TrixBox Made Easy: A step-by-step guide to installing and running your home and office VoIP system

The acronym PSTN stands for Public Switched Telephone Network. PSTN is the network that traditional phone systems used and was generally controlled by the telecommunication companies. This is the network our calls are travelling over when we pick up our handset and dial a number. This network spans the world and there are many different interfaces to it:

The trouble with PSTN is that it's very static and in most countries it is strictly controlled by the telecommunication companies. If a business wants to make a lot of internal calls using the PSTN, it is by no means a cheap way to communicate. ISDN/T1/E1 are most commonly found at the external interface of a company's communication network, with all the internal communications going through internal lines that are controlled by an internal telephone system.

A PBX is an acronym for a Private Branch eXchange, which provides for the internal telephone system. Telephone exchanges were initially under the control of the telephone providers, such as AT&T in the US or British Telecom in the UK. These companies handled all line provisioning and call routing between the businesses and the public. Initially, the routing of calls was done by a team of operators (usually female) sitting in the offices of the telephone companies and routing calls by plugging and unplugging cables to connect one caller to another. Eventually, as the reliance and the demands of this service grew, technology evolved to the point where we had automatic systems managing these calls.

As the modern telephone networks began to take shape, private companies saw a greater reliance on telephone communication. Many decided to implement their own services so that they could handle calls internal to the organization. Usually, the equipment was leased or bought from the telephone companies mentioned previously, so they were quite happy to help with these services. These companies also got to charge for the lines and calls connecting the company externally, and so they could profit from this too. As we saw in the previous section, the more expensive digital lines were now being used only as a means of communicating outside the building, rather than using externally provided lines for all communication.

At this point, it became obvious that there was a need for these companies to install their own telephone equipment to route internal calls and, in some cases, to make sure calls going out or coming into the company went via the correct routes. For example, you don't want Alice in accounting calling Bob in HR through a line that leaves the company and crosses continents if they sit within the same building. Therefore, there is a requirement for a PBX to effectively manage calls and ensure that they go via the most cost effective and reliable routes in order to keep the company communicating internally between departments and employees, and externally with customers and suppliers.

In its basic form, a PBX is the interface between the public telephone network and the private network within the company. Since most companies need fewer phones lines than the number of employees they have, they can get away by having a few outgoing lines but many internal extensions so that employees can converse internally. This costs little more than the maintenance of the PBX and internal cabling, and there are no line rentals or other call charges being paid to the telecommunications provider. The PBX then handles all of the routing in and out of the company using the lines effectively. The PBX also handles calls within the company so that a call from one internal phone to another does not have to go out onto the phone circuits and back in.

As PBXs became more common, businesses and their employees required more features and functionality such as voicemail, call parking, call transfers, music on-hold, IVR menus, least-cost routing, and an Automatic Call Distributor (ACD) in order to provide for calling groups. With the increase in demand for communications in all aspects of a business, the features required in a phone system become more complex and more expensive. If modern companies had to rely on the telecommunications provider for all these features, the cost of communication could become prohibitively high.

The Traditional PBX System

It is not hard to spot a traditional PBX system. It is usually a large box full of mechanical switches and relays mounted on a wall in 'the phone room'. When a company's requirement changes, they generally contact their PBX provider who will charge varying rates to make hardware and configuration changes to fit the new requirements. With PBXs being very complicated and each differing from the others greatly, it can take a considerable level of training and experience to provide the support for a busy PBX system. This leads to most PBX customers relying on their PBX suppliers for, often expensive, support. So while by bringing the communications internally businesses could benefit from savings on line rentals, they still often had a reliance on their providers for support. Often, the companies selling and supporting the PBXs were the same telecommunication companies providing the external lines.

With a traditional PBX system we would also almost always purchase our phone system from the same manufacturer as the PBX system, usually with very few options to choose from when it comes to contract options and hardware such as telephone handsets or headsets. Adding features like voicemail can usually be an expensive add-on to the base system, sometimes requiring an entirely new piece of equipment! A traditional PBX system has the following structure:

Although some legacy PBX systems now have options for network access and VoIP functionality, these options are often very expensive upgrades and they generally lack the features and configuration options in the newer VoIP systems.

Hybrid PBX System

A hybrid PBX system combines the features of a traditional PBX system with VoIP functionality. In some cases, the VoIP functionality may just be the way the PBX communicates with the phones. Some other VoIP functionalities may include the ability to have remote extensions or Soft Phones, and the ability to use Internet Telephone Service Providers (ITSPs) and not just the traditional public telephone network. The main added benefit is the combined functionality, as we can keep all our existing lines and numbers and add in VoIP for substantial savings where possible.

The Asterisk PBX system is a full hybrid system combining numerous types of connections to the public telephone network as well as VoIP functionality including:

• Use of industry-standard SIP-compliant phones

• Remote extensions using either SIP-compliant phones, or Soft Phones

• Support for IAX (Inter-Asterisk eXchange)

• Bridging remote Asterisk systems together to act as a single system

Following is an example of a hybrid PBX system:

We have covered, in brief, how a traditional PBX system could lack some of the features of a Voice over Internet Protocol system and discovered some of the basics of the PSTN. We can now take a look at VoIP in a little bit more detail to get an idea of what the benefits are.

Firstly, it's important to realize that VoIP doesn't entirely replace the PSTN (although it could). VoIP is yet another, cheaper, and easier way to connect to the PSTN. You can make and receive calls that are initiated and terminated entirely across VoIP and you can call a standard PSTN number from VoIP and vice-versa, as long as your ITSP (Internet Telephony Service Provider) supports it or if you link your VoIP system to the PSTN yourself. Both of these are options to consider with TrixBox.

A VoIP system can use a variety of protocols and we will cover each of those protocols relevant to TrixBox as we come across them. VoIP is a catch-all term for these protocols and refers to transferring voice data over the Internet.

As the Internet grew and became a more flexible system than the PSTN, it became apparent that it was possible and, in many cases, preferable to use the Internet for carrying voice as well as data. There were a few limitations that had to be overcome before this could be feasible. For example, data connections can tolerate some latency in communication but latency in voice can be very annoying as it leads to gaps in conversation and constant repetition. Watching a news broadcast from a reporter using a satellite phone is a very good example of how frustrating and error prone this form of communication can be. As Internet connection latency decreased and speeds increased, voice communication has become more viable.

There is a tendency to think of VoIP as a new technology. However, it is almost two decades old and has only recently become so popular because there are now a few good pieces of software that use this technology. There are also many companies investing in VoIP, since the data lines that provide Internet services are now at a level where they are usually reliable enough to be used for voice communication. Customers and employees expect these data lines to be low-latency, clear, and always available. While many Internet services still have problems, the situation is certainly much better than it was in the late 80s and early 90s when VoIP was first touted as the killer technology. It wasn't quite there then, but is certainly getting there now.

In this chapter, we have covered the basic background to the traditional telephone network and have introduced VoIP as a cost-saving and flexible system for managing our telephone system. We've introduced some of the terminology we will be using throughout the book. There is, however, a more detailed list in Appendix A. We should have an understanding of why VoIP is growing in popularity and why it may be a sensible choice for our needs. The chapter also provided us with a good understanding of what a PBX is and what makes a VoIP PBX different from a traditional PBX system.