SIP Trunking: OCS – What Is It? Why Do I Need It? How Do I Deploy It?

Posted: April 16, 2012 in SIP
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A SIP trunk is a direct connection between your organization and an Internet telephony service provider (ITSP). It enables you to extend voice over IP (VoIP) telephony beyond your organization’s firewall without the need for an IP-PSTN gateway. This simpler configuration is easier and less expensive to design, operate, maintain, and upgrade. And because ITSPs deliver services (notably long distance) at substantial savings, your investment in SIP trunking can give a quick and substantial return on investment.

In addition to VoIP calls, SIP trunks can also carry instant messages, multimedia conferences, user presence information, Enhanced 9-1-1 (E9-1-1) emergency calls, and other SIP-based, real-time communications services. Microsoft Office Communications Server 2007 R2 users can place local and long distance calls with caller ID and call hold over SIP trunks. As SIP-based communication technologies advance and SIP trunking becomes commonplace, additional services are likely to become available.

With all of the benefits that SIP trunking provides (especially the substantial cost savings), it’s no wonder that SIP trunking is a hot topic in unified communications. This article gives an overview of this popular new technology and its benefits, information about how it’s deployed, and answers to questions about SIP trunking that people frequently ask Microsoft product managers and engineers.

What is SIP trunking?

This paper uses SIP trunking to describe using the Session Initiation Protocol (SIP) and Real-Time Protocol (RTP) to connect Office Communications Server 2007 R2 directly to an ITSP over an Internet Protocol (IP) connection for the purpose of originating and terminating telephone calls.

SIP is used to create and control the communications sessions that are the basis of VoIP telephony. SIP is also used for instant messaging, presence updates, multimedia, conferencing, and other real-time services that can traverse a SIP trunk. RTP manages the actual voice data in a VoIP calls.

The “trunk” in SIP trunking is a term from circuit-switched telecommunications. It refers to a dedicated physical line that connects switching equipment. A SIP trunk is similar to its circuit-switched predecessor in that it is a connection between telecommunications systems, but it is different because the connection it describes is a virtual connection. This connection can be made over a line that is used only for SIP trunking, over a dedicated line that carries SIP trunking with other IP traffic, or over the Internet on a virtual private network (VPN).

Why do I need SIP Trunking? What are the benefits?

Deploying SIP trunking can be a big step towards simplifying your organization’s telecommunications and towards preparing for the latest real-time communications enhancements, but the biggest motivation for most organizations is immediate and substantial cost savings.

  • Save money on long distance service
    Long distance service typically costs significantly less with a SIP trunk connection.
  • Eliminate IP-PSTN gateways (or even your entire PBX)
    Because SIP trunks connect directly to your ITSP without traversing the publicly switched telephone network, you can get rid of IP-PSTN gateways and their attendant cost and complexity. Some ITSPs will even host a PBX for you, taking over both the PBX hardware and user administration if you choose, with substantial cost savings from reduced complexity, maintenance, and administration.
  • Eliminate a redundant network
    Deploying SIP trunking is a logical step towards the goal of having a single, IP-based network, rather than redundant telephone and data networks.
  • Eliminate BRI and PRI subscription fees
    With a SIP trunk connected directly to an Internet telephony service provider, you can dispense with costly BRIs and PRIs, replacing them with service that can cost significantly less. Furthermore, with SIP trunking you don’t need to buy lines in blocks of 24 or 32. Instead, you can buy the bandwidth you need, in smaller increments, and at better prices.
  • Extend the capabilities of Office Communications Server with new services from ITSPs
    With a SIP trunk in place, you can extend existing capabilities with additional services, such as E9-1-1 emergency calling. In the future, we expect ITSPs will to new services, such as greater integration with mobile phones and presence information on devices that are not running Office Communicator.

How Do I Deploy SIP Trunking?

A SIP trunk can connect your organization to an ITSP through either an IP-PBX or a Mediation Server. A Mediation Server performs encrypting, decrypting, and data translation between an Office Communications Server deployment on one side and the world beyond on the other.

This paper addresses only connecting through a Mediation Server. For details about deploying a SIP trunk with an IP-PBX, refer to the documentation provided by your PBX manufacturer and your ITSP.

Determining which Type of Connection to Use for your SIP Trunk

A SIP trunk can be deployed over the Internet or over a line that is dedicated to your organization and that may or may not share bandwidth with other IP traffic. A dedicated line (sometimes called a “leased line”-typically a T1 or fiber optic line) between your organization and the ITSP usually costs the most, but it can typically carry the most simultaneous calls and offers the highest security and reliability. If your organization has lower call volumes or less stringent security and availability requirements, another connection type might make sense for you. The following table highlights the pros and cons of each connection type.

Table 1. Comparison of SIP Trunking Connection Types

Connection type Advantages Disadvantages
Dedicated line (“leased line”) with no other traffic Most reliable

More secure

VPN not required

Highest call-carrying capacity

Most expensive
Dedicated line (“leased line”) shared with other IP traffic, often using Multiprotocol Label Switching (MPLS) More secure

VPN not required

Excessive IP traffic can interfere with VoIP operation unless VoIP traffic is given priority
Public (Internet) Least expensive Least secure

VPN required

Least reliable

Lowest call-carrying capacity

Recommendations for Setting up SIP Trunking Network Connections

To improve security, we recommend the following practices when you set up SIP trunking network connections with Office Communications Server 2007 R2:

  • Set up a VLAN with static routing between the Mediation Server and the router. If you use a VPN server, set up a VLAN between the VPN server and the Mediation Server.
  • Do not allow broadcast or multicast packets to be transferred from the router to the VLAN. If you use a VPN server, do not allow broadcast or multicast packets to be transferred from the VPN server to the VLAN.
  • Block any routing rules that route traffic from the router to anywhere but the Mediation Server. If you use a VPN server, block any routing rules that route traffic from the VPN server to anywhere but the Mediation Server.
  • If you use a VPN server, encrypt data on the VPN by using Generic Routing Encapsulation (GRE).
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