Transcript Multiplexing 3/9/2009

Multiplexing
3/9/2009
Multiplexing

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
Multiple links on 1 physical line
Common on long-haul, high capacity, links
More cost effective when multiplexing is used
Include
 frequency division multiplexing,
 time division multiplexing,
 code division multiplexing.
Multiplexing Techniques
FDM
TDM
CDM
Advantages and Disadvantages
Frequency Division Multiplexing
FDM
 Analog signaling is used to transmits
the signals.
 Examples:
 Broadcast radio and television,
 cable television,
 the AMPS cellular phone systems
 The oldest multiplexing technique.
 It involves analog signaling, it is more
susceptible to noise.
FDM –
Analog
Signals
Summed
Summed
Basic Operation
Subcarriers
 Multiple signals can
be modulated using
different frequencies
 Signals with
different frequencies
can be Multiplexed
together
 The multiplexed
signal has a center
frequency and
bandwidth

Larger than total
BW of all
multiplexed signals
Shifted in
frequency
No
overlap
B=Total BW
FDM –
Example
 Transmitted TV signal
 Total BW is 6 MHZ
 Audio carrier operating at
fca at f0+5.75
 Color subcarrier operating
at fcc at f0+4.799545
MHz
 Video subcarrier operating
at fvc at f0+1.25 MHz
 CATV has a bandwidth of
about 500 MHZ
 Many channels can be
multiplexed together!
FDM –
Multiplexing three voice signals
 Voice signal has a
range of 300-3400
KHz
 Recall FM
Carrier
Freq.
[1  x(t )] cos 2f c t
x(t )  cos 2f mt
 cos 2f ct  cos 2 ( f c  f m )t  cos 2 ( f c  f m )t
Lower
Sideband
Fc-Bw
Upper
Sideband
Fc+Bw
s1
Analog Carrier Systems
 long-distance links use an FDM hierarchy
 AT&T (USA) and ITU-T (International) variants
 Group


12 voice channels (4kHz each) = 48kHz
in range 60kHz to 108kHz
 Supergroup


FDM of 5 group signals supports 60 channels
on carriers between 420kHz and 612 kHz
 Mastergroup

FDM of 10 supergroups supports 600 channels
 so original signal can be modulated many times
Analog Carrier Systems
4KHz
each
Range
Wavelength Division
Multiplexing


FDM with multiple beams of light at
different frequency
carried over optical fiber links
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architecture similar to other FDM
systems
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commercial systems with 160
channels of 10 Gbps
lab demo of 256 channels 39.8
Gbps
multiplexer consolidates laser
sources (1550nm) for transmission
over single fiber
Optical amplifiers amplify all
wavelengths
Demux separates channels at the
destination
also have Dense Wavelength
Division Multiplexing (DWDM)
Synchronous Time Division
Multiplexing
 The original time division
multiplexing.
 The multiplexor accepts input from
attached devices in a round-robin
fashion and transmit the data in a
never ending pattern.
 T-1 and ISDN telephone lines are
common examples of synchronous
time division multiplexing.
Synchronous Time Division
Multiplexing - TDM
TDM –
Basic Operation
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
 corresponding source must be quenched
 leaving empty slots
 error control
 errors detected & handled on individual channel
Example –
Only one station is transmitting!
Another Example
-Byte interleaving
-Characters are shuffled!
-The receiver reassembles each channel
Pulse Stuffing
 Reasons
 synchronizing data sources
 having clocks in different sources drifting
 having data rates from different sources not related by
simple rational number
 Pulse Stuffing a common solution
 have outgoing data rate (excluding framing bits) higher
than sum of incoming rates
 stuff extra dummy bits or pulses into each incoming
signal until it matches local clock
 stuffed pulses inserted at fixed locations in frame and
removed at demultiplexer
Example –
TDM of analog and digital sources with different transmission rates
Analog
Analog
Sampler
A/D
Converter
1
Example –
TDM of analog and digital sources with different transmission rates
Conversing
Analog
To Digital
Analog
Sampler
A/D
Converter
1
Example –
TDM of analog and digital sources with different transmission rates
Digital
R
Digital
Digital Signal
Pulse Stuffing
Digital
R’
Example –
TDM of analog and digital sources with different transmission rates
16 Bits
32 Bits
2 Bit Each
Note: Voice signal, hence minimum BW requirement is 4KHz per bit
Digital Carrier Systems
 long-distance links use an TDM
hierarchy
 AT&T (USA) and ITU-T variants
 US system based on DS-1 format
 can carry mixed voice and data signals
 24 channels used for total data rate
1.544Mbps
 each voice channel contains one word
of digitized data (PCM, 8000 samples
per sec)
 same format for 56kbps digital data
 can interleave DS-1 channels for
higher rates

e.g., DS-2 is four DS-1 at 6.312Mbps
DS-1 Transmission Format
; Bit 8 indicated voice or data
(6x8KHz or 5x9.6KHz or 10x4.8 KHz) – The first bit is used to indicate the subrate
Statistical TDM
 in Synch TDM many slots are wasted
 Statistical TDM allocates time slots
dynamically based on demand
 multiplexer scans input lines and collects
data until frame full
 line data rate lower than aggregate input line
rates
 may have problems during peak periods
 must buffer inputs
Comparing Synchronous and Statistical TDM
Frame Format
Each slot has channel ID and possibly message length information
http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/m2/temari/seccio9/t1e1.htm
Performance of Statistical TDM
 Refer to your notes.
Asymmetrical Digital Subscriber
Line (ADSL)
 link between subscriber and network
 uses currently installed twisted pair
cable
 is Asymmetric - bigger downstream than
up
 uses Frequency division multiplexing
 has a range of up to 5.5km
 Its underlying technology is Discrete
Multitone (DMT)
http://www.cs.tut.fi/tlt/stuff/adsl/node6.html#kuspektri
High-bit-rate Digital Subscriber Line (HDSL) –
Connecting the subscriber to the PSTN
http://www.thenetworkencyclopedia.com/d2.asp?ref=872
Discrete Multitone (DMT)
 A multiplexing technique commonly found in digital
subscriber line (DSL) systems
 The basic idea of DMT is to split the available
bandwidth into a large number of subchannels
 DMT then combines hundreds of different signals, or
subchannels, into one stream
 Each subchannel is quadrature amplitude modulated
(recall - eight phase angles, four with double
amplitudes)
 Theoretically, 256 subchannels, each transmitting 60
kbps, yields 15.36 Mbps. Unfortunately, there is
noise.
Discrete Multitone (DMT)
 Idea: If some subchannel can not carry any data, it can
be turned off and the use of available bandwidth is
optimized
High attenuation at higher
frequencies
http://www.cs.tut.fi/tlt/stuff/adsl/node22.html#kudmtex
Discrete Multitone (DMT) – How…
 On initialization, the DMT
modem sends test signals to
each subchannel to determine
its S/N
 Channels with higher S/N
receive more bits
 Each subchannel is quadrature
amplitude modulated (recall eight phase angles, four with
double amplitudes)