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A Session Initiation Protocol (SIP) trunk is, at its essence, the virtual equivalent of a traditional business phone line. WhatIs.com defines SIP trunking as the use of voice over IP (VoIP) to facilitate the connection of a private branch exchange (PBX) to the Internet. While traditional voice services require a physical interface, be it a copper analog or digital Primary Rate Interface (PRI) circuit, a SIP trunking service leverages IP-based data networks to deliver voice, video and rich media services. This separation from physical circuits enables SIP-based voice services to be delivered dynamically. For example, while a PRI circuit is limited to 23 channels of concurrent voice calls, a SIP trunking service is limited only by the amount of bandwidth available on a customer's wide-area-network connection.
Because SIP is natively IP-based, IP PBXes and media servers can often interface directly with SIP trunks -- although a session border controller might be considered for protection and translation services. Media or VoIP gateway devices, on the other hand, typically are deployed when a business wants to take advantage of SIP trunking services while maintaining its legacy PBX or UC platforms. In this case, gateways will present the SIP trunks as analog or digital PRI trunks to the legacy hardware.
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