6.7 Call Control


6.7.1  Mobile-originated Call Setup


To initiate a call, the UE needs to establish communication with the network via radio interface signaling and send a message that contains the called subscriber number, i.e. the SETUP message on the Iu interface. The CN will establish a communication channel to the UE and use the obtained called party UERN to create an IAM/IAI message before sending the message to the called party. Note that the IAM/IAI message is not involved in the intra-office calls and it only exists on the E interface.

1)      The UE sends a CHANNEL REQUEST message to the network over the random access channel.
2)      The network responds with an IMMEDIATE ASSIGNMENT message so that the UE can occupy the specified dedicated channel.
3)      The UE sends a CM SERVICE REQUEST message to the CN.
4)      The network initiates the authentication and ciphering procedures.
5)      After sending the SECURITY MODE COMPLETE message, the UE sends the SETUP message to the CN to initiate the call setup procedure.
6)      The CN responds with a CALL PROCEEDING message.
7)      In the case of immediate assignment, a communication channel must be assigned to the UE before the CN initiates the call setup procedure to the fixed network.
8)      When the called party rings, the network must send an ALERTING message to the calling UE upon receipt of the ALERTING message from the called party. Meanwhile, it also needs to send a ringback tone to the calling party.
9)      When the called party answers, it will send a CONNECT message to the network and the network will forward this message to the calling party.
10)    When the calling UE returns a CONNECT ACKNOWLEDGE message, the call setup procedure is complete.

6.7.2  Mobile-terminated Call Setup


A mobile-terminated call procedure is executed when the mobile subscriber is called. And the call setup procedure is initiated by the network.
If the incoming call is allowed for setup after CN receives the IAM/IAI message or gets the MSRN from within the local office, the CN must page the UE via radio interface signaling. When the UE responds with the PAGING RESPONSE message, the CN will establish a communication channel to the UE upon receipt of this message.
1)      The CN sends a PAGING message to the RNS and the RNS broadcasts this PAGING message over the paging channel. For details, refer to the paging procedure described in Section 6.6.4.
2)      The called UE detects this paging and sends a channel request to the RNS. The RNS returns an immediate assignment command to indicate the UE to use the specified signaling channel.
3)      The UE sends a PAGING RESPONSE message on the signaling channel. Upon receipt of the PAGING RESPONSE message from the UE, the CN initiates the authentication and ciphering procedures (these two security procedures are optional and can be configured through the MAP function procedure).
4)      The CN sends a SETUP message to the RNS. The SETUP message carries the bearer capability of the call and the calling number.
5)      Upon receipt of the SETUP message from the RNS, the UE returns a CALL CONFIRMED message. If the negotiated bearer capability parameter changes, the CALL CONFIRMED message shall carry the bearer capability information.
6)      Upon receipt of the CALL CONFIRMED message from the RNS, the CN sends an RAB ASSIGNMENT REQ message to the RNS to request for radio channel assignment while the RNS sends an assignment message to the UE so as to switch the UE to a specified communication channel. After the UE is switched to the specified communication channel, it sends the RB ASSIGNMENT COMPLETE message to the RNS.
7)      The RNS sends a RAB ASSIGNMENT RESPONSE message to the CN.
8)      The UE sends an ALERTING message to indicate that the called subscriber should ring now.
9)      When the called party answers, the called UE sends a CONNECT message via the RNS to the CN.
10)    The CN returns a CONNECT ACKNOWLEDGE message to the UE to end the call setup procedure.

6.7.3  RAB Procedure

1. RAB management

It is defined that the RAB (Radio Access Bearer) shall be established between the UE and the CN. Different RABs are used for the different subscriber data, CN service capabilities and QoS requirements in the service request of the UE.
The RAB ID is related to the NAS binding information. For example, the RAB ID in the RANAP layer of the CS domain is numerically the same as the SI in the CC sublayer. The SI is allocated by the UE and the CN corresponds with the SIs to the RAB IDs one by one during the allocation of RAB IDs. In the point of view of a UE, the RAB ID is global in terms of RB (Radio Bearer) and Iu bearer and one RAB ID corresponds to a unique connection instance in the user plane (an Iu UP instance).
The CN controls the establishment, modification and release of RABs. It initiates the establishment, modification and release procedures of RABs, while the UTRAN executes the establishment, modification and release procedures of RABs. It is one function of the UTRAN to initiate the RAB release request (this function is triggered when the UTRAN cannot maintain the RAB with the UE).
During RAB establishment, the CN maps the RABs to the Uu interface bearers while the UTRAN maps the RABs to the Uu interface transport bearers and the Iu interface transport bearers.
If the AAL2 bearer is to be used in the CS domain, the UTRAN will initiate the setup and release of AAL2 connections.
The priority of RABs are determined by the CN according to the subscription and QoS information. The CN specifies the priority, preempt capability and queuing feature of an RAB during the RAB establishment and modification messages. The UTRAN executes the RAB queuing and resource preemption.

