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NTT Docomo: VoLTE Roaming and Interconnection Standard Technology
NTT Docomo: VoLTE Roaming and Interconnection Standard Technology
NTT Docomo: VoLTE Roaming and Interconnection Standard Technology
NTT Docomo: VoLTE Roaming and Interconnection Standard Technology
NTT Docomo: VoLTE Roaming and Interconnection Standard Technology
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NTT Docomo: VoLTE Roaming and Interconnection Standard Technology


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  • 1. VoLTE 3GPP Roaming Further Development of LTE/LTE-Advanced – LTE Release 10/11 Standardization Trends – NTT DOCOMO Technical Journal VoLTE Roaming and Interconnection Standard Technology In 3GPP Release 11, the VoLTE roaming and interconnection architecture was standardized in cooperation with the GSMA Association. The new architecture is able to implement voice call charging in the same way as circuit-switched voice roaming and interconnection models by routing both C-Plane messages and voice data on the same path. This was not possible with the earlier VoLTE roaming and interconnection architecture. Core Network Development Department Itsuma Tanaka of the existing VoLTE roaming and 1. Introduction Voice over LTE (VoLTE) is a tech- This article compares existing cir- 2. Existing Circuit-Switched Voice Roaming Model nology for providing existing voice and cuit-switched voice roaming and inter- Existing circuit-switched voice SMS services on LTE , which does not connection models with the earlier roaming and interconnection apply a have a circuit-switched domain [1]. It VoLTE roaming and interconnection charging model called “cascaded charg- is implemented using the IP Multime- models, describe issues that needed to ing”[2]. This model is described below. dia Subsystem (IMS) , which is a stan- be resolved by 3GPP and GSMA. Then Figure 1 shows the circuit-switched dard 3GPP technology for IP-based it is described the new VoLTE roaming voice interconnection model, with Sub- multimedia services. and interconnection architectures stan- scriber (a), using Operator A, placing a dardized to resolve these issues. call to Subscriber (b), who uses Operator interconnection architecture. *1 *2 3GPP Release 11 (“Rel. 11”) specifies VoLTE roaming and interconnection architecture, created in collabora- Operator A *3 Operator B message tion with the GSM Association (GSMA) . Voice interconnect Voice interconnect network C-Plane network Subscriber (a) Subscriber (b) Voice call Rel. 11 adds standardized necessary functions for implementing roaming and interconnection in a manner similar Interconnect equipment Switch Figure 1 Circuit-switched voice interconnection scheme to existing circuit-switched voice on top ©2013 NTT DOCOMO, INC. Copies of articles may be reproduced only for personal, noncommercial use, provided that the name NTT DOCOMO Technical Journal, the name(s) of the author(s), the title and date of the article appear in the copies. NTT DOCOMO Technical Journal Vol. 15 No. 2 *1 Circuit switched domain: A functional block of a network offering circuit switched services. 37
  • 2. VoLTE Roaming and Interconnection Standard Technology B. There are also voice interconnect in this case is done between operators networks between the two operators. and voice interconnect networks in order, as with voice interconnection. uses information in the Control Plane Combining these two cases yields the 3.1 Overview of Existing (C-Plane) message, and parties are circuit-switched voice roaming and billed, in-turn, from the call receiver’s interconnection scheme as shown in VoLTE is based on IMS, which is operator (Operator B), to the voice Figure 3. In this case, Subscriber (a) an IP-based system implemented using interconnect network, and the caller’s from Operator A is roaming to Operator the Session Initiation Protocol (SIP) operator (Operator A). C, and Subscriber (b) from Operator B standardized by the Internet Engineer- is roaming to Operator D. ing Task Force (IETF) . IMS compris- An example of roaming with the VoLTE Roaming Model *4 *5 circuit-switched roaming scheme is Subscriber (a) pays for voice com- es SIP control equipment including shown in Figure 2. In this model, Sub- munication on the segment including Proxy Call/Session Control Function scriber (a1) from Operator A has a Operators C and B, while Subscriber (P-CSCF) , the Serving Call/Session roaming connection to Operator C in (b) pays for the receiving segment, Control Function (S-CSCF) and the another country. In this case, for exam- between Operators B and D. Application Server (AS) [3]. *6 *7 *8 ple, Subscriber (a1) then calls Sub- Calculation of charges for each The existing VoLTE roaming and scriber (a2) from Operator A, or a sub- operator and the voice interconnect net- interconnection model is shown in scriber on another operator. Between work is done, as in the cases in Fig. 1 Figure4. The P-CSCF is implemented Operators A and C, there are two types and 2, according to the information in by Operator C, but the S-CSCF and AS, of