voice options for lte - · pdf filevoice options for lte ... or utran. 1kineto has ......

A Kineto White Paper

Kineto Wireless, Inc.1601 McCarthy Blvd.Milpitas, CA 95035

Tel: +1 408 546 0660

© 2009 Kineto Wireless, Inc. All rights reserved. www.kineto.com

Voice Options for LTE

A comparison of VoLGA and CS Fallback technologies for delivering voice services to LTE devices.

December 2009

December 2009 © 2009 Kineto Wireless, Inc. All rights reserved.


IntroductionMobile operators around the world have embraced long-term evolution (LTE) as the basis for the next-generation mobile Internet. As an access technology, LTE offers a significantly lower cost per bit over existing technologies, while providing a very high-speed, low-latency broadband network ideal for delivering real-time services like packet voice and video.

While many operators view LTE initially as a data-only service offer, ideally suited to address the recent explosive growth in mobile data traffic, it is clear that the economics of developing and managing a new macro-network require operators to encourage a more rapid migration of mainstream subscribers to LTE to provide an adequate return on investment.

Thus, sooner rather than later, operators are being confronted with the need to deliver a wider range of services over LTE with the support for voice services being paramount.

LTE Voice Options

For many operators, the target architecture for voice over LTE is IMS telephony. In order to realize that target, a group of companies recently announced the ‘One Voice’ initiative. One Voice is a crucial first step in bringing commercial IMS mobile telephony services to market, providing a framework for a common baseline set of services across multiple operators.

The roadmap operators follow to IMS will be based on their individual business, economic and regional competitive requirements, which of course vary widely. So while some operators will target initial deployment of voice over LTE services with IMS, there will be other operators looking to leverage the LTE network for voice prior to investing in IMS.

For these operators, there are two ‘pre-IMS’ options to deliver voice (as well as other CS-based services, such as SMS) to LTE devices. One option is known as Circuit-Switched Fallback (CSFB). The other approach is called Voice over LTE via Generic Access (VoLGA). These options may also need to be considered by those operators that deploy initially with an IMS voice solution, in order to support inbound LTE roaming from networks where IMS is not yet enabled or supported.

This whitepaper examines these two alternatives for providing voice in the LTE network prior to IMS and demonstrates that, with VoLGA, operators do have the option to deliver voice natively and inexpensively over the LTE packet network at a quality level that their subscribers demand, while establishing a clear path toward IMS telephony.

One Voice and IMS TelephonyLed by LTE pioneer Verizon Wireless, a group of companies recently announced the “One Voice” Initiative. In a paper published by the group, they acknowledged that IMS telephony has been in the standards setting phase for many years and had become overly complex. The initiative sought to define a ‘minimum’ feature set, or a starting point, for IMS telephony.

The initiative is a similar to the RCS project initiated for IMS data services. Rather than trying to ‘boil the ocean’ by embracing all the features and capabilities available in these IMS specifications, it instead carves out the minimum feature set to start and work to achieve interoperability around those capabilities.

Given the current state of IMS telephony, establishing leadership was necessary for getting the industry moving towards a common goal. For an industry, this was more than a ‘good idea;’ it’s actually a required step on the path towards commercial deployments.

From their published document, the One Voice initiative does not seek to mandate IMS telephony as the only solution for voice over LTE, nor does it endorse any interim approaches. It simply represents another step in the development of an

IMS telephony solution and ecosystem.



Circuit Switched Fall Back (CSFB)In the CSFB approach, LTE handsets revert, or “fall back,” to an existing 2G or 3G radio network whenever a subscriber needs to place or receive a voice call.

As LTE offers the most efficient service delivery network and provides the best data service experience, it is reasonable to expect that the operator will wish LTE capable handsets to always attach to the LTE network as the preferred radio access environment.

However, whenever a subscriber needs to make or receive a voice call, the handset will drop the LTE connection and automatically ‘fall back’ to an existing 2G or 3G network. Once the handset is attached to the fall-back network, the call can be connected. At the conclusion of the call, the device will re-associate and register with the LTE network.

While this approach sounds simple, the reality is that it is a costly, ill-conceived technology that requires new feature investment in the ‘legacy’ 2G/3G network, burdens the LTE environment with functions required only in support of circuit switched voice and has no synergy with any aspect of an IMS voice service, meaning that an investment in CSFB has no longer lasting benefit in the network.

User Experience

The CSFB approach provides a poor user experience due mainly to the fact that it does not deliver voice services natively over the LTE network.

First, CSFB introduces a significant call-setup delay1. This delay is caused by the procedures necessary to cause the handset to fall back to make or receive a voice call and is in addition to any standard delays associated with paging or establishing and connecting the actual voice call. Call setup times are a key performance indicator for subscriber satisfaction with the voice service.

Second, CSFB relies upon the presence of a 2G/3G fallback network and that the fall back to that network is successful. By significantly increasing the number of inter-system handovers, a common source of dropped calls, key service quality metrics (for example, the percentage of successful voice call completions) are likely to degrade.

