1. What is an IP Phone?
    An IP phone is a telephone with an integrated Ethernet connection. Although users speak into a traditional analog handset (or headset) on the IP phone, the IP phone digitizes the spoken voice, packetizes it, and sends it out over a data network (via the IP phone’s Ethernet port).
  2. What is a call agent?
    A call agent is a repository for a VoIP network’s dial plan. For example, when a user dials a number from an IP phone, the call agent analyzes the dialed digits and determines how to route the call toward the destination.
  3. In VoIP what is a gateway?
    A gateway in a VoIP network acts as a translator between two different telephony signaling environments. In the  figure, both gateways interconnect a VoIP network with the PSTN. Also, the gateway on the right interconnects a traditional PBX with a VoIP network.
  4. What is a PBX?
    A Private Branch Exchange (PBX) is a privately owned telephone switch traditionally used in corporate telephony systems. Although a PBX is not typically considered a VoIP device, it can connect into a VoIP network through a gateway, as shown in the figure.
  5. What is an analog phone?
    An analog phone is a traditional telephone, like you might have in your home.  Even though an analog phone is not typically considered a VoIP device, it can connect into a VoIP network via a VoIP or, as shown in the figure, via a PBX, which is connected to a VoIP network.
  6. What is SIP?
    Session Initiation Protocol (SIP) is a VoIP signaling protocol used to set up, maintain, and tear down VoIP phone calls. Notice in the figure that SIP is spoken between the IP phone and the call agent to establish a call. The call agent then uses SIP to signal a local gateway to route the call, and that gateway uses SIP (across an IP WAN) to signal the remote gateway (on the right) about the incoming call.
  7. What is RTP?
    Real-time Transport Protocol (RTP) is a Layer 4 protocol that carries voice (and interactive video). Notice in the figure that the bidirectional RTP stream does not flow through the call agent.
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