Voice in LTE – CSFB trials and tribulations

As LTE is a packet data-only network, operators use two main solutions to provide voice to their subscribers with LTE devices: VoLTE, which we previously discussed in our blog posts, and Circuit Switched Fallback (CSFB).

CSFB is often seen as a ‘temporary’ solution, until there are enough VoLTE devices on the market, and consists of 4G users being handed over to legacy 2G/3G networks to use voice services. However, CSFB also has major challenges, such as the fact that subscribers can lose their 4G data connectivity after their CSFB call ended. This is also a troublesome aspect for operators with LTE-only networks, as they have roaming agreements with MNOs for voice services over 2G or 3G, and will be forced to pay higher fees once their subscribers don’t return to their 4G networks.

CSFB allows operators with newly created LTE networks to exploit their legacy networks or to use MVNO agreements, and provide voice capabilities without major investments or fundamental changes to their circuit switched (CS) core networks. CSFB moves a subscriber from the LTE core network to the CS core network through the SGs interface during call setup (the SGs interface is added to the LTE architecture and allows mobility management and paging procedures between the MME and the MSC). Normally, one would expect the subscriber will return to the LTE network once the call has ended. The reality, however, is otherwise.

Circuit Switched Fallback (CSFB)
Circuit Switched Fallback (CSFB)

Among CSFB’s main issues, we can name:

  • data traffic suspends during the handover between networks
  • data rates decrease dramatically during the CSFB call’s answer and hang-up moments
  • mobile apps terminate during the CSFB voice call
  • data transfer is suspended during the call if the 2G/3G networks don’t support dual transfer mode
  • most importantly, once the voice call has ended, the subscriber cannot return to the home LTE network, especially in the case of MVNO agreements and not when the operator has both the LTE and CS networks

Studies have shown that behavior patterns such as those listed above depend on the data packet size and the running data packet interval.

Operators with LTE-only networks need to use roaming agreements with other MNOs to enable CSFB. Therefore, they are the ones who will bare the data traffic costs when their subscribers remain stuck in 2G/3G networks, sometimes even for hours.

The main impediment in proposing a solution that will work for all operators and will prevent such problems is that CSFB standards don’t give any insight into how devices are supposed to return to the LTE home network. One solution non-MVNOs typically adopt is to set up rules for the handover back to 4G or for the cell reselection procedure.

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