You’d be forgiven for thinking that there’s something wrong with you if you haven’t rushed headlong down the LAN telephony path. Sometimes it seems it would be easier to deploy it than keep explaining why you haven’t. Admitting to not having an IP telephony deployment—not even a pilot!—to people you meet at IT seminars and trade shows is a bit like admitting you don’t own a television, and telephone your friends for a chat instead of sending emails.

There are good reasons to implement VoIP, whether it’s a full-blown IP telephony deployment or not (and we’ll look at what the significance of the two are in a minute). There are also times when it’s best not to, or when it might suit part of your business, but not all. So let’s start with some basic criteria to determine what we’re talking about.

VoIP vs IP telephony
You’ll find a lot of people talking about Voice over IP and IP telephony (or LAN telephony) interchangeably. This is wrong. Just to get the terminology sorted out, so you can talk to vendors without them patronising you, Voice over IP, Voice over Frame, and Voice over ATM, are technologies that have been around for a long time to let you use your existing WAN infrastructure to carry voice traffic between PBXs, or to provide long-line extension services from a remote handset to a PBX.

It’s a cost-saving exercise, nothing more. Nothing in the telephone handset or PBX side of things changes one iota. You still have analogue (or proprietary digital feature set) phones, and your PBX keeps all its existing line and trunk cards, numbering plans and route maps. Instead of a trunk card, say, connecting to a dedicated circuit, or to the PSTN, it plugs into a gateway that encapsulates the voice traffic into IP (or frame relay or ATM) sends it across your WAN and pops it out at the far end. You use one circuit instead of two and save yourself carrier costs.

Time Division Multiplexers (remember GDC, Newbridge and the like?) did this well over a decade ago, but you had to dedicate timeslots to the voice, whether anyone was making a call or not. The difference with the VoIP setup is that the bandwidth can be used by your data traffic if there’s no voice.

Of course it is a bit more complicated than this, since you have to configure your network for the prioritisation, low delays and minimal jitter that voice traffic requires but that’s only on a couple of devices. To be honest, if you’re not doing it, you probably should be.

IP telephony is a much bigger concept. If you embrace it completely (and you don’t have to as we’ll see), it’s goodbye to your exiting PBXs, key systems and handsets. The IP part of the equation is no longer just gateway to gateway but right from the handset on your desk. PBXs are replaced with softswitches—servers that handle all call routing, management and reporting functions. Gateways manage connections to other sites, and to the PSTN, and the handsets are now fully functioning IP stations that can almost take the place of a PC, for some users.

Circuit vs Packet Switching
Where traditional voice calls involved the connection-oriented setting up of a circuit end-to-end for the duration of the call, IP calls are packet-based. In the circuit switched environment, phones are connected directly to the switch (PBX) that will take care of all signalling, call establishment and voice exchange traffic. Calls are placed via the PBX and it is permanently in the voice path throughout the length of the call.

The softswitch takes part in call signalling: a station needs to ask it how to find the station it wants to communicate with, but once given the destination IP address, the two stations communicate directly, via IP, and the softswitch is no longer in the path. There is no permanent circuit between the two — the IP packets carrying voice payload are dynamically routed through the network just as any other IP traffic.

Comparative architectures
The PBX and IP telephony functional architectures are more or less identical: it’s the way they are connected together that is significantly different.

With a PBX, the call control, switching and user/trunk connections are consolidated, usually into one large chassis, often comprising of multiple racks of shelves. All communication between them is via a high-speed proprietary backplane. Redundancy is provided by means of multiple processors, line cards, PSUs, etc.

The softswitch architecture distributes these functions throughout the network. Call control is done via the softswitch — instead of multiple CPUs, redundancy is achieved by two or more separate softswitches. Switching and user connectivity is done by the existing LAN infrastructure — this is where you have to be careful, as the main prerequisite of IP telephony is a suitable LAN, and if you don’t have one, it’s going to add to the complexity and the cost — and trunk connections, via gateways, which can be either dedicated to the task or added to existing WAN hardware. These gateways could also connect your IP telephony system to your existing PBX, via one of its trunks, so you can bring in an IP telephony system without having to get rid of your existing phone system and migrate slowly from one to the other over time.

All of these are connected via standard LAN switches and cabling. We’ll look in detail at some of the configuration and tuning you’ll need to do in the next article. This does mean, however, that your two redundant softswitches can be deployed in two separate comms rooms at opposite ends of the building for physical separation in case of say a flood in one comms room.

Because they’re only involved in call set up and tear down (in fact, once a call is established, the softswitch could fail completely and the people on the call wouldn’t know about it), softswitches don’t require the massive processing power of PBXs designed for similar numbers of users/calls. Hence the reason most softswitches are server-based — a one rack unit (RU) high server that can handle several thousand users isn’t uncommon — which is handy if you’re tight for space. It also makes the relative cost significantly less — though always remember that that has to be offset against the cost of handsets.

It’s important you understand the differences between basic VoIP and IP telephony, and also some of the comparisons between PBX-based telephony and its IP alternative. Now we can start delving into the subject a bit deeper.