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SIP Trunking Design and Implementation Considerations

Chapter Description

This chapter highlights many of the network design and implementation considerations you should work through while planning or installing a SIP trunk for production purposes in your network.

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IP Connectivity Options

Several different types of service providers offer SIP trunks. Enterprises can find offers from service providers to transport just their data services or just their voice services, or both. When the data and voice services are delivered by different providers, each traffic type is typically delivered over a separate physical medium. Also, when the SIP trunk carries high traffic, for example 1000 sessions or more, a separate physical medium for the SIP trunk is often used.

Consider two aspects of IP connectivity when connecting to a SIP trunk:

  • Physical medium of delivery
  • IP addressing

Physical Delivery and Connectivity

A dedicated physical connection for a SIP trunk is not uncommon for larger enterprises. The physical delivery for these types of terminations is often optical fiber (OC-3 or higher) and through a series of multiplexing, switching, and routing equipment eventually terminates as a gigabit Ethernet connection onto the enterprise's Session Border Controller (SBC). This connection model is shown in the left panel of Figure 7-1.

Figure 7-1

Figure 7-1 Physical Delivery of a SIP Trunk

For smaller businesses this dedicated delivery model is not cost-effective and these organizations predominantly get their voice and data services from the same provider over the same physical connection. The connection in this case might be Digital Subscriber Loop (DSL), cable, Integrated Service Digital Network (ISDN), T1/E1, 3G wireless, or any other medium capable of carrying QoS-enabled IP traffic. This connection might connect directly to the customer's routing equipment, or the provider might drop off a Customer Premise Equipment (CPE) access device (for example, a DSL, cable modem, or a low-end router), which connects with Ethernet to the customer's routing or switching equipment. This connection model is shown in the "Small Business" model in Figure 7-1 using an Integrated Access Device (IAD).

It is important to note that only service providers that have complete control over the QoS of the physical connection can offer business-class voice services over a SIP trunk. VoIP services that ride on non-QoS enabled networks owned by a separate Internet provider cannot provide guarantees of quality levels because they do not control the sequencing of packets on the physical medium of delivery into your premises.

Physical connection options include Ethernet, DSL, cable, wireless (3G cellular), and traditional T1/E1.

IP Addressing

The configuration of a SIP trunk requires coordination between the enterprise to configure its border element and the service provider's border element before starting to exchange SIP traffic. The provider allocates either explicit IP addresses or access via Domain Name System (DNS). For dedicated voice-only connections, most providers allocate two addresses per SIP trunk, whereas some offer more. If only two addresses are provided, these can often be used (per agreement with the service provider) in either a primary-secondary failover or a load-balancing algorithm.

For integrated data and SIP trunk services, there is often a single IP address. Several service providers that offer both data and voice over a single IP interface also offer Multiprotocol Label Switching (MPLS) services and require that voice be sent with an MPLS label. This setup enables the service provider to terminate voice traffic, whereas data traffic marked with a different label can be tunneled through the network.

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