LTE voice solutions CS fallback

CS Fallback 

overview:

lthere are two standard solutions to provide CS services for E-UTRAN UEs:

CS Fallback has a simpler network architecture compared with VoIP over IMS.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these services LTE reuses CS infrastructure when the UE is served by E-UTRAN. 

A CS Fallback enabled terminal is redirected to 2G/3G network after it initiates a CS service such as voice calls.

CS Fallbcak procedure 

To support CS Fallback, the SGs interface is required, so as to let the MME perform a UE location update over the SGs interface so that the core network of the UTRAN or GERAN learns about UE location.

After a UE is powered on in the E-UTRAN, it initiates a combined EPS/IMSI attach procedure.

If a UE is camping on an E-UTRAN cell, it periodically initiates a combined TAU/LAU procedure, which allows for simultaneous UE location updates both in the MME and in the core network of the UTRAN or GERAN.

The Combined EPS/IMSI Attach Procedure is shown in the prvious snapshot:

After the RRC connection setup, the UE sends an Attach Request message to the MME, requesting a combined EPS/IMSI attach procedure. This message also indicates whether the CS Fallback or SMS over SGs function is required.
The MME allocates an LAI to the UE, and then it finds the MSC/VLR for the UE based on the TAI-LAI mapping. If multiple PLMNs are available for the CS domain, the MME selects a CS PLMN based on the selected PLMN information reported by the eNodeB. Then, the MME sends the MSC/VLR a Location Update Request message  over the SGs interface so that the core network of the UTRAN or GERAN learns about the UE location, which contains the new LAI, IMSI, MME name, and location update type.
The MSC/VLR performs the location update procedure in the CS domain.
The MSC/VLR responds with a “Location Update Accept” message that contains information about the VLR and temporary mobile subscriber identity (TMSI). The location update procedure is successful.
At last, the UE is informed that the combined EPS/IMSI attach procedure is successful by RRC Connection Reconfiguration message. (If the network supports SMS over SGs but not CS Fallback, the message transmitted to the UE contains the information element (IE) SMS-only. The message indicates that the combined EPS/IMSI attach procedure is successful but only SMS services are supported.)

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CS Fallback to UTRAN

Based on the capabilities of UEs and networks, the following mechanisms are available for an eNodeB to perform CS Fallback to UTRAN

-CS Fallback based on PS redirection

Flash CS Fallback(with RIM)

-CS Fallback based on PS handover

Flash CS Fallback(Blind handover)

CS Fallback Based on PS Redirection(UMTS)

- Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure:

- The UE sends the MME an NAS message Extended Service Request to initiate a CS service.

- The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to the eNodeB).

- The eNodeB sends an RRC Connection Release message to instruct the UE to perform a redirection. The message contains information about a target UTRAN frequency. If flash CS Fallback is available, the RRC Connection release message includes information about a target UTRAN frequency,PSC and their associated system information, In this way, the UE can quickly access the target UTRAN without the need to perform the procedure for acquiring system information of the target UTRAN cell. Then, the UE can directly initiate a CS service in the UTRAN cell.

- Then, the eNodeB initiates an S1 UE context release procedure.

- The UE may initiate an LAU, a combined RAU/LAU, or both an RAU and an LAU in the target cell and initiates a CS call establishment procedure in the target UTRAN cell.

RAN Information Management (RIM) Procedure

To support Flash CS Fallback, eNodeB requires exchange information between E-UTRAN and GERAN/UTRAN through the core networks

- Flash CS Fallback is defined in 3Gpp R9 .With this function, SIB can be included into the ”RRC connection Release” during the redirection procedure. This is achived by the RIM procedure. with RIM, eNodeB can get information from GERAN/UMTS.

- The RIM procedure supports two information exchange modes:

 Single Report and Multiple Report. 

- In Single Report mode, the source sends a request, and then the target responds with a single report.

- In Multiple Report mode, the target responds with a report after receiving a request from the source, and it also sends a report to the source each time the system information changes.

