Leonardo Zanoni Pedrini
Telecom Consultant at Telefônica Brasil (Telefônica | Vivo)

All Articles by Leonardo Zanoni Pedrini

1. VoLGA - Voice over LTE via Generic Access

The first implementation alternative that emerged was the VoLGA, tried to use what are already available, with minimal changes required.

To use the infrastructure of legacy 2G/3G networks, VoLGA introduces a new network entity, the VNC (VoLGA Network Controller), which basically functions as a 2G BSC, communicating with a GSM MSC (Mobile Switching Center) and as one 3G RNC communicating with a UMTS MSC (Mobile Switching Center).

When we have a new call (be it originated or terminated), it is managed by the MSC of legacy network. VNC is who mediates the voice signal and its related messages between the MSC and the LTE network.

Although it is possible to carry out the delivery of voice and SMS services to users of LTE, the VoLGA was unsuccessful. This is because, as we have seen, exclusive investments are needed for this purpose. At the same time however, global efforts to VoLTE increased (eg investments in IMS), and thus this alternative eventually failed into disuse.

2. CSFB - Circuit Switched Fall Back

On one hand operators follow seeking a complete LTE infrastructure (with full IMS) to meet multimedia services and also purely LTE voice, this is not a topology that is available in the short and even medium term.

While that reality doesn't come, we must use the legacy network when there is the need of voice and SMS delivery to LTE users.

And the most common alternative to this is the CSFB (CS Fall Back), an interim solution until we have full support for voice over LTE.

At CSFB scheme, whenever there is a demand for a new voice call, the LTE user is 'backed' for a CS legacy network, assuming that this provides an overlapping coverage. In other words, with CSFB, a voice call is never active in LTE, but in legacy networks.

At the end of the call in the legacy network, the UE can re-register the LTE network.

It goes something like this: the UE is registered (also) in the legacy network. When it got a call, the legacy network tells to LTE network: 'I have a call to the UE, can you ask it to come here and make the call?'

To CSFB be possible, users must be using dual mode devices, i.e. able to operate both in LTE network and in the legacy network.

To support CSFB, a new interface is introduced: the SGs, connecting the MME to the legacy MSC.

As the CSFB is currently the most widely used option by several operators, let's see some basic scenarios of it (CSFB).

2.1. CSFB - Registration and Location

When the CSFB UE is turned on, it registers itself in the two networks: LTE and legacy network (CS). And to allow quick transfer to the legacy network either 2G or 3G when necessary, the LTE network needs to know the location of the UE.

For this, the MME, which tracks the location of the UE in the LTE network, continuously provides location information to the legacy MSC, using the new SGs interface.

The set of SGs messages then supports management of mobility, paging and SMS.

2.2. CSFB - Originated Call

We will continue, and assume that the UE is initially covered by the LTE network, and that there is an active IP connection.

When the UE decides to originate a voice call, it sends an SRM (Service Request Message) to the MME (more specifically the ESR - Extended Service Request).

The MME checks whether the UE is CSFB capable, and notifies the eNodeB to transfer the UE to the legacy network.

Before performing the UE transfer, the eNodeB can ask it to make RF measures on neighboring 2G/3G network. The eNodeB then decides the best network for the UE and performs the transfer.

Once the UE camp in 2G/3G network, it starts the call procedure as usual - the UE starts the call control procedures in legacy network.

2.3. CSFB – Call + Data Connection in LTE

And what happens if I have an active data connection in the IP LTE network, and decide to make a voice call?

There are two options:

• The data are also transferred to the legacy network, or

• The data are temporarily suspended, until I return to the LTE network.

Although the first option seems the best, we must take into account that the transmission of IP data is also transferred: it can operate at much lower speeds (legacy systems). In addition, it may be that the legacy networks deny the IP session due to lack of resources or for not being able to process it.

The S3 interface is used to carry out the PS session handover for 3G (in this case, the DTM - Dual Transfer Mode must exist, but this details escapes form our theme).

There are no 4G data handover supported to 2G - in this case, the data is suspended.

The eRABs 4G are released when the UE performs the CSFB.

An important information is that the S3 is a 'new' interface between MME and SGSN on GTPCv2. And to support it, the SGSN needs to be updated (most carriers do not want to do this without a strong justification).

And Gn interface is already on GTPCv1, which is the native GTP version for 3G networks. So in this case only the MME needs to be updated, and as it is a relatively new node, it is probably easier to do. Not to mention that the new SGSN may have native support for S3. 

2.4. CSFB - Terminated Call

Finally, we have the case of a terminated call for LTE user.

The call request arrives first to the MSC where the UE was previously registered.

When the MSC receives call request, it sends paging messages to the related MME via SGs interface.

This message is forwarded to the UE, which is still connected to the LTE network.

If the user accepts the call, it sends an SRM (Service Request Message) to the MME.

The then MME notifies the eNodeB to transfer the UE for the legacy network, and the eNodeB then decide the best network for the UE to make the call.

2.5. CSFB – What happens after the end of the CS call?

We have seen that the 4G eRABs are released when the UE performs the CSFB. But what happens when the UE ends the CS call?

About what should follow next (if the UE should return or not to LTE as soon end the call CS), there is no specific rule.

Anyway, the main possibilities are:

• The upper layers forcing the 'reselection' to LTE so that the UE enters idle mode in legacy network.

• The operator send LTE 'redirection' information in RRC connection release message of legacy 3G network after the call is finished. This will result again in reselection to LTE.

The lower layers (AS - Access Stratum in this case URRC or GRR) reselect to LTE if the reselection criterion is satisfied. In most cases, operators have their parameters set such that the reselection to LTE happens if there is a good LTE coverage area overlapping the legacy network.