2. RAB access control

Upon receipt of an RAB establishment or modification request (the RAB QoS is mapped via the BC IE according to the R99 CS specification), the CS verifies if the subscriber is allowed to use the RAB in the request parameter. If yes, the CN will accept the request; otherwise it will reject the request.
When the UTRAN receives an RAB establishment or modification request from the CN, the admission control entity will judge if the request should be accepted or rejected according to the current radio resource conditions.       

3. RAB establishment, release and modification procedures

The purpose of the RAB Assignment procedure is to modify and/or release the established RAB, and/or establish a new RAB. The procedure is connection-oriented.
The CN first sends an RAB Assignment Request message to the RNC and the CN then starts the timer TRABAssgt. In an RAB Assignment Request message, the CN may request the UTRAN to establish/modify/release one or several RABs. The message contains the following information:
The list of RABs to be established or modified, along with the bearer feature;
The list of RABs to be released.
The RAB ID is unique in each Iu connection. If the message received by the RNC includes an existing RAB ID, the RNC will regard that the RAB is to be modified (except release).
The RNC will receive RAB release messages at any time and will always respond. If the RNC receives a message to release an RAB while it is establishing or modifying this RAB, it will stop the RAB configuration procedure, release all resources related to this RAB and return a response.
Upon receipt of the message, the UTRAN will execute the requested RAB configuration and then send an RAB Assignment Response message to the CN to report the result of request. One RAB Assignment Response message may contain the information of one or more RABs as follows:
The RABs that have been successfully established, modified or released;
The RABs that fail to be established, modified or released;
The RABs in queuing.
If no RABs are queued, the CN will strop the timer TRABAssgt and then the RAB Assignment procedure will end on the UTRAN side.
When the RABs to be established or modified are queued, the UTRAN will start the timer TQUEUING that specifies the maximum queuing time before establishment and modification of the RABs and will monitor all the queued RABs. The queued RABs may have the following results:
Establishment or modification success;
Establishment or modification failure;
Failure due to expiry of the TQUEUING timer.
In the first RAB Assignment Response message, the UTRAN reports the state of all the RABs involved in the RAB ASSIGNMENT REQUEST message. The UTRAN then reports the state of the queued RABs in the following RAB Assignment Response message, except the RABs with TQUEUING expiry. When knowing that the modification or establishment of all the queued RABs has succeeded or failed, the UTRAN stops the timer TQUEUING and the RAB Assignment procedure ends at the CN and the UTRAN simultaneously.
When the CN receives a response indicating that the RABs are queued, it hopes that the UTRAN can provide the result of RABs queuing before expiry of the timer TRABAssgt; otherwise, it will regard that the RAB Assignment procedure has ended and the configuration of those RABs not reported has failed.
Upon expiry of the timer TQUEUING, all the queued RABs in the UTRAN will end the procedure of queuing and the UTRAN will use an RAB Assignment Response message to report the states of all the queued RABs. At the same time, the whole procedure is stopped on the CN side.

4. RAB establishment procedure

The following figure depicts the procedure of RAB establishment via UTRAN between the CN and the UE.

