Making%20Connections%20Efficient:%20Multiplexing%20and%20Compression

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Lecture 05

• Making Connections Efficient Multiplexing and
  Compression

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Making Connections Efficient

• Under simplest conditions, medium can carry only
  one signal at any moment in time
• But, this approach is not efficient. In
  practice, multiple signals share a medium.
• The technique used to place multi-signals onto a
  medium is called multiplexing
• Different multiplexing techniques

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Making Connections Efficient

• Most common multiplexing techniques
• Frequency division multiplexing
• Time division multiplexing
• Wavelength Division Multiplexing
• Discrete Multitone
• Code division multiplexing
• Comparison

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Making Connections Efficient

• Data Compression concept
• Application examples

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Frequency Division Multiplexing (FDM)

• it is a technique to pack several analog signals
  onto a telephone wire
• General concept
• It works this way because a telephone signal is
  carried signal range of 0 to 4000 hz
• A twisted pair of wire can carry 1 million hz.
• how multiplexing is done?

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FIGURE 5-22 Frequency multiplexed voice
signals.
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Frequency Division Multiplexing (FDM) (cont.)

• we need to transmit a sine wave in the new freq
  range, the change of the carrier wave is called
  Modulation
• similar token, we would have frequency modulation
  ((FM), amplitude modulation (AM), phase
  modulation (PM).
• at the other end, we need to demodulation so that
  signals are can unscrambled back to the original
  form

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FIGURE 5-23 Amplitude and frequency modulation.
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Frequency Division Multiplexing (FDM) (cont.)

• In tel system, the modulation occurs at the
  central office, and demodulation takes place at
  the serving central office near the user
  home/office. (same as MODEM at home?)
• multiplexing equipment (multiplexers) at central
  office grouped as shown in
• Figure 5-24
• 12 voice channels grouped as a base group
• 5 base groups into a super group
• 10 super groups into a master group

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FIGURE 5-24 The hierarchy of voice channels as
they are multiplexed together.
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Time division Multiplexing (TDM)

• is a technique uses to divide a circuits
  capacity into time slots so that data could be
  transmitted in a long distance on a single
  circuit without the need of the regeneration (why
  important?)
• Digital signaling is used exclusively
• Time division multiplexing comes in two basic
  forms
• Synchronous time division multiplexing
• Statistical time division multiplexing

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Synchronous Time Division Multiplexing

• Sharing of the signal is accomplished by dividing
  available transmission time on a medium among
  users
• A TDM takes one character from each terminal and
  group them into a frame before transmit them on
  the circuit
• At the end, another TDM breaks down the frame and
  direct individual message to respective receivers

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(TDM)

• TDM effect is totally a transparent to users,
  terminal and computer
• total of terminals could be packed into a TDM
  is depended on capability of a circuit
• Eg
• If a circuit has a speed of 9600 bps, then
• it may carry 4 x 2400 pbs or 8 x 1200 pbs etc

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(TDM)

• it takes one bit from each terminal instead of
  one character and transmit a frame in bit
• Different between FDM and TDM
• T-1 and ISDN telephone lines are common examples
  of synchronous time division multiplexing. (what
  is T-1 and ISDN?)
• Similar applied to Sonet system

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Synchronous Time Division Multiplexing (continued)
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Mechanical data-transmission procedure
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Synchronous Time Division Multiplexing (continued)

• If one device generates data at faster rate than
  other devices, then the multiplexor must either
  sample the incoming data stream from that device
  more often than it samples the other devices, or
  buffer the faster incoming stream
• If a device has nothing to transmit, the
  multiplexor must still insert something (by
  reserving a blank!) into the multiplexed stream,
  So that the receiver may stay synchronized with
  the incoming data stream, the transmitting
  multiplexor can insert alternating 1s and 0s into
  the data stream

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Synchronous Time Division Multiplexing (continued)
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FIGURE 9-14 FDM channels have full time use of
a limited range of frequencies. TDM channels can
use the full range of frequencies but only during
predetermined time slots.
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T-1 Multiplexing

• The T-1 multiplexor stream is a continuous series
  of frames

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ISDN Multiplexing

• The ISDN multiplexor stream is also a continuous
  series of frames
• Each frame contains various control and sync info

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SONET/SDH Multiplexing

• Likewise, SONET incorporates a continuous series
  of frames

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2) Statistical TDM (STDM)

• does not assign a specific time slot for each
  terminal, but transmit terminals address along
  with each character or message of data
• i.e. A statistical multiplexor transmits the data
  from active workstations only
• If a workstation is not active, no space is
  wasted in the multiplexed stream
• it avoids of having empty slot in a frame, and
  attempt to allocate next terminal has data to
  send so that efficiency rate is improved
• See Figure 9-16

