simultaneous transmission of multiple signals on shared path (bandwidth)

Frequency-division multiplexing (FDM) 

• analog process
• Each signal has carrier of different frequency
• Carriers are spaced to avoid overlap when modulated (guard bands)
• actual signal is composite of all modulated carriers
• demultiplexing uses filters to separate the individual modulated carriers
• example: cable TV (6 Mhz channels mux’ed onto coax)

• Developed for fiber optics
• Same concept as FDM, except signals are digital
• Each distinguished wavelength (color) represents a digital channel
• All wavelengths xmit simultaneously w/o interference
• Therefore each channel gets "full fiber"

Time-division multiplexing (TDM) 

• digital process
• signals are interleaved; each signal gets full path bandwidth for period of time
• analogous to timesharing O.S.
• time partitioned into fixed length frames
• frame partitioned into fixed length time slots (device gets entire bandwidth)
• each frame includes framing bit for MUX/DEMUX synchronization
• synchronous TDM: frame contains fixed reserved time slot for each input device (round-robin)
• asynchronous TDM: each time slot available to any device ready to send
• synch adv: no overhead (sender address not transmitted)
• synch dis: slot wasted if nothing to send, # input signals limited to # slots
• asynch adv/dis: opposite

MUX and telephone service 

• mux and bandwidth go together like soup and sandwich
• analog switched service (POTS) hierarchy:
• you get bandwidth 0-4KHz (minus some)
• your line mux’ed with 11 others into 48KHz group
• 5 groups mux’ed into 240KHz supergroup
• 10 supergroups mux’ed into 2.52MHz mastergroup
• 6 mastergroups mux’ed into 16.98Mhz jumbo group
• digital signal (DS) service hierarchy:
• DS-0: 64Kbps channel
• DS-1: 1.544 Mbps service (or partition into 24 DS-0 + framing bits)
• DS-2: 6.312 Mbps (or partition into 4 DS-1 or 96 DS-0, etc)
• DS-3: 44.376 Mbps (or partition into 7 DS-2, 28 DS-1, 672 DS-0, etc)
• DS-4: 274.176 Mbps (or partition into 6 DS-3, blah, blah, blah)
• lines that carry DS-x service are called T-x (x=1,2,3,4)
• analog on T-1 using PCM
• frame: 1 framing bit plus 8-bit PCM sample for each of 24 channels
• total frame length= 8*24+1 = 193 bits
• at 8000 frames (samples) / second, total is 193*8000 = 1.544Mbps
• digital on T-1 example: ISDN
• ISDN primary rate interface (PRI)
• 23 "B" channels (@64 Kbps) + 1 "D" channel (16 or 64 Kbps)
• See ISDN notes for more info

Related Home Pages: notes | CSC 465 | Peter Sanderson | Computer Science | SMSU

Peter Sanderson ( pete@csc.smsu.edu )