VoLTE Mobility and FallBack

7/24/2019 VoLTE Mobility and FallBack

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It is important that VoLTE calls are maintained even when the UE switches between

radio cells. For LTELTE handovers the MME and SGW will manage the process by

performing a regular PS handover. In the future it may be possible to perform an

inter-system handover between LTE and HSPA for voice calls.

During the preparation phase of the handover the MME will identify the SGSN of the

target system and use the S3 interface to request resources from the target system.

The SGSN and RNC will create the necessary bearers for the handover and pass the

bearer and radio-related information back to the LTE system. It is important the that

new bearer has the same QoS parameters mapped from the LTE EPS bearer.

The handover execution phase includes the transmission of the handover command

to the UE, including all the radio and network-related parameters required. After

successful handover the MME will release the bearer in the E-UTRAN and instruct theSGW to forward voice packets to the SGSN via the S4 interface.


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The concept behind SRVCC is to allow PS voice sessions to be handed off seamlessly

to the CS network. The SRVCC procedure describes the handover of the voice-based

connections, and if possible the handover of any ongoing data connections to the 3G

PS network. When the target network is GSM CS based it will not be possible to

handoff the data connections; they must be suspended in this case.

The general process begins with the mobile sending measurement reports to the

E-UTRAN, which will detect the requirement for a handover to a non-LTE radio cell. If

there is a voice bearer (QCI-1) in place then the SRVCC process will begin.


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A prerequisite for SRVCC is that the CS network includes an MSC server that supports

the SRVCC function and that there is an Sv interface in place between the MSC and

the MME.

The SRVCC MSC supports two functions in respect of the VCC process. It performs call

set-up and reserves resources towards the CS network (Iu-CS or A interfaces). It also

establishes the call on behalf of the UE towards a specific address provided by the

MME; that address will identify the AS that was part of the original call.


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When the handover decision is made and the identified target is within the CS

domain, the MME will signal the MSC with information such as the codecs in use and

an

STR-SR (Session Transfer number).

If the target RAN is not connected directly to the SRVCC-enhanced MSC, it will

perform a relocation to the MSC connected to that RAN. If there are also data bearers

to be handed over, that process will take place in parallel to the voice handover.

Once CS resources have been reserved in the CS domain the SRVCC MSC will signal

the IMS AS with details of the session that is being handed over. This may require that

the connection is established via a media gateway, in which case coded information,

IP address and port numbers will be required to configure the MGW.

The mobile will be commanded to handover to the CS RAN and the IMS session on

the PS side can be released. When the handover is successfully detected on the CS

side the MSC will connected the mobile to the CS resources.


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CSFB (Circuit Switched Fallback) allows the use of existing CS services even when a

mobile is camped on the LTE system. This may include the sending and receiving of

SMS messages and the making and receiving of voice calls. In either case the general

process is that the mobile, whilst camped on the LTE system, will fall back to the CS

system to perform the services outlined above.

It is necessary for both the mobiles and the core network to explicitly support the fall

back service and both will signal their capability and preferences at registration. In

the case of a voice-centric device connecting to the LTE network that does not

support fall back, the mobile will camp only on the voice network.

There are architectural changes that must be made to both the existing CS network

and LTE. In particular, the SGs interface between the MME and MSC server is

important in that it allows for combined update/registration and the forwarding ofpaging indication for incoming calls.

Procedures supported by the SGs interface include:

Alert and UE activity

Location Update

Paging


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EPS attach and IMSI detach

UL and DL unitdata


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When the UE performs the initial attach to the EPS it will indicate that it requires a

combined attached to take place (the UE knows that this is possible from broadcasted

system information). The MME will determine which MSC/VLR the attach is to be

sent to and send a location update. The update is acknowledged to the UE.

This combined procedure also reduces the number of signalling events and messages

that need to be sent to the EPS and CS core.


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The mobile-originating CSFB procedure beings with the UE sending an extended

service request to the MME indicating that the CSFB is required. The MME then

informs the eNB that the mobile has requested the CSFB service. The eNB can then

redirect (handover) the mobile to a base station that supports the CS service. Once

the mobile has established itself on the CS RAN it will perform a normal CS call set-up

via the RAN and MSC.

The call set-up may be rejected by the MSC, prompting the mobile to perform a

location update, after which the call set-up will proceed as normal. This may increase

the amount of time required for the call set-up, but it may be reduced by making use

of a FoW (Follow on Bit), which tells the circuit-switched network to keep signalling

connections alive toward the RAN.


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In cases where the UE has an ongoing PS connection with the EPS and a CS call is

made toward the mobile, the SGs interface can be used to forward the paging

message to the mobile via the MME. This paging message can include the CLI, hence

the user can decide on the presentation of the CLI whether to accept the call.

The UE responds with the Extended Service Request, which triggers a handover

process to the CS RAN. If there any PS bearers active this may also trigger the

handover of those bearers if the target system has sufficient resources to handle the

additional connections.

In cases where the target RAN is 2G, the existing bearers will have to be suspended

until the voice call is complete and the UE returns to the LTE RAN.

VoLTE Mobility and FallBack

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