Switching Division

Migration of Softswitch to IMS (Release 1)

Abstract

Telecom Network Operators are in the process of migration to NGN (Next Generation Network), to provide multimedia and innovative value added service to their customers. Many operators have already deployed softswitchs in their network. With the commencement of IMS (IP Multimedia Subsystem), it appears that softswitches will soon be replaced by IMS platforms. This paper describes how a network based on softswitch architecture can be migrated to IMS architecture.

1. Introduction IMS is a further development of NGN technology. A softswitch is more or less like a traditional digital exchange based on software switching to provide IP based Tele com. services. Softswitch architecture separates service control from service access and uses an IP-based core layer in the switching network. It implements service logic to control external trunking gateways, access gateways and remote access servers. Softswitches run on commercial computers and operating systems, and they provide open applications programming interfaces. IMS is a standardised network architecture that uses SIP protocol. It was originally designed by the wireless standards body 3GPP for evolving mobile networks beyond GSM. ETSI/TISPAN enhanced it for fixed line also. It can support services across any access technology. Now it has been adopted by telecom standardisation bodies, major service providers and equipment manufactures. The reason for migration from softswitch to IMS is that IMS is an open, standardised, operator friendly, multimedia architecture for mobile, wireless and fixed line services. Now the question is whether to make a one or two-step transition to IMS. By moving directly from TDM to an IMS-based architecture, operators can skip the intermediary step of installing a softswitch. However, a two-step migration from TDM to an NGN softswitch environment and then finally to IMS ensures that operators will be able to deliver feature parity, while giving the IMS standard more time to harden before moving it into their networks. Operators that have yet to make a serious investment in the transformation of their Class 5 networks see more advantage in moving directly to IMS.

2. Technology Evolution from softswitch architecture to IMS is depicted in figure 1. IMS further decomposes softswitch functions and adds a few new concepts. Call control, user’s database and services, which are the typical functions of softswitch, are controlled by separate units in IMS. CSCF (Call Session Control Function) handles session establishment, modification and release of IP multimedia sessions using the SIP/SDP protocol suite. Services features are separated from call control and handled by application servers. Subscriber’s database function is separated from service logic function and handled by HSS using open subscriber directory interface.

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The IMS has following advantages over softswitch. • Network efficiency increase. • Rapid service development. • Unified subscriber management. • Single service delivery platform for hybrid networks. • True fixed/mobile service convergence.

Services Users

Call Control

Function

Access and Transport

Softswitch

Services Users

Call/Session Control Function

IMS

Access and Transport

Figure 1: Comparison between softswitch and IMS

Services

3. Logical Network Architecture of IMS

The logical network architecture of IMS is shown in Fig 2. End users are connected to the IMS network in various ways, using standard Internet Protocol (IP). The SIP terminals can be connected directly on IMS network, even when they are roaming in another network or country, the only requirement is that they can use SIP. Legacy terminals are connected through Access Gateways (AGW). The AGW is controlled by Access Gateway Control Function (AGCF) through the Megaco/H.248 protocol. The AGCF interacts with the rest of the IMS core through IMS standard session initiation protocol (SIP). The AGCF provides IMS-based PSTN/ISDN emulation, which is transparent to the end user (same equipment, same look and feel). CSCF handles session establishment, modification and release of call sessions using the SIP/SDP protocol suite. CSCF can be configured as a standalone configurations or any combination of P-CSCF, I-CSCF and S-CSCF. The P-CSCF is the first contact point for the UE in IMS. The P-CSCF receives the REGISTER message from the terminal and forwards it to an appropriate I-CSCF or S-CSCF (based upon information received from DNS). The P-CSCF receives all outbound SIP requests from the UE’s in its domain, regardless of last destination and sends them to the I-CSCF or S-CSCF for further processing. It also receives all

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inbound SIP requests addressed to the UE’s in its domain and sends them to the UE. The P-CSCF performs the following tasks: • Keeps track of registrations and active call sessions. • Determines home service domain IP address. • Enforces min/max registration times.

