hye alls..!!.cont my reflection on our latest n freshly from oven lect notes.previous post we hve dscussd bout CABLING which is all about the ntworking through cable.so now..diz week,we  proceed with UNBOUNDED/UNGUIDED MEDIA or WIRELESS MEDIA..

wat??no idea bout it??..chill up!. =) just keep reading-*networking dat run with out any physical connectors between the two devices-cable,wire.etc.usually the transmission is send through the atmosphere(air/water)-electromagnetic wave.

there are three types of wireless media:

~radio waves

~micro waves

~infrared waves.

RADIO WAVES

It has frequency between 10 K Hz to 1 G Hz.
types of radio waves:

• Short waves
• VHF (Very High Frequency)
• UHF (Ultra High Frequency)

1)SHORT WAVES:-

There are different types of antennas used for radio waves. Radio waves transmission can be divided into following categories.

• LOW POWER, SINGLE FREQUENCY.As the name shows this system transmits from one frequency and has low power out. The normal operating ranges on these devices are 20 to 25 meter. CHARACTERISTICS LOW POWER , SINGLE FREQUENCY:-
• Low cost
• Simple installation with pre-configured
• 1 M bps to 10 M bps capacity
• High attenuation
• Low immunity to EMI

• HIGH POWER, SINGLE FREQUENCY
  This is similar to low power single frequency. These devices can communicate over greater distances.

          CHARACTERISTICS HIGH POWER, SINGLE FREQUENCY:-

• Moderate cost
• Easier to install than low power single frequency
• 1 Mbps to 10 Mbps of capacity
• Low attenuation for long distances
• Low immunity to EMI

2) Ultra-high frequency (UHF)    
      **designates the ITU radio frequency range of electromagnetic waves between 300 MHz and 3 GHz (3,000 MHz), also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres (10 cm to 1 metre). Radio waves with frequencies above the UHF band fall into the SHF (super-high frequency) and EHF (extremely high frequency) bands, all of which fall into the microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands.

The main advantage!! of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception antennas is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands.

The major disadvantage!! of UHF is its limited broadcast range and reception, often called line-of-sight between the TV station's transmission antenna and customer's reception antenna, as opposed to VHF's very long broadcast range and reception, which is less restricted by line of sight.

MICRO WAVES
Micro waves travels at high frequency than radio waves and provide through put as a wireless network media. It is line of sight transmission (Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country) Micro wave transmission requires the sender to be inside of the receiver.    

 The types of Micro waves.

1. · Terrestrial Micro waves
2. · Satellite Micro waves

1. Terrestrial Micro waves:-

Terrestrial Micro waves are used are used to transmit wireless signals across a few miles. Terrestrial system requires that direct parabolic antennas can be pointed to each other. These systems operate in a low Giga Hertz range.

Characteristics of Terrestrial Micro waves:-

• Moderate to high cost.
• Moderately difficult installation
• 1 M bps to 10 M bps capacity
• Variable attenuation
• Low immunity to EMI

2)Satellite Micro waves

why we need to use these highly EXPENSIVE stuff?? huh..The main problem with aero wave communication is the curvature of the earth, mountains & other structure often block the line of side. Due to this reason, many repeats are required for long distance which increases the cost of data transmission between the two points.
This problem is recommended by using satellites.
**Satellite micro wave transmission is used to transmit signals through out the world. These system use satellites in orbit about 50,000 Km above the earth. Satellite dishes are used to send the signals to the satellite where it is again send back down to the receiver satellite. These transmissions also use directional parabolic antenna’ with in line of side.
In satellite communication micro wave signals at 6 GHz is transmitted from a transmitter on the earth through the satellite position in space. By the time signal reaches the satellites becomes weaker due to 50,000 Km distance. The satellite amplifies week signals and transmits it back to the earth at the frequency less than 6 GHz.

• High cost
• Extremely difficult and hare installation.
• Variable attenuation.
• Low immunity to EMI
• High security needed because a signal send to satellite is broadcasts through all receivers with in satellite.
• Operates on a number of frequency bands, called transponder channels.
• Receives transmissions on one frequency band (uplink), and transmits on another frequency (downlink).
• Satellite period of rotation equals the earth's period of rotation At height of 34,784 km.
• Used for TV distribution, long-distance telephone, and business networks

INFRARED.

Infrared frequencies are just below visible light. These high frequencies allow high sped data transmission. This technology is similar to the use of a remote control for a TV. Infrared transmission can be affected by objects obstructing sender or receiver. These transmissions fall into two categories.

1. Point to point
2. Broadcast

(1) Point to Point: - Point to point infrared transmission signal directly between two systems. Many lap top system use point to pint transmission. These systems require direct alignment between many devices.

Characteristics of Point to point:-

• Wide range of cost
• Moderately easy installation.
• 100 k bps to 16 Mb of capacity.
• Variable attenuation.
• High immunity to EMI

(2) Broad Cast: - These infrared transmission use sprayed signal, one broad cast in all directions instead of direct beam. This help to reduce the problems of proper alignment and abstraction. It also allows multiple receiver of signal

Characteristics of Broad Cast:-

• In expensive.
• Single installation.
• 1M bps capacity.
• Variable attenuation.

**too long to read it all???..hehehe.okeyh now let see how all the unguided media applied in our life..

~CELLULAR PHONE.

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. In a typical analog cell-phone system in the United States, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, like this:


Each cell has a base station that consists of a tower and a small building containing the radio equipment (more on base stations later).
 
A single cell in an analog system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:
 

• A cell-phone carrier typically gets 832 radio frequencies to use in a city.
• Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this on the next page.)
• Therefore, each cell has about 56 voice channels available.

In other words, in any cell, 56 people can be talking on their cell phone at one time. With digital transmission methods, the number of available channels increases. For example, a TDMA-based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

• The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure above, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.
• The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region. Now let's analyses what happens we as you (and your cell phone) move from cell to cell.