs. If any of the intended recipients is / are unable to provide the service, it will be indicated in the confirmation primitive(s).After setting up L1 including the start of Tx / Rx in Node B, the NW-RRC sends a RADIO BEARER SETUP message to its peer entity (acknowledged or unacknowledged transmission optional for the NW). This message contains L1, MAC and RLC parameters. After receiving the message, the UE-RRC configures L1 and MAC.

When L1 synchronisation is indicated, the UE sends a RADIO BEARER SETUP COMPLETE message in acknowledged-mode back to the network. The NW-RRC configures MAC and RLC on the network side.

The UE-RRC creates a new RLC entity associated with the new radio bearer. The applicable method of RLC establishment may depend on RLC transfer mode. The RLC connection can be either implicitly established, or explicit signalling can be applied.

Finally, an RB Establish Indication primitive is sent by UE-RRC and an RB Establish Confirmation primitive is issued by the RNC-RRC.

Figure 5: Radio Bearer Establishment with Dedicated Physical Channel Activation

6.2.1.1.2 Radio Bearer Establishment with Unsynchronised Dedicated Physical Channel Modification

Figure 6: Radio Bearer Establishment with Unsynchronised Dedicated Physical Channel Modification

The establishment of a radio bearer, when unsynchronised physical channel modification is applicable, is shown in figure 6. If the old and new physical layer configurations are compatible in the sense that they can coexist in the peer entities, an unsynchronised procedure for radio bearer establishment can be applied. In this case no fixed activation time is required.

The modifications on the physical layer in the network are done in response to a CPHY_ modify request. Failure to comply is indicated in the confirmation primitive. In an error-free case the RADIO BEARER SETUP message on L3 is transmitted. Acknowledged or unacknowledged transmission is a network option. Configuration changes on the UE-side proceed after this message has been received. Reception of the RADIO BEARER SETUP COMPLETE message triggers configuration changes in MAC and RLC in the network.

6.2.1.1.3 Radio Bearer Establishment with Synchronised Dedicated Physical Channel Modification

In this case the CPHY-RL-Modify request doesn’t immediately cause any changes in the physical layer configuration, it only checks the availability of the requested configuration and makes a "reservation". After the confirmations have been received from all applicable Node Bs, the RRC chooses the appropriate "activation time" when the new configuration can be activated. This information is signalled to MAC, RLC and also the physical layer (CPHY_Commit request primitive).

After the RADIO BEARER SETUP message (acknowledged transmission on L2 required) between peer L3 entities the setup proceeds on the UE-side. The new configuration is now available both on the UE and the network side, and at the scheduled activation time the new configuration is assumed by all applicable peer entities.

In case the old and the new physical channel configurations are incompatible with each other (due to different DPCCH format, TFCI patterns or similar differences), the modification on physical layer and L2 require exact synchronisation between the UE and the NW, as shown in figure 7.

Figure 7: Radio Bearer Establishment with Synchronised Dedicated Physical Channel Modification

6.2.1.1.4 Radio Bearer Establishment without Dedicated Physical Channel

Figure 8: Radio Bearer Establishment without Dedicated Physical Channel

For some radio bearers dedicated radio resources are not permanently associated. Therefore the setting up of the physical resource is separate from the actual radio bearer setup, which involves only RLC and MAC.

MAC can be initially configured to operate either on existing dedicated transport and physical channels or on common channels.

6.2.1.1.5 Void

6.2.1.2 Radio Bearer Release

Similar as for Radio Bearer Establishment procedure, the Radio Bearer Release can include physical channel modification or physical channel deactivation depending on the differences between new and old QoS parameters. These can also be both synchronised and unsynchronised.

The Radio Bearer Release procedure is initiated when the release is requested from the RRC layer on the NW side. This request contains a bearer reference, and on retrieval a RB Release Confirm primitive is immediately returned to the Non-Access Stratum.

