In the United States, the inter-exchange carriers (IXCs) generally handle traffic crossing boundaries known as Local Access and Transport Areas (LATAs). A LATA is physical location within the United States that outlines the boundaries of Local Exchange Carriers (LECs). LECs are capable of providing local transport within LATAs.
When a toll free number is dialed by a customer, a Responsible Organization (RESPORG) is assigned by the IXCs to own and maintain that number. In general, the RESPORG is an IXC who will be responsible for the delivery of the majority of the toll free services.
Looking at it with a more in-dept view, when a toll free number is dialed, every digit is analyzed and processed by the LEC. The call is identified by the service switching point (SSP). The SSP makes it possible to send call information to the signal transfer point (STP). Further, the STP asks the service control point (SCP) about the destination of the call.
It all boils down to the customer's choice which is processed by the LEC and an IXC is assigned. It is possible to share toll free numbers among IXCs. One of the reasons why a customer would do this is for disaster recovery or for bargaining a rate among different carriers. For example, a customer may want to direct 50% of his calls to one carrier and the other 50% to another carrier as per his convenience.
After determining an IXC, the LEC sends the call to the IXC's point of presence (POP). After the receiving the call, the IXC's SCP decides where to pass this call. The SCP is the brain of the long distance network when it comes to routing. The protocol used for this call routing and control is known as Signalling System 7 (SS7).
About the Author: Jatin Chawla is a freelance journalist. Jatin writes for
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