Introduction: Voice over Long-Term Evolution – VoLTE

LTE or 4G is a standard for high-speed wireless communication for mobile phones and data terminals defined by GSMA (Global System for Mobile Communications Association). It is based on the IMS (IP Multimedia Subsystem), and the VOICE Service is delivered as data flows within the LTE data bearer (VoLTE).

This means that there is no dependency with the the “OLD” voice network circuit-switched 2G or 3G (legacy). Moreover, as last analysis, in the next future there will not be any need to keep any legacy networks. Furthermore, VoLTE has up to three times more voice and data capacity than 3G UMTS and up to six times more than 2G GSM. Finally, it frees up bandwidth because VoLTE’s packets headers are smaller than those of unoptimized VoIP/LTE.

Circuit Switch Fall Back – CSFB

The Circuit Switched Fallback, or CSFB, is a Service Handover Procedure, of LTE protocol, devised to deliver VOICE and SMS to LTE devices with a circuit switch network (3G or 2G).

LTE doesn’t support circuit switched calls, because it is an IP based network. The 3GPP (3rd Generation Partnership Project) then defines the CSFB when using Circuit Switched-Based Services to deliver Voice or SMS before the coming of VoLTE (Voice over LTE).

What It Does?

Before the release of VoLTE, in the network updates period from 2G/3G to 4G, most operators worldwide are than relying CSFB for the provision of circuit services. However, to operationalize this feature, it was necessary to make updates on the network. In effect, it was introduced a new interface, called SGs, between MSC and MME.

When an LTE UE has a need to perform a voice call (either incoming or outgoing), an LTE handset falls back from the LTE network to either 3G WCDMA or 2G GSM and establishes a traditional circuit switched connection.

This allows an LTE subscriber to benefit from the well-established voice services available on existing networks. However, the approach has some disadvantages:

• Call setup times are extended, because of the need to switch radio networks as well as establishing a circuit switched call.
• 2G and 3G radio technologies use spectrum less efficiently than LTE, so it is wasteful of capacity to use GSM or WCDMA when LTE coverage is available.
• It is quite possible that a handset operating with a strong LTE signal, for example indoors at 800MHz, might fall back to a weak 3G signal at 2.1GHz. This could result in a poor quality voice call even if the handset was showing a perfectly good LTE signal immediately prior to making the call. A quirk of deploying LTE in one of its lower frequency bands is that it can improve the indoor coverage of mobile data services, which are often already well catered for by WiFi, but not voice services. In fact a Skype call over LTE might achieve better quality than a 3G voice call.

How Does It Works?

When a UE starts a call, the MME (Mobile Management Entity) connects to the MSC (Mobile Switching Center) Server via a SGs interface enabling the UE to be both CS- and PS-registered.  If it is an LTE network attempting to connect to a legacy network, the MME recognises this and subsequently routes the call through a 2G or 3G network.

This double registration allows to fallback from LTE to circuit switched doing a CSFB.

It is important to note that this process only works when an LTE network’s coverage area is also covered by GSM, UMTS or CDMA/WCDMA networks.

MME node has essential importance because, in addition to tracking the user at level of TA (Tracking Area) for the management of mobility in EPS, it manages also  the association between TA and LA (Location Area) for interworking with legacy network CS. The MME is also responsible to notify the start of the CSFB procedure when required.

Let’s describe the flow:

• The UE requestes a radio access E-UTRAN to move temporarily on 2G or 3G network in cases of call originated and terminated in LTE coverage
• MME asks HSS if UE is properly registered in LTE
• MSC ask HLR if UE is properly registered in CS (2G/3G)
• MME node tracks the user to the TA level (tracking) for the management of mobility in EPS, manages the association between TA e LA (Location Area) for interworking with legacy network CS
• MSC node is essentially the same as that within GSM, and it manages the circuit switched calls under way
• The SGs interface between MSC / VLR and MME manages UE can take advantage of packet services through LTE and legacy services
• The eNodeB orders to the UE a vertical handover to access legacy network by the Redirect RRC procedure. The fallback technology is configured at the eNodeB accordingly with the operator’s choices and it is communicated to the device through the LTE radio interface
• The UE, on the eNodeB request, make a fallback to legacy network and then run the CALL SETUP using the CS network
• When the voice call end, the UE re-selects LTE access and log back on to the LTE network services

Of course, if the CSFB procedure will starts during an active data session (Multi-RAB), the shift to 2G/3G inevitably will produce a degradation if the EU will fall in 3G, or interruption if the EU will fall in 2G.