This post particularly describes the gateways in IMS which communication back and forth with a legacy endpoints.To get a overview of IMS itself click here and to get a detailed description of IMS and its architecture click here .
What is IM-SSF ?
IP Multimedia Service Switching Function is a gateway to provide IN service such s legacy VPN ( Virtual Private Network ).
IMSSF
What is R-IM-SSF ?
Reverse IP Multimedia Service Switching Function Works on reverse principle to connect IN network to IMS services using IMS services such as FMFM ( find me follow me ) .
More link on telecom transformation , migration and inter-opereability :
Telecom Service Broking supplements value-added telecom services ( VAS) by blending several existing services. For example Call Screening with Find me follow me service, or Call Forwarding with Voice Mail etc, or a logical combination of all of them.
Service Broker combining 4 services inot a single calloflow
Service Broker is part of a unified platform for allowing carriers, mobile operators, and cable operators to rapidly create, manage, and deliver converged video, voice, and data service bundles across multiple networks and devices. Service Broker works as mediator layer between Network Environment and Application environment layer. It manage the new services, existing services & combine them with each other in loose coupling. I have defined Service Broker , Service Harmonization and Servic orchestration here – Service Broker 1
It contains detailed and full fleshed architecture diagrams of Service Broker and how service provider acts as a central Node for Services invocation and services composition. It clearly shows Service Broker provides Services Orchestration / Interaction , service development, third party integration and acts like a protocol gateway .
Today’s voice services are predominantly delivered via Service Control Points (SCP) or Intelligent Network (IN) Application Servers that rely on IN protocols (such as INAP and CAMEL) for complex call control. Those services tend to be highly stable and profitable and also highly customized, and are therefore not easily moved to SIP-based application servers.
Recreating the functionality of deployed IN-based applications requires a exhaustive survey, documentation, and duplication of used features and capabilities, a task that may not be easily achieved. CSPs will need to deliver these same services (down to feature sets and even quirky behavior) on LTE subscribers to ensure migrated users have the same level of service and experience.
As CSPs evolve their networks for LTE, the resulting networks present tremendous challenges in voice services and application delivery. Realizing this opportunity, the telecom software industry has come forward with a purpose-built network element: the Service Broker, a solution specifically designed to overcome network architecture challenges and ensure voice service delivery from any network domain to any other network domain.
Service Brokers are placed between the application layer and the control layer, with the purpose of delivering and extending the reach of applications to all network domains of the CSP. They do this by performing the signaling, media and call control interworking between the applications and different network domains. Implicit in the successful deployment of service brokers is the capability of delivering the required interworking without necessitating changes to either the applications or the networks.
Service Brokers provide many functionality. Among them the most often delivered features are:
• IMS Service Broker • IN-IN Trigger Management • Real-Time Charging • Protocol/Call Flow Management/Call Screening Management • Subscriber Data Management Interaction • Media Resource Brokering • Service Orchestration
The Service Broker’s ability to perform orchestration and combination of discrete voice applications and services into new combined offerings (voice mash-ups) is particularly exciting. With this capability CSPs are able of creating new revenue-producing offers to subscribers where they previously were not available: CRBT ( Ring Back Tome ) and Pre Paid, Find Me / Follow Me combined with Voice VPNs, etc.
Service Brokers also provide the capability of generating Real-Time Charging events, either programmatically (via an API) or automatically as part of service delivery. The challenge facing CPSs is delivering new, innovative services that seamlessly integrate into existing billing platforms. Doing so often means normalizing charging events or even transforming charging events from one technology to another, as is the case in IN to IMS migration.
In conclusion, the Service Broker is responsible for orchestrating and delivering combinational services and generating a charging event upon successful start/completion of those enhanced services.
Convergence : Telephone networks and computer networks converging into single digital network using Internet standards. Components in a Network Client computer Server computer Network interfaces (NICs) Connection medium Network operating system Hub or switch Routers- Device used to route packets of data through different networks, ensuring that data sent gets to the correct address Figure :simple computer…
Packet Switched and/or Circuit swicthed Communication The earlier models were distributed between legacy circuit switched networks and evolving packet switched networks With the massive improvents in quality of network srevices packet switched comunication protocls became more resilent and replaced the circuit swicthed protcols for realtime communication. LTE ( Long Term Evolution ) LTE evolved its…
– signalling at the protocol level (such as SIP, MGCP and SS7)
•For telephony, data and wireless communications networks, the Java APIs defined through.
– service portability
– network independence
– open development
•A Service Logic Execution Environment (SLEE) is high-throughput, low-latency, event-processing application environment.
•JAIN SLEE is designed specifically to allow implementations of a standard to meet the stringent requirements of communications applications (such as network-signaling applications).
Goals of JAIN SLEE are:
– Portable services and network independence.
– Hosting on an extensible platform.
– services and SLEE platform available from many vendors.
Key Features are :
•Industry standard :- JSLEE is the industry-agreed standard for an application server that meets the specific needs of telecommunications networks.
•Network independence:-The JSLEE programming model enables network independence for the application developer. The model is independent of any particular network protocol, API or network topology.
•Converged services:- JSLEE provides the means to create genuinely converged services, which can run across multiple network technologies.
•Network migrations :-As JSLEE provides a generic, horizontal platform across many protocols, independent of the network technology, it provides the ideal enabler technology for smooth transition between networks.
