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RF wave propagation is highly regulated by all the worlds governments. Some places are very serious about throwing a person in jail for unlawful, that is non-government approved, radiation. RF spectrum is finite with fierce competition for bandwidth. And RF wave generation, propagation, and reception requires expensive facilities, equipment, and maintenance. Just ask the CATV service providers.
While there is a role for RF wave propagation in our communications systems of the future, ATM and ADSL will make some forms of RF communications, such as CATV, obsolete. Dont need that expensive coaxial delivery system anymore, and with ATM multicasting capability, satellite delivery systems, with their limited delivery capabilities for video products, are a technology dinosaur. And we dont need the expensive microwave hops used for relaying CATV video products from source to destination anymore.
Light wave technology is really a simple idea. Take a light wave, make it pulsate in relation to the information to be conveyed, and ship it over some medium from source to destination. The implementation of that simple idea in the United States is called Synchronous Optical Network (SONET). In Europe, light wave technology is called Synchronous Digital Hierarchy (SDH). While pushing light waves down a tube is a simple idea, the underlying technology to accomplish the simple is very complex. But, simply put, SONET/SDH is light waves pulsating with passionate electrical pectorals, passing through fiber cable from source to destination, albeit at a very fast speed. Why SONET/SDH? Speed, speed, and more speed. Speed is synonymous with bandwidth. And remember, I said bandwidth was green gold. See Table 3-2 for the various SONET/SDH transmission rates.
STS Level | OC Level | Frequency (Mbps) |
STS-1 | OC-1 | 51.84 |
STS-3 | OC-3 | 155.52 |
STS-9 | OC-9 | 466.52 |
STS-12 | OC-12 | 622.08 |
STS-18 | OC-18 | 933.12 |
STS-24 | OC-24 | 1244.16 |
STS-36 | OC-36 | 1866.24 |
STS-48 | OC-48 | 2488.32 |
Note: STS is the acronym for Synchronous Transport Signal and OC is the acronym for Optical Carrier.
Table 3-2. SONET/SDH transmission rates
The light waves are contained within and guided on their journey from source to destination by a special type of material called an optical fiber. See Figure 3-11.
Figure 3-12. Simplified switching scheme
Figure 3-13. Typical digital switching element
The fiber is made from silicon and resembles a narrow glass tube. The light wave travels from one end to the other by reflecting off the inside wall of the fiber tube. The light wave can be stimulated to carry signals with a bandwidth up to 2.4 Gbps (OC-48).
Figure 3-14. Four-stage Delta-2 switching matrix
As long as there were just two telephones in the world comprising the network, there was no need to provide any switching capability. As soon as a third telephone was added to the network (perhaps within the week after discovering the technology?), switching became an issue. Three telephones give any one user the choice of two possible connections. Some specialized circuitry was required to provide the correct path depending upon the choice made by the caller. So, the birth of the switch followed soon after the discovery of the telephone.
Originally, switches located in a central office (CO) were mechanical contraptions with the circuit connection provided by a telephone switchboard operator. A switchboard had rows of plugs arranged in pairs, called cord pairs, over rows of jacks. The plugs were used to mechanically connect two jacks together to form a completed connection, or circuit. A user rang the operator and advised her of the party to be called. The operator would then use the cord pair to connect the two parties.
In 1891, Almon B. Strowger patented his mechanical switch that formed the building blocks of an automatic switching system, eventually replacing the plug and jack mechanical switchboard and putting many operators out of work, a familiar story. A Strowger switch, connected to each incoming line, responded to dialed digits by rotating a certain amount with each digit. After completing the final rotation when the last digit was received, the incoming line would be connected to the correct outgoing line. The Strowger switch was so successful that in 1978, 53 percent of the Bell System exchanges in service, with 23,000,000 subscribers, still used Strowger switching.
Due to certain significant limitations of Strowger switching, common control of electromechanical switch matrices called crossbar switching was introduced. Crossbar switches were slow, complex electromechanical switching devices that required significant amounts of current to activate.
Common control refers to the designation of some device that receives the dialed information, interprets the number, then actuates the proper circuit elements to make the physical connection. Also, the common control breaks the connection when one party hangs up. When the phone is picked up by the calling party the common control is assigned to listen for an incoming call to the CO. The common control provides the dial tone to the calling party, then accepts the dialed digits. After receiving the dialed digits, the common control determines the path through the switch matrix by either hard-wired or stored program (computer) rules. The rules determine which two points to close to complete the path. This particular method of identifying a connection is very simple and handy. It is the most common way of detecting when and what key is pressed on a computer keyboard.
Reed relays were developed to reduce the large currents required to activate the crossbar switch and to increase the reliability of switches. Reed relays are small, glass encapsulated electromechanical switching devices. The common control selects which relays to close to complete the physical path.
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