interconnect network, one for voice the C-Plane messages used to set up the which perform actual call control, are and one for signaling. Voice charging voice call, and the duration of the call. implemented by Operator A. There are also multiple IP eXchanges (IPX) *9 between the operators, which form the Operator A international IP interconnect network Subscriber (a2) Signal 信号 Interconnect 中継網 network Signal 信号 Interconnect 中継網 network Location registration and other control signals Voice Interconnect network Control signal Voice Interconnect network Subscriber (a2) subscribes to operator A Voice call NTT DOCOMO Technical Journal Charging for voice communication 3. Existing VoLTE Roaming Model that transmits the SIP and voice data [4]. In circuit-switched networks, call control is achieved using switches belonging to the operator where the subscriber is located, but a major differ- Operator C ence between this and VoLTE is that call control is achieved with SIP servers Subscriber (a1) (S-CSCF, AS) on the home network (the subscriber’s contracted operator). HSS Interconnect equipment Switch HSS:Home Subscriber Server Also, since VoLTE is an IP based system, there is no need for the C-Plane Figure 2 Circuit-switched voice roaming model messages (SIP) to use the same route that the actual voice data uses, as was *2 IMS: A communication system standardized by 3GPP for implementing multimedia services. IMS used IP and the SIP Internet telephony protocol to integrate the communication services of the fixed telephone and mobile communication networks. 38 *3 GSMA: An association that supports and manages activities of the mobile industry, such as formulating roaming rules. The largest mobile communications industry association in the world, with members in related businesses including mobile communications providers, IPX operators, and terminal, equipment and software vendors. *4 SIP: A call control protocol defined by the IETF and used for IP telephony with VoIP, among other applications. NTT DOCOMO Technical Journal Vol. 15 No. 2
  • 3. Operator A Operator B Voice Interconnect network Signal 信号 Interconnect 中継網 network NTT DOCOMO Technical Journal Signal 信号 Interconnect 中継網 network Voice Interconnect network Location registration and other control signals Voice Interconnect network Control signal Voice Interconnect network l cal ice Vo Voice Interconnect network Voice call Voice Interconnect network Signal Interconnect network Control signal Signal Interconnect network Operator D Operator C Subscriber (a) Location registration and other control signals Subscriber (b) HSS Interconnect equipment Switch Figure 3 Circuit-switched voice roaming and interconnection model IPX Operator A IPX Operator B to identify what users placed voice calls SIP signal S-CSCF/AS SIP signal IPX Operator C over what periods of time. Thus, IPX S-CSCF/AS IPX SIP signal does not necessarily pass through the IPX handling the voice data operators not carrying SIP signals are IPX time basis. IPX VoLTE is positioned as an extenP-CSCF P-CSCF not able to charge for voice calls on a SIP signal The operator of the IPX cannot charge for voice traffic in time units if the SIP signal does not pass through it. IPX through the same IPX, it is not possible Operator D IPX sion to existing voice services at the GSMA, and the GSMA has agreed that Voice call implementing existing voice charging, Subscriber (a) Subscriber (b) Figure 4 Existing VoLTE roaming and interconnection model including IPX operators, is a business requirement [5]. Given this GSMA requirement, the 3GPP studied signal- true for earlier schemes, and communication from Subscriber (a) to Subscriber 3.2 Issues with Existing VoLTE Architecture ing and voice routing schemes that are equivalent to existing circuit switching in order to implement existing voice (b) goes through IPX and can be routed Existing VoLTE architecture allows directly, without needing to pass for more efficient voice data routing, through the home network. With cir- without requiring it to pass through the cuit-switched schemes, the actual voice home network. However, from the per- data always uses the same route as the spective of IPX operators, since SIP C-Plane messages. signaling does not necessarily pass For Rel. 11, a work item called *5 IETF: A standardization organization that develops and promotes standards for Internet technology. The technology specifications formulated here are published as Request For Comment documents (RFCs). *6 P-CSCF: A SIP relay server that performs the roles of SIP transmission as well as linking with the LTE core network, and QoS control. *7 S-CSCF: A SIP server that performs terminal session control and user authentication. *8 AS: A server that executes an application to provide a service. *9 IPX: An exchange network that has evolved from GRX and that provides QoS function. NTT DOCOMO Technical Journal Vol. 15 No. 2 charging for VoLTE voice calls. 4. Rel. 11 VoLTE Roaming Model 39
