SIP Trunk 101

Session Initiation Protocol (SIP) trunks are a digital method of connecting to phone calls and other digital communications over the Internet.
The term trunk in SIP trunking refers to a virtual phone line used to make phone calls to any phone number holder over the Internet.

SIP trunks are not only for voice calls, they can also be used for multimedia communications that include voice, video and messaging services.SIP trunks will replace SIP trunks will replace analogue and ISDN telephone lines, and many countries are already planning to discontinue the use of ISDN services.

Background on  the creation of SIP Trunk

1.Traditional Phone Network

In a traditional telephone network, as shown in Pic1, all internal calls are made through an internal PBX, and communications between the business and the outside world are made through trunk lines provided by the PSTN telephone operator.

2.SIP & PSTN

With the development of IP technology, more and more enterprises are deployingSIP-based IP-PBXs, as shown in Pic2, for internal calls. However, for external calls, enterprises still need to rent a trunk line from a PSTN telephone operator. The disadvantage of this type of network is that the enterprise needs to maintain both the SIP network and the PSTN network, making it more difficult to manage.

With the massive deployment of SIP-enabled IP-PBXs in enterprises and the increasing number of ITSPs (Internet Telephone ServiceProviders) providing voice communications infrastructure over SIP, there is an urgent need for as solution that can improve the quality of service.

(Internet Telephone Service Provider), there is an urgent need for a SIP trunking technology that can provide a trunk line similar to traditional PSTN trunks to connect enterprise IP-PBXs.

SIP trunk technology, similar to traditional PSTN trunking, is needed to connect the enterprise IP-PBX to the external Internet Telephone Service Provider (ITSP) for network-wide IP. A typical network diagram of which is shown in Pic 3.

SIP Trunk as a feature can be embedded in a gateway or firewall deployed at the edge of the enterprise network. Devices that can implement SIP Trunking are called SIP Trunking Devices, or TG Gateways.

SIP Trunk  Features

1.Only a unique, secure, QoS-guaranteed SIP Trunk link is established between the SIP Trunk device and the ITSP.
This link is used to carry multiple concurrent calls for the enterprise, and the operator only needs to authenticate the link, not each SIP call carried on the link.

2.Internal corporate communications are handled by the corporate IP-PBX.
All outbound traffic is routed to the ITSP via SIP Trunk, which in turn is sent to the PSTN network, eliminating the need to maintain traditional PSTN trunk and saving hardware and maintenance costs.

3.Enterprises can select and connect to multiple ITSPs at will by setting the destination address, making full use of ITSPs around the world and saving on call costs.

4.With the deployment of SIP Trunking devices, SIP protocols can be used across the network, allowing better support for voice, conferencing, instant messaging and other communication features

5.SIP Trunk devices are different from SIP proxy servers in that they receive a call request from a subscriber and initiate a new call request to the ITSP on behalf of the subscriber.

During the forwarding process, the SIP Trunk device not only relays the signaling messages, but also relays the RTP media messages. Throughout the process, the devices on both ends of the SIP Trunk device (internal and external to the enterprise) think that it is the SIP Trunk device itself that is interacting with them.

The SIP Trunk device is different from a SIP proxy server.

SIP Trunk Classic Application

Network configuration using SIP Trunk

1.The SIP Trunk device is deployed between an enterprise IP-PBX and an ITSP.

2.All internal calls to the enterprise are made through the enterprise IP-PBX and all external calls to the enterprise are sent to the ITSP via the SIP Trunking.

Related Protocols

The following protocol specifications are relevant to SIP Trunk

RFC3261

RFC3515

SIP connectTechnical Recommendation v1.1