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Exercise 10 Fiber Optic And Wireless Networking

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Objectives for Exercise 10 At the end of this Exercise, you will be able to: ■ Describe the construction of a fiber optic cable. ■ State the difference between a fiber’s core and cladding material. ■ Explain the terms “total internal reflection” and “critical angle.” ■ Discuss the advantages and disadvantages of single mode fiber optic cable.

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Objectives (Continued) Compare the advantages and disadvantages of fiber optic cable to copper cable. ■ List the three wireless LAN technologies. ■ Explain the major differences between IR and RF technologies. ■ Describe how wireless LANs fit into networking schemes. ■

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History of Optical Fiber In 1870, British physicist John Tyndal observed that light could be bent in a stream of water. ■ In 1880, Alexander Graham Bell invented a photophone that transmitted voice signals on beams of light. ■ In the 1970s, Corning Glass & Bell Labs developed the first practical optical fiber. ■

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What is Optical Fiber? ■

Communication grade optical fibers are ultra-pure strands of glass the thickness of a human hair. – It is basically comprised of two components; the core and its cladding. – Core and cladding, differing only in their refractive index, form an “optical waveguide.” – This propagates photons, much as wire propagates electrons, but much faster.

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Optical Fiber Construction ■

This shows the basic construction of an optical fiber. – The core and cladding are both glass with different indices of refraction.

Core

Cladding

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Fiber Optic Cable ■

This shows the optical fiber integrated into a typical fiber optic cable. *CAUTION* When pulling this cable, apply tension ONLY to the strength member.

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How Does Optical Fiber Work? ■

Some important points necessary to the understanding of this subject are: – – – – – –

Index of refraction Critical angle Total internal reflection Propagation modes Bandwidth Types of fibers available

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Total Internal Reflection ■

You can think of an optical fiber as a hall of mirrors. In fiber, notice that the border between core and cladding serves as the mirrors.

Hall of Mirrors

Optical Fiber

Core

Cladding

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Fiber Modes and Indices ■

Optical fiber comes in two major types, multimode and singlemode.

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Multimode is also available in two different types, step index or graded index. – Graded index is more commonly used in communication applications.

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Step and Graded Index Fiber Step index fiber provides a single, abrupt change in its index of refraction. ■ Unlike step index, the cladding of graded index has a nonuniform index of refraction. ■

Step Index Fiber

Graded Index Fiber

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Critical Angle The critical angle is that angle at which a ray is not allowed to pass through the barrier between core and cladding. ■ At angles greater than the critical angle, all energy is reflected back into the core. ■

< Critical Angle

Critical Angle

> Critical Angle

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Multimode Fiber ■

Multimode fiber can accept a fairly large range of input angles. Each takes a different amount of time to travel the same distance. Each path is a mode. – Note that the output pulse is wider and weaker than the input pulse. This effect is known as modal dispersion. – Also note the orange ray, which strikes the interface at less than the critical angle, is lost.

Input

Output

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Singlemode Fiber ■

This fiber accepts only one input angle, therefore the output closely matches the input. – This cable is used for very long cable runs. It is much more expensive to buy and install.

Input

Output

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Bandwidth ■

Bandwidth is the fiber’s capacity to carry data. – The greater the bandwidth, the greater its carrying capacity.

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Fiber bandwidth is specified as frequency (MHz) divided by distance (km) at specific wavelengths. – If a cable is specified at 800 MHz, its bandwidth at 2 kilometers would be reduced to 400 MHz.

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Advantages of Fiber Optics Greater capacity than copper: Can carry hundreds of times more information. ■ No electrical interference: Immune to EMI, RFI, and cross-talk. ■ Security: Does not radiate fields and cannot be tapped without detection. ■ Low transmission losses: Allows for very long cable runs between repeaters. ■

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Disadvantages of Fiber Optics Electrical/Optical conversions: Expensive to convert electrical signals to light and back again. ■ Special installation: Requires special tools, techniques, and training. (Crimping, wire wrapping, and soldering do not apply.) ■ Expense: Cable, associated components and manpower are currently more expensive. ■

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Wireless Communications ■

Wireless communications are an adjunct to wired LANs, not a replacement.

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Current systems use either: – –

Infrared light, or Radio frequency waves.

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Common Characteristics ■

These technologies differ from copper and fiber cable in that they do not require physical connections. They, instead, rely on waves traveling through free space. – Mainly used to connect two buildings or remote users where cables cannot be run. – Useful where it is difficult to run cable. – Air is a more uncontrollable medium than cable.

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Infrared (IR) Technology IR is simply a range of light waves just below the visible spectrum. ■ Like visible light, it can not pass through walls or ceilings but can reflect off flat surfaces. ■ It is not subject to EMI or RFI interference. ■ Two types of IR systems are currently available, direct IR and diffuse IR. ■

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Direct Infrared Connections This is similar to the method used for TV remote controls. ■ Requires the transmitter and receiver to be in direct line-of-sight. ■ Transmits infrared in a 30-degree cone. ■ Receiving unit must be within this cone. ■ Uses two levels of intensity to represent digital data “1” and “0.” ■

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Direct IR ■

This illustrates the directivity required for direct IR operation.

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Diffuse Infrared Connections ■

Does not need direct line-of-sight but is limited to a single room.

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Instead of a focused beam, it floods a room with IR, similar to a light bulb. – Since all receivers in the area receive the same signals, units need to have unique identifiers.

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Diffuse Infrared LAN Connections ■

This illustrates how additional nodes can connect to a wired LAN using diffuse IR.

LAN

LAN

LAN

Additional Infra-Red LAN Nodes

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How RF LANs Work Radio waves consist of a high frequency, low power carrier, modulated by the data signal. ■ This signal is received at the other end where the carrier frequency is stripped away, leaving only the original data signals. ■ These waves can be transmitted up to half a mile, subject to FCC regulations. ■

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Radio Frequency Technologies The FCC has allotted bands of frequencies for use by wireless LANs. Applications using these frequencies include: ■ Narrowband ■ Spread Spectrum Frequency Hopping (FHSS) ■ Spread Spectrum Direct Sequence (DSSS) ■

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Narrowband Narrowband systems transmit and receive on discrete frequencies within the band. ■ Cross-talk is avoided by sharply tuning each receiver to its assigned frequency. ■ This is comparable to cable TV where the cable may contain 50 channels but the TV receiver filters out all but the channel it is tuned to. ■

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Narrowband Signal Plot Notice this occupies a very small portion of the allocated frequency band. ■ Has relatively high power output but very low data rate. ■ Annual FCC license is required for each transmitter site. ■

Allocated Spectrum

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Spread Spectrum These technologies use a wider portion of the allocated frequency band than Narrowband systems. ■ Two types of Spread Spectrum systems are currently in use. These are: ■

– Frequency-Hopping Spread Spectrum and – Direct Sequence Spread Spectrum. ■

Spread spectrum systems do not require FCC licensing due to their low power.

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Frequency-Hopping Spread Spectrum ■

Frequency-Hopping uses a narrowband carrier, which shifts frequency in a predetermined pattern known to both the transmitter and receiver.

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If synchronized, a single logical channel is maintained. To an unsynchronized receiver, the signal appears to be random noise.

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FHSS Signal Plot Transmitted signal hops from frequency to frequency. The frequency-shift pattern must be known to both the transmitter and the receiver. This 2.4 GHz band contains 83 discrete 1 MHz channels.

FHSS 2400 MHz

2483 MHz

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Direct-Sequence Spread Spectrum ■

DSSS uses a complex modulation/ demodulation technique to spread its power over a wide portion of allocated bandwidth.

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To a receiver not equipped with the proper DSSS codes, the signal appears to be only low power, wide band noise.

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Now It’s Your Turn

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