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But why 64 Kbps ISDN? Commercially deployable digital technology was based on 16-bit processors. How high can a 16-bit processor count? How about 64,000, rounded off? The actual number is 65,536 but it is customary to write the number as 64 Kbps. This original 64 Kbps ISDN is now referred to as Narrowband ISDN (N-ISDN) to distinguish it from Broadband ISDN (B-ISDN).

Why is ISDN important to ATM and ADSL? Ever hear of “legacy systems”? Well, ISDN is the legacy system that ATM and ADSL must deal with in todays teleco systems.

Broadband ISDN

ISDN at 64 Kbps was okay for the text-based traffic and the digitally encoded voice circuits that originally used the ISDN networks. But faster speed processors (whoever thought of desktop 300 MHz processors 20 years ago?), 32-bit processors, the proliferation of personal computers, and the Internet virtually made (N-) ISDN obsolete in a couple of years.

The original ISDN is a (bit) bucket too small for the Age of Gates with the attendant multi-megabit data needs. See Figure 3-9. Now, we want data in really big (byte) buckets. One economical solution that does not trash our legacy ISDN systems is to use multiple ISDN bit buckets for the byte-hungry, image-based applications. If 25 ISDN buckets are combined, then the resultant data barrel is 1.536 Mbps (64 Kpbs/ bucket x 24 buckets) deep. See Figure 3-10.

Telecos take the 1.536 Mbps barrel, stretch it by adding 8 Kbps for traffic management purposes, and call it Broadband ISDN, also known as DS-1 at 1.544 Mbps.

Copper Wire

Copper wire was the original electrical transmission medium for the industrial revolution. As revolutions go, the industrial could not have had a better partner. Copper has desirable characteristics that make it very useful for industrial revolutions. Copper is cheap (well, it used to be), has good electrical conductivity, weathers very well, and is easy to work with. Seems like the industrialists were waiting with bated breath for someone to invent something that could use copper as a major component of the product. Because the telephone and copper wire went together like bread and butter, bacon and eggs, Elvis and 7-Elevens...you get the idea. As soon as the telephone was invented, the race to literally (copper) wire the world was under way.

Copper wire is everywhere. There are 600 million homes and businesses worldwide wired with copper. It is estimated 80 percent of wired premises are within 1,800 feet of a CO or access node. The 80 percent/1,800 feet is a U.S. estimate, but let us extend the figure around the globe, not an unreasonable feat. Then let us assume the average distance from CO to home is half of 1,800 feet, or 900 feet, or approximately 0.2 miles. Ball park, 120 million miles of copper are snaking around underground and overhead. That is a lot of copper wire. All this wire and all these premises are candidates for ADSL modems.

Wire Type Max Frequency
STP 155 Mbps
UTP-3 25.6 Mbps/51.84 Mbps
UTP-5 155 Mbps
Coaxial 45 Mbps/155 Mbps

Table 3-1. Maximum copper transmission frequencies

Copper wire has various characteristics that influence electrical signals including ADSL signals. Telephone wire from the access to the premises, as I have mentioned once or twice (and will mention once or twice more), is a twisted pair of wires. The twist is to reduce signal coupling between the two wires and the additional surrounding wires when the twisted pair is placed in large bundles. The twisted pair can be shielded, known as shielded twisted pair (STP), or unshielded, known as unshielded twisted pair (UTP).

Shielding, or the lack of it, length, size, and age are important considerations for determining what flavor of DSL technology is best for any particular application and location. All copper wire is not made equal. Table 3-1 shows the maximum practical frequency for data transmission by typical telephony wire type. Coaxial cable, of the flavor used in CATV premise wiring, is tossed in for comparison. Most homes have the UTP-3 flavor. Well, how can twisted pair at 25.6 Mbps compete with CATVs 155 Mbps? Read on, until the end (of the book) is near.

RF Propagation

While copper wire and telephony have had a 100-year marriage made in heaven, the marriage has not been totally without discord. Sometimes, after the honeymoon glow was a faded memory, Granny Bell wanted to go places Papa Copper could not take her. But Granny had a secret suitor waiting to do her bidding.

Radio frequency (RF) wave propagation technology was under development at the turn of the century. By the end of World War I radio amateurs were demonstrating the practicality of long-distance communications using radio frequency wave propagation through the atmosphere. After launching these things called radio waves into space and receiving the signal many miles away, as if by magic, the usefulness of radio frequency wave propagation in long-distance communications systems was self-evident.

RF communications technology was initially relatively low frequency, voice-based transmission systems. RF transmission links were particularly useful for over-the-horizon (out of sight) communications with remote locations. Today, aircraft flights from Alaska to the Orient must still rely upon such antiquated technology for a safe ride. But RF transmission technology has kept pace with the rest of the developments in communication systems. RF transmission rates range from the very slow (16 Kbps) to the very fast (Gbps).


Figure 3-11.  Traveling light waves

Currently, Direct Broadcast Satellite (DBS), microwave, personal communications (paging systems), and wireless systems (cellular) use RF propagation to get data from source to destination. The upper bounds of RF propagation keep moving up in frequency as new enabling technologies are discovered and refinements in current processes are made. Continued advances in RF technology may yield 300 GHz transmission systems. With current digital encoding schemes, the total amount of bits that can be transmitted without interference from 0 to 300 GHz is 9.6 tera bps (9,600,000 Mbps). That is the equivalent bandwidth of 3,858 OC-48 fiber connections. While RF has an upper bound limiting the amount of bits that can be transported, fiber based systems are only limited by the demand for additional bandwidth, as additional fibers can be connected as demand warrants.


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