Next generation networks



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5.5Two types of NGN


There are two different types of the NGN: IMS based NGN and Call server based NGN. Both of two has been developed according to the ITU-T recommendations, especially based on those NGN principles [b-ITU-T Y.2018], requirements [b-ITU-T Y.2201] and functional architectures [b-ITU-T Y.2012], however approaches are different. Key differences between two types are identified the way of supporting PSTN/ISDN services such as voice services including supplementary services. Taken into account support of services which available over PSTN/ISDN, “IMS based NGN” more focused on mobile networks aspects while “Call server based NGN” focused on fixed networks aspects.

IMS (IP Multimedia Subsystem) [b-ITU-T Y.2021]

IMS, that is, IP Multimedia Subsystem, has been specified by the 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2). The IMS has been adopted into the NGN to support session-based services, and other services based on session initiation protocol (SIP). Thus IMS based NGN means how the IMS can be used in the NGN context following the fundamental principles described in [ITU-T Y.2001] and [ITU-T Y.2011]. Core functions of IMS based NGN has been realized in providing the IP multimedia service component into the NGN functional architecture.

IMS is a collection of core network functional entities utilizes SIP-based control for the support of SIP-based services. IMS supports the registration of the user and the terminal device at a particular location in the network including authentication and other security arrangements. The services supported by IMS may include multimedia session services and some non-session services such as presence services or message exchange services. IMS supports operator-provided services including operation and interworking with a variety of external networks via defined a number of network reference points. IMS also supports defined reference points for the collection of accounting data in support of charging and billing operations.

The NGN IMS component supports the provision of SIP-based multimedia services to the NGN terminals and the provision of PSTN/ISDN simulation services. NGN IMS supports the following:



  • control of IP connectivity access networks (QoS, admission control, authentication, etc.);

  • coordination of multiple control components to a single core transport for resource control;

  • interworking and interoperability with legacy and other networks;

  • mutual de-coupling of the applications from the session/call control and the transport;

  • access technology independence of session/call control and applications.

Functional entities of an IMS may be used by an operator in support of transit network scenarios. The routing may be performed, depending on the entity performing the routing, and depending on the traffic case, signaling information, configuration data, and/or database lookup.
Call server-based NGN [b-ITU-T Y.2271]

Call server-based NGN mainly focused on how to support services available over the PSTN/ISDN through the NGN. Exact name of this is “Call server-based PSTN/ISDN emulation” identified as one of the service components of the NGN and provides PSTN/ ISDN basic and supplementary services. Call server-based PSTN/ISDN emulation service component co-exists and interworks with other components such as the IP Multimedia component (IMS) and the streaming component (e.g., IPTV) including interworks with the existing networks.

"Call server-based" indicates that service control logic and service execution environment is primarily located in a Call/Session Control Server (CSCS; briefly called “Call server”) which has responsibility of network entity for service delivery. This function relates to the service switching function (SSF) in PSTN/ISDN. The Call server-based service control concept is completely opposite with the IMS-based approach, because the application server houses the service control logic and service execution environment behind Call server entities.

Call server-based PSTN/ISDN emulation satisfies the following basic requirements:



  • In terms of service provisioning, it inherits PSTN/ISDN basic and supplementary services, and provides IN (Intelligent Network) services;

  • In terms of user-network connection, it supports existing PSTN/ISDN UNI interfaces.

The Call server-based PSTN/ISDN emulation component should support:

  • PSTN/ISDN teleservices and supplementary services as per I.240 and I.250 series of ITU-T Recommendations;

  • Capabilities provided by the application server (AS);

  • Capabilities provided by traditional IN;

  • Public interest services.

Consequently, using Call server-based NGN, PSTN/ISDN end users may use existing services and existing terminals without the knowledge of network being replaced by the NGN. This feature of Call server-based NGN is indicated one of the best ways for migration of PSTN/ISDN networks to the NGN.

6Gaps between Legacy Networks and NGN


This clause analyzes gaps between legacy networks and NGN in terms of technologies used, capabilities, architectures including services and systems aspects. Gaps analyzed in this section would be helpful to identify migration scenarios as well as strategies.

6.1Technology Gaps


Legacy networks use various different technologies according to the services as shown in Figure 10.



Figure 10 – Various technologies used in Legacy Networks

PSTN uses 64 kbit/s based channelized circuit for layer 3 as a key technology which caused to support 3.1 kHz audio service (that is voice). Because of a circuit consisted with a series of 64kbit streams, it requires fixed time based control such as switching and transmission. In addition, PSTN requires echo cancellation using A/ law according to the standards to support voice service caused by far end-to-end delivery of voice signals in telecommunication environment. Finally switching technology used in PSTN has been developed “digit based switching” and formed of number based addressing. Because of PSTN used as a global telecommunication infrastructure to connect the people with voice and 3.1 kHz Audio data services (e.g., FAX and Modem based services), this number based addressing mechanism of PSTN has been developed in global way so managed by ITU, called ITU-T E.164 numbering.

PSDN is a traditional packet network using ITU-T X.25 packet. ITU-T X.25 packet technology has been developed mainly for data services, thus very significant amount of technology addressed on securing delivery of data which contained in each bit of the packet. Because of the status of physical mediums (e.g., copper and air waves) as well as transmission technology were not enough to provide secure delivery of all bits without problem (e.g., lost bits, delay delivery, error of bit value and others), many of supplementary technologies such as HEC (Header Error Control) error control and handshaking should be incorporated into the X.25 packet and relevant processing of the packet.

Internet is another packet network and is little difficult to called as a part of legacy networks because it uses IP. But taken into account service features provided, this Technical Paper classified Internet as a legacy network. Because of Internet basically provides communication services with “Best effort” which cannot guarantee quality of services at all, thus services are only available according to the given status of any parts of the networks, sometimes are not available even the connection is still maintained.

One of the key technical characteristics of the NGN is identified of using a single transport technology that is IP, which placed between services and underline various transmission technologies. This single transport technology takes a role to decouple services from underline technologies such as switching (or routing) and transmission. In other word, IP provides a single bridge for various services to use different transmission technologies according to meet service requirements. Because of this, the NGN covers non-real time as well as real time multimedia services including voice services, even though QoS in the NGN has been strongly correlated with the underline capabilities such as level of broadband connectivity.

By considering such differences of technologies used in legacy networks and the NGN, followings are identified as issues to resolve when interworking and/or migration be considered:



  • Between PSTN (including ISDN circuit mode) and the NGN: support of echo cancellation, adaptation of numbering schemes, provision of accounting and charging capabilities and packet assembly and dis-assembly;

  • Between PSDN (including ISDN packet mode) and the NGN: support of adaptation of different formats of packetization, addressing schemes and accounting capability;

  • Between Internet and the NGN: support of QoS and security including other aspects of managed capabilities such as accounting and managements of services and users.



Figure 11 – Technical gaps of Legacy Networks and NGN

Figure 11 summarized these technical gaps between legacy networks and the NGN focusing on underline technologies, while services are integrated smoothly into the NGN. However it is required further considerations from service provisioning aspects such as SLAs (service level agreements), accounting and charging and others.





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