Transmission Media

1. The electromagnetic spectrum has voice-band frequencies that are generally transmitted as current over metal cables, such as twisted-pair or coaxial cable. Radio frequencies can travel through air or space but require specific transmitting and receiving mechanisms. Visible light is harnessed using fiber-optic cable.

2. Guided Media provide a conduit from one device to another.

   • Twisted pair cable
     • Unshielded twisted-pair (UTP) cable is the most common type of telecommunication medium in use today. Its frequency range is suitable for transmitting both data and voice. The two wires are twisted around each other at regular intervals (between 2 and 12 twists per foot), each wire is closer to a noise source for half the time and farther away for the other half. Therefore, the cumulative effect of the interference is equal on both wires.

     • Advantages of UTP are its cost and ease of use.
       • Category 1 is the basic twisted-pair cabling used in telephone systems. This level of quality is fine for voice but inadequate for all but low-speed data communications.

       • Category 2 is suitable for voice and for data transmission of up to 4 Mbps.

       • Category 3 is required to have at least three twists per foot and can be used for data transmission of up to 10 Mbps. it is now the standard cable for most telephone systems.

       • Category 4 must also have at least three twists per foot as well as other conditions to bring the possible transmission rate to 16 Mbps.

       • Category 5 is used for data transmission up to 100 Mbps.

     • UTP connectors are snap-in plugs like those used with telephone jacks.

     • Shielded twisted-pair (STP) cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors. The metal casing prevents the penetration of electromagnetic noise. It also eliminates crosstalk.

   • Coaxial cable carries signals of higher frequency ranges than twisted-pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two (also usually copper).
     • Different coaxial cable designs are categorized by their radio government (RG) ratings. Each RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing.
       • RG-8 is used in thick Ethernet.

       • RG-9 is used in thick Ethernet.

       • RG-11 is used in thick Ethernet.

       • RG-58 is used in thin Ethernet.

       • RG-59 is used for TV.

     • Bayonet network connectors (BNC) are barrel connectors and are the most popular type of connectors. All coaxial connectors have a single pin protruding from the center of the male connector that slides into a ferrule in the female connector. Two other commonly used types of connectors are T-connectors and terminators. Terminators are required for bus topologies where one main cable acts as a backbone with branches to several devices but does not itself terminate in a device.

   • Optical fiber is made of glass or plastic and transmits signals in the form of light.
     • Light, a form of electromagnetic energy, travels at 300,000 kilometers/second, or approximately 186,000 miles/second in a vacuum. The speed decreases as the medium through which the light travels becomes denser.

     • If a ray of light traveling through one substance suddenly enters another (more or less dense) substance, its speed changes abruptly, causing the ray to change direction. This change is called refraction.

     • When light travels into a more dense medium, the angle of incidence is greater than the angle of refraction; and when light travels into a less dense medium, the angle of incidence is less than the angle of refraction.

     • When the change in the incident angle results in a refracted angle of 90 degrees, with the refracted beam now lying along the horizontal. The incident angle at this point is known as the critical angle.

     • When the angle of incidence becomes greater than the critical angle, a new phenomenon occurs called reflection. In this case the angle of incidence is always equal to the angle of reflection.

     • Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.

     • Propagation modes
       • Multimode step-index uses multiple beams of light. The density of the core remains constant from the center to the edges. A beam of light moves through the constant density in a straight line until it reaches the interface of the core and the cladding. At the interface there is an abrupt change o a lower density that alters the angle of the beam's motion.

       • Multimode graded-index uses fiber with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. Each density difference causes each beam to refract into a curve. Varying the refraction varies the distance each beam travels in a given period of time, resulting in different beams intersection at regular intervals. Careful placement of the receiver at one of these intersections allows the signal to be reconstructed with far greater precision.

       • Single mode 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. Propagation of different beams is almost identical and delays are negligible.

     • Optical fibers are defined by the ratio of the diameter of their core to the diameter of their cladding, both expressed in microns.

     • A core is surrounded by cladding, forming the fiber. In most cases, the fiber is covered by a buffer layer that protects it from moisture. The entire cable is encased in an outer jacket.

     • The light source can be either a light-emitting diode (LED) or an injection laser diode (ILD). LEDs are limited to short-distance use. Lasers can be focused to a very narrow range, allowing control over the angle of incidence. They preserve the character of the signal over considerable distances.

     • Advantages

       • Noise resistance
       • Less signal attenuation
       • Higher bandwidth

     • Disadvantages

       • Cost
       • Installation/maintenance
       • Fragility

3. Unquided media or wireless communication transport electromagnetic waves without using a physical conductor.

   • The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands, each regulated by government authorities.

   • In surface propagation radio waves travel through the lowest portion of the atmosphere, hugging the earth.

   • Tropospheric propagation can work two ways. Either a signal can be directed in a straight line from antenna to antenna (line-of-sight), or it can be broadcast at an angle into the upper layers of the troposhere where it is reflected back to the earth's surface.

   • In ionospheric propagation higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. The density difference between the troposhere and the ionosphere causes each radio wave to speed up and change direction, heading back to earth.

   • In line-of-sight propagation, very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth.

   • Space propagation utilizes satellite relays in place of atmospheric refraction.

