The Anatomy of a Voice Call | Voice Mobility Technologies

Placing a call may seem very simple to us modern users of voice mobility. Remove the phone from your pocket, purse, automobile cup holder, or wherever you may keep your phone, find the name of the person you wish to call, and press the Send or Yes button to dial the caller. However, underneath that simple experience lies a vast wealth of technology, with electronic gears turning to produce a sound that not only sounds like a human, but is actually recognizable as the person on the other end of the phone, even if you are driving at 65 miles an hour down the interstate.
The voice call is made of a number of moving parts. Figure 1 illustrates the basic example.

Figure 1: The Anatomy of a Voice Call
Each handset contains all of the technology necessary to place a phone call. The phone must have a microphone to capture the voice call and a speaker to play out the audio from the other party. But beyond that, the phone must have a way of converting the analog voice information into encoded digital signals, using audio codecs with possible compression to ensure that high-fidelity voice quality can be carried over lower-bandwidth links. The phone must also have one or more wireless radios, complete with technology stacks and engines that allow the phone to connect to each of the networks, or even to hand off between the networks. These stacks must fully understand and adequately implement the necessary protocols to provide high voice quality over the network, something easy for voice-oriented technologies but much harder for data-oriented packet-switched networks. Finally, the phones must also support a rich telephony application. This application needs to look like, act like, and have the features of a common telephone, and yet, for voice mobility, must now often store the addresses and phone numbers of crucial contact information, provide access into the enterprise directories, allow for dialing as if in the office, and be manageable by network administrators. Many of these applications are not necessarily native to the phone itself, but created by third-party vendors to tie multiple telephone systems together to appear as one. The telephony applications must support all of the necessary protocols to create a phone call, set up the audio channels, and deal with advanced features such as voicemail and three-way calling.
The call itself is composed of two separate flows of information. The more obvious flow is what is known as the bearer channel. The bearer channel, a term borrowed fromIntegrated Services Digital Network (ISDN) technology, carries the voice of the callers. Looking back to analog telephones, there is only one channel, an analog audio channel that provides both the callers' voices and the necessary tones to tell the caller what to do. Dial tones, busy signals, and other beeps and clicks are used to communicate the number being dialed and the state of the call. In voice mobility networks, however, the audio is kept separate from the communication that is used to set up the call and keep it running. This is known as out-of-band signaling. Voice can be encoded in a variety of ways, and the bearer channel is more likely to be carried in a number of digital formats. The communication about what the phone call is for, how it is being set up, and what state it is in is encoded in the signaling channel. The signaling protocol is used to dial out to set up the call, provide feedback as to whether the other phone is ringing or busy, and to finally set up the bearer channel when the call goes through.
The signaling information needs to go somewhere, and that somewhere is to a telephone switch. For private telephone networks, where the handsets are maintained by the enterprise or organization who owns the network, these switches are also owned by the enterprise and are called private branch exchanges (PBX). The term exchange and switchare interchangeable, and these devices are the electronic equivalent of the old manual switchboard, creating connections between two calls. The PBX (or PBXs) for analog systems are proper switches, with internal analog phone lines that run from the handset directly into the PBX. But for Internet Protocol (IP)-based telephone networks, the PBX is more of the central call manager, providing for the list of extensions—users, basically, as well as the telephone numbers each user has—and the dialing plan—the notion of how to route calls based on the phone numbers dialed. PBXs also implement all of the advanced features of voice mobility calling, including voicemail, call forwarding, and multiple ring.
The PBXs define the internal voice network, but calls need to be able to be routed back onto the real, public telephone network, to allow calls in and out from the outside world. IP PBXs rely on a module or a separate device, known as a media gateway, which has the responsibility of converting the signaling and bearer traffic into a format and onto a wire that is provided by the telephone company. Media gateways thus bridge the inside world to the outside.
Let's get a better look at each component.

The People and Their Devices: Phones

Phones come in a number of shapes and sizes. Some have the latest features for consumers, such as music playing, video recording and camera functions, global positioning, and touch screens with tactile feedback. Some are designed for enterprise users, and have large screens, with strong email access and integration, spreadsheet and document editing capabilities, and large storage for use as a computer away from the laptop. Others are simple and rugged, meant for use in physically demanding environments where the phones need to withstand a beating. Some have nearly no buttons at all, and are even designed for nearly hands-free operation.
All in all, each phone may seem wildly different from the next. But, underneath, they are made of the same stuff: a microphone to pick up voice; a speaker to play it back; maybe another speaker for speakerphone operation or to play ringtones, so as not to deafen the user who happens to have accidentally hung up on a conversation and is being called back; some way to dial; some way to see or hear who is calling; a battery for mobility; and one or more radios to connect back to the network.
Within those components, the description becomes even more common. There is a digital sampler and a codec engine, to convert voice into digital data and back. There is a CPU somewhere, orchestrating everything, along with memory and some nonvolatile storage. The radios have antennas, all folded neatly into the small device. Voice operates the same on every one of these devices, and users will become just as irritated by poor audio quality as they will be pleased by good quality. The best voice mobility network is the one that users forget is even there, or is anything unique or special. They get out of the way, so to speak, and let the voice mobility user do her work.

The Separate Channels: Signaling and Bearer

In the analog telephone days, there was only one line per extension. This analog line has to do everything. It carries the voice, but it also has to ring the phone, send busy or dial tones, and handle the beeps corresponding to each button being pressed.
With digital phone calling, the signaling and bearer channels are separated. All the beeping, humming, and chirping that is meant to tell the caller what is going on with the call is removed from the audio stream and sent separately in the signaling channel. The bearer channel holds the human voice, and nothing else. The advantage of this having been done for IP-based voice mobility networks is that it allows the call setup part of the network to operate differently from the voice encoding and decoding part. IP PBXs may be configured to never carry a single voice packet, because their job is simply to figure out how to route calls—much like how Internet DNS servers are so critical in figuring out what"" refers to without carrying a byte of Google's web traffic. Media gateways can be created that specialize in conversion of media formats, and then only need to implement the basic signaling protocols, and do not need to be concerned with advanced PBX features.

