The main purpose of a telecom network is to interconnect telephone users, devices and services. Since all users does not need to use the network all of the time, many network resources can be shared among several users.

A large exchange is a means to share switching re­sources by keeping many such resources in one common pool, thus providing statistical gains. A large exchange also will reduce network costs by enabling the sharing of net­work resources and reducing the number of lines, trans­mission capacity and external signaling needed if split into smaller exchange nodes or several specialized nodes.

The concept of an exchange can be related to the func­tions of a network node where many telecommunication lines are connected. However, the traditional functions as­sociated with an exchange are these days sometimes dis­tributed and located also to other node types in the net­work.

Functional and Structural Perspectives on Networks

A practical approach is to look at networks from the opera­tor’s perspective. From this point ofview a telecommunica­tions network can be divided into four major networks: an access network, a transport network, a signaling and con­trol network, and a management network. In more recently developed networks one often complements this view with service and media networks.

However, for different purposes and reasons, one may also talk about other types of networks that are related to each other in different ways. A reason to do this is that one often needs to look at networks both from functional and from structural perspectives.

In abstract functional perspectives, one views the net­work from the external environment as if it were a black box and focuses mainly on the services that the network provides. Functionally, a network can be defined as a set of points at which a set of functions are provided and at which certain properties of each such function can be measured. A network function at one level may be implemented by using a function or a set of functions at the adjacent lower level (see Fig. 1).

In structural perspectives, one takes a closer, more in­ternal, detailed and structural (white box) view and focuses on matters related to structurally and physically measur­able properties, including how the network functions can be partitioned, distributed in space, and allocated to net­work nodes. Structurally, one can hence define a network as a set of network nodes, each with a set of allocated node functions, placed and interconnected via logical intercon­nection points in this way enabling the node functions to cooperate so as to implement the network functions (or net­work services) to be provided.

Using the functional perspective of networks, one can distinguish some network concepts related to telecommu­nication exchanges. For example: an access network pro­vides access related functions at a set of network access points; a service network provides service (e. g., call) re­lated functions at a set of service points; a mobile network provides location and mobility related functions at a set of location points; a signaling network provides signaling functions at a set of signaling points; a connection net­work provides connection or switch related functions at a set of connection points; a management network provides management functions at a set of management points; a transport network provides transport functions at a set of transport points; and so on a media network provides in­formation functions at a set of media content provisioning points.

Figure 1. Network using functions from a lower-level net­work.

The functions at the network points mentioned above are functionally separated but can for economical or other reasons be colocated in the same physical node or site. The
networks are related to and use each other in many ways. For example: the signaling network is used to enable func­tional inter-work between the other networks; a subscriber can, by sending signals via the access network, obtain ac­cess to functions provided by the service network that es­tablishes these services using (by sending of signals to) the connection network(s); the transport network is used by all the other networks as a bearer for transport of raw data.

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