SIP Provider Trunking

The final integration method isn’t so much integration with an existing PBX as it is a way to provide voice services between the end users without one of the other methods. SIP provider trunking uses an ITSP (Internet Telephony Service Provider) to deliver voice services across IP to an organization with Lync Server 2013, similar to how a service provider provisions Internet access.

The advantages to SIP trunking come in the form of flexibility, redundancy, and capacity. Failover of Direct Inward Dial (DID) numbers between geographic regions using TDM infrastructure is usually impossible or incredibly expensive, but because SIP trunks are simply an IP connection, they can point at multiple locations for primary and backup delivery points. Depending on the provider, the trunk capacity can also be adjusted very quickly without having to install or remove a physical circuit connection. This helps to cut down on the lead time to deploy new services.

If integration with an existing PBX is not possible with any of the other means, or if an organization wants to move away from the legacy PBX, an ITSP can replace those services. In this situation Enterprise Voice users can communicate with users still hosted on the PBX, but only by traversing the PSTN. Figure 17.10 shows how this is not an optimal call path for users who might be physically sitting next to each other on different systems, but does provide a connectivity option if no others exist. After users are migrated to Lync, an organization’s PBX and existing TDM contracts can be deactivated, allowing for all voice services to be delivered via the ITSP.

Figure 17.10. SIP trunking.

SIP trunks can be delivered to an organization via a few different methods:

• Internet—Organizations create the SIP trunk between the ITSP’s public IP address and a public IP address owned by them. This generally requires a firewall that is SIP aware and can modify the content inside the incoming and outgoing SIP messages dynamically. A traditional firewall NAT is not sufficient for SIP trunking. The primary disadvantage of this method is that traffic is subject to the Internet, which has no Quality of Service and which a business has no control over. Audio packets also might not be encrypted, which could allow an attacker to capture and replay a sensitive phone conversation, or collect transmitted signaling information such as account numbers.

• Site-to-Site VPN—Instead of using an unencrypted connection across the Internet, this method relies on a site-to-site VPN tunnel between the ITSP and a business. This can help secure the calls through encryption, but adds additional overhead to each packet and does nothing to solve the Quality of Service issue.

• MPLS—The most reliable method for an ITSP connection is to connect to the provider’s MPLS network. In this scenario the SIP trunk appears as a logical connection across a private WAN circuit. Quality of Service can be enabled on the connection and there is no need for firewall traversal and NAT inspection for SIP. This is the best of both worlds, but also usually comes at the highest price. Organizations should strive to leverage this method for the least complex and highest quality connection.