ay also be used as a temporary expedi- configuration of star-connected exchanges is usually augmented by ent when unexpected development in one part of an exchange area direct routes interconnecting exchanges where a high community of uses up all the cable pairs leading to it. In this way, service can be interest generates sufficient traffic. There is thus a backbone route provided to new subscribers before an additional cable can be laid. joining each switching centre at the lowest level to the highest-level If remote concentrators are used, an additional level of switching is centre via intermediate centres, together with transverse branches introduced into level (2) of the hierarchy. between centres at the same level. There may also be some direct Network architecture 29/7 routes between centres at different levels, which violate this pattern, ever, the transit network was necessary because of the very large if there is sufficient traffic to justify them. number of different possible connections which could not be made The number of levels in the hierarchy of a national network over direct circuits between GSCs, although each of these generated depends on the relative costs of transmission and switching. A relatively little traffic. country which is small and densely populated will have relatively The introduction of digital switching and transmission has had a short distances between its primary centres and large amounts of profound effect on the economics of network provision. As a result, traffic between them. A country which is large and sparsely popu- the architectures of integrated digital networks (IDNs) now being lated will have longer distances and less traffic between its primary developed differ greatly from the analogue networks described centres. Consequently, the cost per circuit of providing direct routes above. The changes are considered in section 29.5. between primary centres is much less in the former case than the latter. As a result, a small densely-populated country will have more 29.2.1 