ZyXEL LTE5121 User Guide - Page 180
Authorization Requirements, SIP Servers, SIP User Agent, SIP Proxy Server
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Chapter 17 VoIP 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. Registration is initiated by the User Agent Client (UAC) running in the VoIP gateway (the LTE Device). The gateway must be configured with information letting it know where to send the REGISTER message, as well as the relevant user and authorization data. A SIP registration has a limited lifespan. The User Agent Client must renew its registration within this lifespan. If it does not do so, the registration data will be deleted from the SIP registrar's database and the connection broken. The LTE Device attempts to register all enabled subscriber ports when it is switched on. When you enable a subscriber port that was previously disabled, the LTE Device attempts to register the port immediately. Authorization Requirements SIP registrations (and subsequent SIP requests) require a username and password for authorization. These credentials are validated via a challenge / response system using the HTTP digest mechanism (as detailed in RFC 3261, "SIP: Session Initiation Protocol"). SIP Servers 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. SIP User Agent A SIP user agent can make and receive VoIP telephone calls. This means that SIP can be used for peer-to-peer communications even though it is a client-server protocol. In the following figure, either A or B can act as a SIP user agent client to initiate a call. A and B can also both act as a SIP user agent to receive the call. Figure 118 SIP User Agent A B 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 client device A to call someone who is using client device C. 180 LTE-5121 User's Guide