Lastly, CSFB adds complexity to delivering combinational voice and data services, such as those provided by the IMS Rich Communications Suite (RCS) initiative. RCS is a suite of packet data services (e.g. live address book, instant messaging, presence) leveraging an operator’s IMS data infrastructure that is designed to provide additional value to subscribers.

RCS can be combined with today’s voice network to provide ‘rich communications’ beyond simple voice but relies on concurrent voice and data communication to reach its full service potential. As a result, RCS services with CSFB voice changes from an always-on service to a mostly-on service. If the fall back lands on a 2G network, then real-time RCS communication for that handset and its RCS peers would be delayed or lost for the duration of the call as most 2G systems are not deployed with support for simultaneous voice and data services.

In general, the CSFB approach of pushing all voice calls back to 2G or 3G will provide a user experience that is much poorer than that which they receive today in the GERAN or UTRAN.

1 Kineto has characterized the call-setup delays associated with CS Fallback in a separate paper. Please contact the company for a detailed analysis.



Deployment Cost and Complexity

To support CSFB, an operator’s existing network of mobile switching centers (MSCs) must be upgraded with a new capability to maintain a CS domain control plane link to the devices when they are camped on LTE. For a long time, the goal was to keep the LTE network as a pure all-IP network but with CSFB, CS domain technology becomes embedded in the new LTE network. The new interface must be implemented to allow the MSC to communicate with handsets through the mobile management entity (MME). In turn, the MMEs and eNodeB must be upgraded to support the CSFB functionality.

An operators’ key objective in applying today’s MSC resources to deliver voice over LTE is to minimize modification and avoid investment in existing resources, whereas upgrades to MSCs are invariably costly in both capex and opex. Therefore, a CSFB-required MSC upgrade is counter to the desire to minimize investment in legacy systems.

CSFB dramatically increases the number of signaling transactions2 in the Core Network. RAN as the inter-system signaling load in the network becomes dependent upon the frequency of calls made or received while in LTE coverage, rather than on normal inter-system user mobility. Each time a voice call is made or received, the LTE handset must perform a handover to the 2G or 3G network, attach to that network to conduct the call and then subsequently move back to the LTE network once the call is completed. This will necessitate capacity improvements for signaling in the RAN (more RNCs may be required) and in the Core Network (increased VLR/HLR transaction processing capacity will be needed).

CSFB adds complexity for unique operator situations. For an operator with an LTE license and an MVNO relationship for 3G and/or 2G coverage, CSFB requires all voice traffic to be carried through the alternative provider. Or, for operators looking to provide blanket LTE coverage in a specific area, there must be fall-back coverage capable of supporting CSFB.

Finally, for those operators planning to deploy LTE femtocells along with their LTE macro network, CSFB makes little sense. Why deploy an LTE femtocell if all voice calls must fall back to the outdoor 2G or 3G network when the premise for the femtocell in the first instance is to improve indoor coverage? It is certainly possible to imagine a dual-radio LTE/3G femtocell, however the cost and complexity in such a device would be a barrier to a healthy LTE femtocell business case.

While positioned as a ‘quick and easy’ way to support voice for LTE operators, it is clear that CSFB is nothing of the sort and actually adds significant cost and complexity in the network.

CSFB Summary

The CSFB objective is to provide a low-cost, low-risk and timely approach for mobile operators to support voice services on LTE devices and networks. An investigation into the issues surrounding the implementation of CSFB suggests that this goal is far from attained. Even the potential for some modifications to CSFB in 3GPP Release 10 (pushing CSFB availability even farther out) will likely not reduce the impacts to the operator’s network or to the user experience.

It is clear that CSFB is neither low cost nor low risk, nor does it offer any advantage in the planned evolution to IMS telephony.

2 Kineto has characterized the signaling load impact in a separate paper. Please contact the company for additional information.



VoLGA: True Voice Over LTEIn March 2009, a group of leading mobile operators and equipment vendors, comprised of Alcatel-Lucent, Deutsche Telekom (formerly T-Mobile International), Ericsson, Huawei, Kineto Wireless, LG Electronics, Motorola, Nortel, Samsung, Starent Networks and ZTE, announced the formation of the VoLGA Forum.

The goal of the Forum was to create a set of specifications for enabling the delivery of voice and SMS services over LTE based on the existing 3GPP GAN standard. In September 2009, the group announced the completion of the VoLGA specification, which is publicly available on its website.

In the VoLGA approach, rather than forcing a handset to fall back to the overlay 2G or 3G network, voice services are packetized and delivered natively over LTE bearers. As VoLGA enables inbound and outbound voice calls to be carried over LTE, it avoids the majority of problems that are created by CSFB.

Circuit Services

Pres. Share

WAP MMS

VMSSMSTel. ...

