Zyxel IES-1000 EE [105/368] Sip servers and clients

Zyxel IES-1000 EE [105/368] Sip servers and clients
Chapter 17 VoIP
VOP1224-61 User’s Guide
105
17.1.2.2 SIP Service Domain
The SIP service domain of the VoIP service provider (the company that lets you
make phone calls over the Internet) 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.
17.1.3 SIP Servers and Clients
SIP is a client-server protocol. A SIP client is an application program or device that
sends SIP requests. A SIP server responds to the SIP requests.
When you use SIP to make a VoIP call, it originates at a client and terminates at a
server. A SIP client could be a computer or a SIP phone. One device can act as
both a SIP client and a SIP server.
17.1.3.1 SIP User Agent
A SIP user agent can make and receive VoIP telephone calls. A User Agent Client
(UAC) is a logical entity that initiates a SIP request, and a User Agent Server
(UAS) is a logical entity that creates a response to a SIP request. These logical
entities last only as long as the duration of the transaction they have initiated or
responded to. So, a piece of software can act as the UAC in one call and the UAS
in another.
17.1.3.2 SIP Proxy Server
A SIP proxy server receives requests from clients and forwards them to another
server.
In the following example, you want to use a telephone connected to client device
A to call someone who is using a telephone connected to client device C. In this
example, A is the UAC and C is the UAS.
1 The client device (A in the figure) sends a call invitation to the SIP proxy server
(B).

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