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Figure 3-8 shows an increment of time divided into 24 slots. If the time period shown is one second, then each slot occupies 41.67 milliseconds of time. Each bit of each slot then occupies 5.2 milliseconds.

How does this relate to telephone circuits? A voice circuit is sampled at 8 KHz, or every 125 microseconds (1/8 KHz). The voice sampling data rate is 64 Kbps (8 Khz / sample times 8 bits / sample). The DS-1 transmission rate is 1.544 Mbps. There are 25 voice slots, each 64 Kbps, in the DS-1 rate. So, if we fully loaded a 1.544 Mbps data stream with voice circuits, we could carry 25 phone conversations simultaneously. Simultaneously in this sense does not mean at the exact same instant of time, but together as a data stream. In actual practice, there are 24 voice circuits carried over a DS-1 line. The 25th slot is used by the telephone companies (telecos) for system management. Have you ever clearly heard a strange, one-sided conversation of short duration over the telephone? You guessed it. The mean stationmaster unloaded the wrong car on your circuit and while you could hear the other party, they could not hear you. This situation was fairly common in the '70s when multiplexing was in its infancy.

What does all this have to do with ATM and ADSL? The previous TDM example used voice circuits as the model for explaining TDM. However, any digital data signal can be time multiplexed. The very existence of ATM and ADSL is dependent upon the ability to multiplex signals. ATM is commonly called statistical multiplexing because the telecos gamble with the ATM TDM time slots. Sometimes you win and sometimes you lose, and if you want to beat the house percentage, do not skip over the remaining chapters. ADSL uses FDM techniques to get the message to the home.

ISDN Basics

Integrated Services Digital Network (ISDN) was originally conceived as an implementation of digital technology that integrated the separate analog switching, transmission, and user premises equipment and interfaces into a “seamless” network. To understand “seamless,” perhaps we should first understand what a “seam” is.

In the old days, CO switching equipment, transmission facilities, and user equipment were analog and considered as separate entities. For signals, especially digital signals as we entered the Age of Gates, to transit the telephone company networks from home (source) to CO to long-haul carrier (long-distance teleco) to CO to home (destination), the signal had to pass through the various equipment “seams.” Each of these seams represents a beginning/ending in a stage of the journey of the signal from source to destination. So a seam is the interconnection, or interface, between each stage of the signals journey. Each of these seams might require the equipment to operate on the signal in some manner that changes fundamental signal relationships such as level, timing, and/or frequency, or even the type of signal (light wave, electrical, electromagnetic wave, etc.).

The idea of seams and stages in the transmission path is illustrated with a person setting out on a journey around the country using various modes of transportation. The first stage of the trip requires the person to walk from the home out to an automobile. The first seam occurs when the person opens the car door and gets inside the auto. The second stage of the journey occurs when the person drives to the airport. The second seam occurs when he exits the car at the airport. The third stage occurs when he walks into the airport. Another seam occurs when the fellow gets on the airplane. And the next stage of the journey occurs when the airplane takes off and then lands at the destination. And the journey continues with stages and seams for each change in transportation mode. Yet, a trip by automobile from source to destination would only require two seams, getting into and out of the automobile at the source and destination, respectively.

Slow analog voice circuits could deal with mechanical switch gear and analog transmission techniques relatively easily. But digital signals were appalled at the antiquated methods and equipment used for analog signals. Since digital signals could not deal very well with the seams in the transmission systems, the desire to develop a digital signal standard that would provide “seamless” telephony service was born. That is where the Integrated comes from in ISDN. It was originally IDN, or Integrated Digital Network, meaning a “seamless” digital network. As folks talked about the concept, the idea of multiservices (meaning voice, video, data) was conceived. So, we finally wound up with the Services part of ISDN.

ISDN is a transmission and switching concept that provides a “seamless” digital signal path from source to destination. Assuming, of course, that all the telecos in the transmission path have ISDN compatible equipment installed, “seamless” connectivity is end-to-end connectivity. Not all telecos have ISDN equipment installed, especially in the former Soviet Union and third world countries. (From this point forward the term “seamless” will be used without the quotes.)

An important enabling legacy technology for ISDN is light wave transmission. Previously, without light wave technology, only very expensive coaxial delivery systems could transport ISDN data rates. See Table 3-2 for SONET/SDH transmission rates. Light wave technology, with the attendant fiber-based transmission facilities, provided a sufficiently fast transmission medium for the ISDN data rates. Now, ATM and ADSL enable ISDN services over copper wire.

Narrowband ISDN

Narrowband ISDN is the term applied to ISDN services with a 64 Kbps data transmission rate. When ISDN was originally conceived, 64 Kbps seemed terribly fast..... Please stop laughing..... :)..... OK, thats enoughyou are starting to dribble. In those caveman, or cave-PC days, modems operated at a blindingly fast 200 bps, making 64 Kbps seem like a rocket. Just be glad you did not live in those dark ages and have to communicate with a 50 bps teletype.


Figure 3-9  A bit bucket too small

In those long ago days, all of the countrys information databases were text based. If you kept the message short and sweet, 50 bps got the job done, more or less. Ever see a teletype (playboy) “picture”? My, how the machines could labor trying to reproduce a “graphic” image.


Figure 3-10.  A bit barrel just right(for now)


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