Multiplexing

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Multiplexing
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6 ? ???(Multiplexing)

• Dividing a link into channels

• Word link refers to the physical path
• Channel refers to the portion of a link that
  carries a transmission
• between a given pair of lines.

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???(Multiplexing)

• ???(Multiplexing)
• is the set of techniques that allows the
  simultaneous transmission of multiple signals
  across a single data link.

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6.1 FDM

• ????(Multiplexer)
• transmission streams combine into a single
  stream(many to one)
• ?????(Demultiplexer)
• stream separates into its component
  transmission(one to many) and directs them to
  their intended receiving devices

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8.2 Many to One/One to Many

• Categories of Multiplexing

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FDM

• FDM(Frequency-Division Multiplexing)
• is an analog technique that can be applied when
  the bandwidth of a link is greater than the
  combined bandwidths of the signals to be
  transmitted

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FDM (contd)

• FDM process
• each telephone generates a signal of a similar
  frequency range
• these signals are modulated onto different
  carrier frequencies(f1, f2, f3)

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FDM (contd)

• FDM multiplexing process, time-domain

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FDM(contd)

• FDM multiplexing process, frequency-domain

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FDM(contd)

• Demultiplexing
• separates the individual signals from their
  carries and passes them to the waiting receivers.

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FDM(contd)

• FDM demultiplexing process, time-domain

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FDM(contd)

• FDM demultiplexing, frequency-domain

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Example 1
Example 1
Assume that a voice channel occupies a bandwidth
of 4 KHz. We need to combine three voice channels
into a link with a bandwidth of 12 KHz, from 20
to 32 KHz. Show the configuration using the
frequency domain without the use of guard bands.
Solution
Shift (modulate) each of the three voice channels
to a different bandwidth, as shown in Figure 6.6.
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Example 1 (contd)
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Example 2
Example 2
Five channels, each with a 100-KHz bandwidth, are
to be multiplexed together. What is the minimum
bandwidth of the link if there is a need for a
guard band of 10 KHz between the channels to
prevent interference?
Solution
For five channels, we need at least four guard
bands. This means that the required bandwidth is
at least 5 x 100 4 x 10 540
KHz, as shown in Figure 6.7.
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Example 2 (contd)
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Example 3
Example 3
Four data channels (digital), each transmitting
at 1 Mbps, use a satellite channel of 1 MHz.
Design an appropriate configuration using FDM
Solution
The satellite channel is analog. We divide it
into four channels, each channel having a 250-KHz
bandwidth. Each digital channel of 1 Mbps is
modulated such that each 4 bits are modulated to
1 Hz. One solution is 16-QAM modulation. Figure
6.8 shows one possible configuration.
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Example 3 (contd)
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FDM(contd)

• Example Cable Television
• coaxial cable has a bandwidth of approximately
  500Mhz
• individual television channel require about 6Mhz
  of bandwidth for transmission
• can carry 83 channels theoretically

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Analog Hierarchy
To maximize the efficiency of their
infrastructure, telephone companies have
traditionally multiplexed signals from lower
bandwidth lines onto higher bandwidth lines.
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Applications of FDM
Example 4
The Advanced Mobile Phone System (AMPS) uses two
bands. The first band, 824 to 849 MHz, is used
for sending and 869 to 894 MHz is used for
receiving. Each user has a bandwidth of 30 KHz in
each direction. The 3-KHz voice is modulated
using FM, creating 30 KHz of modulated signal.
How many people can use their cellular phones
simultaneously?
Solution
Each band is 25 MHz. If we divide 25 MHz into 30
KHz, we get 833.33. In reality, the band is
divided into 832 channels.
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6.2 Wave Division Multiplexing (WDM)

• WDM is conceptually same as FDM
• except that the multiplexing and demultiplexing
  involve light signals transmitted through
  fiber-optic channels

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WDM (contd)

• WDM is an analog multiplexing technique to
  combine optical signals.

Very narrow bands of light from different sources
are combined to make a wider band of light
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WDM (contd)

• Combining and splitting of light sources are
  easily handled by a prism
• Prism bends a beam of light based on the angle
  of incidence and the frequency.
• One application is the SONET.

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6.3 TDM

• TDM(Time-Division Multiplexing)
• is a digital process that can be applied when
  the data rate capacity of the transmission medium
  is greater than the data rate required by the
  sending and receiving device
• TDM is a digital multiplexing technique to
  combine data.

