As organisations increasingly recognise and require the benefits voice over Internet Protocol (VoIP) offers, they stop asking "Why?" and start asking "How?". A successful voice over IP implementation requires a keen understanding of how voice services will integrate with the existing network.

Organisations must know how voice and data traffic will coexist, what infrastructure additions will be required and whether major network re-designs will be necessary. This article takes a macroscopic look at how VoIP technology integrates with existing network infrastructure.

The first and foremost consideration with any VoIP implementation is the cumulative network infrastructure (switches, routers, firewalls and the like) to which you will add VoIP. If you currently run an IP network, or even a hybrid IP network, VoIP technology lets you pass voice traffic, encapsulated in IP packets, through your existing network hardware.

While the utilisation of existing infrastructure is a key VoIP benefit, you must allow for your current network's capabilities. You should know whether your switches can place voice and data networks into separate VLANs. You should also understand the Quality of Service (QoS) capabilities your switches and routers offer.

Along with your network hardware's supported features, a successful VoIP integration will depend on the hardware's physical capacity. Adding an additional traffic type to you existing network will increase network resource consumption. Bandwidth requirements will rise and with them memory and CPU utilisation. Minimum hardware requirements should be followed to ensure successful delivery of both voice and data traffic. For example, a 100Mb switched connection to the desktop is a common requirement. When your voice conversation depends on the underlying network's availability, you must ensure that traffic can be fully supported; otherwise, the voice communications quality will deteriorate as dropped packets cause choppy audio and speech delays. Data communications are resilient to delayed or lost packets, voice is not.

Before beginning a voice and data integration project, you should have accurate baseline data for the existing network's performance. Evaluate possible bottlenecks, network errors, and average bandwidth usage before putting phones on the network. This should be done before you begin evaluating a VoIP solution. Reliable baseline data will help you choose the VoIP implementation that provides the most bang for your buck.

Legacy telephony equipment
Unless you are deploying VoIP in a completely new environment, your VoIP solution will likely need to coexist, at least for a time, with your existing PBX or key switch telephone system. VoIP and traditional phones can coexist separately, allowing for a "flash" cutover -- where you move from the old system to VoIP on a scheduled date. If you prefer a staggered migration, you can run both systems simultaneously. While a staggered approach lets you test the VoIP waters before taking the plunge, it does require that you link your VoIP solution with your existing phone system.

How you link your VoIP solution and your existing phone system will depend on each system's specific characteristics, but you can smooth the process with a few common migration techniques. Many companies use a dial plan with non-overlapping phone numbers to simplify call routing. If your existing system uses 8XXX and your VoIP system uses 3XXX, both systems will know how to route specific calls.

You can also connect geographically distant legacy phone systems with VoIP technology over a WAN link. This allows you to test a VoIP solution and save on long distance costs. Some VoIP systems are merely "IP-enabled" PBX's that natively perform this function.

A variety of PBX's and key systems exist, but almost any two systems can be made to communication with each other. You typically create the link by emulating either a station-side or a trunk-side connection. Either system can then translate the phone numbers to achieve interoperability. With large-scale VoIP rollouts this can be a critical process. It is also important to verify the signaling types supported by both the legacy and VoIP systems. Legacy phone system often include both a standalone voicemail system and a PBX. You should verify that your new VoIP solution will support your existing voicemail system. Voicemail systems use specific messaging protocols and not every VoIP system supports all the available protocols. Some VoIP solutions may require a new voicemail system and this condition may sway your choice of VoIP provider.

Public Switched Telephone Network (PSTN) connectivity
Even if your entire enterprise is going completely VoIP, at some point you will need to reach the outside world. Small sites may only have a few business lines. Larger organisations may have one or many digital Primary Rate Interface (PRI) circuits. Unless you plan to change your outbound connection, you should choose a VoIP solution that works with your existing telephone circuit.

VoIP equipment is designed to work with commonly available telephone circuit types. You should search for an ideal solution that allows you simply unplug your exiting circuit from the PBX and connect it directly to the VoIP gateway. To ensure interoperability you should also confirm compatibility of signaling types, framing, and line coding.

In you plan to concurrently operate your legacy and VoIP systems, you will likely need separate Public Switched Telephone Network (PSTN) lines for each. It is unlikely both systems would be able to share the same local carrier lines. If you use a PRI circuit and are using Direct Inward Dial (DID), you can port the new phone number to the new PRI as you go. The approach allows some users to use the new system with their existing phone numbers and other users to operate on the old system as before.

Integrating VoIP with your existing network takes a significant amount of careful planning. You must ensure the existing network infrastructure can support the additional bandwidth and performance requirements. Ideally, the equipment should be able to logically separate voice and data networks, applying specific levels of service to each. Concurrently running your VoIP solution with your legacy phone system is a good way to test the VoIP waters.

This staggered approach will require linking the two systems and you should verify interoperability of PBX or key systems and voicemail prior to deployment. It is highly likely that you will also require additional PSTN connections while you simultaneously operate the two networks. While seamlessly integrating VoIP with your existing network can be a complex process, careful planning will help you reach the end goal of a IP-based telephony network that provides additional features and cost savings.