Transcript EE 3760 chapter 7 - Seattle Pacific University

Multiplexing:
Sharing a single medium between multiple users
Based on Chapter 8 of William Stallings, Data and
Computer Communication, 7th Ed.
Kevin Bolding
Electrical Engineering
Seattle Pacific University
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Multiplexing
No. 1
Sharing
• Multiplexing is all about sharing
• Multiple users want to use the same medium
• Cost savings
• Fewer wires/fibers
• Use of large capacity links
• Statistical usage
• Necessity
• Airwaves are not private property!
• So, how can we share?
• Any way that we can filter out everybody else’s signal
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Multiplexing
No. 2
Methods of Multiplexing
• Frequency (wavelength) division
• Each channel gets a portion of the total bandwidth
• Use band-pass filtering
• Time division
• Each channel gets the whole bandwidth for a portion
of the time
• Use time-slot filtering – Synchronous
• Use demand-driven techniques - Asynchronous
• Code division
• Each channel has an individual digital code
• Transmits on many bands at once (spread-spectrum)
• Uses digital processing to filter out signals
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Multiplexing
No. 3
Frequency Division Multiplexing
Also called Wavelength
Division Multiplexing
(WDM)
• FDM can be used any time a channel’s required
bandwidth is less than the medium’s total bandwidth
• Simply assign each channel a portion of the bandwidth
Based on Stallings, Fig. 8.5
Single speech signal
0 0.3
kHz
3.4 4
note: dual sidebands
0
60
64
AM Modulated to 64kHz
kHz
68
transmit only
one sideband
0
60
64
68
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72
kHz
Multiplexing
Multiplexed with
other signals
No. 4
Time Division Multiplexing
• Use all of the bandwidth for each channel
• Divide the usage based on time slots
• Normally used only with digital data
Mux
• Synchronous TDM
• Each channel has a fixed, regularly occurring slot
• It’s 4:03:00.03982, this must be channel 3…
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Multiplexing
No. 5
North American TDM Standards
AT&T
SONET
Name
Voice
Mbps
Channels
Name
Data
Payload
Rate (Mbps) Rate (Mbps)
DS-0
1
0.064
OC-1
51.84
50
DS-1(T1) 24
1.544
DS-1c
48
3.152
OC-3
OC-12
155.52
622.08
150
601
DS-2
96
6.312
OC-24
OC-48
1244.16
2488.32
1202
2405
OC-192 9953.28
9621
OC-768 39813.12
38485
OC-3072
153944
DS-3(T3) 672
44.736
DS-4
274.176
4032
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159252.4
Multiplexing
No. 6
Asynchronous TDM
• Synchronous TDM reserves space for the maximum
channel rate
• Always allocated, even if input stream is idle
• Wiser allocation:
• Allocate a slot for a channel only when it is needed
• Issues
• How do we know what channel a slot is for?
• Put a header in each slot (packet)
• How do we manage all of the different needs of input
streams?