The I-CSCF performs SIP routing. The I-CSCF is the entry point for all connections destined to a subscriber of that network operator. The I-CSCF assigns a S-CSCF during initial registration and routes the terminating session signalling to the allocated S-CSCF. The I-CSCF supports the ability to select an appropriate S-CSCF for a subscriber, during the registration procedure. The I-CSCF contacts the HSS to gain the address of the S-CSCF and then forwards the SIP message to an appropriate S-CSCF. The S-CSCF holds both registration and session states and performs the call/session control services for the UE. It contains a call/session state as needed by the network operator for support of services. The S-CSCF interacts with the HSS to obtain subscriber data and to exchange authentication information using DIAMETER messages. The S-CSCF decides whether an application server is required to receive information related to an inbound SIP session request to ensure appropriate service handling. The decision at the S-CSCF is based on information received from the HSS.

Figure 2: Logical network architecture of IMS

AS HSS

Legacy Terminal

BGCF

MGW

P-CSCF SLF

S-CSCF I-CSCF

Service/ Application

Plane

I-BGF Core

Transport Other IP Networks

I-BCF

AGCF

Control/ Signalling

Plane

Media/Transport Plane

Charging Functions

SGW MGCF

PSTN

Media Signalling

AGW

SIP Terminal

MRFC

MRFP NASS RACS

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BGCF: The BGCF is the logical entity within the IP network that manages the sessions initiated in the IP network and terminated in a circuit switched network. The BGCF Selects the MGCF in the network in which the interworking with PSTN domain is to occur and forwards the SIP signalling to that MGCF. Interconnect Border Gateway Function (I-BGF): It acts as a gateway between two IP transport networks. Media Gateway Control Function (MGCF): It manages the call control protocol conversion between SIP and ISUP. It is interfaced with signalling gateway and media gateway. Its functions are like those of a MGC (Media Gateway Controller) Signalling gateway (SGW): It provides the signalling interface between IP network and PSTN signalling network. It transforms the lower layer protocols as SCTP (which is an IP based transport protocol like TCP) into MTP (which is a SS7 protocol) to pass ISUP from the MGCF to the CS network Media Gateway: It interfaces the PSTN for the media flow from IPSTN to IP or vice- versa. It provides functions such as media conversion (circuit to packet, packet to circuit) and echo control etc. Media Resource Function Processor (MRFP): The MRFP provides all of the work related to media functions based on the instructions of the MRFC. Some of the functions are:

• Mixing of media streams of various conference participants in a conference call.

• Providing media stream for announcement messages and IVR functionalities. • Capability of transcoding the codecs

Media Resource Function Controller (MRFC): It acts as a SIP user agent to the S-CSCF and controls MRFP. Home Subscriber Server (HSS): The HSS is the database containing the subscriber related information to support call/session handling. A Home Network may contain one or more number of HSS. In case of multiple HSS, the Subscribers Location Function (SLF) selects the proper HSS. Application server (AS): AS hosts and executes services. It interfaces with S-CSCF using SIP and with HSS using DIAMETER protocol. Charging Functions: As the name suggests, this provides data collection and billing mediation functions for online and offline charging.

4. Migration steps Softswitch to IMS transitional program enables a service provider to utilize a large portion of its investment in NGN. Initially the IMS base architecture may be deployed in parallel with the softswitch architecture to introduce multimedia-based applications into the operator’s service mix. Phase 1: As the first step, the softswitch is decomposed into two logical components – a subscriber facing unit and a PSTN facing unit. The subscriber facing unit in

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softswitch is upgraded to AGCF (Access Gateway Control Function) and PSTN facing unit is upgraded to MGCF (Media Gateway Controller Function) to interwork with IMS as shown in Figure 3. By separating the softswitch into these components, the network can be more easily scaled for better overall network efficiencies. More AGCFs can be added as required, allowing the network to scale with increase in subscribers. Similarly, More PSTN trunks can be added as traffic increases. Once PSTN and subscriber control functions are separated, the IMS elements, CSCF and BGCF functions can be introduced. BGCF is the interface for interconnecting IMS with legacy PSTN networks.

