13.1.1 General

Communications are a critical component in the efficiency of business organisations. The ability to connect geographically separate sites is a key requirement. For example, consider a banking organisation that wishes to connect its branches with its head office. Its branches can vary in size tremendously and can be located throughout the world. The ability to make voice calls and transfer data amongst all locations in a secure manner is critical to the success of the business. Other requirements include the ability to:

• make and receive calls with members of the public;

• adopt flexible numbering plans;

• use equipment from more than one supplier;

• adopt a flexible network architecture;

• control and customise services.

Requirements of this nature can be met in two ways. One approach is to provide a set of network elements and resources that are available to a customer on a dedicated basis. This is termed a 'Private network'. In this case, the customer can own or rent the network elements. Using private automatic branch exchanges (PABXs) at the various sites, a wide range of features can be employed to improve communication between users at the various locations. In the past, the range of supplementary services available on private networks has been greater than those on public networks. Private networks also permitted additional control and customisation of services.

However, dedicated private networks can be expensive to operate, because many customers have periods when a large proportion of capacity is not being used. Efficient capacity allocation between...