VoIP Architecture - A Closer Look

VoIP Architecture & Hardware

Introduction

Having established the process for voice communications across any data network, the next challenge is to provide sophisticated call control (call set up, tear down, etc.) capabilities.

The act of setting up, tearing down and routing calls requires intelligence that goes beyond the simple transmission issues we have covered so far. A lot of effort has been put into standardizing these rules so that IP voice systems can accomplish call processing in the same predictable manner web servers deliver information to web browsers anywhere in the world. For now, we will focus on the core VoIP architecture required.

Soft-switch

To set up a call, the system must act on signals from the calling phone. One way to accomplish this is with specialized call processing software within your VoIP architecture that tracks and manages call progress, and handles conversion between the addressing schemes used on a data network (IP addresses) and telephone numbers (defined in ITU E.164). There are different names for this function within the core VoIP architecture: call server, call processor, gatekeeper, media gateway controller, or softswitch.

Think of this device as an automated operator, handling all the tasks the switchboard operator used to handle.

Telephone	Call Server/Softswitch
I've gone off the hook	OK, here's your dialtone
Here's the phone number I dialed	OK, I'm routing this, here's some progress info
I've just put this caller on hold	OK, I'll remember that call for you; here's another dial tone
I've replaced the receiver	OK, I'll store the call detail record and terminate the call

These examples – which are typical of signals that might be sent between a telephone and a call server – show how a call server within your VoIP architecture can perform the same functions as those of a PBX. So if the call server or softswitch can manage call set up, call routing and call tear down - does that mean we now have a fully functional IP-based alternative to the PBX?

IP Gateway

Not quite. We are still missing an important interface with the legacy PBX within our VoIP architecture. Specifically, we need a gateway between the IP world and the legacy circuit-switched world. The gateway accomplishes this with three components:

1. Trunk or line interface on one side

2. VoIP transmission capability on the other side

3. In between, the gateway must have the necessary logic to convert between the two media formats and ask the call server for help setting up the call

In practice, it often makes sense to combine the functionality of call processing and gateway into single VoIP architecture elements – but for the purposes of this discussion, they are being treated as separate components.

IP Phones

While a complete VoIP architecture with a softswitch and specialized media gateways can potentially support existing, traditional analog handsets, in practice, most implementations only support IP phones. Phones can either be hardware devices that plug into the Ethernet network (and look just like a normal legacy telephone) or soft- phones that run on the user’s PC.

IP phones actually provide the functionality of a single user gateway, converting the analog speech pattern into digitized voice packets which are then sent over the VoIP architecture and out into the IP network. Here are some of the characteristics you should consider when selecting IP phones:

1. Which signaling standard is used?

2. Does the phone provide a second Ethernet port so that a PC can use the same uplink as the phone (offering savings on cabling cost)?

3. Does the phone support Power over Ethernet so it would work without interruption during a power outage?

4. Does the phone provide a mechanism to classify traffic so that voice can be prioritized through the network?

5. Does the phone provide easy access to advanced features through an intuitive interface?

6. Is the phone easy to install and configure?

7. Does the phone deliver good sound quality?

For further detailed information on the different applications of a VoIP phone, click here.

Specialist Phones

As a mature market, differentiation was critical for the legacy PBX vendors; and over time the phones or handsets became the focal point for vendor competition. Specialist phones were developed:

• Operator consoles

• Administrative assistants

• Key systems

• Conference phones

• And a range of phones for the different hierarchical levels within an organization.

Today’s IP phones have software hooks for customization and are far more flexible than their legacy counter-parts. Even the simplest of telephone designs can be extended with applications that reside on the user’s PC, forming a core element of the VoIP architecture.

Today’s IP phone provides an intuitive interface with access to application-rich features and is also capable of leveraging recent improvements in sound quality to provide a better experience for users and the people with whom they communicate.

Summary

Using these components within the VoIP architecture,:

1. Softswitch or media gateway controller

2. IP Gateways

3. IP Phones

we have the necessary components to build a complete VoIP system. Let’s review how these components work within the VoIP architecture during a call:

• A phone transmits state changes (off hook, on hook, etc.) to the call server or softswitch.

• The softswitch sets up calls, finds routes, keeps track of everyone’s state.

• The softswitch automatically converts between telephone numbers and IP addresses.

• Once a call route is established, the softswitch gets out of the way so that the path for the voice stream is independent of the softswitch. (This is important because it prevents delay from being introduced into the conversation.)

• If the call is leaving the IP network and being routed to the PSTN or a legacy PBX, a gateway within our VoIP architecture converts the IP packets back into the appropriate media stream for the trunk.

• If the call is being sent to another IP device, the call may be managed by multiple softswitches. But eventually, the VoIP packets reach the called party’s phone and are converted back into voice.

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