Chapter 3: Conducted and Wireless Media

The world of computer network would not exist if there were no medium by which to transfer data. All communications media can be divided into two categories: physical conducted media and radiated or wireless media.

Conducted Media

such as telephone lines and fiber optic cables. There are three existing types of conducted media:(1) twisted pair wire, (2)coaxial cable, and (3)fiber optic cable

1. Twisted Pair Wire

• Comes as two or more pairs of single-conductor copper wires that have been twisted around each other. 
• Twisting of two wires around each other keeps the wire from running parallel and helps reduce crosstalk- a current signal in one wire can transfer unwanted signal the one next to it.
• Twisted pair wire is categorized from 1-7, abbreviated as CAT 1-7. CAT 5e is the most common type used today.

• Fairly inexpensive and good for short distance
• Easily pick up noise signals which results in higher error rates

2. Coaxial cable

• The simple form with  single wire (usually copper) wrapped in a foam insulation, surrounded by a braided metal shield, and then covered in a plastic jacket.
• Baseband coaxial: used digital signaling in which the cable carries only one channel of digital data.
• Broadband coaxial: typically transmits analog signals and is capable of supporting multiple channels of data simultaneously. 
• Also available in thick coaxial cable that ranges from 6-18mm in diameter and carries broadband signals. Thin coaxial cable that is approximately 4mm in diameter carries baseband signals because it has limited noise isolation.
• Cables that employ copper conductors can easily be tapped. However it is to be remembered that no system can ever be perfectly secure. Even fiber-optic lines can be tapped without detection.
• Coaxial cable is more expensive than twisted pair cable and the thicker the cable, the harder it is to work with.

3. Fiber-optic cable

• a thin glass cable, a little thicker than a human hair, surrounded by a plastic coating and packaged into an insulated cable and is surrounded by Aramid yarn and a strong plastic jacket  to protect it from bending, heat and stress.
• Photo diode is placed at the transmitting end and quickly switched on and off to produce light pulses, which then travel down the glass cable and are detected by an optic sensor called photo receptor on the receiving end.
• Fibers do not leak light and are quite difficult to tap, which give the best security out of the three types of cable.
• Fiber optic cable is capable of transmitting data at more than 100 Gbps over long distance.
• However, it still experiences noise as the light pulses bound around inside glass cable.
• it can only travel in one direction, so in order to support two way transmission, two fiber optic cables are needed
• it is the more expensive than twisted wire but less than coaxial

Summary chart of all cables:

Wireless Media

The term wireless refers to the communication or transmission of information over a distance without requiring wires, cables or any other electrical conductors. The Communication is set and the information is transmitted through the air, without requiring any cables, by using electromagnetic waves like radio frequencies, cellphones, infrared, terrestrial microwaves, satellite, etc. It is important that each application use its assigned frequencies.

There are eight different areas of wireless communication systems:

1. Terrestrial microwave transmission:

• Transmit tightly focused beams on radio signals from one ground-based microwave transmission antenna to another
• most common applications are telephone communication and business intercommunication
• Transmitter towers are placed roughly 15-30 miles from each other.
• Objects such as buildings, forests, hills can obstruct the path of transmitting signals.

2. Satellite microwave transmission:

• Similar to terrestrial microwave system except that the signal travels from a ground station on Earth to a satellite and back to another ground station.
• Satellite system can also transmit a signal pathway around the earth by bouncing it from one satellite to another.
• Satellite orbits around the earth in 4 possible ranges:
  (a) Low-earth-orbit (LEO) satellites: closest to earth, from 100 miles from the surface and as far as 1000 miles away. They are used primarily for wireless transfer of electronic mail, worldwide mobile telephone networks, spying, remote sensing, and video conferencing.
  (b) Middle-earth-orbit (MEO) satellites: can be found roughly from 1000 to 3000 miles from earth. They are used primarily for global positioning system (GPS) surface navigation applications.
  (c) Geosynchronous-earth-orbit (GEO) satellites: are found 22,300 miles from earth and always positioned over the same point on earth. Thus, two ground stations can conduct continuous transmission back and forth. They are most used for signal relays for broadcast, cable and direct television, meteorology, government intelligence, and mobile maritime telephone.
  (d) Highly elliptical orbit (HEO) satellites: are used by government for spying and by scientific agencies for observing celestial bodies.