IMS

TMG +SG

MGCF

AGCF

AGW

PSTN

Softswitch

P-CSCF

BGCF

S-CSCF

I-CSCF

UE

Figure 3: Phase 1 of migration Phase 2: Add SIP-Based Services: To retain existing customers and attract new customers, new SIP-based services can now be rapidly introduced and delivered by deploying new Application Servers (AS). IMS introduces the 3GPP specified ISC interface, which is a SIP-based interface for interfacing to application servers. Using these constructs, multiple application servers from multiple vendors can be interconnected over the IMS ISC interface. Application servers can be for faster rollout of services. Phase 3: Next phase of the migration to IMS is to focus on the business needs related to the expansion of the commercial subscriber base. These customers require a high-quality, business-grade experience and an expanded feature set with capabilities such as conference calling and integrated voicemail and messaging. To address this business goal, build on the IMS environment, SIP endpoints may be

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added. These SIP endpoints are interfaced with P-CSCF. The P-CSCF component should be connected to the policy servers to provide business grade QoS and security functions.

TMG +SG

MGCF

AS

PSTN

Softswitch

IMS

ISC

Figure 4: Inserting Application Server

BGCF

P-CSCF

I-CSCF

S-CSCF

AGCF

UE

GW

Phase 4: Fixed/Mobile Convergence. Moving toward fixed/mobile convergence (FMC), a service provider can address several business needs relating to the introduction of “triple play on the move.” New applications will require high-speed networks to deliver all the three services (data, voice and video) on three devices TV, PCs, and handsets. Accomplishing this phase involves the support for dual-mode handsets, and the introduction of two servers (see Figure 5). The dual-mode devices can communicate over the cellular network, or act as a new endpoint on the IP network. The Home Subscriber Server (HSS), the last missing piece of the IMS architecture, is introduced. It is needed to manage subscriber data uniformly between the cellular and IP worlds. The Handoff Server is also introduced in this phase. It runs on top of the ISC interface, and provides a seamless experience when subscribers move from the cellular network to a Wi-Fi network. The AGCF remains the functional centre of the network, but with the introduction of the HSS, has added the Cx and Sh interfaces defined by the IMS, taking it a step further

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to becoming a complete SCSCF. By continuing to take advantage of the AGCF in each phase, Service Operators accomplish a truly evolutionary move to IMS.

Figure 5: Inserting HSS and Handoff Server for Fixed/Mobile Convergence

TMG +SG

MGCF

AS

PSTN

Softswitch

IMS

ISC

BGCF

P-CSCF

I-CSCF

S-CSCF

AGCF

UE

GW

Handoff Server

HSS

5. Conclusion: IMS architecture promises launching of new services at a short notice by introducing a common subscriber data base and standardised interfaces for application servers. IMS also allows telecom service provides to take full advantage of their existing IP core network.

The IMS component provides many opportunities for cost savings including bypassing the PSTN, more flexibly scaling the network, and more quickly integrating new application servers.

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Abbreviations

AGCF Access Gateway Control Function AGW Access Gateway

AS Application Server BGCF Breakout Gateway Control Function CAPEX Capital Expenditure CSCF Call Session Control Function DNS Domain Name Server DSL Digital Subscriber Line HSS Home Subscriber Server IMS IP Multimedia Subsystem IBCF Interconnect Border Control Function I-BGF Interconnect Border Gateway Function I-CSCF Interrogating CSCF ISC IMS Service Control IP Internet Protocol ISDN Integrated Services Digital Network ISUP ISDN User Part ITU-T Telecommunication Standardisation Sector of

International Telecommunication Union MGCF Media Gateway Control Function

MGW Multimedia Gateway MRFC Media Resource Function Controller MRFP Media Resource Function Processor NAT Network Address Translation NGN Next Generation Network OPEX Operational Expenditure

OSA Open Service Access P-CSCF Proxy CSCF PLMN Public Land Mobile Network

POTS Plain Old Telephone Service PSTN Public Switched Telephone Network QoS Quality of Service SBC Session Border Controller SCP Service Control point

S-CSCF Serving CSCF SG Signalling Gateway SIP Session Initiation Protocol SLF Subscriber Location Function TISPAN Telecommunication and Internet converged

Services and Protocols for Advanced Networking TDM Time Division Multiplex TMG Trunk Media Gateway UE User Equipment

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