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IMS pioneer: how SK Telecom has blazed a trail

South Korean operator SK Telecom has been providing IMS-based multimedia services since 2005. Having started with a video call service, the operator has launched a steady stream of IMS services including mobile messaging, fixed Internet telephony, a converged fixed-mobile service, high-definition video calling and a social networking talk service. SK Telecom has also joined with Korean mobile carriers KT and LG U+ to enable subscribers to use the mobile messenger service across networks. The operator claims such interconnection of IMS services between operators remains the only example to date. It has resulted in the mobile messenger service being used by 14 million subscribers. One of the strengths of IMS is that it allows for experimentation and customisation, says

Woo-Yong Choi, senior manager of the core network development team at SK Telecom. For example, the operator conducted a study and found that some users felt uncomfortable being seen on a video call. With SK Telecom’s RCS-based HD video calling service, a user starts a circuit-switched voice call and can choose from a list of multimedia features such as real-time video, photos, a sketch book and location information. The user can also choose whether to just receive video or use two-way video sharing. For 3G circuit-switched video calls, voice and video traffic are transmitted at 64 kilobits-

per-second (Kbps). SK Telecom’s HD video calls use the 3G packet network and WiFi to transmit at 100-200 Kbps, resulting in a fourfold improvement in video resolution.

year ago. “One year ago it was still circuit- switched fall-back,” says Myers—in other words, voice implemented not on LTE but using the existing 2G/3G circuit-switched infrastructure. “Now operators are accel- erating their plans, launching VoLTE much more quickly once LTE is in place.” But operators face significant challenges

as they move to implement IMS for mobile. They will also need to ensure interopera- bility between their IMS-enabled core communications services. There are two approaches for adopting

VoLTE. A minority of operators plan to have sufficient LTE coverage such that a user can remain using VoLTE without needing to hand over to any other cellu- lar voice technology. This is the strategy of two US operators, Verizon Wireless and MetroPCS, for example. Other operators will need to keep the

voice call active as a user moves from an LTE cell to a 3G or 2G cell. “This is tech- nically tough, and it has exercised the minds of the operators, vendors, the 3GPP and the GSMA,” says Warren. The challenge is that two events must

be supported simultaneously. As the radio interface is handed over, the nature of the voice bearer changes from IP to a circuit- switched one. This requires a handover from the IMS domain to the mobile switching centre server, says Warren. One solution operators are investigat-

ing is to separate the two events by delaying the bearer handover, in this case moving from LTE to a 3G HSPA


connection first. Such a connection has sufficient bandwidth to support the IP bearer, allowing the handover to circuit- switched infrastructure to occur later. A1 is considering such issues. “We think

it is important to implement VoLTE,” says Furtl. “And if you implement VoLTE, [is it necessary to have] a fallback solution for GSM or is it possible to start the call in LTE and finish it in HSPA?” SK Telecom is still undecided about its

plans to adopt VoLTE. But Woo-Yong Choi, senior manager of the core network development team at SK Telecom, believes the implementation would be straightforward. “The VoLTE implemen- tation will not be difficult as our company can use its existing IMS infrastructure and technology,” he says. Magnus Furustam, vice president,

product area core & IMS at Ericsson, says getting such coexistence to work between the circuit-switched and packet-switched cores will be challenging, but that it is less a technical problem than one of project planning. Clever at NSN highlights another

networking challenge operators face when moving to IMS. In time-division multi- plexed networks a peak load might be three times the regular load; in IP networks the surge can be 100 times the load. The IMS core needs to be designed to tackle such unpredictable traffic, he says: “It is a huge effort and does not come for free.” What is of more concern for operators, says Furustam, is how they protect their

most important business: voice and SMS services that still typically represent 75% of their revenues. Operators must grapple with how to advance their services in a world where all sorts of communications applications can be downloaded in a second, says Furustam: “That is becom- ing more of a real, real concern.” Another important component that

builds on IMS and which will play its part alongside VoLTE is an enhanced version of the Rich Communication Suite (RCS- e), defined by five European operators and which is being promoted by the GSMA. RCS-e will enable users to send instant messages, video and files, making use of the handset’s address book. Deutsche Telekom, Orange, Telecom Italia, Telefonica and Vodafone announced at Mobile World Congress earlier this year plans to launch RCS-e services by the end of this year or in early 2012. “The things that have stopped RCS

happening are the business case, and in some areas the complexity of the service itself,” says Warren at the GSMA. Indeed, work is about to start to harmonize RCS-e with the latest version of RCS to create the functionality required using a reduced set of protocols. But for RCS/RCS-e enabled services to be used across opera- tor boundaries in the same way as voice calls and SMS, it is essential for the oper- ator community to develop around VoLTE and RCS, says Warren. “All the while, the Internet players will be further developing their offerings.” SK Telecom highlights the threat posed

by free smartphone applications that directly compete with IMS-based serv- ices provided by mobile carriers. “This is the biggest challenge to IMS,” says Choi, who says that mobile operators’ future business models will be shaped on devel- opments based on RCS. But Choi implies that success will

largely be down to the operators. “The biggest strength of IMS is that after it is launched the development of additional services becomes very simple,” he says. “The return-on-investment will be high if one decides to adopt IMS and develop many additional services instead of stop- ping at just one or two.” n September 2011

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