IMS

Packet Services

GERAN UTRAN

Elevates entire CS core to function as a packet service to LTE devices

BSC RNC RNC

A/lu -CS

GANC

LTE

EPC

1

Enables operators to focus IMS investment on new RSC-type service

2

Enables operators to bring telephony revenues sooner to LTE investment

3

Leverages growing investment in low- cost, feature-rich Release 4 voice core

VoLGA: Extend Voice Services over LTE

http://www.volga-forum.com





Available Today

In December 2009, Deutsche Telekom (DT) announced it had demonstrated two independent VoLGA infrastructure systems together with two VoLGA client vendors providing separate, interoperable VoLGA solutions within the DT LTE test network in Germany. These solutions, from Forum members Alcatel-Lucent and Kineto Wireless, serve to showcase the advantages of VoLGA as a technology for voice over LTE.

User Experience

VoLGA delivers the voice experience that subscribers expect from a next-generation network technology, such as LTE. Voice calls take place over LTE rather than forcing handsets to fall back to a 2G or 3G network and, as shown during the DT testing, VoLGA users experience call setup times that are better than subscribers receive today in 2G or 3G.

VoLGA extends the services available from existing voice platforms to the LTE network. All telephony services available over 3G/GSM are available in LTE, including all supplementary services, emergency services, as well as advanced Intelligent Network applications, such as pre-paid, toll free calling and international roaming.

VoLGA provides a commercial grade mobile voice service today with a user experience that outperforms the 2G/3G environment.

Deployment Cost and Complexity As demonstrated by DT, deploying a VoLGA solution requires no modification or upgrade to an operators existing 2G/3G core network. VoLGA systems require just the introduction of a VoLGA controller to bridge the control and user planes between the LTE network and existing voice service platforms. As a result, a VoLGA system is faster to deploy and significantly lower in cost and complexity than CSFB.

Finally, VoLGA systems transport voice as VoIP in LTE, meaning it is not necessary to switch RANs to make a voice call. Consequently, there’s no significant increase in the amount of inter-system signaling traffic in the network. This also means that VoLGA properly supports voice services over the packet bearers in an LTE femtocell environment.

Solution Timing and Risk

Today, two infrastructure equipment manufacturers have functioning VoLGA controllers available, and additional companies have announced their intentions to develop VoLGA-based products. For operators testing their nascent LTE networks, VoLGA technology is clearly available now for evaluation.

VoLGA is a simplified version of the 3GPPs GAN standard, adapted to take advantage of the characteristics of LTE; the mobile operators’ own broadband access network. Therefore, the majority of LTE handset manufacturers that already have a solid understanding of GAN can employ their existing technology to provide VoLGA support in their devices.

Most operators wish to minimize investment in older systems, especially for new features that have limited scope. Investments made in the LTE network for VoLGA are equally relevant investments for voice support in the IMS.



A Better Path to IMS

Compared with CSFB, VoLGA offers a better path towards an IMS voice network. VoLGA relies on many of the same underlying technologies as IMS voice:

• The same technology for carrying VoIP, RFC 4867

• The same technology for managing QoS, Policy and Charging Control (PCC)

• Network initiated handover using the Sv interface on the LTE network

• LTE network support for Robust Header Compression (RoHC)

• Concurrent support for IMS data services like RCS over LTE

• Support for voice services when using LTE femtocells

VoLGA supports combinational services like IMS RCS natively on LTE. The RCS packet services run over LTE and can easily be combined with VoLGA voice services to deliver a rich communications experience for LTE users. Thus operators can begin investing in IMS for data-only services, add RCS services to complement VoLGA telephony and ultimately evolve to a full IMS environment for telephony and data services.

VoLGA offers a clear path to Voice over IMS as it enables operators to evaluate the performance of RTP-based voice services in the LTE access network and to begin earlier testing of procedures, such as handover using SR-VCC methods and QoS enabling of voice flows prior to their availability in IMS. CSFB technology provides no such affinity.

Conclusion It is clear that most operators target architecture for voice over LTE is IMS, and many of these operators will move to IMS for their initial deployment. However, it is also clear that there will be an interim period for many other operators during which LTE is deployed but they have not yet invested in a migration to IMS for voice. For these operators, and interim solution is needed. Even for those operators that go straight to IMS, there remains the problem of how to support voice services for roaming visitors in the LTE network if the visiting subscriber’s home network is not yet IMS enabled.

Two choices are available to support interim voice services for users attached to the 4G LTE network.

CSFB will eventually offer the operator a complex, costly network upgrade and deliver a user experience that will be poorer than that of today’s 2G network. CSFB has no potential for re-use when the operator introduces voice support in the IMS.

VoLGA today offers a less-intrusive, lower-cost and lower-risk approach and delivers a user experience that improves upon current 2G and 3G networks. An investment in VoLGA has significant synergy with voice over IMS, ensuring that investments made to support VoLGA in LTE are re-used later on.

In comparing VoLGA with CSFB, it’s clear that VoLGA provides a higher quality user experience at a lower cost and with far lower risk. VoLGA builds on a ready-made ecosystem of vendors to provide a complete range of VoLGA-based products enabling mobile operators to weave their core revenue generating service and voice into all elements of their LTE strategy.



A Kineto White Paper

Kineto Wireless, Inc.1601 McCarthy Blvd.Milpitas, CA 95035

Tel: +1 408 546 0660www.kineto.com

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