There are two basic categories of transmission media: guided and 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.
Best Microsoft MCTS Certification, Microsoft MCITP Training at certkingdom.com
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.
Know we are discussing the types of guided media:
1. 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. We are going to extend the traditional definition of open wire 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.
2. 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 a wire used for the -ve 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 (ISO 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).
Un-Shielded 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).
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 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 the 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.
3. Coaxial Cable: Coaxial cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.
The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thin-net arid Thick-net. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than with twisted pair cable.
4. Optical Fiber: 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 125 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. An example of this is when we look into a pond of water.
Advantages of Optical Fiber 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 Disadvantages of Optical Fiber Physical vibration will show up as signal noise
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.
5. Microwave System: These systems have no wire connection, so the transmission has low error rate, making it reliable, but expensive. As there is no wire connections, so it can be used for large area transmissions. They system transmits a signal in a line of sight path between relay stations, approximately at a distance of 30 miles to boost up the signal. Satellites are also used in microwave communication..
6. Communication Satellite Communication satellites are an important medium used in microwave communications. There are hundred of communications satellites today in the “parking orbit”. They are solar powered space relay station to link earth stations. The elite receive a signal from earth station,, amplifies it and retransmit it to the other earth station thousand of miles away.
7. Waveguides Waveguides are metal tubes that confine and serve as paths for very high frequency radio waves. Waveguides have proven feasible and are being implemented on an experimental basis, but their large-scale implementation depends’ upon a greater demand for additional transmission capacity. BELL the US laboratories has introduced a Waveguide system that can carry as many as 200000 voice grade channels.
8. Laser: Lasers use a much higher portion of the electromagnetic spectrum than by-radio transmission, therefore, they offer tremendous potential for sending a large amount of data without tying up the already crowded radio frequency spectrum. The use of optical frequency and the need for line-of -sight paths present unique problems for laser implementations. Short distance intracity links among tall buildings equipped with rooftop transmitters and receivers appear to be a one way resolve this problem.