Guest Author:  Pieter Veenstra – Architect Voice Solutions – KPN.

profile picture Pieter Veenstra KPN

Pieter Veenstra is lead architect at KPN with over 25 years of experience in the architectural design of voice solutions.  Past roles at KPN included automated test bed design, protocol specification of SS7, and 15 years representing KPN in ETSI and ITU-T standardization with 6 years as chairman of an ETSI working group. These days he is the lead architect in IMS projects and giving presentations at international conferences. Pieter is also guest lecturer at the Technical University of Delft on Service Architecture in Next Generation Networks and holds an MSc degrees in Electrical Engineering from the Eindhoven University of Technology, The Netherlands.

KPN Experience with Centralized Routing

The concept of a Centralized Routing Function (CRF) is regarded by KPN as a vital asset for operators with both an existing SS7 controlled PSTN and an ENUM and/or SIP controlled VoIP/IMS network. In such a context the CRF offers the practical means for a gradual migration of subscribers and services from the traditional SS7 working methods to the SIP based practices in NextGen IP networks.

IN history in PSTN

KPN adopted a pre-CRF approach in the PSTN network in Netherlands with the strategic decision in 1996 to deploy an IN solution for number portability.  At that time, it was a rather revolutionary decision to modify the network the make real-time routing decisions in the PSTN dependent on data in a central network database.  Over the years this routing concept in the KPN PSTN was further developed in subsequent steps to support multiple centralized routing services:

  • Number Portability – offers customers number retention when switching between operator
  • Service Portability – simplifies migration of customers between POTS and ISDN services
  • Technical Portability – enables migration of subscribers to new switching systems for replacements and extensions of switches
  • Carrier (Pre)Selection – supports subscriber specific service handling and network routing
  • Full centralized routing – all real-time call routing controlled by a central IN database.

CRF introduction with SIP and IMS

With the implementation of VoIP networks, similar call routing decisions and porting processes needed to be supported. With the notion that operators like KPN need to offer an integrated PSTN and IMS call routing practice, the CRF concept was recognized within KPN as the means to build an integrated solution. The following key design principles were identified:

  • Federation
    Enabling 100% re-use of the porting information in the IN database also for the CRF routing decisions in ENUM and/or SIP controlled session routing in the VoIP domain. From an operator view this federation capability allows maximum re-use of the existing number porting processes in place within IN that have proven their operational value over the years.
  • Formatting
    Distinct and incompatible number representations exist in SS7 (e.g. the Nature of Address indication) and in ENUM and/or SIP (e.g. the global number format). In this respect the CRF may assist to offer the specific number routing formats to both the PSTN domain and the IMS/VoIP domain (and thus interworking between the specific format conventions in SS7 and those in ENUM and SIP).
  • Separation
    As the business logic in the VoIP domain will be different from the traditional business logic in the PSTN, it was recognized to build a complete new CRF logic for the VoIP domain with more flexible and specific routing capabilities (e.g. for transcoding selection). Hence the new CRF is developed in full separation and without any impact to the existing IN.

With these principles in place the CRF can be viewed as a very vital element in an operator network to build a seamless and central real-time routing capabilities that are used by both the PSTN and IMS/VoIP domains. It may also be noted that operational processes will be simplified and improved with only one single source where the routing information is stored and updated. Please see the following picture as an example with the CRF represented as Routing Engine in the context of an IP Interconnect service offering either a SIP or ENUM service view.


Future CRF evolutions steps.

In future the role of the CRF may be further extended according to the following lines:

  • Federation with the mobile STP in the Mobile CS network that performs a similar routing function as the IN database in the PSTN.
  • Making the connection with the porting processes by which the CRF may take over the role of the IN and mobile STP for routing.
  • To offer a federated routing capability to the Diameter Routing Agents (DRA) when customer related routing decisions will be needed in the advancing Diameter signaling domain.
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