Service control is not part of the network but uses it so that the media component of the service transferred between the terminals and the servers with the expected quality of service and with a minimum of constraints regarding the location of customers and providers.
Control of resources and the transfer network
As described in previous sections control of the network includes a resource control function which has the objective of controlling, when requested by applications, the reservation and the configuration of all the resources needed for these applications to run smoothly, in accordance with the parameters that express the required QoS. These resources may be associated with information transfer: either directly (to attribute an address, establish a channel, reserve a transmission rate)or indirectly (via the intervention of a server, etc). These resources may also be linked, for example, with the dialogue between user and application (via the intervention of a voice server, etc.) or with security functions (implementation of a firewall, etc). It is noticeable that the control of resources comprises a “global” part, located in the network control, and a “decentralized” part, at the level of the network elements themselves.
As regards the control of transfer resources in the NGN, three main aspects should be mentioned.
-The consideration of differentiated QoS, which implies that the network elements know, flow by flow, which level of QoS to observe, in such a way as to be able to implement the appropriate mechanisms
-The possibility of modifications during a single session, which might mean a change of the bit rate allocated to a flow that has already been established, the processing of a complementary flow or the routing of a given flow via an intermediate entity such as a transcoding device.
-The convergence of transfer resources onto a single control interface whatever the applications involved. In conventional networks, the control of transfer resources uses one of two quite distinct channels according to the origin of the request: network operator for a management application or user for a communication service. This has led to the need to manage two different interfaces each with specific protocols: the “equipment-management” interface (considered a weak or average real-time constraint) and the equipment-call processing’ interface (a severe real-time constraint). However, in the NGN convergence between the fields of management and control of services(illustrated, from the end user’s point of view, by the fact that he will himself increasingly have direct access to management facilities) should also lead to the convergence of the control of resources towards a single interface at the equipment level.
From Service Control to resource control
With the aim of ensuring the connection between users and the terminals and servers involved in a telecommunications service, it is necessary to decide which paths the media flows between these terminals will be routed along. This may or may not mean building real new paths by manipulating addresses and indications of directions, and deciding whether or not to set aside network capacity, or establishing priorities between flows sharing the same path, according to the characteristics of the media flows that support the service requested. Establishing this ‘connectivity’ and managing its evolution comes under the title of “network control”, whose prime role is to give directives that will allow flows to follow the right paths under satisfactory conditions. This role is known more precisely by the term “resource control”. It must, of course, take into account the techniques used on the path being established. It is a different task to establish a path through a 64kbps switched digital network than to do the same through an end-to-end IP network, where it is enough to indicate to a SIP terminal, for example, which IP address should be used to reach the requested party, and perhaps to implement the appropriate mechanisms to guarantee the required level of QoS.
One of the features of NGN principles in network control is to distinguish between resource control and “call control” or “session control”. The aim here is to separate.
- The implementation of actual resources, specific to transfer techniques, that are necessary for establishing physical connectivity, with the required QoS, between network inputs and outputs, these being represented by physical addresses. This is the role of resource control.
- The actions, on a more generic level, ideally independent of transfer techniques, that manipulate the various possible representations of these addresses, and that determine the general characteristics of the connection to be made, particularly in terms of bit rate and QoS. This is the role of call control or session control.
The first distinction, proposed within the framework of NGN, has never really been implemented before. In telephone networks, for example, resource control and call control are integrated into the “call-processing” of the switches.,
A second distinction, also included in the NGN, separates everything to do with network control, i.e. the more or less simple connection (in a session) of terminals, from everything to do with the control of the services provided during these sessions. Control of services here can be extended to include even the consideration of complex information necessary for the complete establishment of the session. This may concern the identification and authentication of the users, their presence and availability, network and even geographic localization, their means of payment (especially in the case of prepayment), etc. This distinction may be considered as an extension of the IN (Intelligent Network) concepts but experience shows it to be difficult to define and even more difficult to stabilize, In fact, certain services can be processed only at the level of call control. This is the case, for example, for a simple telephone or videophone communication. For other services, the definition may appear to be quite a simple matter, as in the case of a pre-paid multimedia messaging service. However, it is easy to imagine that this distinction between network control and service control would be less easy to define in complex applications involving several actors, network operators and service providers. The availability of open interfaces, which has not been achieved in the IN context applied to telephony, can be seen to be even more necessary in multi-service networks like the NGN.