DATA COMMUNICATION Lecture-27

Recap of Lecture 26 Guided Media Optical Fiber Cable Unguided Media Radio Frequency Allocation Propagation of Radio Waves

Overview of Lecture 27 Frequency Ranges Microwave Communication Satellite Communication Cellular Telephony

Frequency Ranges The type of propagation used in radio transmission depends upon the frequency of the signal

Very Low Frequency (VLF)

Low Frequency (LF)

Middle Frequency (MF)

High Frequency (HF)

Very High Frequency (VHF)

Ultra High Frequency (UHF)

Ultra High Frequency (UHF)

Super High Frequency (SHF)

Extremely High Frequency (EHF)

Terrestrial Microwave Microwaves do not follow the curvature of earth Line-of-sight transmission Height allows the signal to travel farther

Terrestrial Microwave

Terrestrial Microwave Antennas

Satellite Communication Line-of-sight microwave communication using satellite Satellite acts as a very tall antenna and a repeater

Satellite Communication

Geosynchronous Satellites

Satellite Frequency Bands Each satellite sends and receives over two bands Uplink: From the earth to the satellite Downlink: From the satellite to the earth

Satellite Frequency Bands Band Downlink Uplink C 3.7-4.2 GHz 5.925-6.425 GHz Ku 11.7-12.2 GH 14-14.5 GHz Ka 17.7-21 GHz 27.5-31 GHz

Cellular Telephony Each service area is divided into small ranges called cells Each cell office is controlled by a switching office called MTSO

Cellular Telephony

Operations of Cellular Telephony Transmitting Mobile phone sends the number to the closest cell office Cell office  MTSO  Telephone office MTSO assigns an unused voice channel

Operations of Cellular Telephony Receiving Telephone office sends the signal to MTSO MTSO sends queries to each cell (paging) If mobile phone is found and available, assigns a channel

Operations of Cellular Telephony Handoff MTSO monitors the signal level every few seconds If the strength diminishes, MTSO seeks a new cell and changes the channel carrying the call

Transmission Impairments Transmission media are not perfect What is sent is not what is received

Transmission Impairments

Attenuation Attenuation means loss of energy Some of electrical energy is converted to heat

Attenuation

Decibel (dB) Relative strengths of two signals or a signal at two points dB = 10 log10 (P2/P1) P2 and P1 are signal powers Negative dB means attenuation Positive dB means amplification

Example 7.1 Imagine a signal travels through a transmission medium and its power is reduced to half. This means P2=(1/2)P1. Calculate Attenuation? Solution: 10log10(P2/ P1)= 10log10(0.5 P1/ P1) =10(-0.3)= -3 dB

Distortion Distortion means that the signal changes its form or shape Distortion occurs in a composite signal

Summary Frequency Ranges Microwave Communication Satellite Communication Cellular Telephony Transmission Impairments

Suggested Reading Section 7.2, 7.3 “Data Communications and Networking” 2nd Edition by Behrouz A. Forouzan