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May 2007
Volume 10 / Number 5
Enterprise View

SIP Trunking: Is It Right for Your Enterprise?

By Steven Johnson, Columns: Enterprise View

SIP trunks offer businesses a unique opportunity to rapidly reduce their communications expenses, while leveraging the benefits of IP communications. They create one converged network for both data and voice, increasing a business’ flexibility and future-proofing against an ever-changing communications environment.

A SIP trunk is the use of SIP to set up communications over the Internet between a customer location and an Internet Telephony Service Provider (ITSP) which transfers the SIP calls to the PSTN. Unlike in traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk is a logical connection from one point to another over the public Internet.


The Benefits of SIP Trunking

SIP trunking eliminates the need to purchase BRIs (Basic Rate Interfaces), PRIs (Primary Rate Interfaces) and local PSTN gateways. This means that capacity can be increased without the expense of adding another full E1 or T1 and without upgrading the hardware to support the additional connections.

SIP trunking also reduces costs by eliminating the need for separate voice and data connections, and expands the potential for communications convergence using both voice and data together.

A further advantage is that by outsourcing PSTN connectivity calls may be terminated close to the called party, resulting in reduced long distance charges.


The Need for Standards

SIP is the standard of choice for IP communications today and has been adopted by the major PBX and phone vendors and service providers. Connecting to a service provider network is further improved by use of the SIPconnect(SM) standard. This set of best practices was developed by the SIP Forum to ensure a seamless connection between the enterprise and the service provider. SIPconnect meets a critical need by eliminating many of the unknowns and incompatibilities of interfacing two networks for delivering quality voice and other services.



A relatively small investment is necessary to benefit from the use of SIP-based communications and to enjoy the cost savings that accrue from using SIP trunks. For the enterprise, converting to VoIP usually involves the purchase of an IP-PBX, IP phones or soft clients, and a SIP-aware firewall or edge device to maintain security while admitting VoIP traffic.


Delivery And Quality of Service

Although the IP pipeline can carry much more traffic than a traditional connection, it is important to employ proper quality of service (QoS). Voice and video are very susceptible to delay, which means that some QoS procedures should be in place to guarantee priority delivery of these packets vs. other information downloaded to the converged network.

Despite some perceptions to the contrary, the core network of the Internet is often not a bottleneck today. The last mile and the customer network can be. With the right QoS prioritization and admission control at the enterprise edge, this issue can be eliminated.



Using a firewall that’s specifically designed to handle SIP communications will provide the best defense against unwanted activity. Full SIP proxy technology allows for advanced filtering, verification and routing, as well as dynamic control of the opening and closing of media ports. Some products offer encryption of the signaling using Transport Layer Security (TLS) and of the media using Secure RTP (SRTP) or other algorithms.

Authentication with the service provider is also critical. While some IP-PBX equipment can support this natively, others cannot. A full SIP proxy firewall or other edge device may offer this capability as well.

Another security-related issue is redundancy. A fully SIP-capable firewall or customer premises device can provide a robust system for securing full VoIP redundancy, as traffic can be routed to a back-up carrier if the primary carrier is unavailable.



SIP trunking offers enterprises the benefits of converged communications and saves substantial expense by eliminating the need to purchase BRIs, PRIs or PSTN gateways. It also reduces expenses by terminating calls closer to the called party. A robust enterprise solution combined with a SIP trunk from an ITSP realizes the promise of safe global connectivity, over the Internet, so long envisioned by the voice over IP pioneers.

Steven Johnson is President of Ingate Systems. (news - alert) For more information, visit the company online at


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