EL2006 Data Communication Week 6 Multiplexing

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EL2006 Data Communication Week 6 Multiplexing

• LIU Jiakang/Philip Tranter
• BIT/UCLAN

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Contents

• Multiplex / Demultiplexing
• Frequency Division Multiplexing, FDM
• Wave-Division Multiplexing, WDM
• Time Division Multiplexing, TDM
• Application Telephone System
• Digital Subscriber Line (DSL)
• Cable Modem/TV
• Note this material is from chapters 6 9 of the
  book and website

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Summary

• This week, we will discuss the Multiplexing
  technique, which is defined as sets of techniques
  to allow the simultaneous transmission of
  multiple signals across a single link.
• Since FDM in telephone system is no longer in
  use, the discussion is limited. We will
  concentrate on TDM digital hierarchy.
• Asynchronous TDM is more difficult to understand.

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1 Multiplex / Demultiplexing
Basic Definitions Types of Multiplexing
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Basic Definitions

• Multiplexer(MUX) device to combine signals to
  one composite signal.
• Demultiplexer(DEMUX) device to separate the
  composite signal into signal components
• Path refers to the physical link.
• Channel refers to a portion of a path.

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Types of Multiplexing

• There are two basic types of multiplexing in use
  Frequency Division Multiplexing (FDM) Time
  Division Multiplexing (TDM).
• Wavelength Division Multiplexing (WDM) is one
  special kinds of FDM used in fiber optical
  transmission.
• TDM can be further divided into synchronous TDM
  and asynchronous TDM.

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Categories of multiplexing
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2 Frequency Division Multiplex
Multiplexing Process Demultiplexing
Process Concept of Guard Bands
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FDM Definition

• Frequency Division Multiplexing (FDM) is the
  technique to combine signals from different
  bands.
• Each signal modulates a separate carrier
  frequency.

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FDM Multiplexing time domain
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FDM Demultiplexing time domain
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FDM Multiplexing frequency domain
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FDM Demultiplexing frequency domain
animation
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Concept of Guard Band
To prevent channel overlapping, they must be
separated by unused bandwidth called Guard Band.
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3 Wavelength Division Multiplexing
Note It is sometimes called Wave-Division
Multiplex. Wave refers to wavelength of the
fiber.
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WDM

• WDM is a special case for FDM used in fiber
  optics. Different signals are separated by
  wavelength, not frequency.
• More common use of WDM is Dense WDM (DWDM), which
  combines hundreds of optical sources together
  into one fiber.

Light waves
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Implementation of WDM

• An optical prism is used for WDM multiplexing and
  demultiplexing. Different wavelengths are
  refracted (bent) by different amounts

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4 Time Division Multiplexing
TDM Basics Interleaving Synchronous
TDM Asynchronous TDM
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TDM Definition

• Time Division Multiplex (TDM) is the process of
  combining signals together in the time domain.

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TDM Basics

• The basic principle of TDM is Sampling Theorem.
• The basic unit in TDM is Time Slot.
• The data in TDM is grouped into Frames, which
  consists of one cycle of time slots.

T
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Interleaving

• Interleaving is the process of multiplexing.
• In TDM, synchronization between the sender and
  receiver is very important.

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Synchronous TDM

• In synchronous TDM, each source is pre-assigned a
  fixed location of time slot.
• Each source can and only can send information at
  the time slot given to it. If a source has no
  data to send, its time slot remains empty. This
  is one kinds of waste.
• If n sources are grouped together, the total data
  rate of the path is n times the original data
  rate of each source.

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Synchronous TDM Example
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Synchronous TDM - Multiplexing
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Synchronous TDM - Demultiplexing
animation
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TDM Example
We have four sources, each creating 250
characters per second. If the interleaved unit is
a character and 1 synchronizing bit is added to
each frame, find (1) the data rate of each
source, (2) the duration of each character in
each source, (3) the frame rate, (4) the duration
of each frame, (5) the number of bits in each
frame, and (6) the data rate of the link.
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Solution to TDM Example
Solution
We can answer the questions as follows 1. The
data rate of each source is 2000 bps 2 Kbps. 2.
The duration of a character is 1/250 s, or 4
ms. 3. The link needs to send 250 frames per
second. 4. The duration of each frame is 1/250 s,
or 4 ms. 5. Each frame is 4 x 8 1 33
bits. 6. The data rate of the link is 250 x 33,
or 8250 bps.
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Asynchronous TDM

• In asynchronous TDM or statistical TDM, only
  sources containing data will be sent with time
  slot. Therefore, asynchronous TDM can avoid
  bandwidth waste in synchronous TDM.
• But, in order to distinguish data from different
  sources, address should be added into the frame
  structure, increasing the overhead of the
  transmission.

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Asynchronous TDM Example
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Address and Overhead

• As shown below, address is added before the data
  from each source.
• It is practical only when the data size for each
  time slot is relatively larger than the address.

In this example, the addressing information takes
up as much space as the actual data - inefficient
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5 TDM Application Telephone System
Digital Signal (DS) Service T Lines E Lines
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TDM in Telephone System

• FDM was used in telephone system, but now the
  telephone lines (except the subscriber line) are
  all in digital form, so FDM is not in use now.
• There are two types of TDM used in telephone
  system. In US, T-1 line with basic rate of 1.544
  Mbps is used, while in Europe and China, E-1 Line
  of 2.048 Mbps is in use.

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Digital Signal (DS) Hierarchy in US
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T Line and E Line
Table 6.1 DS and T lines rates
Table 6.2 E line rates
There are 24 voice channels in T-1 line, while 30
voice channels out of 32 channels in E-1 line.
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T-1 Frame Structure
One frame 24 8 1 bits Frame rate 193
8000 1.544 Mbps
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6 Digital Subscriber Line
DSL Technology ADSL
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DSL Technology

• Digital Subscriber Line (DSL) refers to the
  digitization of subscriber line from telephone
  office to user premises.
• There are many forms of DSL technologies, ISDN
  (Integrated Services Digital Networks) is one of
  them.
• The most popular DSL is Asymmetric Digital
  Subscriber Line (ADSL).

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ADSL

• The bandwidth of the telephone line twisted-pair
  is approximately 1 MHz.
• This bandwidth is divided into three bands one
  for regular telephone service (POTS plain Old
  Telephone Service), one for upstream, and one for
  downstream.
• The modulation used in ADSL is Discrete Multitone
  Technique (DMT).

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Concept of DMT
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ADSL Modem and DSLAM
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7 Cable Modem/TV
Cable TV system Cable Modem
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HFC System(Hybrid Fiber-Coaxial)
Communication can be 2-way Fiber for long
distance/high rate Coax for short distance/low
rate
(Regional Cable Head)
1,000 subscribers
400,000 subscribers
40,000 subscribers
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Cable TV Bandwidth

• Data upstream and downstream are added to the
  regular video band for TV signal.
• Therefore cable modem can be used to send and
  receive data using the cable TV system.

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Cable Modem and CMTS
CMTS Cable Modem Transmission System
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Summary for Week 6

• Multiplexing is the simultaneous transmission of
  multiple signals across a single data link.
• There are two main types of multiplexing TDM and
  FDM. TDM includes synchronous and asynchronous
  TDM.
• In telephone system, there are two types of TDM
  in use T line and E line.
• ADSL is the most common DSL in telephone.
• Cable modem is used in cable TV system, which can
  also support domestic Internet access.