Multiplexing No. 1 Seattle Pacific University Multiplexing: Sharing a single medium between multiple users Kevin Bolding Electrical Engineering Seattle.

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.


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


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


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)


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…


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


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


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


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


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


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


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 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

Multiplexing No. 1 Seattle Pacific University Multiplexing: Sharing a single medium between multiple users Kevin Bolding Electrical Engineering Seattle.

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