SIP/VOIP transformation towards IMS (Total IP)

The telecommunications industry has been going through a significant transformation over the past few years. At the outset incumbent operators used to focus on mainly basic voice services and still remained profitable due to the limited number of players in the space and requirement of huge amounts as initial investment.

However, with the advent of competitive vendors, rise in consumer base, and introduction of cost effective IP based technologies a major revolution has come about. This has enabled operators to come out of their traditional business models to maintain and enhance subscriber base by providing better and cheaper voice, multimedia and data services in order to grab the biggest possible share in this multi- billion dollar industry.

The evolution in Telecom industry has been accelerating all the time. The Next-Generation Operators wants to keep pace with the rapidly changing technology by, adapting to market needs and looking at the system and business process from multiple perspectives concurrently. Communication Service Providers (CSPs) need to consider several factors in mind before proposing any solution. They need to deploy solutions which are highly automated, highly flexible, caters to customer needs coupled with ultra low operating costs.

Upgrading a softswitrch solutions to IMS

The Softswitch is decomposed into two logical components of a subscriber-facing unit and a PSTN-facing unit. 

  • Subscriber facing unit in Softswitch is upgraded to AGCF (Access Gateway Control Function) 
  • PSTN facing unit is upgraded to MGCF (Media Gateway Controller Function) to interwork with IMS as shown.

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 an 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.

New SIP-based services can now be rapidly rolled out by deploying new Application Servers (AS) and its integrations to other SBC for UCC( unified communication and colloboartion ) systems. IMS has 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.

Intelligent Networks( IN)

Telecom networks (2014) are made up of integrated service digital network (ISDN), the public switched telephone network (PSTN) ,the Public Land Mobility Network (PLMN) and many others. Intelligent networks (IN) ensures that call control is handed over to a control platform. The control platform determines how the establishment of this call shall continue. Applying IN to any of these networks has in common that call establishment is intercepted at a designated node in the network

By hosting new services on the new platform and combining new and old services CSP‟s aim to provide service bundles that would generate new revenue streams. This process is largely dependant on IMS ( IP Multimedia Subsystem ) architecture .

Transformation towards IMS (Total IP)
Transformation towards IMS (Total IP)

Optimization in operator landscape evolve as result of synergistic technologies that come together to address the innovation and cost optimization needs of operator for better user experience. In following sections different technological evolutions that are affecting overall operator ecosystems have been discussed with focus towards Service Layer.

Fixed/mobile convergence(FMC) with IMS

“Fixed Mobile Convergence is a transition point in the telecommunications industry that will finally remove the distinctions between fixed and mobile networks, providing a superior experience to customers by creating seamless services using a combination of fixed broadband and local access wireless technologies to meet their needs in homes, offices, other buildings and on the go.”

 Fixed-Mobile Convergence Alliance (FMCA)  2004

System can communicate over the cellular network, or act as a new endpoint on the IP network. Home Subscriber Server (HSS) manages subscriber data uniformly between the cellular and IP worlds. The Handoff Server 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.

Legacy to IP transformation

This section broadly covered the aspects of migration from legacy IN solution to new age JAINSLEE framework based one. Applies to Legacy IN hosting voice based services mostly  such as VPN, Access Screening ,Number Portability, SIP-Trunking,Call Gapping.

Most operator environments have seen a rise in the number of service delivery platforms. Also complexity of telecom networks have increased manifold hence CSPs are facing multiple challenges. Increased efforts and costs are required for maintaining all the SDP platforms. These platforms are generally of different vendors and cater to different technologies thereby greatly increase chances of limiting the scalability and flexibility of the operator landscape. More effort required for sustaining the life cycle of the platform and challenges in integrating non compatible SDPs due to proprietary design have been stumbling blocks in the progress of CSPs across the world.

To overcome these challenges there is trend in the market to move towards SDP consolidation wherein instead of maintaining several SDPs with their proprietary design CSPs prefer maintaining a single or less number of SDPs having standardized interfaces.

SDP consolidation
SDP consolidation (1)
SDP consolidation (2)

As illustrated in the above figure there is a transition that is taking place in the industry towards consolidation of service delivery session control. This would provide a cost effective sustenance of existing applications and the rapid creation and deployment of new services leading to increased revenue recognition by CSPs.

  • Agile Development
  • Innovative services
  • open SOA based architectures
  • IN/NGN Platform and Services
  • Reuse of existing investments in legacy service platforms
  • low cost of new service development
  • faster time to market
  • Monetize investment in Network Infrastructure uplift – SIP trunking, VoLTE etc.

