Computer Networks

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Computer Networks

• Chapter 6 - Multiplexing

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Multiplexing

• The term multiplexing is used whenever it is
  necessary to share a single device among several
  devices
• In this chapter multiplexing denotes a set of
  techniques that provide sharing of a single high
  bandwidth channel among a number of individual
  users.
• The reasons for using multiplexing
• Media with high bandwidth are expensive
• Sharing is necessary to justify their use

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Categories of Channel Multiplexing

• FDM (Frequency Division Multiplex)
• The individual user signals are shifted to
  different frequency channels
• WDM (Wavelength Division Multiplex)
• Conceptually the same as FDM, except that the
  multiplexing involves optical signals transmitted
  over fiber-optic channels
• TDM (Time Division Multiplex)
• Each user signal is assigned a particular time
  slot for transmission

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Other Types of Multiplexing

• Statistical multiplexing
• The channel is shared in time but without
  particular time slots assigned. Sharing is on the
  as needed basis.
• DMT (Discrete Multitone)
• Found in digital subscriber lines. It combines
  hundreds of different signals that are QAM
  modulated into a single stream
• CDM (Code Division Multiplex)
• An advanced technique that allows multiple
  devices to transmit on the same frequency at the
  same time

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FDM Multiplexing Process
Time domain
Frequency domain
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FDM Demultiplexing Process
Time domain
Frequency domain
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The Analog Hierarchy
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Using FDM

• The oldest multiplexing technique. Susceptible to
  noise
• Used in the old days of telephone network to make
  efficient use of the long distance trunks
• Used in AM and FM radio broadcast to provide the
  use of many stations on a single channel without
  each interfering with the other
• The first generation (1G) of mobile phones used
  FDM with analog frequency modulation which
  required 2x30Khz per user

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Example

• FDM multiplexing is used for voice channels
  (each requiring 4000 Hz) with 200-Hz guard band
  between each signal source. If the total
  bandwidth is 29.6 KHz, how many channels can be
  multiplexed?
• Answer If n is the number of channels that can
  be multiplexed then the total bandwidth required
  i expressed with the following equation.
• nx4000 (n-1)x200 29600
• Solving this equation for n will show that 7
  channels can be multiplexed.
• n (29600-200)/(4000200) 29400/42007

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TDM Time Division Multiplex

• Digital process that is nowadays used for
  long-distance telephone lines.
• A portion of time is allocated to each voice
  channel
• The voice signal is first transformed into bits
  using PCM
• The data flow is divided into units called
  frames. The frame contains as many bits as the
  number of channels multiplexed

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TDM The Multiplexing Process
The duration of the bit on the high speed channel
is 3 times shorter, assuming that three channels
are multiplexed.
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TDM -Interleaving
Even when a channel has nothing to send, its time
slot cannot be used by another channel Synchroniza
tion between sender and receiver is important
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Example

• 20 voice signals are to be multiplexed and
  transmitted over twisted pair.
• What is the bandwidth required, in bps, if
  synchronous time division multiplexing is used,
  if we use the standard analogue-to-digital
  sampling rate, if each voice signal has a
  bandwidth of 4000Hz, and if each sample is
  converted into an 8-bit value?
• Answer Each voice signal is sampled at 2 times
  the frequency 8000 samples per second.
• Each sample is an 8 bit value so
• 8000 samples per sec 8 bits 64,000bps
• 20 signals 64,000bps 1,280,000bps.

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TDM - Flexibility

• It is possible to assign several slots to a fast
  transmitting device
• The channel needs to have the speed that is an
  integer times higher than the speeds of the
  regular channel
• If this cannot be accomplished, extra dummy bits
  are added. This process is called bit padding

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Digital Hierarchy

• Telephone companies group TDM slots in higher
  rate services that are standardized.

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American vs. European Standards
European
American
Line Rate (Mbps) Voice Channels
E1 2.048 32
E2 8.448 4xE1
E3 34.368 4xE2
E4 139.264 4xE3
Line Rate (Mbps) Voice Channels
T1 1.544 24
T2 6.312 96
T3 44.375 672
T4 274.176 178 T1
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Statistical Multiplexing

• Statistical multiplexing is mainly used with
  computer networks
• Only the data from active sources are transmitted
  using the full capacity of the channel
• Bits are grouped into frames, but they can be
  with different size.
• If there is more traffic than the high speed link
  can transmit, the frames are queued and are
  served using the first-come firs-served
  principle.

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Statistical Multiplex -Buffering the Packets
Packets for one output
Data
Hdr
R
X(t)
R
Data
Hdr
Link rate, R
R
Packet buffer
Data
Hdr

• Bursty flows share the same link efficiently
• When too many packets arrive simultaneously, the
  buffer could not have enough space to keep all of
  them
• Dropping packets is possible when packet buffer
  is full