The above example shows the procedure of establishing an RAB (DCH) in the DCH RRC state after the RRC connection has been established.
l   Time
In the CS domain, upon receipt of a service request from the UE (Caller SETUP, called CALL CONFIRM and CONNECT messages, etc.), the CN sends an RAB Assignment Request message to indicate that a new AS bearer channel is needed to bear the NAS subscriber data and thus initiates the procedure of bearer channel establishment.
l   Procedure
1)      The CN decides what kind of RABs are to be used according to the subscriber data, CN service capability and QoS requirements in the service request of the UE. It sends an RANAP message, i.e. the Radio Access Bearer Assignment Request (Setup) message, to request the RAB establishment. In the message, the RAB ID is filled with the SI value. The key parameters of the CS domain include the RAB parameter, the user plane mode, the ATM address of the local user plane and the IU BINDING ID.
2)      The Serving RNC uses the ALCAP protocol to initialize the Iu interface data transport bearer establishment.
In the case of using the AAL2 bearer in the CS domain (this procedure is not needed in the case of the PS domain), the SUGR parameter will be used in the AAL2 connection establishment request message to transparently transmit the BINDING ID to the CN, so as to complete the binding between the RABs and the data transport bearers. This request message also contains the following key parameters:
Peer ATM address, path ID, Channel ID (CID), path feature and channel feature.
3)      After the Serving RNC reconfigures the radio links with such equipment as Node B and completes the uplink-downlink synchronization, it uses an RRC message (i.e. Radio Access Bearer Setup) to carry the subflow and subflow combination parameters among the RAB parameters as well as the RAB ID before sending the message to the UE.
4)      Upon receipt of the success acknowledgement RRC message (i.e. Radio Bearer Setup Complete) from the UE and the establishment success information of the ALCAP procedure, the Serving RNC acknowledges the RAB establishment success to the CN. It sends an RANAP message (Radio Bearer Assignment Response) to the CN.
5)      If the user plane is in the support mode, the UTRAN initializes the Iu interface user plane via the Initialization message after the result is reported.
Note:
The procedures of interaction with the Drift RNC and the Drift Node B are not indicated in the above figure.
For the RACH/FACH – DCH, RACH/FACH - RACH/FACH and PS domain non-synchronous modes, the procedures are similar.

5. RAB release procedure

l   Trigger time
This procedure is initiated in the CS domain when all the transactions over the RAB in the CC layer end or the RNC requests to release the RAB.
l    Procedure
1)      The CN sends an RANAP message, i.e. Radio Access Bearer Assignment Request (Release), to initiate the RAB release procedure together with the RAB ID specified.
2)      The Serving RNC acknowledges the request message with another RANAP message, i.e. Radio Access Bearer Assignment Response.
3)      The Serving RNC uses the ALCAP protocol. In the case of the AAL2 bearer, it uses the AAL2 release message to initiate the release procedure of the Iu data transport bearer between itself and the CN (this procedure is not needed in the PS domain).
4)      The Serving RNC sends an RRC message (Radio Bearer Release) to the UE to initiate the bearer release procedure after releasing the link with such equipment as Node B.
5)      The Serving RNC receives the acknowledgement RRC message (Radio Bearer Release Complete) from the UE and the whole release procedure thus ends.

6. RAB modification procedure

l   Trigger condition
When the UE is handed over or the UE rate changes, the CN reconfigures the traffic channel to support the service attribute change.
l   Procedure
1)      The CN sends an RANAP message, i.e. the Radio Access Bearer Assignment Request (Modify), to request the RAB modification. The request message contains the RAB ID that indicates which RAB is to be modified and such key parameters of the CS domain as RAB parameters.
2)      The Serving RNC selects which parameters are to be modified and which programs are to be initiated.
3)      The SRNC uses the ALCAP protocol to modify the channel characteristics of the Iu interface data transport bearers.
4)      Upon successful modification procedure of the Iu interface transport control plane, the Serving RNC modifies the radio links together with such equipment as Node B. It then sends the subflow and subflow combination parameters among the RAB parameters as well as RAB ID in the RRC message (Radio Bearer Reconfiguration) to the UE.
5, 6)          Upon receipt of the success acknowledgement RRC message (i.e. Radio Bearer Setup Complete) from the UE, the Serving RNC acknowledges the RAB modification success to the CN. It sends an RANAP message (Radio Bearer Assignment Response) to the CN.
7)    If the user plane is in the support mode, the UTRAN initializes the Iu interface user plane via the Initialization message after the result is reported.

