1
3.9 Time-Division Multiplexing
? Definition,Time-division multiplexing (TDM) is the
time interleaving of samples from several sources so
that the information from these sources can be
transmitted serially over a single communication
channel,
2
3.9 Time-Division Multiplexing
3
3.9 Time-Division Multiplexing
? in the transmitter
the pulse width of
the TDM PAM,
the pulse width of
the TDM PCM,
3Ts
Ts
4
3.9 Time-Division Multiplexing
? At the receiver the decommutator has to be
synchronized with the incoming waveform so that the
PAM samples corresponding to source 1 will appear
on the channel 1 output,This is called frame
synchronization,which is very important for TDM
system,
? Low-pass filters are used to reconstruct the analog
signals from the PAM samples,
? Because of ISI resulting from poor channel filtering,
PCM samples from one channel may appear on
another channel,often called crosstalk,even though
perfect bit and frame synchronization were
maintained,
5
3.9 Time-Division Multiplexing
? Synchronous system,all devices have same clock,by
data signal with clock,by separate clocking line)
? Asynchronous system,receiver clock is started
aperiodically and no synchronization with a master
clock,
Digital
system
Synchronous
system
Asynchronous
system
Frame synchronization -- grouping data
Word synchronization (start up signaling)
bit synchronization -- regenerating data
6
3.9 Time-Division Multiplexing
(Frame Synchronization)
? Methods for frame sync,
? (1) Sending a frame sync signal from the transmitter
over a separate channel
? (2) Deriving the frame sync from the TDM signal itself,
? TDM frame sync format
C h an n el
N d at a
s 1 s 2 … … s k C h an n el
1 d at a
C h an n el
2 d at a
… C h an n el
N d at a
s 1 s 2 …
I n f orm at ion
w ord s
s yn c w ord s I n f orm at ion
w ord s
s yn c
w ord s
F ram e
F ig,3 - 36 T D M s yn c f or m at
7
3.9 Time-Division Multiplexing
? Frame synchronization word,A segmented bits data
stream which obeys same rules,Usually,it should be
unique in the data stream,or at least,the appear
probability is very small in information words
? SW ----the sync word,usually it appears in the TDM
data stream periodly,So if we locate the sw,then we
could separate Ai from data stream,at same time,it
can be located at other Ai and Aj
8
3.9 Time-Division Multiplexing
(principle of frame synchronizer circuit )
? Coincident
detector is a
K-input
AND gate,
Amplifer
and
filter
Sampler and
decision
circuit
TDM input
Bit
synchronizer Stage 1
bT
sT
Stage
2
Stage
3
Stage
i
…
Frame synchronizer
shift register
2s 3s is
Coincident detector
Figure 3.37 Frame synchronizer with TDM receiver
format end
Z(t)
Regenerated TDM data
Frame sync
1s
9
3.9 Time-Division Multiplexing
? In some situation,when there is word in receiver data
stream happening to match the bits in the sync word,
False sync output pulses will occur,Its probability is,
9 0 )-(3 2= - KfP
? Since the output of the coincident detector is a
digitized crosscorrelation of the sync word with the
passing K-bit word stored in the shift register,The
sync word’s autocorrelation function must have the
properties,Rs(0)=1 and R(K)=0 for k not equal to 0,
? Example if Pf=4*10-5 is the allowed probability of false
sync,then a K=15-bit sync word is required,That is,a
15-stage shift register is needed for the frame
synchronizer in the receiver
10
3.9 Time-Division Multiplexing
? coincident,at each clock,a new binary bit of TDM
data stream is clocked into the shift register,then,the
data in it is corss-correlated with the expected unique
sync word,If there is no difference,the coincident
detector will output a binary 1 with Tb width,or will
output a binary 0 with Tb width,
SW A1 A2 A3 SW
k bits k bits k bits k bits k bits
3k-bits information (known)
sync impulse sync impulse
Received
TDM data
Frame
sync output
11
3.9 Time-Division Multiplexing
? if there is a false sync word (FSW)
SW A1 A2 A3 SW A1 A2 A3 SW A1 A2 FSW SW A1 A2 A3 SW A1 A2 A3 SW A1 FSW A3 SW
FSW FSW
Received
TDM data
Frame
sync output
? Example3-6,Design of a Time-Division Multiplexer
Input,11 sources
Source 1,Analog,2-kHz bandwidth
Source 2,Analog,4-kHz bandwidth
Source 3,Analog,2-kHz bandwidth
Source 4-11,Digital,synchronous at 7,200 bit/s
12
3.9 Time-Division Multiplexing
Suppose,1,4-bitPCM word for each analog source
2,synchronous multiplexer
Solution,
? For analog source,the sampling frequency,fs1=4kHz,
fs2=8kHz,fs3=4kHz,So we can design a multiplexer at
a rotating speed fs 4kHz,then the total output data of
analog source is 64k bits/s,it can segmented eight 8k
bit/s,
? For digital source,pulse stuffing is used to raise the
digital source rate to 8k bit/s
13
3.9 Time-Division Multiplexing
? Fig.3-39 TDM with analog and digital inputs
TMD PCM
output signal
128kbps
Source 1
2kHz
Source 2
4kHz
Source 3
2kHz
Pulse
stuffing Source 4 7.2kbps
digital Pulse
stuffing Source 5 7.2kbps
digital
Pulse
stuffing Source 11 7.2kbps
digital
………..
