Practical VoIP Using VOCAL

System Architecture

When we started designing VOCAL, we had three primary goals in mind:

• Build a distributed architecture.

• Build a system that was scalable.

• Ensure no single point of failure.

A distributed architecture suited our aim to open source VOCAL as it provided components that developers from the community could build upon or build into their projects. Scaling the system meant assigning one type of server, which became known as the Marshal server, with the task of being a single point of contact for the subscribers and enabling duplicates of this server to be added to the system as the subscriber population grew. Our original idea was to achieve load balancing by assigning each additional Marshal server with a specific population of subscribers. Our original plan also called for a multihost system with redundancy for all call control servers to avoid a single point of failure.

Data Types

In its most basic form, a Voice over IP system is a set of data combined with the capacity to process calls. There is persistent data , such as the provisioning databases of users and server configurations, as well as the dynamic data that is potentially different for each call. The call control servers handle the following types of data:

Registration

When a user connects to her service provider, the system needs to add her address to a list of active endpoints.

Security

The system requires a perimeter to allow qualified users in and keep intruders out. Security is multifaceted and ...