New L1 and L2 parameters may be chosen for remaining radio bearers if any. A RADIO BEARER RELEASE message is sent from the RRC layer in the network to its peer entity in the UE. This message includes possible new L1, MAC and RLC parameters for remaining radio bearers and identification of the radio bearer to be released (note). An RB Release Indication is sent by the UE-RRC.

NOTE: In synchronised case a specific activation time would be needed for the change of L1 and L2 configuration to avoid data loss.

The RRC on the UE side configures L1 and MAC, and releases the RLC entity associated to the released radio bearer. After receiving a RADIO BEARER RELEASE COMPLETE message from the UE, the NW-RRC does a similar reconfiguration also on the network side.

6.2.1.2.1 Radio Bearer Release with Unsynchronised Dedicated Physical Channel Modification

The example in figure 10 shows the case where release can be executed as an unsynchronised physical channel modification, i.e. without physical channel deactivation.

After notifying upper layers of the release, a RADIO BEARER RELEASE message (acknowledged or unacknowledged transmission optional for the network) is sent to the UE triggering the reconfiguration in the UE. When this is finalised the UE sends a RADIO BEARER RELEASE COMPLETE message to the network, after which the reconfiguration is executed in the network.

Figure 10: Radio Bearer Release with Unsynchronised Dedicated Physical Channel Modification

6.2.1.3 Radio Bearer Reconfiguration

For Radio Bearer Reconfiguration, both synchronised and unsynchronised procedures are applicable. The unsynchronised procedure is shown as an example.

6.2.1.3.1 Unsynchronised Radio Bearer Reconfiguration

Because of the unsynchronised nature of the procedure in figure 11, there is no activation time and no separate commit request for the Node B physical layer is needed. The possibility for executing the requested modification will be reported in the confirmation primitives from the physical layer. If the modification involves the release of an old configuration, the release can be postponed to the end of the procedure. After the reception of a RADIO BEARER RECONFIGURATION from the RNC-RRC (acknowledged or unacknowledged transmission optional for the network), the UE executes the modifications on L1 and L2.

Upon reception of a RADIO BEARER RECONFIGURATION COMPLETE message from the UE-RRC, the NW-RRC executes the modifications on L1 and L2. Finally the old configuration, if any, is released from Node B-L1.

Figure 11: Unsynchronised Radio Bearer Reconfiguration

6.2.2 Transport Channel Reconfiguration

For transport channel reconfiguration, both synchronised and unsynchronised procedures are applicable.

6.2.2.1 Unsynchronised Transport Format Set Reconfiguration

Figure 12 illustrates an example of a procedure for a change of the Transport Format Set for one transport channel. This is done with the Transport Channel Reconfiguration procedure.

A change of the transport format set for a transport channel is triggered in the RRC layer in the network. A TRANSPORT CHANNEL RECONFIGURATION message is sent from the RRC layer in the network to its peer entity (acknowledged or unacknowledged transmission is a network option). This message contains the new transport format set and a new transport format combination Set, i.e. new parameters for L1 and MAC (note). When this message is received in the UE a reconfiguration of L1 and MAC is done. A similar reconfiguration is also done on the network side after the reception of a TRANSPORT CHANNEL RECONFIGURATION COMPLETE message.

NOTE: In a synchronised procedure a specific activation time is needed for the change of L1 and L2 configuration to avoid data loss.

During the reconfiguration of the transport format set for a transport channel, radio traffic on this channel could be halted temporarily since the UE and the network are not necessarily aligned in their configuration. This traffic can resume after the COMPLETE-message.

Figure 12: Unsynchronised Transport Format Set Reconfiguration

6.2.3 Physical Channel Reconfiguration

For physical channel reconfiguration, both synchronised and unsynchronised procedures are applicable.

6.2.3.1 UE-Originated DCH Activation

Figure 13 illustrates an example of a procedure for a switch from common channels (CELL_FACH) to dedicated (CELL_DCH) channels.

