No Jitter is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

What Is a SIP Trunk?

The IETF draft, written by SIP guru Jonathan Rosenberg of Cisco, notes that the term "trunk" comes from the legacy circuit-switched world, and isn't clearly defined in the context of SIP. Rosenberg writes:

A SIP trunk has been used to describe all of the following:

  • A service provided by service providers to enterprises, used for the purpose of interconnection to the PSTN, as a replacement for circuit based connectivity.

  • A SIP port on an enterprise server, for the purposes of interconnection to other server-based systems, such as voicemail servers, call centers, and application servers.

  • A SIP-based interconnection between IP-PBXs, for the purposes of replacing traditional TDM tie lines.
  • He then proposes this definition of "SIP Trunk:"

    A SIP trunk is a virtual SIP entity on a server (UAS [User Agent Server], UAC [User Agent Client] or proxy) constrained by a predefined set of polices and rules that determine how to process requests.

    As that term "virtual SIP entity" suggests, Rosenberg conceives of a SIP trunk as something fundamentally different than the old telephony term. Specifically, in the TDM world, a trunk connects two switches in a one-to-one relationship. Rosenberg's definition posits that a SIP trunk "represents functionality provided on [a] server, possibly to a large number of other clients that connect"--in other words, it can support a one-to-many configuration.

    But Rosenberg's draft proposes that this definition is truer to the actual functionality of a trunk in the TDM world:

    The definition posits that, in the TDM world, the important part of the concept of a trunk was the policies, features and functionality that were bound to it - not the fact that it was a physical connection to another switch. For this reason, a SIP trunk is defined entirely by a set of policies, features and functionality that are invoked when SIP requests are sent to that trunk.

    How does a clear definition of "SIP trunk" advance the cause of actually being able to buy SIP trunks ubiquitously? I don't think it's the lack of a clear definition and standardized concept that's kept the carriers from offering SIP trunks; it's clearly been a business decision based on their concern about losing revenue from PRIs.

    But hopefully, this draft, together with efforts like the SIP Forum's SIPconnect, will help buyers understand and ask the right technical questions about SIP Trunks if and when the market brings enough pressure to bear on the major carriers to make SIP trunk offerings ubiquitous. When that'll happen is anyone's guess.

  • A service provided by service providers to enterprises, used for the purpose of interconnection to the PSTN, as a replacement for circuit based connectivity.

  • A SIP port on an enterprise server, for the purposes of interconnection to other server-based systems, such as voicemail servers, call centers, and application servers.

  • A SIP-based interconnection between IP-PBXs, for the purposes of replacing traditional TDM tie lines.

    He then proposes this definition of "SIP Trunk:"

    A SIP trunk is a virtual SIP entity on a server (UAS [User Agent Server], UAC [User Agent Client] or proxy) constrained by a predefined set of polices and rules that determine how to process requests.

    As that term "virtual SIP entity" suggests, Rosenberg conceives of a SIP trunk as something fundamentally different than the old telephony term. Specifically, in the TDM world, a trunk connects two switches in a one-to-one relationship. Rosenberg's definition posits that a SIP trunk "represents functionality provided on [a] server, possibly to a large number of other clients that connect"--in other words, it can support a one-to-many configuration.

    But Rosenberg's draft proposes that this definition is truer to the actual functionality of a trunk in the TDM world:

    The definition posits that, in the TDM world, the important part of the concept of a trunk was the policies, features and functionality that were bound to it - not the fact that it was a physical connection to another switch. For this reason, a SIP trunk is defined entirely by a set of policies, features and functionality that are invoked when SIP requests are sent to that trunk.

    How does a clear definition of "SIP trunk" advance the cause of actually being able to buy SIP trunks ubiquitously? I don't think it's the lack of a clear definition and standardized concept that's kept the carriers from offering SIP trunks; it's clearly been a business decision based on their concern about losing revenue from PRIs.

    But hopefully, this draft, together with efforts like the SIP Forum's SIPconnect, will help buyers understand and ask the right technical questions about SIP Trunks if and when the market brings enough pressure to bear on the major carriers to make SIP trunk offerings ubiquitous. When that'll happen is anyone's guess.

  • A SIP port on an enterprise server, for the purposes of interconnection to other server-based systems, such as voicemail servers, call centers, and application servers.

  • A SIP-based interconnection between IP-PBXs, for the purposes of replacing traditional TDM tie lines.

    He then proposes this definition of "SIP Trunk:"

    A SIP trunk is a virtual SIP entity on a server (UAS [User Agent Server], UAC [User Agent Client] or proxy) constrained by a predefined set of polices and rules that determine how to process requests.