  • 4. VoLTE Roaming and Interconnection Standard Technology “Roaming Architecture for Voice over IMS with Local Breakout” (RAVEL) passed from an S-CSCF from other (1) When Subscriber (a) presses the Call button on the terminal, the *11 INVITE signals. was agreed upon and the architecture terminal sends an INVITE signal (7) Having determined that the INVITE was studied within this framework. to the P-CSCF, which is a request to was passed from an S-CSCF, the initiate a call. TRF uses SIP Uniform Resource A feature of the existing circuitswitched voice model was that voice (2) The P-CSCF includes *12 Identifiers (SIP URIs) the and other associated TRF address to the destination information configured the same path, and RAVEL agreed on a INVITE. in the INVITE to decide the model to emulate this for VoLTE as (3) The P-CSCF then forwards the destination operator (Operator B) INVITE to the S-CSCF/AS on the and the IPXs to use. The example in Figure 5 shows a RAVEL VoLTE home network (Operator A). Here it Fig. 5 shows IPXs, but existing roaming and interconnection model and specifies the route that the voice voice interconnect networks can session setup procedures [3]. data must take. also be used as the interconnection well. With this architecture, a new entity (4) The S-CSCF/AS performs voice called the Transit & Routing Function network. call control. (8) The INVITE is then transmitted to (TRF) is introduced, which brings the (5) The S-CSCF sets the Loopback Subscriber (b) by ordinary VoLTE SIP signals back from the network Indicator in the INVITE and sends calling processes, completing where the caller is currently connected it to the specified TRF as a VoLTE call configuration. In the (Operator C) to the home network per- response. call configuration process, Operator forming call processing (Operator A), (6) Having received the INVITE from B configures the voice data route to function that the S-CSCF, the TRF checks be the same as that used for the SIP routes the voice data and SIP signals whether there is a Loopback signaling. Note that the details of over the same path. Indicator. The Loopback Indicator VoLTE call configuration were is used by the TRF to distinguish described in an earlier issue of this INVITE signals for voice calls journal [1], so we omit them here. *10 and provides an anchor The calling process is described below. Operator A Operator B (4) Service control (6) Check Loopback indicator IPX S-CSCF/AS (3) Transmit INVITE (5) Forward INVITE (Loopback indicator) IPX IPX e ic Vo IPX (2) Assign TRF address S-CSCF/AS Voice call NTT DOCOMO Technical Journal data and control signals were routed on l cal Operator C IPX IPX Operator D P-CSCF (6) Check Loopback indicator (7) Select interconnect network P-CSCF TRF (1) Initiate call Subscriber (a) Subscriber (b) Figure 5 Rel. 11 (RAVEL) VoLTE roaming and interconnection scheme I *10 Anchor: A logical node site for control-signal or user-bearer switching. 40 *11 INVITE: A SIP signal that requests a connection. *12 SIP URI: The SIP addressing scheme used when making a telephone call via SIP protocol. NTT DOCOMO Technical Journal Vol. 15 No. 2
  • 5. Using the above procedure, SIP sig- Operator A IPX path, and each operator and IPX Operator B IPX SIP signal nalling and voice data use the same Voice call S-CSCF/AS S-CSCF/AS IPX SIP signal Applying the above techniques, a IPX IPX Voice call charging. Voice call provider can apply existing voice SIP signal IPX scheme that routes voice calls to Opera- NTT DOCOMO Technical Journal tor A first, and then interconnects with Operator C Operator D Operator B is also specified in Rel. 11 (Figure 6). This scheme is expected to be used for functions such as lawful intercept or to provide assistance to P-CSCF P-CSCF Subscriber (a) Subscriber (b) Figure 6 Rel. 11 (RAVEL) VoLTE roaming and interconnection model II Subscriber (a). standard. Selection of the model will be 5. Conclusion This article has given background left to organizations such as the GSMA or the bilateral operator agreement. References [1] Tanaka et al.: “Overview of GSMA VoLTE Profile,” NTT DOCOMO Technical Journal, Vol. 13, No. 4, pp. 45-51, Mar. 2012. and described voice roaming and inter- NTT DOCOMO has contributed [2] 3GPP TR23.850 V11.0.0: “Study on connection schemes specified in the greatly to study of these architectures at roaming architecture for voice over IP VoLTE architecture in 3GPP Rel. 11. both the 3GPP and the GSMA, as Multimedia Subsystem (IMS) with local breakout,” Dec, 2011. VoLTE operators can use the three chairperson and in other roles. Look- roaming and interconnection architec- ing forward, concrete operational Subsystem (IMS); Stage 2,” Dec. 2012. tures described in Fig. 4 to 6 and speci- guidelines based on the 3GPP specifi- [4] GSMA PRD IR.65 V10.0: “IMS Roaming fied in the 3GPP Rel.11 standard. It is cations will be created at the GSMA, not yet known which one of these mod- and NTT DOCOMO will continue to els will ultimately become the de facto contribute proactively to this study. NTT DOCOMO Technical Journal Vol. 15 No. 2 [3] 3GPP TS23.228 V11.0.0: “IP Multimedia & Interworking Guidelines,” Aug. 2012. [5] 3GPP S2-113523: “VoLTE roaming architecture requirements,” Jul. 2011. 41