A SIP trunk is an IP connection that establishes a SIP communications link between organization and an Internet telephony service provider (ITSP) beyond firewall. Typically, a SIP trunk is used to connect organization’s central site to an ITSP. In some cases, SIP trunking may opt use to connect your branch site to an ITSP.
SIP Trunk vs Direct SIP
|Areas||SIP Trunk||Direct SIP|
|External Entity Connection||
Implement SIP Trunking
To implement SIP trunking, connection must route through Mediation Server, which acts as a proxy for communications sessions between Lync Server clients and the service provider and transcodes media, when necessary.
Each Mediation Server has an internal network interface and an external network interface. The internal interface connects to the Front End Servers. The external interface is commonly called the gateway interface because it has traditionally been used to connect the Mediation Server to a public switched telephone network (PSTN) gateway or an IP-PBX. To implement a SIP trunk, you connect the external interface of the Mediation Server to the external edge component of the ITSP.
Centralized vs. Distributed SIP Trunking
Centralized SIP trunking routes all VoIP traffic, including branch site traffic, through central site. The centralized deployment model is simple, cost-effective, and generally the recommended approach for implementing SIP trunks with Lync Server 2010.
Distributed SIP trunking is a deployment model in which implement a local SIP trunk at one or more branch sites. VoIP traffic is then routed from the branch site directly to a service provider without going through the central site.
Distributed SIP trunking is required only in the following cases:
- The branch site requires survivable phone connectivity (for example, if the WAN goes down).
- Resiliency is required between two central sites.
- The branch site and central site are in different countries/regions.
Supported SIP Trunking Connection Types
Lync Server supports the following connection types for SIP trunking:
- Multiprotocol Label Switching (MPLS) is a private network that directs and carries data from one network node to the next. The bandwidth in an MPLS network is shared with other subscribers, and each data packet is assigned a label to distinguish one subscriber’s data from another’s. This connection type does not require VPN. A potential drawback is that excessive IP traffic can interfere with VoIP operation unless VoIP traffic is given priority.
- A private connection with no other traffic—for example, a leased fiber-optic connection or T1 line—is typically the most reliable and secures connection type. This connection type provides the highest call-carrying capacity, but it is typically the most expensive. VPN is not required. Private connections are appropriate for organizations with high call volumes or stringent security and availability requirements.
- The Internet is the least expensive connection type, but it is also the least reliable. Internet connection is the only Lync Server SIP trunking connection type that requires VPN.