Technical Reference - ZyXEL Communications EX5501-B0 User Manual

Chapter 22 VoIP

22.9 Technical Reference

This section contains background material relevant to the VoIP screens.
VoIP
VoIP is the sending of voice signals over Internet Protocol. This allows you to make phone calls and send
faxes over the Internet at a fraction of the cost of using the traditional circuit-switched telephone
network. You can also use servers to run telephone service applications like PBX services and voice mail.
Internet Telephony Service Provider (ITSP) companies provide VoIP service.
Circuit-switched telephone networks require 64 kilobits per second (Kbps) in each direction to handle a
telephone call. VoIP can use advanced voice coding techniques with compression to reduce the
required bandwidth.
SIP
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol that handles the
setting up, altering and tearing down of voice and multimedia sessions over the Internet.
SIP signaling is separate from the media for which it handles sessions. The media that is exchanged
during the session can use a different path from that of the signaling. SIP handles telephone calls and
can interface with traditional circuit-switched telephone networks.
SIP Identities
A SIP account uses an identity (sometimes referred to as a SIP address). A complete SIP identity is called
a SIP URI (Uniform Resource Identifier). A SIP account's URI identifies the SIP account in a way similar to
the way an e-mail address identifies an e-mail account. The format of a SIP identity is SIP-Number@SIP-
Service-Domain.
SIP Number
The SIP number is the part of the SIP URI that comes before the "@" symbol. A SIP number can use letters
like in an email address (johndoe@your-ITSP.com for example) or numbers like a telephone number
(1122334455@VoIP-provider.com for example).
SIP Service Domain
The SIP service domain of the VoIP service provider is the domain name in a SIP URI. For example, if the
SIP address is 1122334455@VoIP-provider.com, then "VoIP-provider.com" is the SIP service domain.
SIP Registration
Each Zyxel Device is an individual SIP User Agent (UA). To provide voice service, it has a public IP address
for SIP and RTP protocols to communicate with other servers.
A SIP user agent has to register with the SIP registrar and must provide information about the users it
represents, as well as its current IP address (for the routing of incoming SIP requests). After successful
registration, the SIP server knows that the users (identified by their dedicated SIP URIs) are represented by
the UA, and knows the IP address to which the SIP requests and responses should be sent.
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