8kbps
8kbps
8kbps
4-bit A/D fs=4kHz
f2=8kHz
14
3.9 Time-Division Multiplexing
TDM
Used in computer system,data rate,
1.2 2.4 3.6 4.8 7.2 9.6 14.4 19.2 28.8 kbps
10M,100M bps
Used in
common
carriers
CCITT digital TDM hierarchy
64k ? 2.048M ? 8.448M ? 34.368M ? 139.264M ? 565.148M 30 4 4 4 4
VF 1 30 120 480 1920 7680
TMD standards for north America (Japan)
24 DS0 ? DS1 ? DS2 ? DS3 ? DS4 ? DS5 4 8 6 2
VF 1 24 96 672 4032 8064
64k 1.544M 6.312M 44.736M 274.176M 560.160M
T1 T2 T3 T4 T5
E1 E2 E3 E4 E5
15
3.9 Time-Division Multiplexing
Channel 1
PCM word
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Channel 2
PCM word
1 2 3 4 5 6 7 8
Channel 24
PCM word
Framing bit(1 bit) 193bit=8 24+1
125us
…………………………
T1 TDM format for one frame (single)
Sampling frequency (analog source),8kHz
Each sampler coded into,8-bit PCM word
The period of each frame,125 us
The frame rate, 8000 frame/s
The T1 data rate,(193bit/frame)
8000frame/s=1.544Mbps
The duration of each bit,0.6477us
Each voice channel bit rate,8bit/sample 8kHz/s=64kbps
16
3.11 Pulse Time Modulation,Pulse Width
Modulation and Pulse Position Modulation
? Pulse time modulation (PTM) is a class of signaling
techniques that encodes the sample values of an
analog signal onto the time axis of a digital signal,
? Categories,
Pulse Width Modulation (PWM)
Pulse Position Modulation (PPM)
? In PWM,which is also called pulse duration
modulation (PDM) sample values of the analog
waveform are used to determine the width of the pulse
signal,
? In PPM the analog sample value determines the
position of the narrow pulse relative to the clocking
time,
17
3.9 Time-Division Multiplexing
18
3.9 Time-Division Multiplexing
19
3.9 Time-Division Multiplexing
20
3.9 Time-Division Multiplexing
21
Summary(1)
? Baseband digital signaling,
? How the information in analog waveforms can be
represented by digital signaling,
? How to compute the spectra for line codes
? How filtering of the digital signal,due to the
communication channel,affects out ability to recover
the digital information at the receiver
? How we can merge the information from several
sources into one digital signal by using TDM
22
Summary(2)
? PCM is an analog-to-digital conversion scheme that
involves three basic operation,
? (1) sampling a bandlimited analog signal ;
? (2) quantizing the analog samples into M discrete
value and
? (3) encoding each smaple value into an n-bit word
where M=2n,
? Two source of noise in the signal that is recovered at
the receiver output,
? (1) quantizing noise ; (2) noise due to receiver bit
detection errors; (3) noise due to aliasing,
23
Summary(3)
? ISI due to improper channel filtering
? Raised cosine-rolloff Nyquist filter
? The minimum bandwidth required to pass a digital
signal without ISI was equal to one-half of the baud
rate,
? The channel bandwidth may be reduced if multilevel
signal techniques are used (for a given data rate R)
3.9 Time-Division Multiplexing
? Definition,Time-division multiplexing (TDM) is the
time interleaving of samples from several sources so
that the information from these sources can be
transmitted serially over a single communication
channel,
2
3.9 Time-Division Multiplexing
3
3.9 Time-Division Multiplexing
? in the transmitter
the pulse width of
the TDM PAM,
the pulse width of
the TDM PCM,
3Ts
Ts
4
3.9 Time-Division Multiplexing
? At the receiver the decommutator has to be
synchronized with the incoming waveform so that the
PAM samples corresponding to source 1 will appear
on the channel 1 output,This is called frame
synchronization,which is very important for TDM
system,
? Low-pass filters are used to reconstruct the analog
signals from the PAM samples,
? Because of ISI resulting from poor channel filtering,
PCM samples from one channel may appear on
another channel,often called crosstalk,even though
perfect bit and frame synchronization were
maintained,
5
3.9 Time-Division Multiplexing
