MAS.632 Conversational Computer Systems - MIT OpenCourseWare

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Figure 10.6 A PBX provides an internal telephone network. Telephones
served directly by the PBX can be connected without using the public telephone
network. The PBX can connect an internal telephone to the central
office switch through trunk lines.
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PBX
Figure 10.7. A PBX may have trunks to other PBXs as well as the central
office, providing private telephone networks.