It is very common for a branch office in a large organization to need less than 1.544 Mbps of bandwidth for data traffic, and a handful of lines (typically three to eight) for long distance telephone service. In this scenario, there are two easy options: use a fractional T1 service and devote three to eight channels for voice "tie lines" to the home office and use the rest for data, or use a full T1 for data, and run the voice traffic over IP. How do you decide?
From a cost perspective, there are many factors at odds with one another. Each organization has different resource constraints, so they weight these factors differently. The major factors to consider are:
a) Depending on your location, the cost of the data circuits may be more or less than the cost of the voice circuits. If your WAN bandwidth isn't fully utilized, this can be an excellent cost savings.
b) If you run voice and data separately using the Fractional T1, you will need separate boxes at each end to handle the voice and data (typically a key-system and a router) and a third device to split the channels. If you use VoIP, you will only need a router at each end, but you will also need something with digital signal processors to convert the analog voice to digital. This could be a card in the router or you could use IP phones and an IP PBX. From a hardware cost perspective, two simple boxes are often cheaper than the single, more complex multi-service router. However, if you can replace a lot of PBXs with a centralized IP Telephony system, VoIP becomes cost-effective quickly.
From a manageability perspective, there are also conflicting factors. With VoIP, there are fewer devices to maintain, but they are more complex and it can be more difficult to get installed. In a full-blown IP Telephony system, VoIP is more complicated to troubleshoot, but "moves, adds and changes" are dramatically easier. However, when you're just trying to decide between a Frac-T1 and VoIP over a full T1, you don't gain any of those benefits, and you have to keep an eye on the performance of your QoS settings. These need to be reviewed occasionally and may need to be tweaked.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.