Digital Protocols - ISDN

Once a signal is digitized, the means and rules of transmission and “de-digitizing” for human understanding come into play.  There are many unique combinations of these means and rules, and each combination forms a specific protocol.

Digitized information is malleable and flexible.  When engineers design protocols, it’s done so all the switches and machines between the caller and the receiver work the same way – they use the same protocol.  The first problem at the beginning of worldwide digital telephony was that the analog technology it was to replace was universal.  The languages, preferences, and goals of regulating bodies all over the world were not.  Protocols compatible with analog plants still in place, while able to exploit digital advantages had to develop.

Such a challenge is more economic and political than technical.  As such, it was a fitting issue for the United Nations to address.  The International Digital Service Network (ISDN) is the result of that effort.  As the name implies, ISDN allows developing nations to digitize their analog plant-in-place when they can afford it, or feel otherwise compelled.  Simultaneously, ISDN allows more technologically invested users to employ the tools of the Digital Age. 

Further on we’ll discuss other points of comparison between ISDN and voice-on-the-Internet schemes.  Now, one point needs emphasis:  ISDN systems are self-powered.  Every line into every phone carries 8 volts of ringing voltage with whatever else it bears.  Back in the day, many customers had phones before they had electrical power.

Digital Protocols – VoIP (Voice over Internet Protocol)

Voice on the telephone is the lowest common denominator for ISDN, and it never needs more than 2.5 kHz of bandwidth. The Internet is the first communications technology to have so much impact that it can justify supplementing (or sidestepping) this standard around the world.

Because ISDN protocols try to provide all to all users, their designs include many compromises.  The most fundamental compromise is this low bandwidth, whose typical symptom for the user is lack of speed.  If you are reading this article via a dial-up telephone (ISDN-compatible) modem, you know exactly what we’re talking about.

The collected wisdom and BS of humanity is storable and retrievable with computers, and that is much stuff.  Voice-grade lines are sufficient for one person to speak with another.  For one computer to communicate with another, they are pathetic.  Video, music, CNC and CAD instructions, three-dimensional layouts and other digital applications we invent include vast amounts of information.  ISDN was first to carry it only because it was already there (and still is), but its capacity to move information among PCs (let alone DataGator IX servers and mainframes) is inadequate.

The Internet developed from the same motives as ISDN: The need for a universal protocol. The differences between them all arise from the differences between their primary customers, either humans or computers.  For computers, Voice over Internet Protocol (VoIP) is designed to share data, not voice.  At best, carriage of voice information on the Internet is vestigial.  As data transmission does not do well on voice lines, delivering an audible signal compatible with human hearing is not the priority of the Internet, and telephony suffers for it.

As mentioned earlier, VoIP-based telephony requires the power that drives the computer, whereas a traditional telephone is self-powered.  Also, it requires that every station that offers telephony have computer access and access to broadband (high-speed) Internet connection.  Notable time delays can result from Internet use of satellite, fiber and microwave to deliver signal – even from the house next door.  Finally, there is the issue of sound quality.

Imagine a piece of gravel rattling in a beer can.  One of the adjectives that might apply to that sound is “sloppy.”  With VoIP, this shows as dropped sections of conversation, jittery-sounding voices, erratic shifts of tone and timbre, and sometimes even the sound of the caller’s voice speaking pure babble.  These are symptomatic of the differences between switching techniques employed by ISDN and VoIP technologies.  ISDN uses circuit switching, described elsewhere on this site.  VoIP ships and switches its digital content in individual “packets,” but these packets intermix intermixed with other packets, often from different sources found throughout the reach of the Internet.

Because each packet is addressed to each unique computer, the computer selects only those packets addressed to it all those that flow by.  The computer assembles them in the same way that it does all the rest of the information it handles.  Once information becomes resident on the computer it is plucked from different sectors of the hard drive, and assembled according to the rules of the operating software.  With VoIP, error correction stops at the eardrum, but error detection continues in the brain.  If information is improperly assembled, these symptoms appear.  Such defects can be overcome by VoIP providers’ properly designed software and interface, and with voice-specific buffering.  Still it’s ironic that so much extra digitization and bandwidth has to be dedicated to successful delivery of a 2.5 kHz signal.