Asynchronous TDM – Use packets (datagrams) instead of time slots
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Multiplexing
No. 7
Code Division Multiplexing
• Instead of allocating discrete time/frequency units,
allow multiple users to use the whole bandwidth
• Use digital coding techniques to separate users
• Each sender has a unique digital code
• All data is encoded with this code; receiver separates
signals by codes
• Spread-spectrum technique
Shannon’s Law: C=B log2(SNR+1)
SS: Large bandwidth, low power
Signal
10x Spreading Code
Encoded signal (10x BW)
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Multiplexing
No. 8
CDMA – Walsh Codes
Hadamard-Walsh codes are mutually orthogonal
After being combined, they can all be separated back out
Walsh functions of order 2 (can combine two sequences)
The (0) code is used to transmit a binary 0, the (1) for a binary 1
W20(0) = +1 +1
W20(1) = -1 -1
W21(0) = +1 -1
W21(1) = -1 +1
Note: 2-times spreading –
Each bit becomes two
chips
To transmit: Sum codes from all channels
Ch. 0 - 0: +1 +1
Ch. 1 - 0: +1 -1
Ch. 0 - 0: +1 +1
Ch. 1 - 1: -1 +1
Sum:
Sum:
2 0
0 +2
Ch. 0 - 1: -1 -1
Ch. 1 - 0: +1 -1
Ch. 0 - 1: -1 -1
Ch. 1 - 1: -1 +1
Sum:
Sum:
0 -2
-2 0
All summed combinations are unique – can separate out the original code
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Multiplexing
No. 9
Larger Walsh Codes
• Walsh codes are (nearly)
mutually orthogonal codes of
any degree
• Some correlation in larger
codes, but minimal
• CDMA uses 64-bit Walsh
codes
• 64x Spreading
• Can support 64
simultaneous transmissions
on the same frequency band
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An 8-way Walsh code
(Note: Use negative of code to send 0)
W80
+ + + + + + + +
W81
+ -
W82
+ + -
-
W8 3
+ -
+ + -
W84
+ + + + -
-
W85
+ -
W86
+ + -
-
W87
+ -
+ -
Multiplexing
+ -
-
+ -
-
+ -
+ -
+ + -
-
-
+
-
-
-
+ -
+
-
-
+ +
+ + -
No. 10
Using Walsh Codes: 8-sender Example
Sending (Modulating) Process
Time for 1 Bit
8 Chips
C0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
C1
1
1
1
-1
1
-1
1
-1
1
-1
1
-1
1
-1
1
-1
1
-1
C2
0
-1
1
1
-1
-1
1
1
-1
-1
-1
-1
1
1
-1
-1
1
1
C3
0
-1
1
-1
-1
1
1
-1
-1
1
-1
1
1
-1
-1
1
1
-1
C4
0
-1
1
1
1
1
-1
-1
-1
-1
-1
-1
-1
-1
1
1
1
1
C5
1
1
1
-1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
-1
1
C6
0
-1
1
1
-1
-1
-1
-1
1
1
-1
-1
1
1
1
1
-1
-1
C7
1
1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
-1
1
1
-1
Walsh matrix: Multiply
Data to
data to send by row.
send by 8
Spreads each bit 8x.
senders
(Binary 1 represented
by +1, Binary 0
represented by -1)
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Each row
represents
8 chips
sent by
that
sender
On the common channel, all
signals are effectively summed
when combined in airwaves
0
-4
4
0
0
4
4
0
This is sent on the channel over
one bit time (8 chip times)
Multiplexing
No. 11
Using Walsh Codes: 8-sender Example
0
-4
4
0
0
4
4
0
1
1
1
1
1
1
1
1
1
-1
1
-1
1
-1
1
-1
1
1
-1
-1
1
1
-1
-1
1
-1
-1
1
1
-1
-1
1
1
1
1
1
-1
-1
-1
-1
1
-1
1
-1
-1
1
-1
1
1
1
-1
-1
-1
-1
1
1
1
-1
-1
1
-1
1
1
-1
This is sent on the channel over
one bit time (8 chip times)
Receiving
(Demodulating)
Process
Walsh matrix:
Multiply
received data
by column.
Sum rows
8  Binary 1
-8  Binary 0
0
-4
4
0
0
4
4
0
8
C0
1
0
4
4
0
0
-4
4
0
8
C1
1
0
-4
-4
0
0
4
-4
0
-8
C2
0
0
4
-4
0
0
-4
-4
0
-8
C3
0
0
-4
4
0
0
-4
-4
0
-8
C4
0
0
4
4
0
0
4
-4
0
8
C5
1
0
-4
-4
0
0
-4
4
0
-8
C6
0
0
4
-4
0
0
4
4
0
8
C7
1
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Each channel recovers the
original bit sent to it
Multiplexing
No. 12
Multiplexing Summary
• Three basic methods of division:
• Frequency
• Time
• Code (digital)
• Can combine methods:
• Frequency-division into large bands, then timedivision within each band
• SONET works this way
• Time-division over a single CDMA channel
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Multiplexing
No. 13