在我们学习SIP协议的过程中，对于一个概念要清楚，那就是SIP中继的概念。为了让大家更好地理解这方面内容，我们采用中英文互译的对照形式，帮助大家了解这方面的知识，具体资料如下：
A SIP Trunk is a logical connection between an IP PBX and a Service Provider’s application servers that allows voice over IP traffic to be exchanged between the two. When a call is placed from an internal phone to an external number, the PBX sends the necessary information to the SIP Trunk provider who establishes the call to the dialed number and acts as an intermediary for the call. All signaling and voice traffic between the PBX and the provider is exchanged using SIP and RTP protocol packets over the IP network.
一个SIP中继是一个IP PBX和服务提供商的应用服务器，使语音的逻辑联系 IP流量 将两者之间的交换。当一个呼叫从内部到外部电话放在号码，交换机发送必要的信息提供者的SIP中继谁建立了调用拨打的号码，并作为为调用中介行为。所有的信号和 语音通信 交换机之间的交换和提供者是通过IP使用SIP和RTP协议包 网络。
If the number being called is a traditional PSTN telephone, the trunk provider routes the IP packets to a PSTN Gateway that is closest to the number being called, to minimize possible long distance charges. The provider can also terminate PSTN numbers, and route incoming calls for those numbers back to the IP PBX over the SIP can also Trunk. This allows businesses to offer local phone numbers in several geographical areas, but service them all from a single location.
如果数字被称为是一种传统的PSTN电话，树干提供IP数据包路由到PSTN网关是最接近的号码被称为，以尽量减少可能的长途电话费。提供者也可以终止的PSTN号码，并在园区路线为这些号码来电返回的IP PBX还可以干。这使得企业能够在几个地理区域提供本地电话号码，但服务从一个位置全部。
If the number being called can be reached over a SIP Trunk, the call does not need to be routed over the PSTN, but can be carried on the IP network end to end. Providers can deliver these calls to their customers for very little cost, and many offer them at no charge. Some service providers have agreements and exchange calls for each others customers directly over their IP networks. Where no such agreements are in place, calls are routed over the PSTN, even though both endpoints may ultimately be reachable over a SIP Trunk.
如果这个数字被称为可以通过一个SIP中继达成共识，呼吁不须在PSTN路由，但可以对IP进行 网络 年底结束。供应商可以提供这些客户的要求非常低的成本，很多免费提供他们。有些服务提供者直接通过IP网络协议，为彼此交换客户要求。如果没有这样的协议落实到位，呼叫路由在PSTN，即使这两个端点最终可能超过一个SIP中继到达。
The SIP Trunk can be provided by the Internet Service Provider, or by an independent Internet Telephony Service Provider. In fact, there can be several parties involved, each one providing a different part of the service needed to deliver the end-to-end communication: Internet access, SIP termination, PSTN gateway, etc.
该 SIP中继 可提供的互联网服务供应商，或由一个独立的 互联网电话服务提供商。事实上，可以有几个当事人，每一个提供了所需提供的终端到终端的通讯：互联网接入，SIP协议终止，PSTN网关等服务的不同部分
Because a SIP Trunk is not a physical connection, there is no explicit limit on the number of calls that can be carried over a single trunk. Each call consumes a certain amount of network bandwidth, so the number of calls is limited by the amount of bandwidth that can flow between the IP PBX and the provider’s equipment.
因为一个SIP中继不是物理连接，也没有就可以在一个单一的电话号码进行明确的主干限制。每个调用都会消耗一定量的 网络 带宽，所以电话的数量是有限的带宽，可以流动之间的IP PBX和供应商的设备的数量。
For marketing reasons, many providers associate a SIP Trunk with a single phone call. When you purchase such a SIP Trunk, it provides the ability to place one call at a time through the service provider’s network. Organizations purchase as many trunks as they need to support their specific needs,typically one trunk per 3-5 employees. Some providers allow several calls to be carried over a single trunk.
对于市场的原因，许多供应商带来一个单一的电话呼叫的SIP中继。当你购买这样一个SIP中继，它提供了能够在一个地方通过一次电话服务供应商的 网络。组织购买尽可能多的树干，因为他们需要支持他们的具体需要，一般每3-5员工主干。有些供应商允许多次打电话给超过一个主干进行。
