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The Mobile

Dispelling LTE Myths
At the recent Leadership meeting of 3GPP, the Technical Specification Group Chairmen sat down to discuss some of the Myths that surround the roll-out of LTE™.

Is LTE Data Only? ...Does it support SMS? ...Is IMS a success? ...Can LTE complete emergency calls?

Here are the answers:

Myth 1: LTE is Data only

Reality: LTE supports voice and efficient support of voice was one of the key considerations in designing LTE. The voice solution for LTE is IMS VoIP and it is fully specified.

The 3GPP solution for voice over LTE is a combination of multiple efforts:
  • The work in Rel 7 to optimize IMS signalling and VoIP encoding so it would be as good or better than CS voice in terms of quality and efficiency,

  • The work in Rel 8 to develop a radio and core network evolution optimized for the transfer of packet data.

  • The work in Rel-7 to add the IMS emergency call requirements and to adapt it to regulatory requirements in LTE and GPRS in Rel-9.

  • The work in Rel-8 to add the always-on IP connectivity requirements in LTE

A key consideration to recognize is that under LTE, voice is just one of many potential media streams that can be communicated. A packet based network and VoIP allows this flexibility while still providing efficient use of radio and network resources.

However, 3GPP recognizes that adoption of both LTE and IMS will not occur overnight. For this reason 3GPP provided a transition solution for voice called CS Fallback. This allows a LTE device to drop back to the legacy 3G or 2G network if IMS VoIP capabilities are not supported. This is viewed as an interim solution to ease the transition to IMS and VoIP.

Myth 2: SMS isn’t supported over LTE

Reality: LTE and EPS will support a rich variety of messaging applications and also SMS is supported over LTE. The solution is twofold, covering both the full IMS case and a transition solution for those networks that do not support IMS.

SMS over IP was fully specified 3GPP Rel 7. It depends on IMS and it is intended to provide compatibility between the existing cellular legacy and the implementations with more elaborate messaging capabilities via SMS and IMS interworking..

For environments without IMS a transition solution was specified. This is called SMS over SGs (previously called the misleading name: SMS over CS). It is a hybrid approach that allows the transmission of native SMS from CS infrastructure over the LTE radio network. SMS over SGs was specified as part of Rel 8. SMS over SGs provides SMS service for mobiles in LTE and since it requires also CS domain infrastructure for the SMS transmission, it is intended to be a transition solution.

Myth 3: IMS isn’t ready for prime time

Reality: IMS has been around a long time. It was first developed as part of Rel 5 in 2002. It is based on IETF protocols such as SIP and SDP that are very mature. These technologies have been embraced by the industry as the signalling mechanism for multimedia applications.

In Rel 7 an effort was made to optimize IMS and the supporting protocols to ensure that voice and other media were supported as efficiently as in circuit switched networks.

IMS is fully specified and mature. The difficulties in rolling out IMS are not due to the protocols or the specifications. The consideration point is not only technical aspects but also shifting the whole industry paradigm from CS services to a truly IP-based environment, i.e. service migration, policies, interoperability and deployment plan included. However, these complexities must be addressed if the idea is to truly provide a richer service environment. This work is ongoing in many forums outside of 3GPP (e.g. Rich Communication Suite).

Myth 4: LTE doesn’t support emergency calls

Reality: VoIP support for emergency calls (including location support) is specified as part of Rel 9. This fulfils the last regulatory requirement separating VoIP from CS in 3GPP networks. A transition solution exists which is falling back to 3G/2G for completing emergency calls. This solution has existed since IMS was introduced (Rel 5).

However, to satisfy the situation of a fallback network not existing, this enhancement was completed in Rel 9. This allows the operator the option of supporting the regulatory requirements for LTE VoIP calls both for phones that can register for normal services and for those in limited service, including the USIM-less case.

Also the emergency call callback from the PSAP and its interaction with the possibly activated supplementary services is specified.

Legacy ServiceTransition SolutionEPS Solution
CS VoiceCS Fallback (Rel 8) IMS VoIP (Rel 7)
SMS SMS over SGs (used to be called SMS over CS) (Rel 8)SMS over IP (Rel 7)
Supplementary ServicesCS Fallback (Rel 8)Multimedia Telephony (Rel 7)
Emergency Calls w Location SupportCS Emergency Calls (Rel 5)IMS Emergency Calls w Location Support (Rel 9)

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