- the RIM procedure in Multiple Report mode is performed as follows: After an E-UTRAN cell is set up, the eNodeB sends a request for system information to neighboring UTRAN cells. After a neighboring UTRAN cell receives a request or the system information changes, this cell sends the system information to the eNodeB.

- If an eNodeB supports flash CS Fallback, it requires the system information of neighboring UTRAN cells to perform a redirection. If the serving cell does not have that information, the eNodeB must initiate an RIM procedure in Single Report mode to acquire the system information.

CS Fallback Based on PS Handover(UMTS)

Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure:

The UE sends the MME an NAS message Extended Service Request to initiate a CS service.

The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to the eNodeB).

The eNodeB initiates the preparation phase for a PS handover. If the preparation is successful, the eNodeB instructs the UE to perform a handover.

After the handover, the UE may initiate an LAU or combined RAU/LAU procedure in the UTRAN.

The UE’s context in EPS is released.

CS Fallback Procedure for terminated Calls(UMTS)

CS Fallback procedure for a terminated call is shown in the slide:
The MSC sends a Paging Request message from the CS domain to the MME over the SGs interface. Then, either of the following occurs:
-If the UE is in idle mode, the MME sends a Paging message to the eNodeB. Then the eNodeB sends a Paging message over the Uu interface to inform the UE of an incoming call from the CS domain, then UE initiates a connection establish procedure.
- If the UE is in active mode（connected）, the MME sends the UE an NAS message to inform the UE of an incoming call from the CS domain.
- The UE sends an Extended Service Request message containing a CS Fallback Indicator after receiving the paging message from the CS domain.
- The MME instructs the eNodeB over the S1 interface to perform CS Fallback.
- The subsequent steps are similar to the originated CS Fallback to UTRAN.
- The service request message from the UTRAN cell to UMTS CN is considered as the Paging Response message.

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CS Fallback to GERAN

Based on the capabilities of UEs and networks, the following mechanisms are available for an eNodeB to perform CS Fallback to GERAN

- CS Fallback based on PS redirection

Flash CS Fallback(with RIM)

- CS Fallback based on PS handover

Flash CS Fallback(Blind handover)

- CS Fallback based on CCO/NACC

-The procedures of CS Fallback to GERAN are similar with those of to UTRAN, just the CCO/NACC is particularly for GSM.

-During CS Fallback based on CCO/NACC, the eNodeB receives a CS Fallback Indicator from the MME, and then it sends a Mobility From EUTRA Command message to the UE over the Uu interface. The message contains information about the operating frequency, ID, and system information of a target GERAN cell. The UE searches for the target cell based on the information it received, and then it performs initial access to the cell to initiate a CS service.

CS Fallback Based on CCO/NACC(GERAN)

- The Cell Change order (CCO) procedure with Network Assisted Cell Change (NACC) is an alternative to the RRC Connection Release with Redirection procedure used for CS Fallback. The main difference is that the UE is moved to the target RAT whilst in RRC Connected Mode, also MME can get some response(UE Context Required) from GSM so as to trigger the UE context release procedure.

- In this CS Fallback procedure, the eNodeB sends a “Mobility From EUTRA Command” message over the Uu interface to indicate the operating frequency and ID of the target GERAN cell. If the source cell has the system information of the target cell, the system information is also carried in the message.

CSFB For SMS and LCS service

SMS :short message

LCS : location service

- SMS services are unknown to the eNodeB because SMS messages are encapsulated in NAS messages. During interworking with the UTRAN, SMS messages are exchanged between the MME and the MSC over the SGs interface. Because a UE does not require fallback to the UTRAN/GERAN to perform an SMS service, the SMS over SGs function can be used in a place covered only by the E-UTRAN.

- After a UE initiates an LCS request, the MME performs an attach or combined TAU/LAU procedure to inform the UE of the LCS capability of the EPS. If the EPS does not support LCS, the UE falls back to the UTRAN to initiate LCS under the control of the EPS. The CSFB procedure is the same as the procedure for CSFB to UTRAN for mobile-originated calls.

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Author : telecom engineer

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