    As that term "virtual SIP entity" suggests, Rosenberg conceives of a SIP trunk as something fundamentally different than the old telephony term. Specifically, in the TDM world, a trunk connects two switches in a one-to-one relationship. Rosenberg's definition posits that a SIP trunk "represents functionality provided on [a] server, possibly to a large number of other clients that connect"--in other words, it can support a one-to-many configuration.

    But Rosenberg's draft proposes that this definition is truer to the actual functionality of a trunk in the TDM world:

    The definition posits that, in the TDM world, the important part of the concept of a trunk was the policies, features and functionality that were bound to it - not the fact that it was a physical connection to another switch. For this reason, a SIP trunk is defined entirely by a set of policies, features and functionality that are invoked when SIP requests are sent to that trunk.

    How does a clear definition of "SIP trunk" advance the cause of actually being able to buy SIP trunks ubiquitously? I don't think it's the lack of a clear definition and standardized concept that's kept the carriers from offering SIP trunks; it's clearly been a business decision based on their concern about losing revenue from PRIs.

    But hopefully, this draft, together with efforts like the SIP Forum's SIPconnect, will help buyers understand and ask the right technical questions about SIP Trunks if and when the market brings enough pressure to bear on the major carriers to make SIP trunk offerings ubiquitous. When that'll happen is anyone's guess.

  • A SIP-based interconnection between IP-PBXs, for the purposes of replacing traditional TDM tie lines.

    He then proposes this definition of "SIP Trunk:"

    A SIP trunk is a virtual SIP entity on a server (UAS [User Agent Server], UAC [User Agent Client] or proxy) constrained by a predefined set of polices and rules that determine how to process requests.

    As that term "virtual SIP entity" suggests, Rosenberg conceives of a SIP trunk as something fundamentally different than the old telephony term. Specifically, in the TDM world, a trunk connects two switches in a one-to-one relationship. Rosenberg's definition posits that a SIP trunk "represents functionality provided on [a] server, possibly to a large number of other clients that connect"--in other words, it can support a one-to-many configuration.

    But Rosenberg's draft proposes that this definition is truer to the actual functionality of a trunk in the TDM world:

    The definition posits that, in the TDM world, the important part of the concept of a trunk was the policies, features and functionality that were bound to it - not the fact that it was a physical connection to another switch. For this reason, a SIP trunk is defined entirely by a set of policies, features and functionality that are invoked when SIP requests are sent to that trunk.

    How does a clear definition of "SIP trunk" advance the cause of actually being able to buy SIP trunks ubiquitously? I don't think it's the lack of a clear definition and standardized concept that's kept the carriers from offering SIP trunks; it's clearly been a business decision based on their concern about losing revenue from PRIs.

    But hopefully, this draft, together with efforts like the SIP Forum's SIPconnect, will help buyers understand and ask the right technical questions about SIP Trunks if and when the market brings enough pressure to bear on the major carriers to make SIP trunk offerings ubiquitous. When that'll happen is anyone's guess.

    He then proposes this definition of "SIP Trunk:"

    A SIP trunk is a virtual SIP entity on a server (UAS [User Agent Server], UAC [User Agent Client] or proxy) constrained by a predefined set of polices and rules that determine how to process requests.

    As that term "virtual SIP entity" suggests, Rosenberg conceives of a SIP trunk as something fundamentally different than the old telephony term. Specifically, in the TDM world, a trunk connects two switches in a one-to-one relationship. Rosenberg's definition posits that a SIP trunk "represents functionality provided on [a] server, possibly to a large number of other clients that connect"--in other words, it can support a one-to-many configuration.

    But Rosenberg's draft proposes that this definition is truer to the actual functionality of a trunk in the TDM world:

    The definition posits that, in the TDM world, the important part of the concept of a trunk was the policies, features and functionality that were bound to it - not the fact that it was a physical connection to another switch. For this reason, a SIP trunk is defined entirely by a set of policies, features and functionality that are invoked when SIP requests are sent to that trunk.

    How does a clear definition of "SIP trunk" advance the cause of actually being able to buy SIP trunks ubiquitously? I don't think it's the lack of a clear definition and standardized concept that's kept the carriers from offering SIP trunks; it's clearly been a business decision based on their concern about losing revenue from PRIs.

    But hopefully, this draft, together with efforts like the SIP Forum's SIPconnect, will help buyers understand and ask the right technical questions about SIP Trunks if and when the market brings enough pressure to bear on the major carriers to make SIP trunk offerings ubiquitous. When that'll happen is anyone's guess.