在我們學習SIP協議的過程中，對于一個概念要清楚，那就是SIP中繼的概念。為了讓大家更好地理解這方面內容，我們采用中英文互譯的對照形式，幫助大家了解這方面的知識，具體資料如下：

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.

一個SIP中繼是一個IP PBX和服務提供商的應用服務器，使語音的邏輯聯系 IP流量 將兩者之間的交換。當一個呼叫從內部到外部電話放在號碼，交換機發送必要的信息提供者的SIP中繼誰建立了調用撥打的號碼，并作為為調用中介行為。所有的信號和 語音通信 交換機之間的交換和提供者是通過IP使用SIP和RTP協議包 網絡。

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.

如果數字被稱為是一種傳統的PSTN電話，樹干提供IP數據包路由到PSTN網關是最接近的號碼被稱為，以盡量減少可能的長途電話費。提供者也可以終止的PSTN號碼，并在園區路線為這些號碼來電返回的IP PBX還可以干。這使得企業能夠在幾個地理區域提供本地電話號碼，但服務從一個位置全部。

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.

如果這個數字被稱為可以通過一個SIP中繼達成共識，呼吁不須在PSTN路由，但可以對IP進行 網絡 年底結束。供應商可以提供這些客戶的要求非常低的成本，很多免費提供他們。有些服務提供者直接通過IP網絡協議，為彼此交換客戶要求。如果沒有這樣的協議落實到位，呼叫路由在PSTN，即使這兩個端點最終可能超過一個SIP中繼到達。

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.

該 SIP中繼 可提供的互聯網服務供應商，或由一個獨立的 互聯網電話服務提供商。事實上，可以有幾個當事人，每一個提供了所需提供的終端到終端的通訊：互聯網接入，SIP協議終止，PSTN網關等服務的不同部分

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.

因為一個SIP中繼不是物理連接，也沒有就可以在一個單一的電話號碼進行明確的主干限制。每個調用都會消耗一定量的 網絡 帶寬，所以電話的數量是有限的帶寬，可以流動之間的IP PBX和供應商的設備的數量。

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.

對于市場的原因，許多供應商帶來一個單一的電話呼叫的SIP中繼。當你購買這樣一個SIP中繼，它提供了能夠在一個地方通過一次電話服務供應商的 網絡。組織購買盡可能多的樹干，因為他們需要支持他們的具體需要，一般每3-5員工主干。有些供應商允許多次打電話給超過一個主干進行。