Voice and Video Services

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Private Branch Exchange

• legacy voice services use the public switched telephone network (PSTN)
  • residential telephone installation would be serviced by:
    • simple box providing a one- or two-line analog interface to the local exchange
      • analog interface is called plain old telephone service (POTS)
    • each line provides a single channel for an incoming or outgoing call
    • a typical business requires tens or hundreds of lines for voice comms, let alone capacity for data comms
      • historically, this requirement would be facilitated by a digital trunk line
        • referred to as a time division multiplexing (TDM) circuit
        • can multiplex separate voice and data channels for transmission over a single cable

A private branch exchange (PBX) is an automated switchboard providing a single connection point for an organization’s voice lines.

• a TDM-based PBX connects to the telecommunications carrier over a digital trunk line, which supports multiple channels (inward and outward calls)
  • supplied as vendor-specific hardware
• allows for the configuration of the internal phone system to direct and route calls to local extensions
• provides other telephony features:
  • call waiting, music on hold, and voicemail

VoIP-Enabled PBX

• TDM-based PBXes are being replaced by hybrid and fully IP/VoIP PBXes
• for internal calls and conferences, a VoIP PBX establishes connections between local VoIP endpoints with data transmitted over the local Ethernet network
• can route incoming and outgoing calls from and to external networks
  • might involve:
    • calls between internal and external VoIP endpoints
    • or with voice telephone network callers and receivers
• supports music on hold and voicemail
• can be implemented as software running on a Windows or Linux server
  • e.g., 3CX, Asterick
• are hardware solutions
  • VoIP PBX runs on a router
  • e.g., Cisco Unified Communications Manager
• placed at the network edge and protected by a firewall
  • internal clients connect to PBX over Ethernet data cabling and switching infrastructure
    • use IP at Network layer for addressing
  • uses the organizations Internet link to connect to a VoIP service provider
    • facilitates inward and outward dialing to voice-based telephone networks