Multiplexing No. 1 Seattle Pacific University Multiplexing: Sharing a single medium between...
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Transcript of Multiplexing No. 1 Seattle Pacific University Multiplexing: Sharing a single medium between...
Multiplexing No. 1Seattle Pacific University
Multiplexing:Sharing a single medium between multiple users
Kevin BoldingElectrical Engineering
Seattle Pacific University
Based on Chapter 8 of William Stallings, Data and Computer Communication, 7th Ed.
Multiplexing No. 2Seattle Pacific University
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
Multiplexing No. 3Seattle Pacific University
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
Multiplexing No. 4Seattle Pacific University
Frequency Division Multiplexing
• 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
kHz0 0.3 3.4 4
Single speech signal
AM Modulated to 64kHz
60 64 68
note: dual sidebands
kHz0
60 64 68
transmit only one sideband
kHz0
Multiplexed with other signals
72
Bas
ed o
n S
talli
ngs,
Fig
. 8.
5Also called WavelengthDivision Multiplexing (WDM)
Multiplexing No. 5Seattle Pacific University
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…
Multiplexing No. 6Seattle Pacific University
North American TDM Standards
Name Voice Mbps Channels
DS-0 1 0.064
DS-1(T1) 24 1.544
DS-1c 48 3.152
DS-2 96 6.312
DS-3(T3) 672 44.736
DS-4 4032 274.176
Name Data Payload Rate (Mbps) Rate (Mbps)
OC-1 51.84 50
OC-3 155.52 150OC-12 622.08 601
OC-24 1244.16 1202OC-48 2488.32 2405
OC-192 9953.28 9621
AT&T SONET
OC-768 39813.12 38485
OC-3072 159252.4 153944
Multiplexing No. 7Seattle Pacific University
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
Multiplexing No. 8Seattle Pacific University
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
Signal
10x Spreading Code
Encoded signal (10x BW)
Shannon’s Law: C=B log2(SNR+1)SS: Large bandwidth, low power
Multiplexing No. 9Seattle Pacific University
CDMA – Walsh Codes
Hadamard-Walsh codes are mutually orthogonalAfter 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 -1W21(0) = +1 -1 W21(1) = -1 +1
To transmit: Sum codes from all channels
Ch. 0 - 0: +1 +1Ch. 1 - 1: -1 +1
Sum: 0 +2
Ch. 0 - 0: +1 +1Ch. 1 - 0: +1 -1
Sum: 2 0
Ch. 0 - 1: -1 -1Ch. 1 - 0: +1 -1
Sum: 0 -2
Ch. 0 - 1: -1 -1Ch. 1 - 1: -1 +1
Sum: -2 0
All summed combinations are unique – can separate out the original code
Note: 2-times spreading – Each bit becomes two chips
Multiplexing No. 10Seattle Pacific University
Larger Walsh Codes
• Walsh codes are (nearly) mutually orthogonal codes of any degree
• Some correlation in larger codes, but minimal
-++-+--+W87
++----++W86
+-+--+-+W85
----++++W84
+--++--+W83
--++--++W82
-+-+-+-+W81
++++++++W80
An 8-way Walsh code(Note: Use negative of code to send 0)
• CDMA uses 64-bit Walsh codes
• 64x Spreading
• Can support 64 simultaneous transmissions on the same frequency band
Multiplexing No. 11Seattle Pacific University
Using Walsh Codes: 8-sender Example
-111-11-1-11
11-1-1-1-111
1-11-1-11-11
-1-1-1-11111
1-1-111-1-11
-1-111-1-111
-11-11-11-11
11111111
Walsh matrix: Multiply data to send by row.Spreads each bit 8x.
044004-40
On the common channel, all signals are effectively summedwhen combined in airwaves
-111-11-1-11
-1-11111-1-1
1-11-1-11-11
1111-1-1-1-1
-111-1-111-1
11-1-111-1-1
-11-11-11-11
11111111
Each row represents 8 chips sent by that sender
Sending (Modulating) Process
Time for 1 Bit
8 Chips
This is sent on the channel over one bit time (8 chip times)
Data to send by 8 senders
1C7
0C6
1C5
0C4
0C3
0C2
1C1
1C0
(Binary 1 represented by +1, Binary 0 represented by -1)
1
-1
1
-1
-1
-1
1
1
Multiplexing No. 12Seattle Pacific University
Using Walsh Codes: 8-sender Example
-111-11-1-11
11-1-1-1-111
1-11-1-11-11
-1-1-1-11111
1-1-111-1-11
-1-111-1-111
-11-11-11-11
11111111
044004-40
Walsh matrix: Multiply received data by column.
Receiving (Demodulating) Process
Sum rows
8 Binary 1-8 Binary 0
1C7
0C6
1C5
0C4
0C3
0C2
1C1
1C0
04400-440
04-400-4-40
0-4400440
0-4-4004-40
0-4-400-440
0-4400-4-40
04-400440
044004-40
8
-8
8
-8
-8
-8
8
8
This is sent on the channel over one bit time (8 chip times)
Each channel recovers the original bit sent to it
Multiplexing No. 13Seattle Pacific University
Multiplexing Summary
• Three basic methods of division:
• Frequency
• Time
• Code (digital)
• Can combine methods:
• Frequency-division into large bands, then time-division within each band
• SONET works this way
• Time-division over a single CDMA channel