The invention of the telephone set in 1876 heralded the beginning of our ability to send voice conversations over long distances. It provided a way to convert the mechanical energy of sound to electrical energy for transmission purposes, and then convert it back to sound again at the far end. This remains the core function of the telephone to this very day. But much has changed since 1876.
Evolving Telephone Transmission Models
The original telephone was an analog device, and the vast majority of telephones remain analog to this day. This means that the conversion from sound to an electrical signal produces an electrical signal that is directly modeled on the pressure waves produced by the human voice. Since those pressure waves continuously change in amplitude, the electrical wave also continuously changes in amplitude, hence its analog nature.
Rendering voice as an electrical signal had several benefits. First, it can travel down a wire at the nearly the speed of light (approximately 186,000 miles per second). Sound travels at the speed of sound (approximately .2 miles per second, or 767 mile per hour). Indeed, if you shouted into a telephone to a person on the other side of the planet, the person on the other side of the planet (about 12,500 miles away) would hear you in about 67 milliseconds (.067 seconds), and the person standing at the other end of a standard football field (300 feet) would hear you about 284 milliseconds (over a quarter of a second). To hear you at the same time as the person on the other side of the globe, they would have to be about 76 feet away.
The problem with analog voice transmission is noise. As a signal travels down a wire, it loses power, something we call attenuation. To traverse a distance like 12,500 miles, we would have to insert amplifiers in the circuit to re-power the signal. Unfortunately, as the signal moves down the cable, it not only loses power, it also picks up noise. Noise is an unavoidable reality in any circuit, and the amplifier cannot distinguish between the noise present in the circuit, and the voice signal itself. It simply re-powers everything. Extend the circuit a sufficient distance, and the cumulative affects of noise can become serious.
In the 1960s, the public switched telephone network (PSTN) began a relentless journey towards digitization. Digitizing voice had several benefits, including eliminating the cumulative effects of noise, using less bandwidth for transmission, reducing the cost of the technology, and rendering voice in a format compatible with data, making it possible to build networks to carry both. Although most of this work was done in the core of the PSTN, some digital telephones also emerged. Unlike their analog counterparts, these phones had two functions: rendering mechanical speech to an electronic representation, and then digitizing that representation for transmission as a series of bits. Both functions needed to be reversed before the person at the other end could hear the conversation.
The latest shift for telephones, with respect to their interface to the network, has been the emergence of IP telephones. These phones not only convert mechanical sound to an electronic representation and digitize it, they then insert those digits into IP packets and send them across a packet network. At the far end, all three functions need to be reversed before sound can again be heard. Packetizing voice made has made it possible to build packet networks that are truly multimedia, or converged.
Early telephones provided very little in the way of signaling capability. The human person simply picked up the phone and either turned a crank or (later) depressed the hook on which the handset hung. Either technique alerted an operator in a central location that a call needed to be placed. The operator connected their headset into the line for that customer, and all signaling was done verbally between the operator and the subscriber.
The evolution to direct dialing required the telephone to have more capability. It was fitted with a dialing device, and several tones were created to signal to the subscriber the progress of the call (i.e., busy signal, dial tone, etc.). The dialing device produced audible clicks on the line, which the switching equipment could count to determine the desired phone number, a process known as dial pulse.
Later, the dial was replaced with push buttons and the pulses with distinct audible tones, known as Dual Tone Multi-Frequency (DTMF). That remains the primary model today. Even cellular handsets produce these tones, although they do not actually signal with them; they are largely there because the user has become accustomed to them and gets concerned when they are not present.
Digital cellular telephones, ISDN telephones, and IP telephones do not actually send the pulses down the circuit to the switching equipment. Instead, these telephones create a small packet that contains information about the desired telephone call. In both cellular telephony and ISDN, this packet is sent down a channel that extends between the telephone and the switching equipment and is dedicated to his function. The actual voice signal is carried in a separate channel. IP telephones also create packets to carry their signaling information. Most of these use a signaling protocol known as the Session Initiation Protocol (SIP), and send these packets over the same IP network that will shortly carry the voice conversation.
Types of Telephone
The number of different types of telephone today is immense. They differ based on whether they are analog or digital. If digital, whether they are circuit-oriented or packet-oriented. They differ in how they interface to the PSTN. Some have a wired connection, others are wireless (e.g., cellular or Wi-Fi telephones). Some have separate handsets, some are one integrated piece of equipment. For those with a handset, some have a wire connecting the handset to the telephone, and some use wireless technology for that connection. Some telephones are simple, unintelligent devices. Some are actually small computers and telephony is only one of the many things they can do. Some telephones can connect to multiple lines simultaneously, some to only one. Some telephones have an integrated voice messaging system, others do not. The variety available in the modern telephone is truly mind boggling.
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