In the UE the traffic volume measurement function decides to send a MEASUREMENT REPORT message to the network. In the network this measurement report could trigger numerous different actions. For example the network could do a change of transport format set, channel type switching or, if the system traffic is high, no action at all. In this case a switch from CELL_FACH to CELL_DCH is initiated.

Whether the report should be sent with acknowledged or unacknowledged data transfer is configured by the network.

First, the modifications on L1 are requested and confirmed on the network side with CPHY-RL-Setup primitives.

The RRC layer on the network side sends a PHYSICAL CHANNEL RECONFIGURATION message to its peer entity in the UE (acknowledged or unacknowledged transmission optional to the network). This message is sent on DCCH mapped to FACH. The message includes information about the new physical channel, such as codes and the period of time for which the DCH is activated (note).

NOTE: This message does not include new transport formats. If a change of these is required due to the change of transport channel, this is done with the separate procedure Transport Channel Reconfiguration. This procedure only handles the change of transport channel.

When the UE has detected synchronisation on the new dedicated channel L2 is configured on the UE side and a PHYSICAL CHANNEL RECONFIGURATION COMPLETE message can be sent on DCCH mapped on DCH to RRC in the network. Triggered by either the NW CPHY_sync_ind or the L3 complete message, the RNC-L1 and L2 configuration changes are executed in the NW.

Figure 13: UE-Originated DCH Activation

6.2.3.2 UE-terminated synchronised DCH Modify

Figure 14: UE-terminated synchronised DCH Modify

Figure 14 illustrates an example of a synchronised procedure for DCH modification. Triggering of this procedure could for example be accomplished by an inactivity timer. The procedure can e.g. release all transport formats of a radio bearer without releasing the DCH, due to another bearer using it. The synchronised procedure is applied in the case when the old and new configurations are not compatible e.g. change of channelisation code.

After the CPHY-RL-Modify requests have been confirmed, an activation time is chosen by NW-RRC. After deciding upon the activation time, the NW-RRC sends a PHYSICAL CHANNEL RECONFIGURATION message as acknowledged data transfer to the UE. In both uplink and downlink this message is sent on DCCH mapped on DCH.

After reception the UE reconfigures L1 and L2 to DCH resources. If a complete message is used it would be sent on DCCH mapped on DCH. In the unsynchronised case this message could trigger a modification of L1 and L2 resources in the network associated with the dedicated channel.

6.2.3.3 UE-terminated DCH Release

Figure 15 illustrates an example of a procedure for a switch from dedicated (CELL_DCH) to common (CELL_FACH) channels. All DCHs used by a UE are released and all dedicated logical channels are transferred to CELL_FACH instead. Triggering of this procedure could for example be an inactivity timer.

A switch from DCH to common channels is decided and a PHYSICAL CHANNEL RECONFIGURATION message is sent (acknowledged or unacknowledged data transfer is a network option) from the RRC layer in the network to the UE. This message is sent on DCCH mapped on DCH.

NOTE 1: This message does not include new transport formats. If a change of these is required due to the change of transport channel, this is done with the separate procedure Transport Channel Reconfiguration. This procedure only handles the change of transport channel.

If the loss of L1 sync is used to detect in the NW that the UE has released the DCHs, as is one possibility in the figure, then there may be a need to configure the Node B-L1 to a short timeout for detecting loss of sync. This is presented by the CPHY-Out-of-Sync-Config primitives in the figure.

After reception the UE reconfigures L1 and L2 to release old DCH resources. The PHYSICAL CHANNEL RECONFIGURATION COMPLETE message to the network is here sent on DCCH mapped on RACH (message acknowledgement on FACH). This message triggers a normal release of L1 and L2 resources in the network associated with the dedicated channel.

NOTE 2: When a Switch to CELL_FACH is done it is important to free the old code as fast as possible so that it can be reused. Therefore instead of waiting for the Physical Channel Reconfiguration Complete message the network can reconfigure L1 and L2 when the acknowledged data confirmation arrives and the network is sure that the UE has received the Physical Channel Reconfiguration message. To be even more certain that the UE has released the old DCH resources the network can wait until after the Out of sync Indication from L1.
These steps including a timer starting when the Physical Channel Reconfiguration is sent, gives the network four different indications that the released DCH is really released, and that resources can be reused.