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FIGURE 9-16 The STDM tries to avoid having
empty slots in a frame, thereby improving the
line use. If a terminal has no data to send in a
particular time period, the STDM will see if the
next terminal has data that can be included in
the time slot. When the STDM at the receiving end
breaks the frame apart, it uses the terminal
address to route the data to the proper device.
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Alternative layout
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STDM

• Its mechanical steps
• STDM requires a storage area (ie buffer) so that
  data can be saved until line can accept for
  transmission
• Typically, it has buffer size up to 32,000 char,
  but may encounter a slighter delay for
  transmission when buffering is occurred
• In STDM, 12 terminals running at 1200 pbs could
  be handled by a 9600 pbs line in most cases

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How it works

• To identify each piece of data, an address is
  included, see Figure 5-10
• If the data is of variable size, a length is also
  included, see Figure 5-11
• More precisely, the transmitted frame contains a
  collection of data groups, see Figure 5-12

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Wavelength Division Multiplexing

• Wavelength division multiplexing multiplexes
  multiple data streams onto a single fiber-optic
  line
• Different wavelength lasers (called lambdas)
  transmit the multiple signals
• Each signal carried on the fiber can be
  transmitted at a different rate from the other
  signals
• Dense wavelength division multiplexing combines
  many (30, 40, 50 or more) onto one fiber
• Coarse wavelength division multiplexing combines
  only a few lambdas

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Wavelength Division Multiplexing (continued)
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Discrete Multitone

• Discrete Multitone (DMT) a multiplexing
  technique commonly found in digital subscriber
  line (DSL) systems
• DMT combines hundreds of different signals, or
  subchannels, into one stream
• Each subchannel is quadrature amplitude modulated
  (recall eight phase angles, four with double
  amplitudes)
• Theoretically, 256 subchannels, each transmitting
  60 kbps, yields 15.36 Mbps
• Unfortunately, there is noise

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Discrete Multitone (continued)
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Code Division Multiplexing

• Also known as code division multiple access
• An advanced technique that allows multiple
  devices to transmit on the same frequencies at
  the same time
• Each mobile device is assigned a unique 64-bit
  code
• To send a binary 1, a mobile device transmits the
  unique code
• To send a binary 0, a mobile devices transmits
  the inverse of the code
• Receiver gets summed signal, multiplies it by
  receiver code, adds up the resulting values
• Interprets as a binary 1 if sum is near 64
• Interprets as a binary 0 if sum is near -64

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Code Division Multiplexing (continued)

• For simplicity, assume 8-bit code
• Example
• Three different mobile devices use the following
  codes
• Mobile A 10111001
• Mobile B 01101110
• Mobile C 11001101
• Assume Mobile A sends a 1, B sends a 0, and C
  sends a 1
• Signal code 1-chip N volt 0-chip -N volt

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Code Division Multiplexing (continued)

• Example (continued)
• Three signals transmitted
• Mobile A sends a 1, or 10111001, or ---
• Mobile B sends a 0, or 10010001, or -----
• Mobile C sends a 1, or 11001101, or ---
• Summed signal received by base station 3, -1,
  -1, 1, 1, -1, -3, 3

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Code Division Multiplexing (continued)

• Example (continued)
• Base station decode for Mobile A
• Signal received 3, -1, -1, 1, 1, -1, -3, 3
• Mobile As code 1, -1, 1, 1, 1, -1, -1, 1
• Product result 3, 1, -1, 1, 1, 1, 3, 3
• Sum of Products 12
• Decode rule For result near 8, data is binary 1

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Code Division Multiplexing (continued)

• Example (continued)
• Base station decode for Mobile B
• Signal received 3, -1, -1, 1, 1, -1, -3, 3
• Mobile As code -1, 1, 1, -1, 1, 1, 1, -1
• Product result -3, -1, -1, -1, 1, -1, -3, -3
• Sum of Products -12
• Decode rule For result near -8, data is binary 0

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Comparison of Multiplexing Techniques
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Comparison of Multiplexing Techniques (continued)
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CompressionLossless versus Lossy

• Compression is another technique used to squeeze
  more data over a communications line
• If you can compress a data file down to one half
  of its original size, file will obviously
  transfer in less time
• Two basic groups of compression
• Lossless when data is uncompressed, original
  data returns
• Lossy when data is uncompressed, you do not
  have the original data
• 1 vs 2

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CompressionLossless versus Lossy (continued)

• Compress a financial file?
• You want lossless
• Compress a video image, movie, or audio file?
• Lossy is OK
• Examples of lossless compression include
• Huffman codes, run-length compression, and
  Lempel-Ziv compression
• Examples of lossy compression include
• MPEG, JPEG, MP3

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Lossless Compression

• Run-length encoding
• Replaces runs of 0s with a count of how many 0s.
• 0000000000000010000000001100000000000000000000111
  00000000000
• (30 0s)
• 14 9 0 20 30 0
  11

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Lossless Compression (continued)