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TDM(contd)
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TDM (contd)
In a TDM, the data rate of the link is n times
faster, and the unit duration is n times shorter.
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TDM (contd)
Example 5
Four 1-Kbps connections are multiplexed together.
A unit is 1 bit. Find (1) the duration of 1 bit
before multiplexing, (2) the transmission rate of
the link, (3) the duration of a time slot, and
(4) the duration of a frame?
Solution
We can answer the questions as follows 1. The
duration of 1 bit is 1/1 Kbps, or 0.001 s (1
ms). 2. The rate of the link is 4 Kbps. 3. The
duration of each time slot 1/4 ms or 250 ms. 4.
The duration of a frame 1 ms.
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TDM(contd)

• TDM can be implemented in two ways
• Synchronous TDM
• Asynchronous TDM

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TDM(contd)

• Synchronous TDM
• the multiplexer allocates exactly the same time
  slot to each device at all times, whether or not
  a device has anything to transmit.

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TDM(contd)

• Frame
• Time slots are grouped into frames
• A frame consists of one complete cycle of time
  slots, including one or more slots dedicated to
  each sending device, plus framing bits.

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TDM(contd)

• Synchronous TDM

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TDM(contd)

• Interleaving
• synchronous TDM can be compared to a very fast
  rotating switch
• switch moves from device to device at a constant
  rate and in a fixed order

6 empty slots out of 24 are being wasted
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TDM(contd)

• Demultiplexer decomposes each frame by discarding
  the framing bits and extracting each character in
  turn
• Synchronous TDM, demultiplexing process

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TDM(contd)
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TDM(contd)
Example 6
Four channels are multiplexed using TDM. If each
channel sends 100 bytes/s and we multiplex 1 byte
per channel, show the frame traveling on the
link, the size of the frame, the duration of a
frame, the frame rate, and the bit rate for the
link.
Solution
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TDM(contd)
Example 7
A multiplexer combines four 100-Kbps channels
using a time slot of 2 bits. Show the output with
four arbitrary inputs. What is the frame rate?
What is the frame duration? What is the bit rate?
What is the bit duration?
Solution
Figure 6.16 shows the output for four arbitrary
inputs.
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TDM(contd)
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TDM(contd)

• Framing bits
• allows the demultiplexer to synchronize with
  the incoming stream so that it can separate the
  time slots accurately
• (ex 01010101 .)

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TDM(contd)

• Synchronous TDM Example

4 characters 1 framing bit
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TDM(contd)

• Asynchronous TDM statistical time-division
  multiplexing

Synchronous or Asynchronous Not flexible or
Flexible
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TDM(contd)

• Examples of asynchronous TDM frames
• a. Case 1 Only three lines sending data

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TDM(contd)

• b. Case 2 Only four lines sending data

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TDM(contd)

• c. Case 3 All five lines sending data

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Multiplexing application(contd)

• Analog Hierarchy
• To maximize the efficiency of their
  infrastructure, telephone companies have
  traditionally multiplexed signals from lower
  bandwidth lines onto higher bandwidth lines.

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Multiplexing application(contd)

• Digital Services
• advantage
• - less sensitive than analog service to noise
• - lower cost

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Multiplexing application(contd)

• DS(Digital Signal) Service
• is a hierarchy of digital signal

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Multiplexing application(contd)

• DS Service
• DS-0 single digital channel of 64Kbps
• DS-1 1,544Mbps, 24?? 64Kbps 8Kbps? overhead
• DS-2 6,312Mbps, 96?? 64Kbps168Kbps? overhead
• DS-3 44,376Mbps, 672?? 64Kbps1.368Mbps?
• overhead
• DS-4 274,176Mbps,4032?? 64Kbps16.128Mbps?
• overhead

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Multiplexing application(contd)

• T Lines

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T Lines for Analog Transmission

• T-1 line for multiplexing telephone lines

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T-1 Frame
193 24 x 8 1(1 bit for synchronization)
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E Lines

• E line rates

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TDM(contd)

• Inverse Multiplexing
• takes the data stream from one high-speed line
  and breaks it into portion that can be sent
  across several lower speed lines simultaneously,
  with no loss in the collective data rate

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TDM(contd)

• Multiplexing and inverse multiplexing

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TDM(contd)

• Why do we need inverse multiplexing ?
• wants to send data, voice, and video each of
  which requires a different data rate.
• example
• voice - 64 Kbps link
• data - 128 Kbps link
• video - 1,544 Mbps link