Data Transmission
Data transmission, digital transmission or digital communication is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint transmission medium. Example of such media are copper wire, optical fibers, wireless communication media, and storage media. The data is often re-presented as an electro-magnetic signal, such as an electrical voltage signal, a radiowave or microwave signal or an infra-red signal.
While analog communications represents a continuously varying signal, a digital transmission can be broken down into discrete messages. The messages are either represented by a sequence of pulses by means of a line code (baseand transmission), or by a limited set of analogue wave forms (passband transmission), using a digital modulation method. According to the most common definition of digital signal, both basseband and passband signals representing bit-definition only considers the baseband signal as digital, and the passband transmission as a form of digital-to-analog conversion.
Data transmitted may be digital massages originating from a data source, for example a computer or a keyboard. It may also be an analog signal such as a phone call or a keyboard. It may also be an analog signal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more Advanced source coding (data compression) schemes. This source coding and decoding is carried out by code equipment.
There are 2 basic categories of Transportation media:
Guided and
Unguided
Guided media
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 to be interested 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 as such are often called unbound media.
There 4 basic types of Guided media:
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 noice interference. This can include multiconductor cables or single wires. This media is susceptible to a large degree of noise and interference and consequently not acceptable for data Transmission except for short distances under 20 fit.
Twisted pair
The wires in twisted pair cabling are twisted together in pairs. Each pair should consist of a wire used for the +ve data signal and a wire used for the -ve data signal. Any noise that appears one 1 wire of the pair would 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 uniopar line coding (TTL logic).
The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduce the noise interference. To further improve noise rejection a fool 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).
Cables with a shield are called shielded twisted pair and commonly abbreviated STP. Cables without a shield are called unshielded twisted pair or UTP. Twisted the wires together result 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 connectors (RJ45,RJ11,etc...)
STP or shielded twisted pair is used with the traditional token Ring cabling or ICS - IBM cabling system. It reguires a custom connector . IBM STP (shielded twisted pair) has a characteristic impedance of 150 ohms.
Coaxial cable
Coaxial cable consists of 2 conductor. The inner conductor is held inside an insulatedor with the other conductor woven around it providing a shield. An insulating protective coating called a jacket cover 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 impedance for coaxial cable are 75 ohms for Cable TV, 50 ohms for Ethernet thinner and thicknet. The excellent control of the transferred then twisted pair cable.
Optical fiber
Optical fiber consists of thin glass fibres that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of very narrow strand glass called the core. Around the core is the concentric layer of glass called the cladding. A typical core diameter is 63.5 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 Jeckets.
An important characteristic of fibre 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.
If the angle of incidence is small, the light Ray's 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 fibres work on the principal that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along it's path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the media!.
There are 3 primary types of transmission mode using optical fiber.
Step index
Grade index
Single mode
Step index
Step index has a large core the light Ray's tends to bounce around, reflecting off the cladding, inside the core. This causes some Ray's to take a longer or shorted path through the core. Some take the direct path with hardly any reflection while other bounce back forth taking a Langer path. The result is that the Ray's arrive at the receiver at different times. The signal becomes longer than the original signal. LED light source are used. Typical core: 62.5 microns
Grade index
Grade 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. The is represented by a curved reflective path in the attached drawing. The result is a better receive signal than step index. LED light source are used.
Single mode
Single mode has separate distinct Refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflection off core. Mode is used for a single source of light ( one color) operation. It requires a laser and the core is very small : 9 microns.
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