There are three main components of a VoIP network: user agents, gateways,
and proxy servers.
- User Agent
- In a VoIP network, any device that can make or receive telephone calls
is called a User Agent (UA). Each User Agent contains a User
Agent Server (UAS) responsible for handling requests from another endpoint, (for example, inbound calls) and a User Agent Client (UAC) which generates
requests, (for example, outbound calls) for other endpoints. Examples of User Agent Clients and User Agent Servers are:
- A SIP hard phone.
- A SIP soft phone.
- Blueworx Voice Response (which simulates a number of phones) for incoming or outgoing calls.
- Gateways
- A gateway is a device which acts as a bridge between VoIP and the PSTN
network. A gateway can take an incoming call from a T1 interface and convert
the signaling into SIP message exchanges, and convert the voice from TDM into
RTP packets.
- Proxy servers
- In a SIP system, a proxy server (used with a registrar and
a location server), can provide the following services:
- Call Routing including URI translation.
- Registration.
- Access (authentication) to a SIP network.
A Proxy server is the means by which calls are routed within
a SIP VoIP network. For example, a telephony gateway might be configured
to send all incoming calls to the SIP proxy server which will then route the
calls to specific endpoints (this can include load balancing or skills-based
routing).