Time Division Multiplexing
Bit versus Byte versus Frame
Interleaving
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Time Division Multiplexing – Bit
Interleaving
Bit Period
4th Bit
3rd Bit
2nd Bit
1st Bit
1
1
1
1
2
2
2
2
1 4 3 2 1 4 3 2 1
3
3
3
3
4
4
4
4
Combined Bit Stream
TDM
• Bit interleaving is simple. There is no buffering and bit
timing is preserved.
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Signal Strength
Analog Voice Signal Encoding with
Pulse Code Modulation (PCM)
Analog
Voice
Signal
8 Bit Sample
8000 per sec
Time
Byte
Byte
125 µs
125 µs
Byte
125 µs
Byte
125 µs
• The DSO digitized voice signal has bytes every 125 microseconds.
OPTI 500, Spring 2011, TDM, WDM Networks
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History of PCM
www.goforich.co.uk
www.edhird.com
• Alexander Graham Bell demonstrated a telephone in
1876
• Alec Reeves conceived pulse code modulation in 1937
OPTI 500, Spring 2011, TDM, WDM Networks
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Time Division Multiplexing – Byte
Interleaving
125 µs
4th Byte
3rd Byte
2nd Byte
1st Byte
1
1
1
1
2
2
2
2
Buffer
MultiLine
Bus
125 µs
1 4 3 2 1 4 3 2 1
3
3
3
3
4
4
4
4
Combined Byte Stream
TDM
• Byte interleaving takes advantage of technology
developed for computers and preserves byte timing
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Time Division Multiplexing – Frame
Interleaving
125 µs
4th Byte
3rd Byte
2nd Byte
1st Byte
1
1
1
1
2
2
2
2
Larger
Buffer
375 µs
1
3
3
3
3
4
4
4
4
31.25 µs
2 2 2 2 1 1 1 1
Combined Byte Stream
TDM
• Frame interleaving requires larger buffers and does not
preserve byte timing
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Staggered Frame Arrival
Frame
Frame
Frame
Time
Division
Multiplexer
MUXed Frame
Frame
• Frames do not arrive at the same time.
• SONET byte interleaves and does not wait for lagging
frames.
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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SONET Synchronous Payload Envelope
Overhead
Payload
90 Columns
H1 H2
J1
Synchronous Payload Envelope (SPE)
Part 1
STS-1
Frame 1
Synchronous Payload Envelope (SPE)
Part 2
STS-1
Frame 2
• The synchronous payload envelope is allowed to extend
over two SONET frames
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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WDM Network Links
λ 1, λ 2, λ 3 … λ n
• Optical Fibers in WDM Networks carry multiple
signals each with a distinct wavelength.
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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WDM Network Equipment
Wavelength
Multiplexer/
Demultiplexer
• Combines optical
signals with
different
wavelengths into
one combined
optical signal
Reconfigurable
Add/Drop Multiplexer
• Same as OADM but
we can change the
wavelengths to be
added or dropped.
Optical Add/Drop
Multiplexer
• Adds or drops
wavelengths
from a WDM link
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
Wavelength Convertor
• Changes the
wavelength of a signal
10
A Linear WDM Network
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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A WDM Ring
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Data Communication Networks
• Data communication networks typically do not
send real time data
• As a result, packet switching is often used
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Circuit Switching
(Telecom Networks)
4 3 2 1
In
2 1
3
4
2 1
4 3
4 3 2 1
Out
4
3
2
1
• When data is “circuit switched” a fixed path is
established for the duration of the transfer
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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Packet Switching
4 3 2 1
2
2 1
In
4
2
1
3
1 4 3 2
Out
4
3
4
3
1
• When data is switched packet by packet,
individual packets (or frames) can follow
separate paths
OPTI 500, Spring 2011, Lecture 4, TDM, WDM Networks
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