03-05-2012, 12:58 PM
Multiplexing
[attachment=21152]
Frequency Division Multiplexing
FDM
Useful bandwidth of medium exceeds required bandwidth of channel
Each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
Analog Carrier Systems
AT&T (USA)
Hierarchy of FDM schemes
Group
12 voice channels (4kHz each) = 48kHz
Range 60kHz to 108kHz
Supergroup
60 channel
FDM of 5 group signals on carriers between 420kHz and 612 kHz
Mastergroup
10 supergroups
WDM Operation
Same general architecture as other FDM
Number of sources generating laser beams at different frequencies
Multiplexer consolidates sources for transmission over single fiber
Optical amplifiers amplify all wavelengths
Typically tens of km apart
Demux separates channels at the destination
Mostly 1550nm wavelength range
Was 200MHz per channel
Now 50GHz
TDM Link Control
No headers and trailers
Data link control protocols not needed
Flow control
Data rate of multiplexed line is fixed
If one channel receiver can not receive data, the others must carry on
The corresponding source must be quenched
This leaves empty slots
Error control
Errors are detected and handled by individual channel systems
[attachment=21152]
Frequency Division Multiplexing
FDM
Useful bandwidth of medium exceeds required bandwidth of channel
Each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
Analog Carrier Systems
AT&T (USA)
Hierarchy of FDM schemes
Group
12 voice channels (4kHz each) = 48kHz
Range 60kHz to 108kHz
Supergroup
60 channel
FDM of 5 group signals on carriers between 420kHz and 612 kHz
Mastergroup
10 supergroups
WDM Operation
Same general architecture as other FDM
Number of sources generating laser beams at different frequencies
Multiplexer consolidates sources for transmission over single fiber
Optical amplifiers amplify all wavelengths
Typically tens of km apart
Demux separates channels at the destination
Mostly 1550nm wavelength range
Was 200MHz per channel
Now 50GHz
TDM Link Control
No headers and trailers
Data link control protocols not needed
Flow control
Data rate of multiplexed line is fixed
If one channel receiver can not receive data, the others must carry on
The corresponding source must be quenched
This leaves empty slots
Error control
Errors are detected and handled by individual channel systems