Multiplexing and Wireless Transmission - Computer Networks

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Today we will learn about Multiplexing and basics of wireless transmission. 

Multiplexing

Its a technique by which different analog and digital streams of transmission can be simultaneously processed over a shared link. All medias of communication i.e air, physical media and light are capable of multiplexing. 

A device called multiplexer divides the physical channel when multiple senders try to send over a single medium and allocated one to each. On the other side, de-multiplexer receives, identifies and sends to different receivers. 

1. Frequency Division Multiplexing

In this frequency is used as a carrier. FDM divides the carrier bandwidth in logical channels and allocates one user to each channel. All channels are divided in a way that they don't overlap with each other. How do they achieve that? - Each channel is separated by guard bands which is a frequency not used by either channel. E.g. Radio channels that you listen to use FDM (Bajate Rahoo.... Red FM)

Multiplexing and Wireless Transmission

2. Time Division Multiplexing

In this, time slots are used for dividing the shared channel among users. Each user can transmit data within the provided time slot. Digital signals are divided in frames equivalent to that of time slot. in TDM, both multiplexer and de-multiplexer are synchronized and switch to next time slot simultaneously. Imagine crudely that Red Fm can play from 9:00 to 10:00 only and 10:00 to 11:00 is fixed for Radio Mirchi. 

Multiplexing and Wireless Transmission

3. Wavelength Division Multiplexing

In this, light is used as signals. As light has different wavelengths multiple optical carrier signals are multiplexed into an optical fibre by using different wavelengths. This is an analog multiplexing technique. 

Multiplexing and Wireless Transmission

4. Code Division Multiplexing

FDM divides the frequency in smaller channels but CDM allow its users full bandwidth and transmit signals all the time using a unique code called chip. Signals travel with these codes independently, inside the whole bandwidth. Receiver knows in advance which chip code signal it has to receive. Again imagine crudely that Red FM uses a code word 'bajatey raho' with which it sends signals and Red FM uses 'Its hot'!!

Wireless Transmission

Wireless communication doesn't involve any physical link between two or more devices. Signals are spread in the air, received and interpreted by antennas. An antenna connected to circuit of a computer converts digital data into wireless signals and on receiving end, another antenna will reconvert it back to digital data.

An electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiations. Some part of this spectrum can be used for wireless communication. An noteworthy point regarding EM waves is that their wavelength and frequencies are inversely proportional to each other. Look at the figure below for classification of EM waves followed by which we will discuss them in brief. 

Multiplexing and Wireless Transmission

1. Radio Transmission

Important points regarding Radio waves are as follows:
  • Can penetrate through walls and structures due to high wavelength and low frequency. 
  • Sub divided into 6 bands, out of which VLF (very low freq), LF and MF can travel on ground along the curvature of earth while HF and VHF bands use Ionosphere for use i.e they are refracted back to earth. 
  • wavelength range: 1 mm - 1,00,000 km, frequency range: 3 Hz to 300 GHz 

2. Microwave Transmission

  • Travel in straight line and receiver must be strictly in line of sight. 
  • Can't penetrate wall like obstacles due to high frequency
  • Depends upon weather conditions
  • wavelength range: 1 mm - 1 m, frequency range: 300 MHz - 300 GHz


3. Infrared Transmission

  • Lie between visible light spectrum and microwaves. 
  • Used for very short range communication such as TV and Remote. 
  • Travel in straight line and can't pass wall like obstacles
  • wavelength range: 700nm - 1 mm,  frequency range: 300 GHz - 430 THz

4. Light Transmission

  • travels strictly in straight line e.g. LASER
  • Can't penetrate obstacles like walls or rain. Also distorted by wind or temperature. 
  • LASER beam is generally 1 mm wide, difficult to tap hence comparatively safe for data transmission. 

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