SIP 背靠背服务器和代理服务器

背靠背服务器和代理服务器

 

一、代理服务器

用户助理发送消息给代理服务器，代理服务器根据被叫的当前位置，路由一个会话邀请，同时还执行呼叫认证，计费和其它比较重要的功能。代理服务器的重要任务是路由会话邀请到被叫。会话邀请通常会穿过一组代理，直到找到被叫的当前位置为止。代理服务器有两个基本的类型，状态无关和状态相关。

状态无关代理服务器，是简单消息转发器。尽管消息通常被安排在事务中，状态无关代理服务器不关心事务。状态无关代理服务器，尽管简单，但是比状态相关代理服务器响应快。他们可以作为简单的负载平衡器，消息转换和路由。状态无关代理服务器不能处理重传消息，不能执行高级路由。例如，不能进行并行呼叫。

状态相关代理服务器比较复杂。当接收到一个请求时，状态相关代理服务器，要创建一个状态，并且保持状态直到事务完成。一些事务，特别是哪些被INVITET创建的事务，保持较长时间，直到被叫摘机或者挂机。因为状态相关代理服务器必须维持状态，因此性能将受到限制。

状态相关代理服务器支持并行呼叫。能接受重传消息。能够执行更复杂的方法，以找到一个用户。例如，能够尝试接触用户办公电话、手机等多个位置。大多数SIP代理服务器，都支持状态相关。

 

二、背靠背服务器

A back-to-back user agent (B2BUA) is a logical network element in Session Initiation Protocol (SIP) applications.^[1] SIP is a signaling protocol to manage multimedia Voice over Internet Protocol (VoIP) telephone calls. A back-to-back user agent operates between both end points of a phone call or communications session and divides the communication channel into two call legs and mediates all SIP signaling between both ends of the call, from call establishment to termination. As all control messages for each call flow through the B2BUA, a service provider may implement value-added features available during the call.

In the originating call leg the B2BUA acts as a user agent server (UAS) and processes the request as a user agent client (UAC) to the destination end, handling the signaling between end points back-to-back. A B2BUA maintains complete state for the calls it handles. Each side of a B2BUA operates as a standard SIP network element as specified in RFC 3261.

A B2BUA may provide the following functions:

• call management (billing, automatic call disconnection, call transfer, etc.)
• network interworking (perhaps with protocol adaptation)
• hiding of network internals (private addresses, network topology, etc.)

Often, B2BUAs are implemented in media gateways to also bridge the media streams for full control over the session.

A signaling gateway, part of a session border controller, is an example of a B2BUA.

 

三、Can SER Be Used As A Back-To-Back User Agent (B2BUA)?

SER is not a B2BUA. It mostly acts as a proxy server. This means it is primarily designed to scale and handle heavy traffic. It does not keep call state and therefore it is unaware that calls are even in progress.

In other words, SER is completely oblivious to the fact that calls are connected. So when SER connects a caller to a callee, SER forgets immediately about the call. By doing so SER can handle large volumes of subscribers. The bad news is that SER cannot disconnect a call or otherwise participate in the call.

If you need this sort of functionality then you need a B2BUA so act as a "middle man". This is very common for prepaid calling card applications. By using a B2BUA, the caller and callee never communicate directly to each other. Instead they communicate only to the B2BUA, which is aware of call state.

 

四、 Proxy Server VS Back-To-Back Server

BACK-TO-BACK USER AGENT (B2BUA) SIP SERVERS POWERING NEXT GENERATION NETWORKS

A Functional and Architectural Look At Back-To-Back User Agent (B2BUA) SIP Servers

http://www.radvision.com/NR/rdonlyres/733FD7B3-053E-409B-A385-4820BEA0EDFD/0/B2BUASIPServersWhitePaper.pdf