Pstn Hierarchy 6i2914

Telephone web: Requires large number of connecting lines: n(n-1)/2 n2/2, for large value of ‘n’. Central office concept: Requires only ‘n’ lines, no matter how large ‘n’ is: Bell System of hiecharchy:  There are five classes of switching offices (numbered in order): 1. Regional center 2. Sectional center 3. Primary center 4.Toll center 5. End office  Class 5 is of lowest hiecharchy and class 1 is of highest. Regional center: These serve three purposes: (a) their connections were the "last resort" for final setup of calls when routes between centers lower in the hierarchy were not available (b) ) they were initially staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion - although these functions were transferred after 1962 to the Network Control/Operations Center and the distributed Network Management Centers (c) they provided collection points (until the development of more advanced computer hardware and software for toll operators) for circuits that would be ed along to one of the international overseas gateways (which operated as special centers outside the formal North American hierarchy Sectional Center: The class 2 office was the Sectional Center (SC). The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. Primary center: Calls being made beyond the limits of a small geographical area where circuits are not connected directly between class 4 toll offices would be ed from the

toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never served dial tone to the . Toll Center: The class 4 office is the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP). A call going between two end offices not directly connected, or whose direct trunks are busy, is routed through the toll center. The toll center is also used to connect to the long-distance network for calls where added costs are incurred, such as operator handled services. This toll center may also be called the tandem office because calls have to through this location to get to another part of the network. End Office (local exchange) It delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer