Single Mode Fiber
- Uses step index fiber and a highly focused source of light that limits beams to a small range of angles all close to the horizontal
- Single Mode fiber is manufactured with a much smaller Diameter than Multimode
- All of the beams arrive at the destination together and can be recombined without distortion to the signal
Fiber Sizes
- Optical Fibers are defined by the ratio of the diameter of their Core to the diameter of their Cladding
- Both the diameters are expressed in Microns (Micrometers)
- A core is surrounded by cladding forming the Fiber.
- In most cases, fiber is covered by a Buffer layer that protects it from moisture.
- Finally the entire cable is encased in an outer jacket
- Both core and cladding can be made of either glass or plastic but must be of different densities
- In addition the inner core must be ultra pure and completely regular in size and shape
- Chemical differences in material and even small variations in the size or shape of the core alter the angle of reflection and distort the signals
- Some applications can handle a certain amount of distortion and their cables can be made cheaply but others depend on complete uniformity
- The outer jacket can be made of several materials including Teflon, Plastic, Fibrous Plastic, metal tubing
Each of these materials have a purpose:
- Plastic is lightweight and cheap but do not provide structural strength and can emit fumes when burnt
- Metal tubing provides strength but is costly
- Teflon is lightweight and can be used in open air but it is expensive and does not increase cable strength
Light Sources for Optical Cable
- For tx to occur the sending device must have a light source and the receiving device with a photosensitive cell (Photodiode)
- Photodiode converts the light into current usable by the computer
The Light source can either be an LED or an ILD
LED:
- Cheaper but provide Unfocused light that strikes the boundaries of channel at uncontrollable angles
- Limited to short distance use
LASER:
- Can be focused to a narrow range allowing control over angle of incidence
Fiber Optic Connectors
- Importance of Connectors
- If connector is over tight, two cores can be compressed and angle of reflection of the signal will be altered
- All of popular connectors are Barrel shaped that come in male and female versions
- The cable has a male connector that fixes into a female connector attached to the device to be connected
Advantages of Optical Fiber
The major advantages of Fiber over twisted pair and coaxial cable are:
Noise Resistance:
- Because fiber uses light rather than electricity, noise is not a factor
- External light the only form of possible interference is blocked from the channel by the Outer jacket
Less Signal Attenuation
- Fiber optic Transmission distance is significantly greater than other media
- A signal can run miles w/o regeneration
Higher Bandwidth
- Can support higher BWs and higher data rates
- High rates are not utilized by absence of signal generation and reception technology
Disadvantages of Optical Fiber
-
COST
- Expensive
- No impurities or imperfections can be tolerated, so manufacturing is costly
- Laser light sources can be expensive
-
INSTALLATION
- Roughness & Cracking of core cannot be tolerated
- All connections must be perfectly alligned
Disadvantages of Optical Fiber
Fragility
- Glass fiber is very fragile
- Can not be used in extreme conditions where hardware portability is required
Unguided Media
- Unguided Media or Wireless Communication transport Electromagnetic waves without a physical conductor
- Instead signals are broadcast through Air and are available to anyone who has a receiver capable of receiving them
Radio Frequency Allocation
- The section of EM spectrum defined as Radio Communication is divided into 8 ranges called BANDS
- BANDS are rated from very low frequency (VLF) to extremely high frequency (EHF)
Propagation of Radio Waves
Radio Wave transmission utilizes five different types of propagation:
Types of Propagation:
- Surface
- Tropospheric
- Ionospheric
- Line-of-Sight
- Space
Radio Technology considers the earth as surrounded by two layers of atmosphere:
TROPOSHERE
- It is the portion of the atmosphere extending outwards approx. 30 miles from the earth’s surface
- It contains what we call as AIR
- Clouds, wind, Temp. variation and weather in general occur in the Troposhere as does jet plane travel
IONOSPHERE
- It is the layer of atmosphere above the troposhere but below space
- It is beyond what we think of as atmosphere
Surface Propagation
- In surface propagation, radio waves travel through the lowest layer of the atmosphere, hugging the earth
- At the lowest frequencies signal emanate in all direction from the tx antenna and follow the curvature of the planet
- Distance depends on the amount of power in the signal
Troposhpheric Propagation
This can work in two ways
Line-of-Sight:
- A signal can be directed in a straight from Antenna to antenna
Broadcast:
- Signal is broadcasted at an angle into the upper layers of the troposphere from where it is reflected back to earth’s surface
- The first method demands that both transmitter and receiver be placed within line-of-sight distances and is limited by the curvature of the earth
- The second method allows greater distances to be covered
Ionoshpheric Propagation
- Higher frequency radio waves are radiated towards the ionosphere where they are reflected back to the earth
- The difference in density between troposhphere and ionosphere causes each radio wave to speed up and change direction bending back to earth
- Allows greater distances to be covered by lower power output
Line of Sight Propagation
- Very high frequencies signals are transmitted in straight line from antenna to antenna
- Antennas must be directional facing each other or either tall enough or close enough to each other to avoid earth’s curvature
- Radio waves can reflect of the objects in the middle and can reach late to the receiver. These late signals distort signal
Space Propagation
- A broadcast signal is received by the orbiting satellites which rebroadcast the signal to the intended receiver on the earth
Summary
- Guided Media
- Optical Fiber Cable
- Unguided Media
- Radio Frequency Allocation
- Propagation of Radio Waves
Readig Sections
- Section 7.1,“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
