There are two aspects needed to identify architectural gaps between two different networks: functional aspect (via vertical architecture) and distribution aspect (via horizontal architecture). Vertical architecture, in general, is useful to define relevant functions from application/service to physical medium and their relationships. Horizontal architecture is useful to know how this network deployed showing distribution of relevant functions and systems according to geographical coverage.
Gaps in Vertical Architecture
Vertical architecture, in general, identified based on OSI (Open System Interconnection) 7 layered model. Following Figure 12 shows vertical architectures of legacy networks and the NGN. As shown in the figure, NGN and legacy networks commonly use most of underline network infrastructures such as physical mediums and transmission systems. One difference has been indicated in transmission layer caused by supporting broadband services of the NGN while legacy networks only support narrowband which is enough to use PDH transmission. Broadband for the NGN has been supported by xDSL initially and other broadband technologies are supported later and being developed now such as WDM, PON and Giga Ethernet.
Figure 12 – Differences in Vertical Architectures
Key area of gaps between legacy networks and the NGN is switching (L3) and transporting (L4) layer technologies. Legacy networks use different switching technologies as well as transport according to the services which supported such as circuit switching for voice, X.25 packet switching for data and IP for information in case of Internet. NGN just uses IP with managed capabilities in the support of QoS and Security and services of real-time and non real-time natures are supported over this managed IP. Therefore these layers (L3 and L4) are the key areas need adaptation between different technologies when design migration of legacy networks to the NGN and interworking between them.
Gaps in Horizontal Architecture
Typical way of deploying legacy networks have been used hierarchy model covering geometric coverage. This model designs networks based on geometric distance with size of customers which are classified as, for example city, village and rural areas. According to these distance and size, legacy networks consisted with several different types of networks which form of hierarchies such as access network including remote access network, local exchange network, regional network and national network. These hierarchies are generally very helpful not only for installation and operation but also system developments. These hierarchies, especially, are quite well fitted with traditional telephone based service provision and network operation, in terms of identification, that is, E.164 number based. Following Figure 13 shows this hierarchical architecture of legacy networks.
Network systems composed of different hierarchy could have different features according the roles given that hierarchy. For example, role of systems in access network mainly collection of user traffics and delivery to/from local exchange. Role of systems in local exchange networks are identifying destination and routing traffics to that destination. Systems in regional networks collection of traffics from each local exchange networks and deliver to other regions which locates far distance. Followings are summary of key features of hierarchy based legacy networks:
Hierarchical topology with 4 to 5 layers, connectivity to the upper next layer and within each layer as a function of economical optimization;
Number of nodes as a function of output data traffic and nodes capacity;
Service handling for media, signalling, control and management at all exchange nodes;
Carrier grade quality with well-defined QoS criteria and standardized engineering rules.
Figure 13 – Hierarchical Architecture of Legacy Networks
This hierarchy based network configuration of legacy networks is a point of gap with the NGN which use of IP as a key technology. This gap caused by the features of IP such as using flat address and dynamic routing taking consideration of providing end to end connectivity.
The NGN network can be logically decomposed into two different sub-networks, as shown in Figure 14. The major components of the NGN sub-networks are as follows [b-ITU-T Y.2012]:
Customer network: a network within a home or an enterprise and connected to the NGN provider's network via a UNI (user-to-network interface);
Access network: collects end-user traffic from the end-user network to the core network. The access network can be further partitioned into different domains, with the intra-domain interface being termed an INNI (Internal Network-Network Interface) and the inter-domain interface being termed a NNI (Network-Network Interface). The access network belongs to the transport stratum;
Core network: The core network belongs to both the transport stratum and the service stratum. The interface between the core network and the access network or between core networks can be an INNI (in the case of partitioning as a single domain) or a NNI.
Figure 14 – Hierarchical Architecture model of NGN
One of very clear differences of services between legacy networks and the NGN should be the level of integration of services. Legacy networks have been limited to integrate various services even though ISDN because of limited capabilities and capacities of underline access networks including transport. Each legacy network has certain set of list of services but limited for example, as followings:
PSTN: support fixed-line based voice services including supplementary services and voice-band data;
PSDN: support narrow-band data services;
Mobile networks (2G): mobile based voice services, SMS and narrow-band data including mobile level (lower quality than fixed) of information services;
ISDN: support fixed-line based voice services including supplementary services and narrow-band data;
Broadband-ISDN: support fixed-line based multimedia services and data including limited connectionless services;
Internet: Best effort based information services.
Because of each legacy network constructed based on different network related technologies (e.g., different transmission and switching), platforms for providing services also build upon differently. These differences between networks, between service platforms and between network and service derived difficulties and hurdles supporting “Openness” which provide fair and open accessibility to networks and service platforms from 3rd party providers as well various service providers and developers.
In contrast of these, NGN constructed based on common transport technology on top of different underline networks and supports various services which cover almost of services available over legacy networks. Thus the NGN widen services covering text to multimedia as well as not only fixed but also mobile features together. In addition, the NGN has more flexibility to support openness of networks and service platforms via OSE (Open Service Environments), thus 3rd party providers and other service providers including developers are easy to access the NGN resources and use capabilities.