PBX systems offer enterprises many advantages over traditional PSTN-based phone systems. But PBXs can come with proprietary technology that limits how the system is integrated across the business. For some enterprises, an open source phone system can provide more flexibility and customization.
Asterisk is a Linux-based open source platform that enables Linux administrators, developers and power users to set up a private voice over IP (VoIP) system in an enterprise. Asterisk is more customizable than other telephony platforms, with features like voicemail and call queues built into the software. An Asterisk-based phone system can also integrate with business technology in ways that more closed or proprietary PBX systems cannot.
Asterisk, however, is complex and can be daunting for new users. In the O'Reilly Media book, Asterisk: The Definitive Guide: Open Source Telephony for the Enterprise, authors Jim Van Meggelen, Russell Bryant and Leif Madsen offer a complete guide to building an open source telephony platform in Asterisk, from building a dial plan to VoIP system security.
Below is an excerpt from the book: Chapter 5, "User Device Configuration." This excerpt provides an overview of endpoints, such as desk phones and softphones, and their advantages and disadvantages in an Asterisk-based phone system.
Hardphones, Softphones, and ATAs
There are three types of endpoints you would typically provide your users to serve as a telephone set. They are popularly referred to as hardphones (or desk phones), softphones, and Analog Terminal Adaptors (ATAs).
A hardphone is a physical device -- an office telephone. It has a handset, numbered buttons, a screen of some sort, and so on. It connects directly to the network and is probably what people are referring to when they talk about a VoIP telephone (or a SIP telephone). It's normally going to sit on your desk, but it could be mounted on a wall, in an elevator, at a side-table, or in a box by the side of the road.
A softphone is a software application that runs on a laptop, desktop, smartphone, or other computing device. The audio must pass through the device's sound system, so you normally need a headset that will work well with telephony applications. Softphone applications have become popular with smartphones because they allow you to connect to telephone networks other than just the cellular network (for example, you can register as an extension on your corporate PBX). The interface of the softphone is often styled to look like a physical telephone, but this is not necessary. WebRTC will allow all sorts of additional capabilities in this area, as it essentially allows a softphone to simply be part of a session within a browser. To the PBX, the softphone looks and behaves exactly the same as a hardphone.
An ATA is designed to allow traditional analog telephones (and other analog devices, such as fax machines, cordless phones, paging amplifiers, and such) to connect to a SIP network, and will typically be a sandwich-sized box that contains an RJ-11 connector for the phone (commonly referred to as an FXS port), an RJ-45 connector for the network, and a power connector. Some ATAs may support more than one phone. Other ATAs may have advanced features in them such as a firewall or an FXO port (an analog port that can connect to a PSTN circuit). From the perspective of the PBX, the ATA looks exactly like a SIP telephone.
Hardphones have the advantage that the handsets have good acoustic properties for voice communications. Any decent-quality telephone is engineered to pick up the frequencies of the human voice, filter out unwanted background noise, and normalize the resulting waveform. People have been using telephones for as long as the telephone network has existed, and we tend to like what is familiar, so having a device that communicates with Asterisk using a familiar interface will be attractive to many users. Also, a hardphone does not require your computer to be running all the time.
Disadvantages to hardphones include the fact that they are not easily portable, and they are expensive relative to the many quality softphones on the market today that are available for free. Also, the extra clutter on your desk may not be desirable if you have limited work space. If you move around a lot and are not generally at the same location, a hardphone is not likely to suit your needs (although, one at each location you regularly visit might be a valid solution).
Softphones solve the portability issue by being installed on a device that is likely already moving with you, such as your laptop or smartphone. Also, their minimal cost (typically free, or around $30 for a fully featured one) is attractive. Since many softphones are free, it is likely that the first telephone set you connect to Asterisk will be a softphone. Also, because softphones are just software, they are easy to install and upgrade, and they commonly have other features that utilize other peripherals, like a webcam for video calling, or perhaps an ability to load files from your desktop for faxing. Another potentially huge advantage of a softphone is that it is often possible to integrate them with other applications running on the device.
Some of the disadvantages of softphones are the not-always-on nature of the devices, the necessity to put on a headset each time you take a call, and the fact that many PCs will at random times during the day choose to do something other than what the user wants them to do, which might cause the softphone to stop working while some background task hogs the CPU. In a mobile device, the softphone can consume resources, affecting battery life, performance, and operating expense.
ATAs have the advantage of allowing you to connect analog devices to your SIP network, such as cordless phones (which are still superior in many cases to more advanced types of wireless phones, and far less expensive), paging amplifiers, ringers, and antique telephones. ATAs can also sometimes be used to connect to old wiring, where a network connection might not function correctly, or to outbuildings (such as a gatehouse), where a standard ethernet connection would never reach.
The main disadvantage of an ATA is that you will not get the same features through an analog line as you would from a SIP telephone. This is technology that is over a century old.
With Asterisk, we don't necessarily need to make the choice between having a softphone, a hardphone, or an ATA; it's entirely possible and quite common to have a single extension number that rings multiple devices at the same time, such as a desk phone, the softphone on a laptop, a cell phone, and perhaps a strobe light in the back of the factory (where there is too much noise for a ringer to be heard).
More than any other endpoint, the softphone is set to evolve into something far more encompassing than a simple telephone application. The emergence of WebRTC may finally deliver that which has been predicted for many long years: the integration of real-time voice into computing (specifically web-based) applications. There are of course many ways to achieve this already, but WebRTC's advantage is that it is an open standard, built right into all browsers with no plug-ins required. The softphone is dead; long live the softphone.
We still like a desk phone for regular telephone calls, though.