Fundamentals of Bandwidth - An Introductory Tutorial

By Tom Cross

Technology Columnist

 

Since most customers and channel partners need better tools to explain bandwidth to customers and themselves, we offer this tutorial.

 

The animated “pictutorial” is available at http://www.techtionary.com and http://blog.tmcnet.com/cross-talk/

 

This is a tutorial on the fundamentals of bandwidth from transmission of sound waves and analog to digital, advanced optical fiber optics and the concept of data “packet” transmission. Voice is a series of waves which your ear analyzes in terms high and low tones such as a high-pitched soprano voice or low-pitched baritone. These waves of highs and lows are called a sine wave. The top of the sine wave is a 1 and the bottom a 0. Analog is a varying sine 1/0 wave signals of different frequencies (FM) and/or amplitude (AM). Analog is used in residential telephone service called POTS (Plain Old Telephone Service) and small business telephone service called 1FB or business lines and trunks. While analog transmission may have many values, digital transmission consists only of 1s and 0s or bits. If you prefer, digital is like a light switch — on or off. Bandwidth is refers to the number of bits transmitted over one time interval or second.

 

Bandwidth determines how much information (voice, data, video) of any kind, can be sent to another location at any given time and how fast that information can get there. There are many types of digital transmission. One kind of digital transmission is called TDM (Time Division Multiplexing) where transmission is divided into time-divided fixed length channels. The PSTN (Public Switched Telephone Network) is a network of TDM circuits: T-1 or DS-1 is 1,544,000 bits per second of 24 channels of 56,000 for voice and data and 8,000 for signaling. ISDN-PRI (Primary Rate Interface) is also 1,544,000 bits per second but organized 23 B (News - Alert)-Channels of 64,000 BPS for voice/data and 1 D-Channel of 64,000 BPS for signaling. T-1-ISDN-PRI is “provisioned” (installed) using two-pair twisted-copper wiring (referred to as a four-wire circuit). However, optical fiber is used with optical pulses and multiple-colored “dense waves” to 10–40 gigabits per second and increasing speeds. While bandwidth is a “stream” of data, groups of 1s and 0s are organized into groups of eight called a byte or octet. Bytes are grouped into packages called packets to perform tasks. There are hundreds of types of packets. Many packets become accepted or standardized by different industries and then referred to as a protocol. For example, IP-Internet Protocol is used to send email, data or voice.

 

In order to take analog audio to digital a CODEC is required. The concept of a CODEC is an important part of VoIP/SIP. The key point of a CODEC is to take noise, music and other forms of audio and transform it into a digital format. Coding-encoding-decoding is the process of sampling quantities and putting them into digital values of voice, music or other sounds. The Nyquist-Shannon sampling theorem states that the sampling frequency must be at least twice as high as the highest input frequency for the result to closely resemble the original signal. A 4,000 Hz voice pattern would be sampled at 8,000 BPS. Organized into 24 separate voice channels with spacing bits (called Framing bits) separating each 24 segments of 8,000 bits becomes a T-1 transmission circuit of 1,544,000 BPS. For example, in MP3s different compression (sampling or quantizing) rates are needed for different music quality levels such as 128 KBPS – CD quality (twice normal bandwidth), 96 KBPS – near-CD quality and 64 KBPS – FM radio quality.

 

This information is included in online/onsite courses SIP 2.0c and $499 for OCS-101 Office Communications Server per person (volume and site license discounts available).

 

*Courses are free to channel partners – see terms and conditions at http://www.techtionary.com/techu/. Discounts are also available to members of the SIP Forum (News - Alert) and MS Partners for $99 per student during May. For customizing, special discounts, website animations, technical/sales training, technical writing and other services, go to http://www.techtionary.com or please call Tom Cross at 303-594-1694 or [email protected].

 

This is also included in TMC (News - Alert) University special course on Microsoft OCS-Office Communications Server at ITEXPO.com. For more information go here: http://www.tmcnet.com/voip/conference/west-08/tmc-university-microsoft-ocs.htm