ors started leasing a dedicated bundle of telephone lines to a customer for a fixed monthly charge. These large organisations could then use leased lines to link their geographically separated buildings, connecting them through a single internal network. Their internal network used a private switch, known as a Private Branch Exchange or PBX. Such a network structure is illustrated for the Carnegie Mellon University (CMU) campuses in Pittsburgh and Adelaide in Figure 3.22. Voice Communications (412) 268-98xx Adelaide 8110-9911 8110-9900 Pittsburgh Leased lines (412) 268-9000 Leased lines Exchange Figure 3.22 Leased telephone lines to connect the CMU campuses in Adelaide and Pittsburgh Private switches were introduced into the telephony landscape early on. With the spread of telephones, large organisations started to install a local network and switchboards for internal communications. These included hotels, hospitals, government offices and large companies. These private switchboards evolved as automatic switches were introduced. The usage of PBXs became widespread in the second half of the 20th century in conjunction with leased line connections between remote offices of companies. The capabilities of PBXs grew with electronic switches, and functions such as call forwarding, voice mail, and conference calls were developed. Telephone operators leased telephone lines to these large organisations as dedicated links of a fixed quantity, for example 20, 50 or 100 telephone circuits, between any two or more remote offices of the organisation. The telephone company charged for these lines on a monthly basis, hence the name 'leased lines'. Leased lines allowed the operator to lock in a customer for a fixed period and fixed revenue. The leasing organisation also benefited by hav- ing the cost of telephone communications between the offices fixed, and usually at less than long-distance charges. The lease charges depended on the number of lines and the distance between the offices. Initial leased lines were physically dedic