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SIP Trunking Connects Enterprises to Carriers for IP Telephony
A telephone trunking option uses Session Initiation Protocol to connect companies to phone carriers.

By: John Shepler

Enterprise VoIP phone systems are often sequestered within the corporate walls. Even though medium and large businesses have been replacing traditional TDM based PBX telephone systems with IP telephony, and even though carriers themselves are often using IP based core networks to transport voice traffic, it's still a switched circuit world. SIP Trunking provides an IP to IP conduit for voice traffic without that intermediate conversion to and from the PSTN (Public Switched Telephone Network).

Each Organization is Unique
Companies base their technology decisions on what will work best in their particular environment. Each organization takes a look at their current deployment of analog or digital phones, their expansion needs, desired telephony features and both capital and operating expenses. The result is ad-hoc solutions that may be anything from a key telephone system with half a dozen lines to an enterprise-wide VoIP system with thousands of handsets connected to a single converged network. Within the corporate walls, it doesn't matter what you use as long as everyone can talk to everyone else. But if you want to make calls outside of the organization, you need compatibility with the phone number you want to call.

SS7 is the Heart of the Public Phone System
Traditionally, that compatibility has been based on analog copper phone lines or T-carrier digital lines, such as ISDN PRI circuits. These standards were established by the telephone companies over 100 years of development. At the core of these standards is a signaling mechanism known as SS7 or Signaling System # 7. SS7 is the protocol that phone companies use to talk to other phone companies. It is the control mechanism to setup and tear down, or initiate and terminate telephone calls.

SIP as an SS7 Alternative for IP Telephony
SS7 is at the heart of the public telephone system. But VoIP has its own signaling protocols. A popular one and the current favorite as the emerging standard for IP telephony is SIP or Session Initiation Protocol. An enterprise VoIP system may use SIP for signaling on the same network that carries voice, video and data traffic. The corporate IP network can transport internal phone calls between remote locations, completely avoiding the public phone system. That is, until someone wants to talk to a party outside of the company. A voice gateway provides connection to the outside telephone lines or trunks to the desired phone carrier.

SIP Trunking Includes the Telephone Carrier
What SIP trunking does is to include the phone carrier within the VoIP network. That also includes access to that carrier's Voice IP network so that phone traffic can remain as VoIP packets to VoIP enabled users without a protocol conversion. Calls to phone numbers off the network are terminated by the carrier rather than the enterprise. As VoIP becomes more prevalent, fewer calls will need to be terminated to the PSTN. The result is cost savings and other features provided by converged voice and data networking.

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