Multiplexing: Sharing a single medium between multiple users

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MultiplexingSharing a single medium between
multiple users

• Based on Chapter 8 of William Stallings, Data and
  Computer Communication, 7th Ed.

Kevin BoldingElectrical EngineeringSeattle
Pacific University
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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 elses
  signal

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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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Frequency Division Multiplexing
Also called WavelengthDivision Multiplexing (WDM)

• FDM can be used any time a channels required
  bandwidth is less than the mediums total
  bandwidth

• Simply assign each channel a portion of the
  bandwidth

Single speech signal
AM Modulated to 64kHz
note dual sidebands
Based on Stallings, Fig. 8.5
transmit only one sideband
Multiplexed with other signals
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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
• Its 40300.03982, this must be channel 3

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North American TDM Standards
ATT
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
OC-3 155.52 150
OC-12 622.08 601
DS-1c 48 3.152
OC-24 1244.16 1202
DS-2 96 6.312
OC-48 2488.32 2405
DS-3(T3) 672 44.736
OC-192 9953.28 9621
DS-4 4032 274.176
OC-768 39813.12 38485
OC-3072 159252.4 153944
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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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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

Shannons Law CB log2(SNR1)SS Large
bandwidth, low power

• Spread-spectrum technique

Signal
10x Spreading Code
Encoded signal (10x BW)
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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 -1W21(0) 1 -1
W21(1) -1 1
To transmit Sum codes from all channels
Note 2-times spreading Each bit becomes two
chips
All summed combinations are unique can separate
out the original code
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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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Using Walsh Codes 8-sender Example
Sending (Modulating) Process
This is sent on the channel over one bit time (8
chip times)
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Using Walsh Codes 8-sender Example
Receiving (Demodulating) Process
This is sent on the channel over one bit time (8
chip times)
Walsh matrix Multiply received data by column.
8 ? Binary 1-8 ? Binary 0
Sum rows
Each channel recovers the original bit sent to it
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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