Services that should be covered  in the Scope of Migration from fixed line to IP telephony are:

  • Virtual Private Network (VPN) : An Intelligent Network (IN) service, which offers the functions of a private telephone network. The basic idea behind this service is that business customers are offered the benefits of a (physical) private network, but spared from owning and maintaining it.
  • Access Screening(ASC): An IN service, which gives the operators the possibility to screen (allow/barring) the incoming traffic and decide the call routing, especially when the subscribers choose an alternate route/carrier/access network (also called Equal Access) for long distance calls on a call by call basis or pre-selected.
  • Number Portability(NP) : An IN service allows subscribers to retain their subscriber number while changing their service provider, location, equipment or type of subscribed telephony service. Both geographic numbers and non-geographic numbers are supported by the NP service.


WebRTC based Unified Communication platform

Using WebRTC Solution for Delivering In Context Voice which provides new monetizing benefits to the Enterprise customers of Service Providers. This includes following components:

  • WebRTC Gateway for implementation for inter-connect with SIP Legacy
  • Enhancement of WebRTC Client with new features like Cloud Address Book, Conferencing & Social Networking hooks.
  • Cloud based solutions

Challenges in Migration to IMS  (Total IP )

Since long I have been advocating the benefits of migration to IMS  from a current fixed line / legacy/ proprietary VOIP / SS7 based system . However I decided to write this post on the challenges in migration to IMS system from a telecom provider’s view.  Though I could think of many , I have jot down the major 4 . they are as follows :

Data Migration challenges

  • Establishing a common data model definition
  • Data migration seamlessly
  • Configuration management
  • Extracting data from multiple sources and vendors , that includes legacy systems
  • Extracting data due to its large scale and volume


  • Creating an effective knowledge share and transfer for live operations
  • Training in fallback plans, standards and policies .

Customer impact

  • Minimized customer outage
  • Enhance customer experience by delivering quality services on schedule
  • Ensuring security of customer’s confidential data
  • Transfer of customer services without any impact.

Testing in replicated environment

  • Physical pre-transfer test
  • Reducing cycle time
  • Verification and validation at every change in data environment
  • Detect production issues early in the test -lifecycle

Fallback plans

  • Pilot program and real network simulation for ensuring preparedness
  • Tracking changes in new network


VPN ( Virtual Public Network ) over SIP

People working at differing locations need a fast, secure and reliable way to share information across computer networks. This is where a way to connect private networks over and top of the public network becomes necessary and Virtual Private Network comes into the picture.



SIP ( Session Initiation Protocol ) for VPN

VOIP across an SSL-based VPN is achieved in good quality by encapsulating the UDP VOIP packets ( SIP and RTP ) in TCP/IP. Data used for defining a VPN like its Groups, its Members and the associated profiles are organized hierarchically. It includes information like who is the operator, subscriber of VPN, group ID and member ID.

vpn+ service broker
VPN and Service Borker

Grouping for VPN Users

Groups created to implement policies and restrictions common to a set of users.These include:

  • Apply permissions to call between the Groups and to the outside world
  • Apply pricing between distinct types of of PNP (Mobile, Fixed, Privileged list)
  • Some numbers assigned a preferential tariff plan. These numbers are not part of the VPN ( Virtual On-Net) .
  • privileged list within a VPN across multiple groups

Service Overview

Lets see how would a SIP based VPN services over telecom application server with Service Broker works. Leveraging the Service Broker to offer voice VPN service to existing Subscribers is an arduous task. The Subscriber shall benefit from reduced charging rates for VPN calls (ON-Net), improved employee connectivity (within the VPN scope) and a consistent user experience across fixed and mobile phones.

VPN services shall be integrated with the R-IM-SSF( reverse IMS gateway to IN) component of the service broker. R-IM-SSF shall provide mediation as well as session and state management capabilities that shall make VPN service available over multiple networks including SS7 and IMS networks.

The subscriber base can be interfaces via a SMP that might also be used to add groups and assign right and privilege to member

Features of VPN application

1.Private numbering plan for both mobile and fixed subscribers (Short number dialing).

2.Distribution of subscriber under a hierarchical Data Model :

  • Subscriber VPN( Enterprise Level)
  • Group of Users ( Group level. Can be either of type Mobile or PABX )
  • State (End user of service)

3.Grouping of a short number on the basis of following types:

  • Member of mobile VPN
  • Privileged user
  • PABX user

4. Forced On-Net call handling, which shall allow user to dial the public number of another On-Net user with On-Net call Features.

5. Virtual On-Net Call Handling which allocates On-Net extension to non VPN users( Privileged list)

6. Off-Net call Handling via exhaust code which shall allow vpn users to access non-vpn public numbers

7.  Prohibit the call based on a set of rules like ( all off-net calls barred).

8. Allow calls based on destination numbers. For example allow off-net calls for numbers provisioned in the white list(allowed list)

9. Outgoing call screening on the basis of time( Time based barring)

VPN voice application on SIP

This solution  describes the service logic design for the new VPN voice application providing the feature set and service data for the Global VPN service. The new application will replace the Ericsson VPN application running on the legacy IN platform with the new SIP based  VPN services being hosted on the Rhino TAS.

