Sending a Signal Between Computers (23)[detail]

OSI 7 layer

APSTNDLP

communication basics

medium, energy form, way to represent energy as data

PSTN

Public Switched Telephone Network

PSTN Structure

Local loops, trunks and switching offices

synch comm

depends on timing signals provided by the sender

asynch comm

no timing signals required

simplex

comm only in one direction always

half duplex

comm in one direction at a time, can be reversed

full duplex

comm in both directions at a time

RS-232-C

-15V to 15V, start bit, idle period

Nyquist equation

maxD = 2.(bw).log2(num volt levsls)

Shannon-Hartley theorem

D = B log2(1 + S/N)

S/N

dB = 10log10(S/N)

Why nyquist

finite transmission capacity of perfect channel

why shannon-hartley

random noise greatly reduces max data rate

Attenuation

signal on wire becoming weaker with distance, resistance converts energy to heat

amplitude modulation

change strength/alter height of sine wave

freq modulation

change wavelength/speed of sine wave

phase shift modulation

change phase/restart sine wave

multiplexing

multiple signals sent over a medium without interferences

freq div multiplex

divides spectrum into freq bands

time div multiplex

users take turns in round robin

code div multiplex

extract desired signal, rejecting else