The SIP Trunking Advisor
Find the Right Voice Solution

As you consider voice solutions, use our online tool to learn about SIP trunking applications, benefits and impacts. As a bonus, download Voice Trunking in an IP World white paper.



The SIP Trunking Advisor

Session Initiation Protocol (SIP) enables delivery of voice, video and messaging services through IP networks, and among companies of 100 or more employees, over 80% consider SIP very important to their business.*

  • Enables unified communications (UC) solutions that integrate voicemail, fax, SMS, email, instant messaging (chat), presence information and collaboration.
  • Transports voice traffic over local and wide-area data networks, delivering internal “on-net” call traffic as IP data.
  • Minimizes carrier trunk and toll charges for telephony, while enabling innovative features.
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SIP Trunks Enable IP PBX Extensions for On-Net Calling and Support for IP Phones.

SIP trunks support “on-net” call traffic delivered as IP data over the company network to other locations. An array of SIP-enabled IP phone types that connect to the company LAN are supported along with IP PBX–based 4-digit digital plans that increase efficiency and reduce costs.

*Source "Market Size Analysis & Forecast 2013-2018" The Eastern Management Group. Sept. 2013.

Primary Rate Interface (PRI) trunks deliver voice traffic through Integrated Services for Digital Network (ISDN) connections with the Public Switched Telephone Network (PSTN). PRI continues to be a viable option for companies with an installed digital PBX or an IP PBX with IP-TDM voice gateways—especially for companies not planning for near-term voice traffic growth or the deployment of unified communications.

Most carriers provide PRI trunks in blocks of 24 channels for the 1.54 Mbps rate including 23 B channels of 64 Kbps for voice and 1 64 Kbps D channel for signaling . A challenge with this fixed-channel allocation is that an organization with growing voice traffic may be required to buy a block of 24 channels, even when far fewer channels are actually required. Fortunately, some innovative carriers now offer fractional PRI services, enabling organizations to purchase digital voice channels in smaller increments, such as 8, 12 or 16. If PRI is the right choice for your organization, look for a provider that offers fractional PRI services.

ISDN Primary Rate Interface

Companies using SIP for business communications still need to exchange voice traffic with the Public Switched Telephone Network (PSTN) through carrier trunk connections. PSTN trunks have traditionally been offered by carriers as ISDN Primary Rate Interface services.

  • Voice gateways like TDM to IP connect an IP PBX with the PSTN and convert IP packets to PSTN traffic over an ISDN PRI circuit, but SIP trunks eliminate this need.
  • SIP trunks supplement or bypass PRI infrastructure to extend SIP benefits beyond the internal IP PBX edge.
  • Today, 38% of businesses use SIP trunks to bypass PRI, and by 2015 expectations put that figure at 58%.
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Business Use of SIP and PRI Trunks 2013–2015

According to Infonetics Research, “North American businesses are using a combination of services; there isn’t going to be a 100% cutover [from PRI] to SIP.” Therefore, service providers that can aid in this evolution by offering both PRI and SIP trunking solutions may prove to be preferred partners.

Source: “SIP Trunking and SBC Strategies: North American Enterprise Survey,” Infonetics Research (March 2013).

A common design guideline suggests it makes sense to centralize SIP or PRI trunks if branches consume less than 20% of wide area network (WAN) bandwidth.

Organizations may choose to distribute their SIP or PRI trunks for PSTN connectivity given several circumstances:

  • When distance adds network delay or degrades IP call quality
  • When usage strains WAN links between sites
  • When branch-location WAN-bandwidth utilization approaches 50%
  • To add redundancy by avoiding a single point of failure for PSTN interconnection
  • To gain more robust and flexible load balancing and call routing options – particularly valuable with significant call center traffic around sales and customer support

In a 2012 survey by IDC, businesses using SIP trunks reported the top factors for deploying IP voice among SIP trunk users: cost savings, building a foundation for unified communications (UC), adding feature functionality, integrating with customer service and collaboration applications, improving productivity, and ease of management for IT staff.

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