Virtual Public Network - voice Application over SIP RA on SLEE
Virtual Public Network – voice Application over SIP RA on SLEE

Service Logic Structure

This section gives the complete flow in the Service from the instance the INVITE is generated in the network till a Session Connected or Session Disconnected is being sent back to the network.

1INVITE is sent from rim-ssf to SIP Resource Adaptor
2Message is processed by the SIP Resource Adaptor.

RA is configured with a Rate Limiter to enable Overload protection for the TQAS Node.Rate Limiter checks if the Overflow threshold has been reached.If the threshold is reached, RA will not send the message to the service.

3SLEE will check the Service Selector of the VPN service if the message can be processed by it. VPN Service has an initial event selector which checks the service key in the SIP Request.
4Service is configured with a rate limiter to protect the platform from overload protection, for even distribution of calls from different networks etc.

Rate Limiter checks if the Overflow threshold has been reached.If the threshold is reached, Service will close the dialog with a prearranged end.This will not send any message back to the network. This will allow a SIPmessage timeout at rim-ssf. If the threshold is not reached the call is further processed by the Service Ingress

layer of the service
5Service Ingress:  This layer of the Service will do call pre-processing based on the Service key.  This layer identifies the originating network of the call and applies any processing to be done, like number formatting, on the data received from the network before it is used by the core application.
6Service Core: This is the layer where Service processing is carried out.  Service identifies the routing plan associated with Application configured for Service key and executes the routing plan. Routing plan contains features to be executed which will further enable the execution of Subscribed feature set.
7Service Deliver: This layer is where the features corresponding to delivery of the call, like destination number formatting are performed.
8Service Egress: This layer is concerned with all network related functions that need to be performed to setup the call, like manipulating the network message parameters.
9Connect Session or  Discontinue session message is sent back to the network

Overflow Protection

Overflow protection aims at allowing the platform to take only the load that is capable of. This can be achieved by defining the Rhino Rate Limiters at the Resource Adaptor and Service Layers. Rate Limiters at Resource Adaptor are generic in that it counts all the SIP messages that are passing through the Resource Adaptor and can respond with only 4xx response in case the call is not allowed. Limiters at Service level have got greater flexibility in that they can count the messages, dialogs etc and can respond with any message to the network like 403 Forbidden or 406 Not Acceptable etc.

Rate Limiters

Limiters can optionally be linked to a single parent limiter and/or multiple child limiters. A limiter only allows a piece of work if all of its ancestors (its parent, and its parent’s parent, and so on) also allow the work. VPN service has got Rate Limiters defined at both Resource Adaptor and Service layers.

  1. RA Limiter

RA Limiter will help protecting the Rhino node from being throttled with too many messages. Threshold should be captured considering the peak load of all the SIP messages for all the services running in the platform.

2. Service Limiter

Service Limiter will be used to allow the configured number of calls for a particular service. Once the threshold is reached Calls are rejected as dialog close with pre-arranged end. This will close the dialog and release the SLEE resources but will not send any message back to the network. This will allow the dialog to time out at rim-SSF. This prevents the callers from too many re-tries/re-dials on the platform.

Call Barring

According to the business needs of the Bouygues Telecom TCS proposes call barring application which can offer the possibility of limiting the calls for each group of a Company depending on the day and time.

When a subscriber VPN is subjected to a barring 1994/96 him certain destinations are always allowed for the subscriber:

  • Off-net: rights that are identical to the rights of calls to “no destination outside the VPN”, as defined in Error! reference Source not found.
  • On-net: no right to call

The same application can be deployed along with the VPN logic at the Service Core block of the VPN service to barr the call of the subscriber as per the Time, Location , Preveliges of the user of VPN group.

VPN performance issues

VPN has less influence on latency, jitter and packet loss. However VPN overhead is high with enabling authentication, encryption, HMAC, anti-replay attack, and initialization vector, and use small RTP size for Codec, the

Counter Metrics for VPN Quality Assurance

For developing a VPN application counters are employed, some of which could be as follows

  • Number of calls On-Net and Off-Net
  • Numbers of Calls VPN
  • Number of calls with Forced On-Net

Calls between endpoints

  • MS to MS Normal (mobile)
  • MS to MS Privilege
  • MS toward PABX

Success Fail rate

  • Number of calls successful without rerouting
  • Number of calls with successful rerouting
  • Number of calls with Failure (Failed = No answer, Busy, Not reachable, Congestion)
  • Number of calls on non-response (No Answer)
  • Number of calls on Not Reachable
  • Number of calls Route Select Failure
  • Number of calls on busy
  • Number of calls barred by VPN service.

other parameters

  • Total number of queries
  • Number of States created/modified
  • Number of change in the rights of calls
  • Number of issuance of observation Reports

terminology :

R-IM-SSF = reverse IMS gateway to IN

SMP is the Provisioning interface for VPN service subscriber