? Synchronous system,all devices have same clock,by
data signal with clock,by separate clocking line)
? Asynchronous system,receiver clock is started
aperiodically and no synchronization with a master
clock,
Digital
system
Synchronous
system
Asynchronous
system
Frame synchronization -- grouping data
Word synchronization (start up signaling)
bit synchronization -- regenerating data
6
3.9 Time-Division Multiplexing
(Frame Synchronization)
? Methods for frame sync,
? (1) Sending a frame sync signal from the transmitter
over a separate channel
? (2) Deriving the frame sync from the TDM signal itself,
? TDM frame sync format
C h an n el
N d at a
s 1 s 2 … … s k C h an n el
1 d at a
C h an n el
2 d at a
… C h an n el
N d at a
s 1 s 2 …
I n f orm at ion
w ord s
s yn c w ord s I n f orm at ion
w ord s
s yn c
w ord s
F ram e
F ig,3 - 36 T D M s yn c f or m at
7
3.9 Time-Division Multiplexing
? Frame synchronization word,A segmented bits data
stream which obeys same rules,Usually,it should be
unique in the data stream,or at least,the appear
probability is very small in information words
? SW ----the sync word,usually it appears in the TDM
data stream periodly,So if we locate the sw,then we
could separate Ai from data stream,at same time,it
can be located at other Ai and Aj
8
3.9 Time-Division Multiplexing
(principle of frame synchronizer circuit )
? Coincident
detector is a
K-input
AND gate,
Amplifer
and
filter
Sampler and
decision
circuit
TDM input
Bit
synchronizer Stage 1
bT
sT
Stage
2
Stage
3
Stage
i
…
Frame synchronizer
shift register
2s 3s is
Coincident detector
Figure 3.37 Frame synchronizer with TDM receiver
format end
Z(t)
Regenerated TDM data
Frame sync
1s
9
3.9 Time-Division Multiplexing
? In some situation,when there is word in receiver data
stream happening to match the bits in the sync word,
False sync output pulses will occur,Its probability is,
9 0 )-(3 2= - KfP
? Since the output of the coincident detector is a
digitized crosscorrelation of the sync word with the
passing K-bit word stored in the shift register,The
sync word’s autocorrelation function must have the
properties,Rs(0)=1 and R(K)=0 for k not equal to 0,
? Example if Pf=4*10-5 is the allowed probability of false
sync,then a K=15-bit sync word is required,That is,a
15-stage shift register is needed for the frame
synchronizer in the receiver
10
3.9 Time-Division Multiplexing
? coincident,at each clock,a new binary bit of TDM
data stream is clocked into the shift register,then,the
data in it is corss-correlated with the expected unique
sync word,If there is no difference,the coincident
detector will output a binary 1 with Tb width,or will
output a binary 0 with Tb width,
SW A1 A2 A3 SW
k bits k bits k bits k bits k bits
3k-bits information (known)
sync impulse sync impulse
Received
TDM data
Frame
sync output
11
3.9 Time-Division Multiplexing
? if there is a false sync word (FSW)
SW A1 A2 A3 SW A1 A2 A3 SW A1 A2 FSW SW A1 A2 A3 SW A1 A2 A3 SW A1 FSW A3 SW
FSW FSW
Received
TDM data
Frame
sync output
? Example3-6,Design of a Time-Division Multiplexer
Input,11 sources
Source 1,Analog,2-kHz bandwidth
Source 2,Analog,4-kHz bandwidth
Source 3,Analog,2-kHz bandwidth
Source 4-11,Digital,synchronous at 7,200 bit/s
12
3.9 Time-Division Multiplexing
Suppose,1,4-bitPCM word for each analog source
2,synchronous multiplexer
Solution,
? For analog source,the sampling frequency,fs1=4kHz,
fs2=8kHz,fs3=4kHz,So we can design a multiplexer at
a rotating speed fs 4kHz,then the total output data of
analog source is 64k bits/s,it can segmented eight 8k
bit/s,
? For digital source,pulse stuffing is used to raise the
digital source rate to 8k bit/s
13
3.9 Time-Division Multiplexing
? Fig.3-39 TDM with analog and digital inputs
TMD PCM
output signal
128kbps
Source 1
2kHz
Source 2
4kHz
Source 3
2kHz
Pulse
stuffing Source 4 7.2kbps
digital Pulse
stuffing Source 5 7.2kbps
digital
Pulse
stuffing Source 11 7.2kbps
digital
………..
