Interexchange carrier
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In the United States, Interexchange carrier (or IXC) is a legal and regulatory term for a telecommunications company, commonly called a long-distance telephone company, such as AT&T, MCI, and Sprint. It is defined as carriers which provide inter LATA (local access and transport area) communication.
An IXC carries traffic (usually voice traffic) between telephone exchanges. Telephone exchanges are usually identified in the United States by the three-digit area code and the first three digits of the phone number. Different exchanges are generally in different geographic locations, such as separate central offices (COs, also called "wire centers").
IXCs used to carry voice traffic on analog lines, but these days, most voice traffic is digitized. Therefore, voice traffic is more typically a data stream. These voice data streams therefore can be intermixed with data traffic too, such as uplinks for DSL. Most commonly, links to and from COs are ATM links carried on optical fiber.