SIP Trunking Saves Money
A potentially huge savings for companies comes from this next idea of using SIP trunking to replace TDM circuits for transport. I have written an article on this topic that was published in a March 2009 blog on No Jitter. Below is a more detailed perspective.

By combining SIP trunks, a gateway and T1 connections into your TDM (time-division multiplexing) PBX, companies can avoid a forklift of their TDM PBX while realizing the ROI of SIP for a fraction of the cost of purchasing a new IP PBX. By adding this step in your design process, you are building the infrastructure for the time when the organization can upgrade to an IP PBX.

The key components in this design are an Internet T1, a Cisco router with advanced IP services IOS, a WIC 1DSU T-1 V2 card, VWIC 2MFT T-1 E1 card and current TDM PBX. This particular SIP trunking implementation that we undertook did in fact need the intermediary step of installing the Cisco gateway due to the fact that our legacy PBX is not inherently compatible with the SIP protocol. Depending on the type of PBX in your organization you may not need the gateway for the PBX to connect to the Internet T1s.

Depending on codec and compression from the SIP provider, companies can have varying amounts of calls over their Internet T1. This scenario has 45 concurrent calls over the Internet T1 using G.729 codec with 30ms sample rate.

For every Internet T1 you plug into the gateway, you can turn down 2 T1/PRIs; this is part of the ROI. The next part of ROI is the cost difference between what carriers charge for TDM services and SIP.

Previously, there were a total of 48 outbound trunks for Long Distance, directed from the PBX to the PSTN. Now, the first 10 outbound LD calls will go out PSTN, (current design choice). The next LD call (11th) will go out IPLD (IP long distance), as long as the first 10 ports are busy. The next group of calls will be split up into routes of 6 trunks each, split between IPLD and HA (High Availability) IPLD routers with HA IPLD receiving half of the capacity. We assigned 6 ports for each route so we do not need to make a gateway change if we decide to make a change in the number of calls going to the PSTN. HA IPLD is designed to be a 50% capacity failover in case IPLD router fails.

Once this is implemented we can reduce the amount of trunks outbound to PSTN and increase the amount of outbound IPLD at will. The PBX will round-robin calls in the event of an unavailable port. Load-balancing best practice for VoIP is by destination, as it is currently configured in the Gateway.

There are 2 IP routes pointing to the outbound IP address for IPLD; one route is assigned to each serial interface. We have load balancing set on the incoming trunks (Internet circuit 1, 2, and 3) and outbound from the PBX to not over-utilize one circuit or card.

Toll fraud is a major concern in any VoIP network. To help combat this issue, this design used COR (class of restriction) set on the dial peers. In this scenario, only calls that are defined as peers can call each other; all others will fail.

The ROI on this project was about two months. I have found out many interesting details working with two providers, finding out that some sales teams are just not up to speed on their own company's services, let alone mixing types of services for a more custom approach to this type of conversion. I had to pull the plug on one vendor when I found out that we would have been the first company in Florida to use their service and they did not yet have enterprise level references. I tried to convert two years ago with them, now I am using another vendor.

Conclusion
The last point when it comes to cost savings is to keep your skill set current; read the trade magazines, case studies, and network with other industry professionals. They may be experiencing similar situations and challenges, and you can learn from each other.

Todd Vinciguerra is a telecommunications analyst at Florida-based Riverside Bank. He can be reached at toddvinciguerra@bellsouth.net. His certifications include CCVP, CCNA, and CCDA.

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