•Global market—global services:-JSLEE-compliant applications, hosted on a JSLEE application server are network agnostic. A single platform can be used across disparate networks
•Robust and reliable:- As with the enterprise application server space, deploying applications on a standard application server that has been tested and deployed in many other networks reduces logic errors, and produces more reliable applications
Interoperable IMS core for heterogeneous access networks
IMS is a an architectural framework for IP based multimedia rich communications. It was standardized by a group called 3GPP formed in 1999. It started as an enabler for 3rd generation mobile networks in European market and later spread to wirelne networks too. IMS became the key to Fixed Mobile Convergence (FMC).
Based on IETF Protocols (such as SIP, RTP, RTSP, COPS, DIAMETER, etc), IMS is now crucial for controlling conmmunication in a IP based Next Genration Network (NGN).
Communication service providers and telecom operators are migrating from circuit-switched networks to IMS technology with the increasing bandwidth (5G) and user expectations.
Early days TDM networks were not robust enough to support emerging technologies and data networking. There was a need to migrate from voic eonly network to Triple play network ( voice , video and data ). Other factors included :
rapid service development
service availiability in both home and roaming network
wireline and wireless convergence
Due to these above mentioned reasons TDM was outdated and IMS gained support .
It offers counteless applications around rich multimedia services on wireless , packet swtched and even tradional circuit switched networks.
Easier to Create and Deploy New Applications and Services
(+)Enhanced applications are easier to develop due to open APIs and common network services.
(+) Third-party developers can offer their own applications and use common network services, sharing profits with minimal risk New services involving concurrent sessions of multimedia (voice, video, and data) during the same call are now possible.
(+) Reduced time-to-market for new services is possible because service providers are not tied to the timescales and functions of their primary NEPs
Capture New Subscribers,Retain Current Subscribers
(+) Better voice quality for business applications, such as conferencing, is possible
(+) Wireless applications (like SMS, and so on) can be offered to wire line or broadband subscribers.
(+) Service providers can more easily offer bundled services.
Lower Operating and Capital Costs
(+) Cost-effective implementation of services across multiple transports, such as Push-To-Talk (PTT), presence and Location-Based Services (LBS), Fixed-Mobile Convergence (FMC), mobile video services, and so on.
(+) Common provisioning, management and billing systems are supported for all networks.
(+) Significantly lower transport costs result when moving from time-switched to packet-switched channels.
(+) Service providers can take advantage of competitive offerings from multiple NEPs for most network elements.
(+) Reduced expenses for delivering licensed content to subscribers of different types of devices, encodings, or networks.
The strongest argument for adoption of IMS is that it follows established standards and open interfaces from 3GPP and ETSI. This makes it suited for interoperability, policy control accross networks, streamlined OSS/BSS, Value Added Services etc.
Abstraction from Underlying Network : IMS is essentially leading towards an open and standardized network and interface,irrespective of underlay network.
Fixed /Mobile Convergence : Inter operability with Circuit Switched (CS) Mobile application Part (MAP)
Roaming : Location awareness between home and visiting network.
Application layer Call Control : IMS application layer has the provision for defining proxy or B2BUA based call flow completion . This leads to operator being able to introduce business logic into call sessions.
IMS is supplemented by SIP (IETF), Diameter (IETF) and H248(ITU-T).The release cycle of IMS is as follows
2002-03-14 Rel-5 : IMS was introduced with SIP. Qos voice over MGW.
2004-12-16 Rel-6 : Services like emergency , voice call continuity , IPCAN ( IP connectivity Access Network )
2005-09-28 Rel-7 : Single Radio Voice Call Continuity , multimedia telephony,eCall ,ICS
2008-12-11 Rel-8 : IMS centralized services , supplementary services and internetworking between IMS and Circuit Switched Networks,charging , QoS
2009-12-10 Rel-9 : IMS emergency numbers on GPRS , EPS(Enhanced packet system) , Custom alert tone , MM broadcast/Multicast
2011-3-23 Rel-10 : home NodeB, M2M, Roaming and Inter UE transfer
Majorly IMS is divided into 3 horizontal layers given below :
Transport / Media Endpoint Layer
Unifies transports and media from analog, digital, or broadband formats to Real-time Transport Protocol (RTP) and SIP protocols. This is accomplished by media gateways and signaling gateways.
It also includes media servers with media processing elements to allow for announcements, in-band signaling, and conferencing. These media servers are shared across all applications (voicemail, interactive response systems, push-to-talk, and so on), maximizing statistical use of the equipment and creating a common base of media services without “hard-coding” these services into the applications.
Session & Control Layer
This layer arranges logical connections between various other network elements. It provides registration of end-points, routing of SIP messages, and overall coordination of media and signaling resources.
IMS core which is part of this layer primarily contains 2 important elements Call Session Control Function (CSCF) and Home Subscriber Server (HSS) database. These are explained below
HSS ( Home Subscriber Server)
It is a database of user profiles and location information . It is responsible for name/address resolution and also authorization/authentication .
CSCF ( Call Session Control Function)
Handles most routing, session and security related operation for SIP messages . It is further divided into 3 parts :
Proxy CSCF: P_CSCF is the first point of contact from any SIP UA. It proxies UE requests to subsystem.
Serving CSCF: S-CSCF is a powerful part of IMS Core as it decides how UE request will be forwarded to the application servers.
Interrogating CSCF: I-CSCF initiates the assignment of a user to an S-CSCF (by querying the HSS) during registration.
Application Services Layer
The Application Services Layer contains multiple Application Servers (AS), such as:
Telephony Application Server (TAS) – for defining custom call flow logic
IP Multimedia Services Switching Function (IM-SSF)
IMS has been mandated as the control architecture for Voice over LTE (VoLTE) networks. Also IMS is being widely adopted to mange traffic for Voice over WiFi (VoWiFi) systems.
Telecom R & D 