Introduction

Choosing the voice technology within 4G radio access is a decision of which path to take, and it is generally not the same path for every operator. This decision depends directly on the level of investment in the infrastructure, the projected rate of return on investment (ROI) and the complexity of the network design.

Although there is no unique recipe for the short-term path, it is possible to design a solution based on an IP Multimedia Subsystem (IMS) as a long-term target architecture for the delivery of voice services. Other approaches draw their benefit from the circuit-switching technologies, providing an easier migration to voice service delivery over LTE.

Voice as a revenue generator

Currently, around 500,000 users are subscribed to LTE access; however, due to the huge potential of this technology, it has been estimated that LTE will reach about 300 million subscribers within the next four years. The key to the success of this technology is to achieve a satisfactory Quality of Service (QoS) level – and consequently high user expectations. The Policy Charging Rule Function (PCRF) server is responsible for ensuring the required QoS level. Another important aspect might be the ability of the vendor to offer a variety of LTE devices quickly.

With regard to users' habits, in the 25 years since the first mobile networks launched, data traffic has finally begun to outstrip voice traffic. Voice revenue is still key to ROI, with more than $600 billion in sales reported in 2009. However, a single one-year period showed a significant decrease in voice usage — around 15% among users in their teens and early twenties (see Figure 1). That change could point toward a future trend of voice bearing secondary importance as compared to messaging (data service) over smart devices (instead of only phones).

FIGURE 1 :

Trends show a decrease in the number of voice users by age (source: The Nielsen Company)

4G implementation choices for voice

For deploying voice services in the 4G network, three main approaches are available (discussed in detail in the following sections):

IMS-based Voice over LTE (VoLTE) – sometimes called “One voice“ by the GSM Association (GSMA)

VoLTE via Generic Access (VoLGA)

Circuit Switching Fallback (CSFB)

Your choice depends on several conditions. If you need to choose between VoLGA and CSFB, for example, VoLGA offers a number of advantages:

VoLGA provides a richer user experience than is typical for the rest of the LTE services

VoLGA requires less investment (no MSC upgrade) and incurs less deployment risk

VoLGA uses the existing roaming and interconnection scheme

VoLGA supports LTE femtocells (microcells, typically for the home/small business modems deployment)

CSFB invests into obsolete technology, unrelated to IMS

CSFB needs user equipment that supports the fallback procedure, while it has a call setup delay longer than 1.5 seconds

CSFB is feasible only with parallel 2G, 3G and 4G radio coverage

VoLTE: IMS-based voice in 4G networks

The target long-term solution for mobile operators delivering voice, Short Message Service (SMS) and multimedia content, VoLTE focuses on IMS. IMS was introduced in the Universal Mobile Telecommunications System (UMTS) 3rd Generation Partnership Project (3GPP) architecture, when Release 5 offered a multimedia service based on the Session Initiation Protocol (SIP). IMS has always been a central, SIP-based call control node. Since it is a 3GPP standardized environment, however, mobile operators might consider a multiple-vendor architecture. Different vendors are performing interoperability tests, for example, within the MultiService Forum. The main scenarios to verify are basic attachment, roaming, non-LTE access, handover and robustness. An IMS-based voice solution can coexist with a CSFB solution when LTE coverage is unavailable.

Using the IMS-based voice solution as the main solution was described in a previous IP Corner article Voice over LTE (http://www.nil.si/ipcorner/VoiceoverLTE).

FIGURE 2 :

Voice and data bearer paths in UMTS and LTE network architectures

VoLGA: Voice over LTE via Generic Access

Voice over LTE via Generic Access (VoLGA) might be considered as a simple midterm migrating step for the implementation of voice and SMS services over the LTE infrastructure. It is described in the specifications of the VoLGA Forum (www.volga-forum.com), a group of companies that seek to standardize this solution. Instead of investing in a new control switching node, as in IMS, VoLGA architecture adds a gateway between the legacy 2G Mobile Switching Center (MSC) or 3G MSC server and the 4G Evolved Packet Core (EPC) of the LTE. This gateway, a single newly introduced network element, is called the VoLGA Access Network Controller (VANC), and MSC needs no upgrades for it.

From the interconnection perspective, VANC acts as a Base Station Controller (BSC) A interface when connecting to an MSC, and as a Radio Network Controller (RNC) lu interface when connecting to an MSC server. MSCs see no difference between VANC-connected LTE devices and the standard 2G or 3G radio-access devices. Which technology, 2G or 3G, would be used depends on the operator's strategy.