Dialing Plans and Digits: The Difference between Five- and Ten-Digit Dialing

For all the advancements in the digital age, with email accounts, instant-messaging handles, avatars, and what not, phones still work with the concept of dialing a series of numbers. But not all numbers are created alike. In the telephone network, someone needed to determine what all of the digits mean. This meaning is known as the dialing plan.
Think of the dialing plan as a series of simple rules that tell the phone system when you are done dialing and where the numbers are to go. In the United States, the dialing plan for our public telephone lines specifies that every phone number is seven digits long. Type an extra digit, and the phone ignores it. However, some calls are not in the same area code. This area code concept is a part of the dialing plan. To get to other blocks of phone numbers, outside of the block of numbers that you can dial the most conveniently, you need to dial a "1", followed by the area code, followed by the seven-digit number. Other calls require even more digits. An international call requires dialing "011" before the country code, and then whatever digits are necessary to place a call in that country. And the first "0" must be followed by the first "1" quickly enough to prevent the phone from thinking you are done, and connecting you to the operator. Finally, some calls, like "411", require only three digits. In the office, things can be a bit more complicated. Many people may have four-digit extensions. Only those four need to be dialed. Some companies may use longer extensions, however, with access codes in front of them. Finally, to dial out to the public network, you may need to dial a "9". But not just any "9" will do! The "9" must be followed by a pause, to let the system present a new, outside-world dial tone, where the rest of the digits can be placed.
The dialing plan defines all of this behavior. Every PBX system provides an incredible amount of depth into how these dialing plans can be created, and whether some of the digits are just part of the extension number and others are meant to shift the call over to another PBX somewhere else (like the "9" did to dial outside, but even the "1" for long distance does the same thing) to figure out the meaning.
A lot is mentioned about having four- or five-digit dialing within voice mobility networks. There is an added convenience, and it is true that users of a PBX may not remember the outside number corresponding to an extension, especially if the rest of the number is different for different extensions. (Picture a system in which the 6xxx extensions are reached from outside the office by dialing 487-6xxx, but the 7xxx extensions are reached by dialing 935-7XXX.)

Why PBXs: PBX Features

PBXs serve as a lot more than just the anchor or administrative server of the phone network. They also provide a long list of features that people have come to expect from enterprise phone lines—features that they probably do not have at home, even with today's rich cellphone feature sets.
PBX vendors compete with each other by making the feature set as useful and fancy as possible. There are a number of important PBX features. Some are listed here:
  • Dial-by-name directory: A computer voice system that allows callers to find out an extension and dial it by interactively pressing a few buttons, usually the first portion of the name. This directory is driven by the autoattendant feature.
  • Autoattendant: The automated telephone operator, represented by a series of recorded prompts. Autoattendants allow users to access and even manage their account on the PBX simply by calling in. Autoattendants are also the anchor for the interactive voice response systems that outside callers might get into a call center line, whose PBX is advanced enough to guide callers through the menu of options.
  • Call forwarding: The user can set the line up to forward to another extension, or an outside line, rather than ring the phone. This is useful for when the user is out of the office. Call forwarding is also done automatically when the user does not answer the phone after a certain number of rings.
  • Find-me/Follow-me/Hunting: These three names for broadly the same feature allow the user to have a number of different alternative phone numbers. When the user does not answer his or her primary line after a certain number of rings, the system hunts down the list, forwarding the call to the next number until it gives up.
  • Simultaneous ring: Instead of hunting through a series of numbers, the PBX can call out to each of them at once. The first one to answer gets the incoming call. This is useful when the user has a desk phone and a mobile phone, or multiple other phones, and might be at any of them.
  • Call transferring: Allows the user to send the answered phone call to another phone.
  • Call park: Allows the call that is already in place to be placed on hold and transferred to another extension, where the user can remove the call from hold. Unlike call transferring, which would ring the other phone and cause the user to have to run until voicemail picks up, call parking allows the user to take more time.
  • Call pickup: Allows a user to answer another user's phone when it is ringing by entering their extension number. It can also be used in the same sense as simultaneous ringing can, in that an incoming call to a department might ring multiple extensions, and the first to pick it up wins.
  • Do-not-disturb: Rejects the call before it rings the phone, usually sending it to voicemail or back to whomever transferred the call. Similarly, a user can often use this feature manually on an incoming call by pressing a button on the phone to terminate the incoming call and bounce it back.
  • Voicemail: Answers the phone and records a message.
  • Hold music: PBXs provide a series of options and selections for the caller to be subjected to while on hold. For some unknown reason, even advanced PBXs often play a short, few-second-long segment of supposedly relaxing music in an endless loop. Administrators can, however, often replace the hold music with a prerecorded selection. This is most useful for queuing of calls in call centers, where the hold music might be interspersed with the autoattendant informing the caller of the expected wait time.
  • Time-based policies: PBXs can change their configuration based on the time of day, routing calls to the autoattendant instead of the corporate operator, for example, after hours.
  • Conference calling: PBXs can join together a limited number of lines for ad hoc conferences, such as three-way calling, for which multiple parties are needed to be on at once.
As you can see, PBXs are designed to have a broad series of functions. Thankfully, PBX features are generally independent of voice mobility networking, in the sense that every PBX has a good number of features, and these features will generally work on IP PBXs, no matter what IP-based protocol the user is using. On the other hand, fixed-mobile convergence (FMC) solutions and PBXs do interact


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