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First things first with VoIP

What you need from the get-go.

One of the biggest challenges in implementing a VoIP solution is making sure you have all the necessary components. This can be especially daunting if you're new to VoIP. This tip gives a quick list of things you'll need. For the sake of brevity, we assume you already have a functional data-network and related environmental concerns, like adequate conditioned power sources and rack-space, and of course, someone to configure it all.

There are two primary uses for VoIP. The first is "toll-bypass" where you have two geographically separate locations connected by voice and data circuits, and you want to combine these circuits so voice rides over your data network, but your users still use their old phones and they remain connected to your old PBX or key-system.

For this scenario, you'll simply need a gateway at each location. A gateway is typically a module inserted in a router, or it can be a dedicated piece of hardware. It is usually physically connected to your PBX and your data network via Ethernet or a WAN interface such as a T1, depending on your needs. This allows it to convert the voice from the PBX into IP packets (and back on the far end). An example is Avaya's appropriately named Media Gateway line.

The second type of VoIP, which we'll call "IP Telephony", is when users have IP phones that are plugged into Ethernet switches. In this scenario, you'll typically need a lot more, but you won't need any legacy equipment:

  • New phones. While it's possible to use existing phones if you keep your existing PBX and tie it into your new IP system, these phones can't take advantage of any of the new features, from mobility, to services, to cost savings from converged infrastructure.
  • A "soft switch". This is typically a software application that runs on a Windows or Unix/Linux server and controls your calls. It provides similar function to your old PBX. Examples are Avaya's Media Servers and Cisco's Call Manager.
  • Gateways. Assuming you want to be able to talk to the rest of the world, you'll need at least one gateway to the PSTN. This device is just like the gateway mentioned above and can range in size from offering a pair of POTS analog lines to multiple T1s.
  • TFTP/DHCP server. Most IP phones need to load their software from a TFTP server, and usually get their IP address from DHCP dynamically. This software is available for free for most platforms, and you can usually run it on the same server as your soft switch although that's not really recommended.
  • Directory Service. Although this isn't technically required, it's awfully handy. It is the equivalent of a phone book, integrated with your data network. It usually uses the LDAP protocol. Examples are Microsoft's Active Directory and Novell's NDS.

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


This was last published in April 2003

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