Figure 15: UE-terminated DCH Release

6.2.4 Transport Format Combination Control

6.2.4.1 Transport Format Combination Limitation

Figure 16: Transport Format Combination Limitation

Figure 16 illustrates an example of a Transport Format Combination Control procedure. A congestion situation occurs and allowed transport format combinations are restricted temporarily. When the congestion is resolved the restriction is removed.

This procedure is initiated with a Transport Format Combination Control message from the network to the UE (acknowledged, unacknowledged or transparent transmission optional to the NW). This message contains a subset of the ordinary Transport Format Combination Set. The UE then continues with a reconfiguration of MAC. MAC sees the TFC subset as a completely new set.

Further, after a while when the congestion is resolved a new Transport Format Combination Control message is sent to the UE from the RRC layer in the network. This message contains a subset that is the entire original set. Again, the UE reconfigures the MAC.

6.2.5 Dynamic Resource Allocation Control of Uplink DCHs

Figure 17: Dynamic Resource Allocation Control of Uplink DCHs

Figure 17 illustrates an example of a Dynamic Resource Allocation Control (DRAC) procedure of uplink DCHs. The CRNC regularly broadcasts the following parameters:

– transmission probability ptr, which indicates the probability for a UE to be allowed to transmit on its DCHs, which are under control by this procedure, during the next period Tvalidity;

– maximum total bit rate allowed to be used by the UE on its DCH which are under controlled by this procedure, during the next allowed period Tvalidity.

Besides these parameters, the RNC has allocated the following parameters to the UE:

– transmission time validity, Tvalidity, which indicates the time duration for which an access for transmission is granted;

– reaccess time Tretry, which indicates the time duration before retrying to access the resources, in case transmission has not been granted.

This procedure is initiated with a SYSTEM INFORMATION message containing the above DRAC parameters regularly broadcast by the CRNC on the FACH. It applies to all UEs capable of simultaneous reception of Secondary CCPCH and DPCH and having DCHs that can be controlled dynamically. The UEs have to listen to this message prior to transmission on these DCHs. The UE RRC checks whether transmission is allowed, and then reconfigures MAC with a new subset of TFCS derived from the maximum total bit rate parameter. This TFCS subset shall control only the DCHs that are under control by this procedure.

In case of soft handover on the uplink DCH, The UE is requested either to listen to broadcast information from its primary cell (the one with the lowest pathloss), or from all cells involved in its Active Set, depending on its class. In the latter case, the UE is expected to react according to the stricter control information.

6.2.6 Variable Rate Transmission of Uplink DCHs

Figure 18: Variable Rate Transmission of Uplink DCHs

Figure 18 illustrates an example of the Variable Rate Transmission procedure of uplink DCHs. With this procedure the QoS of service with variable rate can be maintained and unnecessary interference can be avoided by a temporary reduction of the data rate within the TFCS.

When a connection for a variable rate service is established the RRC assigns the TFCS to MAC. At the radio bearer set-up procedure the maximum allowable Tx power can also be set for each user if it shall be different from the UE capability class.

With the CPHY-Measurement-REQ the power thresholds will be set to the UE. If during a transmission the allowable transmit power is above the set threshold the event will be signalled to the MAC that will decrease the data rate within the set TFCS at the next transmission time interval. In the UE, the PDUs that can not be transmitted in a TTI (i.e. MAC has indicated that some of the available PDUs can not be transmitted) shall be buffered according to the discard configuration set by RRC.

When channel conditions improve and the averaged transmission power falls below the allowable transmission power the physical layer indicates this event to the MAC. If there is enough data to be sent, the MAC in response increases the data rate by increasing the number of transport blocks delivered to L1 and the physical layer increases the total transmission power to the UE by the predefined amount. This allows the data that was buffered during bad channel conditions to be delivered to the UTRAN.