Scope of this topic (VoIP Protocols)

1.3
Beyond VoIP Protocols is a companion reference to IP Telephony: Deploying Voice-over-IP Protocols (for more details see the last page of this topic). Both topics have been written with the goal of supporting those involved in the design and deployment of multimedia VoIP projects, and provide invaluable references for most of the required technology.
Our idea is that during the execution of a project, an engineer frequently needs to have a fairly accurate view of ‘the complete picture’ in order to avoid fundamental mistakes or misunderstandings, and he/she will usually only need a complete, exhaustive reference for a small fraction of the overall technology involved. In our professional lives, we all have spent an enormous amount of time compiling this ‘complete picture’, and we hope that these two topics will significantly reduce the time needed to assimilate the essential information and avoid many of those errors that can be made through only being aware of half the picture. That said, our intention was to provide a complete overview and not every detail on each subject. For instance, when introducing the audio-coding techniques in topic 2, we do provide some background on the ‘Z transform’ in order to give a feel for the power of this technique and become capable of understanding the
codec design diagrams. This background obviously does not replace a complete topic on the Z transform if your intention is to specialize in codec design and audio-processing algorithms. Similarly, if you do design an H.323, SIP, or MGCP device, you will need to actually read the parts of the standards that relate to your specific application, but Beyond VoIP Protocols will help you skip through 90% of the text, as you will already have enough background.
We structured our reference texts as two topics, because most projects involve two phases:
In phase 1, IP Telephony: Deploying Voice-over-IP Protocols can help you combine the various protocols for the target service, and complements the standards by discussing the most common issues that may result from incomplete protocol implementations, or architectures optimized for private networks which fail in a public environment. This first topic focuses on the functional aspects.
In phase 2, you will need to know how many users the application will serve, you will need to select or configure an IP distribution network, you will need to build a business model for the service. In so doing, you will have to answer questions like: ‘How many gateway ports do I need for 150,000 users?’ ‘Is 128 kbit/s upstream sufficient to support VoIP for my DSL users?’ ‘Can this DSLAM support 5,000 users with 10% placing a phone call and 20% watching IP-TV?’ ‘Do I need an IP DSLAM, or will two ATM VCs per DSL user be sufficient to provide support for VoIP and video?’ ‘What is the cost per user of this softswitch, which is sold per simultaneous call?’
Beyond VoIP Protocols will not directly answer 100% of these questions, because every deployment is a unique challenge, but it will provide some useful tools that can be used as building blocks to help formulate a complete deployment strategy. For instance, every question that relates a number of users to a number of ports, calls, or aggregate bandwidth ultimately boils down to an Erlang calculation. The answer to most quality of service-related questions is a properly designed set of differentiated service levels which must obey certain constraints (detailed in topic 4), such as similarity of aggregated streams, proper support by the layer 2 transport level, etc.