Optical Transmission Modes

There are three primary types of transmission modes using optical fiber. They are

1. Step Index
2. Graded Index
3. Single Mode.

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This cause some rays of take longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and fourth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. A typical Core is 62.5 microns.

Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core is 62.5 microns.

Both step index and graded index allow more than one light source to be used (different colors simultaneously), so multiple channels of data can be run at the same time.

Single index mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small of 9 microns.

Frequency is not used to talk about speed any more, instead of that wavelengths are used. The wavelength of light sources is measured in nanometers or 1 billionth of a meter.

Indoor cable specifications:

• LED (Light Emitting Diode) light source
• 3.5 dB/Km attenuation (loses 3.5 dB of signal per kilometer)
• 850 nM - wavelength of light source
• Typically 62.5/125 (core dia/cladding dia)
• Multimode - can run many light sources.

Outdoor cable specifications:

• Laser Light Source
• 1 dM/Km Attenuation (loses 1 dB of signal per kilometer)
• 1170 nM - wavelength of light source
• Monomode (single mode).

1. Advantage of Fiber Optic
   • Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI - Electromagnetic Interference)
   • Security:cannot tap into cable.
   • Large Capacity due to BW (bandwidth)
   • No corrosion
   • Longer distances than copper wire
   • Smaller and lighter than copper wire
   • Faster transmission rate.
2. Disadvantages of Fiber Optic
   • Physical vibration will show up a signal noise.
   • Limited physical are of cable. Bend it too much and it will break.
   • Difficult to splice.

The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

Fiber Optic

Optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron=10^-6 meters). Typically Cladding has a diameter of microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.

An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other.

If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.
Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it-it bounds the medium.

1. Advantages of Fiber Optic Line (Glass Fibers):
   • Smaller
   • Lighter
   • Faster (speed of light)
   • No interference.
2. Disadvantages of Fiber Optic Line (Glass Fibers)
• Expensive
• Harder to install and modify

Coaxial Cable

A single insulated inner wire is surrounded by cylindrical conductor which is covered with a shield; it transmits electromagnetic signals. Coaxial cable is classified into two categories:

• Base Band (uses digital signals)
• Broad Band (uses analog signals) coaxial cable.

1. Advantages of coaxial cable (Round Insulated Wire):
   • Not susceptible to interference
   • Transmits faster.
2. Disadvantages of coaxial cable (Round Insulated Wire):
   • Heavy and bulky
   • Needs booster over distance

1. Base band Coaxial Cable

It has been used for many years in the telephone network in applications with requirements similar to those of a LAN. Both base band and broadband coaxial cable are available. Although their structure is same, their installation and applications differ.

In base band coaxial cable, a central carrier wire is surrounded by a fine woven mesh of copper which forms an outer shell. The space between the wire and the outer shell is insulated to separate the two conductors and to maintain the electrical properties. The entire cable is covered by protective insulation to minimize electrical emissions. The cable is usually approximately 3/8 inch in diameter.

2. Broadband Coaxial Cable

This comes in several different diameters with varying amounts of insulation. The cable may have the same construction as base band coaxial. The central carrier may be surrounded by an aluminium sleeve. The space between the core and the shell is filled with insulation and the whole is enclosed in a protective coat of insulation. Broadband coaxial cable can carry 50 to 100 television channels or thousands of voice and low speed data channels at the rates of 9.2 to 50 Kpbs.

Twisted Pair

The wires in twisted pair cabling are twisted together in pairs. Each pair consists of a wire used for the +ve data signal and wire used for the negative data signal. Any noise that appears on 1 wire of the pair will also occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase (180 degrees - phasor definition of opposite polarity). When the noise appears on both wires, it cancels or nulls itself out at the receiving end. Twisted pair cables are most effectively used in systems that use a balanced line method of transmission:polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTL logic).

1. Advantages of Twisted Pair (Phone Line):
   • Easy to string
   • Cheap
2. Disadvantages of Twisted Pair (Phone Line):
   • Subject to interference =static and garble.

1. Unshielded Twisted Pair

The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference. To further improve noise rejection, a foil or wire braid "shield" is woven around the twisted pairs. This shield can be woven around individual pairs or around a multi-pair conductor (several pairs).

2. Shielded Twisted Pair

Cables with a shield are called shielded twisted pair and are commonly abbreviated STP. Cables without a shield are called unshielded twisted pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.

UTP or unshielded twisted pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc.)

STP or shielded twisted pair is used with traditional Token Ring cabling or ICS-IBM Cabling System. It requires a custom connector. IBM STP (shielded twisted pair) has a characteristic impedance of 150 ohms.

Open Wire

Open wire is traditionally used to describe the electrical wire strung along power poles. There is a single wire strung between poles. No shielding or protection from noise interference is used. The traditional definition of open wire is extended to include any data signal path without shielding or protection from noise interference. This can include multi conductor cables or single wires. This medium is susceptible to a large degree of noise and interference and consequently is not acceptable for data transmission except for short distances under 20 ft.

Introduction to Network Transmission Media

Transmissions lines are the backbone to a network. It is available in two basic varieties:Base band and broadband. Twisted pair wires are Base band communication links whereas coaxial and fiber optic cables are broadband links. There are two basic categories of transmission media:

• Guided
• Unguided.

Guided transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the cabling system. Guided media is also known as bound media. "Cabling" is meant in a generic sense and is not meant to be interpreted as copper wire cabling only.

Unguided transmission media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and are therefore often called unbound media.

There are four types of guided media:

• Open Wire
• Twisted Wire
  • Unshielded Twisted Pair
  • Shielded Twisted Pair
• Coaxial Cable
  • Base Band Coaxial Cable
  • Broad Band Coaxial Cable.
• Optical Fiber.