However, when looking from the 4G EPC side, VANC is an IP-based gateway, connected to a Packet Data Network Gateway (PDN GW), over an SGi interface (the same reference point defined in 3GPP specification TS 23.401, as with IMS). It carries both signaling (legacy 3G protocol over the LTE physical layer) and data (voice) traffic, over the same path (VANC). VANC includes the Security Gateway (SeGW) feature that is responsible for the secure IPSec tunnels toward the user equipment. This encrypted path is necessary to preserve the integrity of the authentication and signaling data. Also, this secure tunnel will be used to transport the registration of the UE to the MSC. The Direct Transfer Application Part (DTAP) protocol within GSM/UMTS transports the signaling messages. VANC translates the Time-Division Multiplexing (TDM) voice to the Real-time Transport Protocol (RTP) stream (the same RTP as is used by SIP).

The Mobility Management Entity (MME) is the main controlling element of the LTE radio access. Sv is the interface that connects the Handover Selection Function (HOSF) to the MME and to the MSC server. HOSF is another feature within VANC, responsible for the identification of the International Mobile Subscriber Identity (IMSI) belonging to the UE, as well as controlling the Handover (HO) — that is, routing the call to the correct serving MSC, based on changes in geographical position of the UE. Consequently, this means that the choice is whether to use Single Radio Voice Call Continuity (SRVCC) over 4G and VANC, or use Circuit Switching (CS) over 2G/3G exclusively when no LTE coverage is available.

FIGURE 3:

Traffic paths over VANC in a VoLGA network architecture

VANC development benefited from previous experience with the 3GPP Generic Access Network (GAN), which enabled access over a Wi-Fi network for dual-mode devices, providing a Fixed-Mobile Convergence (FMC). Wi-Fi access in GAN is replaced with LTE access in the VoLGA solution.

FIGURE 4 :

The high-level role of VANC in a VoLGA network architecture (source: VoLGA Forum, www.volga-forum.com)

CSFB: Circuit Switch Fallback

CSFB is a voice solution standard described in the 3GPP 23.272 specification. It could be considered a temporary step toward the target IMS-based architecture. Based on using the 2G or 3G circuit-switched network's resources for the voice and SMS services, it uses the 4G access mode for the data service.

All the control in such a setup is contained within the device itself. The major drawback of this approach is high latency of the call setup, since the mobile device changes the Radio Access Technology (RAT) prior to establishing a call setup, and again after the call disconnects.

Another problem might be creating the required expensive and demanding software upgrades of the network elements and the new interfaces (SGs) between the MSC server and the MME, with no clear roadmap relationship to the target IMS-based architecture. Furthermore, an ISP with several radio access technologies is forced to maintain them, significantly increasing operational expenses (OPEX). And it is not a logical step to use 2G or 3G technology after investing in the superior 4G technology (LTE).

FIGURE 5 :

CSFB network architecture (source: www.3GPP.ORG)

The newly added interface that interconnects MME and MSC in Figure 5, SGs, is used for mobility control and paging procedures between the EPC and CS sides. SGs is based on the Gs interface between the Serving GPRS Support Node (SGSN) and MSC fundamentals, and it uses the new SGs Application Part (SgsAP) protocol (Figure 6), based on the Base Station System Application Part (BSSAP) and transported over the Stream Control Transmission Protocol (SCTP).

FIGURE 6:

The TS 23.272 specification standardizes the protocols for the new SGs interface

Conclusion

Carriers worldwide are strongly motivated to expand their infrastructures to accommodate LTE access. By the end of 2010, more than 135 networks worldwide had trials in progress or had launched LTE networks. Along with this migration to advanced radio access, voice and SMS service are becoming prominent. To offer voice service, one common long-term strategy is to employ IMS-based VoLTE, but it is not yet a priority investment demand. The necessity of offering 4G upgrades in the complete radio network will surely provide additional incentive. While waiting for its final deployment, operators can offer a midterm or short-term solution in parallel with the radio access upgrade to LTE. (These alternative solutions might delay the full implementation of IMS in some cases.) Yet to be clarified is the deployment mode for the various supplementary voice services (voice VPN, Find Me/Follow Me, Caller RingBack Tone etc.).