6.7.4  Paging Procedure


The paging procedure is a procedure of paging initiated from the CN to the called party. When the CN needs to set up a connection with the called subscriber, it first needs to find the called subscriber via the paging procedure. The purpose of the paging procedure is just to enable the CN to page the called subscriber. The paging procedure is set up via connectionless signaling.
The CN sends the PAGING message to the called subscriber to initiate the paging procedure. The PAGING message should contain sufficient information so that the RNC can find the called subscriber. If the called subscriber is unreachable in one paging, the CN will repeat the paging procedure via the Iu interface. Generally, the number of paging retries and the paging interval can be controlled by the CN.

1. Paging procedure

After the call request information from the caller is processed by the CN, the paging procedure starts if the called subscriber information has been successfully obtained. The CN needs to know the location area information of the called subscriber and get the sufficient paging information parameters before it can initiate the paging procedure to the called subscriber.
If the CN does not get the location area information of the called subscriber, it needs to send the PAGING message through the broadcast procedure to all the RNCs under its control.
The CN sends the PAGING message via the RANAP interface, which processes the PAGING message from the CN. The PAGING message includes such parameters as whether the paging request comes from the CS domain or the PS domain, what reason caused the paging and the specific location area information of the called subscriber. The RANAP sends the PAGING message to the RNC where the location area of the called subscriber resides.
When the PAGING message reaches the RNC, the RNC analyzes the parameters of the PAGING message to get the location area information of the called subscriber and then transmits via PCCH the PAGING information to the UE in that location area. If the called UE detects the PAGING message from the RNC, it starts to execute the NAS signaling procedure.
If the paging is successful, the CN will get the paging response message; otherwise the CN needs to repeat sending the PAGING message via the Iu interface.

2. Paging procedure of the UE in the RRC idle state

When the RRC is idle, the UE may receive paging from the CS or PS domain. Because the UE is now in the idle state, the CN can learn the Location Area Identification (LAI) information of the UE. The paging will be sent via this location area and the LA in this example crosses two RNCs.
1)      The CN initiates the paging information and sends it after passing two RNCs to the called UE. At this time it can be see from the Iu interface that two PAGING messages are continuously send by the CN, they carry the same LAI, except that the DPCs are the two RNCs.
2)      Cell 1 initiates the paging procedure via the Paging Type 1 message.
3)      Cell 2 initiates the paging procedure via the Paging Type 1 message.
The PAGING message reaches RNC1 and RNC2 via RANAP, and the two RNCs transmit the PAGING message to the UE in the specified location area over the PCCH. If the called UE detects the PAGING message from RNC1 or RNC2, it then starts executing the NAS signaling procedure.

3. Paging procedure of the UE in the RRC connected state

When the RRC is in the connected state, the paging procedure of the UE may fall into two cases: The CS domain or the PS domain of the CN. Due to the independency of mobility management, there are two possible solutions:
1)      The UTRAN coordinates the paging request over the existing RRC connection;
2)      The UE coordinates the paging request over the existing RRC connection.
The following example shows the paging procedure of the UE in the RRC connected state (CELL_DCH and CELL_FACH), where the UTRAN coordinates the paging request over the DCCH in the RRC connected state.
1)      The CN sends the PAGING message via RANAP to UE to page the UE.
2)      The Serving RNC sends the Paging Type 2 to the RRC (UE).
& Note:
Paging Type 1 is sent over the PCCH when the UE is idle while Paging Type 2 is sent over the DCCH when the UE is in the RRC connected state. The typical case is that the UE uses the Paging Type 2 to send the PAGING message of the CS domain in the PS service procedure. However, the Paging Type is controlled by the RNC and the CN does not need to know it.

6.7.5  Call Release Procedure


When the mobile subscriber ends the conversation, the calling party or the called onhook message shall be notified to the network side to initiate the call release procedure. The network releases the call by terminating the CS connection between the PLMNs or between the PLMN and other networks.
1)      After the MS hooks on, it sends a DISCONNECT message to the network to initiate the call clearing procedure. The release cause in the message is Normal Call Clearing.
2)      Upon receipt of the DISCONNECT message, the network sends a RELEASE message to the MS.
3)      The MS sends a RELEASE COMPLETE message to the network. If the communication channel is no longer needed, the channel release procedure will be executed.
4)      If the call is the only one over the entire Iu connection, the Iu connection shall be released. The CN sends an IU RELEASE COMMAND message to the RNS to request for releasing the Iu connection. 

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