8kbps
8kbps
8kbps
4-bit A/D fs=4kHz
f2=8kHz
14
3.9 Time-Division Multiplexing
TDM
Used in computer system,data rate,
1.2 2.4 3.6 4.8 7.2 9.6 14.4 19.2 28.8 kbps
10M,100M bps
Used in
common
carriers
CCITT digital TDM hierarchy
64k ? 2.048M ? 8.448M ? 34.368M ? 139.264M ? 565.148M 30 4 4 4 4
VF 1 30 120 480 1920 7680
TMD standards for north America (Japan)
24 DS0 ? DS1 ? DS2 ? DS3 ? DS4 ? DS5 4 8 6 2
VF 1 24 96 672 4032 8064
64k 1.544M 6.312M 44.736M 274.176M 560.160M
T1 T2 T3 T4 T5
E1 E2 E3 E4 E5
15
3.9 Time-Division Multiplexing
Channel 1
PCM word
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Channel 2
PCM word
1 2 3 4 5 6 7 8
Channel 24
PCM word
Framing bit(1 bit) 193bit=8 24+1
125us
…………………………
T1 TDM format for one frame (single)
Sampling frequency (analog source),8kHz
Each sampler coded into,8-bit PCM word
The period of each frame,125 us
The frame rate, 8000 frame/s
The T1 data rate,(193bit/frame)
8000frame/s=1.544Mbps
The duration of each bit,0.6477us
Each voice channel bit rate,8bit/sample 8kHz/s=64kbps
16
3.11 Pulse Time Modulation,Pulse Width
Modulation and Pulse Position Modulation
? Pulse time modulation (PTM) is a class of signaling
techniques that encodes the sample values of an
analog signal onto the time axis of a digital signal,
? Categories,
Pulse Width Modulation (PWM)
Pulse Position Modulation (PPM)
? In PWM,which is also called pulse duration
modulation (PDM) sample values of the analog
waveform are used to determine the width of the pulse
signal,
? In PPM the analog sample value determines the
position of the narrow pulse relative to the clocking
time,
17
3.9 Time-Division Multiplexing
18
3.9 Time-Division Multiplexing
19
3.9 Time-Division Multiplexing
20
3.9 Time-Division Multiplexing
21
Summary(1)
? Baseband digital signaling,
? How the information in analog waveforms can be
represented by digital signaling,
? How to compute the spectra for line codes
? How filtering of the digital signal,due to the
communication channel,affects out ability to recover
the digital information at the receiver
? How we can merge the information from several
sources into one digital signal by using TDM
22
Summary(2)
? PCM is an analog-to-digital conversion scheme that
involves three basic operation,
? (1) sampling a bandlimited analog signal ;
? (2) quantizing the analog samples into M discrete
value and
? (3) encoding each smaple value into an n-bit word
where M=2n,
? Two source of noise in the signal that is recovered at
the receiver output,
? (1) quantizing noise ; (2) noise due to receiver bit
detection errors; (3) noise due to aliasing,
23
Summary(3)
? ISI due to improper channel filtering
? Raised cosine-rolloff Nyquist filter
? The minimum bandwidth required to pass a digital
signal without ISI was equal to one-half of the baud
rate,
? The channel bandwidth may be reduced if multilevel
signal techniques are used (for a given data rate R)
