A SIP (Session Initiation Protocol) connection is a service offered by many ITSP (Internet Telephony Service Providers) that connects a company's PBX to the existing telephone system infrastructure (PSTN) via Internet using the SIP VoIP standard.
Using a SIP connection may simplify administration for the organisation as the SIP connection typically will use the same Internet connection that is used for normal data.
This removes the need to also have a BRI/PRI installed as well, although sharing the same bearer circuit for calls and data raises its own challenges in maintaining call quality.
If the call traffic runs on the same connection with other traffic like Email or Web, voice and even signalling packets may be dropped and the voice stream can get interrupted.
To mitigate this, many companies split voice and data up into two separate internet connections to solve this problem, so that the resource conflict on the Internet access side is avoided. Other devices perform traffic shaping in order to avoid this resource conflict, but they still depend on the merit of the service provider not to drop packets from the Internet to the PBX.
Registration is required if the end user has a dynamic IP address, if the provider does not support static hostnames, or if NAT is used. In order to share several DID numbers on the same registration, the IETF has defined additional headers (for example "P-Preferred-Identity", see RFC 3325). This avoids multiple registrations from one PBX to the same provider. Using this method the PBX can indicate what identity should be presented to the Called party and what identity should be used for authenticating the call. This feature is also useful when the PBX redirects an incoming call to a PSTN number, for example a cell phone, to preserve the original Caller ID.
The increasing concerns about security of calls that run over the public Internet has made SIP encryption more popular. Because VPN is not an option for most service providers, most service providers that offer secure SIP connections use TLS and SRTP for encrypting the traffic. The keys for SRTP are exchanged using RFC 4568 (SDES).
Users should also be aware that a SIP connection can be used as a channel for attacking the company's internal networks, similar to Web and Email attacks. Users should consider installing appropriate security mechanisms to prevent malicious attacks.
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