3. Cellphone:

• Uses radio waves
• In order to support hundreds and thousands of simultaneous users within a metropolitan area, the mobile service areas (MSAs) or market is broken down into adjacent cells that have low power transmitter/ receiver placed on free standing towers.
• a cellphone within a cell communicates to cell tower, which also is connected to the cellular switching office (CTSO), if the cellphone moves from one to another, the CTSO hands off the connection from one cell to another.
• Advanced Mobile Phone service (AMPS): 1st generation cell phone system that used frequency division multiplexing technology which operates like television transmission.
• Digital-advancedMobile phone service (D-AMPS): digital equivalent of analog cell phone service that uses time division multiplexing technology that provides greater signal clarity and security.
• Personal Communications service (PCS): 2nd generation cell phones. 
• 2.5 generation cell phones are capable of receiving and transmitting digital data between a cell phone and internet provider. GMS networks were converted to General Package Radio Service (GPRS) that transmit data at 30 kbps to 40 kbps. 
• 3rd generation of cell phones use Universal Mobile Telecommunications system (UMTS) technology that is capable of supporting downstream data rates of 220 kbps to 320 kbps.
• 4th generation is based on LTE (Long term evolution) technology.

4. Broadband Wireless Systems and Wimax

• Broadband wireless system is the wireless local loop or fixed-point wireless that deliver internet services into homes and business.
• the system bypass the telephone company's local loop by transmitting voice, data, and video over very high radio frequencies.
• Wimax is broadband wireless transmission technology that is based upon a series of IEEE standard. 
• Wimax is designed to deliver high-speed internet access to home and business and compete against DSL and cable modems and is called IEEE 802.16a

5. Bluetooth

• Uses low-power, short-range radio frequencies to communicate two or more devices.
• It is typically limited to distance between 10cm and 10 meters.
• Bluetooth is capable of transmitting through nonmetallic objects.
• Can transfer data at high speed up to roughly 700 kbps.
• Can communicate among multiple devices with short range transmission such as portable music player, headset, PDA to computer, house or cars.

6. Wireless Local Area Networks

• Capable of supporting data rates up to 2 Mbps  and allows wireless workstations up to roughly several hundred feet away to communicate with access point
• IEEE 802.11b or Wireless Fidelity (Wifi) transmits data in the 2.4Ghz frequency range
• IEEE 802.11a transmit data at speed up to 54 Mbps uses the same freqiency 2.4Ghz
• IEEE 802.11g shares same frequency range as IEEE 802.11b
• IEEE 802.11n is capable of supporting a 100 Mbps signals 

7. Free Space Optics, Ultra-Wideband, Infrared, Near-field Communications, and Zigbee

• Free space optics:
  -  uses lasers or infrared transmitting devices to transmit data from 2 buildings over short distances.
  - Lasers lose strength when transmitting through fog, building movement, scintillation, absorption by suspended water molecules in the air and temporary blocking the beam from birds, ect.
• Ultra-wideband:
  - transmit data over a wide range of frequencies rather than limiting transmission to a narrow, fixed band of frequencies.
  - Some of the frequencies are used by other sources such as cellular phone system
  - Proponents: even though a wide range of frequencies is used, it transmit at power levels lolw enough that other sources should not be affected
  - Opponents: argues ultra-wideband transmission does affect others and should be controlled
  - Capable of supporting speeds up to 100 Mbps.
  - Used for activities such as transferring data frm a video camera to computer or sending photos directly from digital camera to printers. The system also help firefighters detect human within burning building
• Infrared transmission:
  - uses focused ray of light in he frequency range (10^12-10^14 Mhz).
  -commonly used in remote control devices and transfer data from devices at rate typically no more than 4 Mbps.
• Near-field Communications:
  - Close proximity data transfer, often with 2 devices touching each other
  - Use magnetic induction as is commonly found in radio frequency ID card readers
• ZigBee:
  - Based on IEEE 802.15.4 standard.
  - It is designed for data transmission between smaller devices that require low data transfer rates (20-250 Kbps) and corresponding low power consumptions.
  - Devices are able to keep power consumption low and devices are not communicating with each other.