   • The type of propagation used in radio transmission depends on the frequency (speed) of the signal.
     • Very Low Frequency (VLF) 3-30KHz waves are propagated as surface waves, usually through air but sometimes through seawater. VLF waves are used mostly for long-range radio navigation and for submarine communication.

     • Low Frequency (LF) 30-300KHz waves are also propagated as surface waves. LF waves are used for long-range radio navigation and for radio beacons or navigational locators.

     • Middle frequency (MF) 300KHz,3MHz signals are propagated in the troposphere. These frequencies are absorbed by the ionosphere. MF frequencies are used for AM radio, maritime radio, radio direction finding (RDF), and emergency frequencies.

     • High frequency (HF) 3MHz - 30 MHz signals use ionospheric propagation. Uses for HF signals include amateur radio (ham radio), citizen's band (CB) radio, international broadcasting, military communication, long-distance aircraft and ship communication, telephone, telegraph, and facsimile.

     • Very high frequency (VHF) 30 MHZ - 300 MHZ waves use line of sight propagation. Uses include VHF television, FM radio, aircraft AM radio, and aircraft navigational aid.

     • Ultrahigh frequency (UHF) 300 MHz - 3 GHz waves always use line-of-sight propagation. Uses include UHF television, mobile telephone, cellular radio, paging, and microwave links.

     • Superhigh frequency (SHF) 3 GHz - 30 GHz waves are transmitted mostly line-of-sight and some space propagation. Uses include terrestrial and satellite microwave and radar communication.

     • Extremely high frequency (EHF) 30 GHz - 300 GHz waves use space propagation. Uses are predominantly scientific and include radar, satellite, and experimental communications.

   • Terrestrial microwave does not follow the curvature of the earth and therefore require line-of-sight transmission and reception equipment. A system of repeaters is installed with each antenna.
     • A parabolic dish antenna is based on the geometry of the parabola: every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that they intersect in a common point called the focus.

     • A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel beams by the curved head.

      

   • Satellite transmission is much like line-of-sight microwave transmission in which one of the stations is a satellite orbiting the earth. This requires that the sending and receiving antennas be locked onto each other's location at all times. The satellite must move at the same speed as the earth so that it seems to remain fixed above a certain spot. These satellites must be in geosynchronous orbits.

   • Cellular telephony is designed to provide stable communications connections between two moving devices or between one mobile unit and one stationary (land) unit. Each cellular service area is divided into small regions called cells. Each cell contains an antenna and is controlled by a small office, called the cell office. Each cell office, in turn, is controlled by a switching office called a mobile telephone switching office (MTSO). The MTSO coordinates communication between all of the cell offices and the telephone central office.

4. Transmission impairement
   • Attenuation means loss of energy. When a signal, simple or complex, travels through a medium, it loses some of its energy so that it can overcome the resistance of the medium. To compensate for this loss, amplifiers are used to amplify the signal. The decibel (dB) measures the relative strengths of two signals or a signal at two different points.

     dB = 10log10P2/P1 where P1 and P2  are the power of a signal at points 1 and 2
     
     

   • Distortion means that the signal changes its form or shape. Distortion occurs in a composite signal, made of different frequencies.

   • There are several types of noise such as thermal noise, induced noise, crosstalk, and impulse noise. Thermal noise is the random motion of electrons in a wire that creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. Crosstalk is the effect of one wire on the other. Impulse noise is a spike (a signal with high energy in a very short period of time) that comes from power lines and lightening.

5. Performance

   • Throughput is the number of bits that can pass a point of reference in one second.

   • Propagation speed measures the distance a signal or a bit can travel through a medium in one second. The propagation speed of electromagnetic signals depends on the medium and the frequency of the signal.

   • Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to another. The propagation time is calculated by dividing the distance by the propagation speed.

     	Propagation time = Distance / Propagation speed
     

6. Wavelength binds the period of the frequency of a simple sine wave to the propagation speed of the medium. Although wavelength can be associated with electrical signals, it is customary to use wavelengths when talking about the transmission of light in an optical fiber. The wavelength is the distance a simple signal can travel in one period.

1. Decimal-to-Binary and Binary-to-Decimal
   • Decimal-to-Binary

     Divide the decimal number by two continuously and save the remainders.  The
     string of remainders left over is the binary equivalent.
     
     For example, to convert 50 to Binary.
     
     50 divided by two is 25, remainder is 0
     
     25 divided by two is 12, remainder is 1
     
     12 divided by two is  6, remainder is 0
     
      6 divided by two is  3, remainder is 0
     
      3 divided by two is  1, remainder is 1
     
      1 divided by two is  0, remainder is 1
     
     Therefore, the binary equivalent is 110010.
     

   • Binary to decimal.

     Organize the binary string right to left.  Each position that is occupied by
     a binary 1 is converted by raising two to the power of its position.
     
     
     For example.
     1 1 0 0 1 0
     21 + 24 + 25 = 50.
     

2. Formulas to remember
   • Nyquist

   data rate = 2Hlog2V bits/sec
   where H = bandwidth      V = the number of bits encoded per signal
   Note: log2x = lnx/ln2 = 1.443lnx
   
   Shannon 

   • C = B log2(1 + S/N)
     where C = data rate (bps)      B = bandwidth (Hz)
     
         S/N = the signal-to-noise ratio
     The signal-to-noise ratio is often expressed in dB, so therefore it must first
     be converted.
     
     dB = 10log10(S/N)
     or
     S/N = 10(dB/10)