• Run-length encoding (continued)
• Now replace each decimal value with a 4-bit
  binary value (nibble)
• Note If you need to code a value larger than 15,
  you need to use two consecutive 4-bit nibbles
• The first is decimal 15, or binary 1111, and the
  second nibble is the remainder
• For example, if the decimal value is 20, you
  would code 1111 0101 which is equivalent to 15 5

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Lossless Compression (continued)

• Run-length encoding (continued)
• If you want to code the value 15, you still need
  two nibbles 1111 0000
• The rule is that if you ever have a nibble of
  1111, you must follow it with another nibble

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Lossy Compression

• Relative or differential encoding
• Video does not compress well using run-length
  encoding
• In one color video frame, not much is alike
• But what about from frame to frame?
• Send a frame, store it in a buffer
• Next frame is just difference from previous frame
• Then store that frame in buffer, etc.

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Lossy Compression (continued)

5 7 6 2 8 6 6 3 5 6 6 5 7 5 5 6 3 2 4 7 8 4 6 8 5
6 4 8 8 5 5 1 2 9 8 6 5 5 6 6 First Frame
5 7 6 2 8 6 6 3 5 6 6 5 7 6 5 6 3 2 3 7 8 4 6 8 5
6 4 8 8 5 5 1 3 9 8 6 5 5 7 6 Second Frame
0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 -1 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 Difference
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Lossy Compression (continued)

• Image Compression
• One image (JPEG) or continuous images (MPEG)
• A color picture can be defined by red/green/blue,
  or luminance/chrominance/chrominance which are
  based on RGB values
• Either way, you have 3 values, each 8 bits, or 24
  bits total (224 colors!)

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Lossy Compression (continued)

• Image Compression (continued)
• A VGA screen is 640 x 480 pixels
• 24 bits x 640 x 480 7,372,800 bits Ouch!
• And video comes at you 30 images per second
  Double Ouch!
• We need compression!
• JPEG (Joint Photographic Experts Group)
• Compresses still images
• Lossy
• JPEG compression consists of 3 phases
• Discrete cosine transformations (DCT)
• Quantization
• Run-length encoding

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Lossy Compression (continued)

• JPEG Step 1 DCT
• Divide image into a series of 8x8 pixel blocks
• If the original image was 640x480 pixels, the new
  picture would be 80 blocks x 60 blocks (next
  slide)
• If BW, each pixel in 8x8 block is an 8-bit value
  (0-255)
• If color, each pixel is a 24-bit value (8 bits
  for red, 8 bits for blue, and 8 bits for green)

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Lossy Compression (continued)
80 blocks
60 blocks
640 x 480 VGA Screen Image Divided into 8 x 8
Pixel Blocks
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Lossy Compression (continued)

• JPEG Step 1 DCT (continued)
• So what does DCT do?
• Takes an 8x8 array (P) and produces a new 8x8
  array (T) using cosines
• T matrix contains a collection of values called
  spatial frequencies
• These spatial frequencies relate directly to how
  much the pixel values change as a function of
  their positions in the block

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Lossy Compression (continued)

• JPEG Step 1 DCT (continued)
• An image with uniform color changes (little fine
  detail) has a P array with closely similar values
  and a corresponding T array with many zero values
• An image with large color changes over a small
  area (lots of fine detail) has a P array with
  widely changing values, and thus a T array with
  many non-zero values

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Lossy Compression (continued)

• JPEG Step 2 -Quantization
• The human eye cant see small differences in
  color
• So take T matrix and divide all values by 10
• Will give us more zero entries
• More 0s means more compression!
• But this is too lossy
• And dividing all values by 10 doesnt take into
  account that upper left of matrix has more action
  (the less subtle features of the image, or low
  spatial frequencies)

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Lossy Compression (continued)
1 3 5 7 9 11 13 15 3 5
7 9 11 13 15 17 5 7 9
11 13 15 17 19 7 9 11 13 15
17 19 21 9 11 13 15 17 19 21
23 11 13 15 17 19 21 23 25 13 15
17 19 21 23 25 27 15 17 19 21 23
25 27 29
U matrix
Qij Round(Tij / Uij), for i 0, 1,
2, 7 and j 0, 1, 2, 7
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Lossy Compression (continued)

• JPEG Step 3 Run-length encoding
• Now take the quantized matrix Q and perform
  run-length encoding on it
• But dont just go across the rows
• Longer runs of zeros if you perform the
  run-length encoding in a diagonal fashion

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Lossy Compression (continued)

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Lossy Compression (continued)

• How do you get the image back?
• Undo run-length encoding
• Multiply matrix Q by matrix U yielding matrix T
• Apply similar cosine calculations to get
  original P matrix back

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Business Multiplexing In Action

• XYZ Corporation has two buildings separated by a
  distance of 300 meters
• A 3-inch diameter tunnel extends underground
  between the two buildings
• Building A has a mainframe computer and Building
  B has 66 terminals
• List some efficient techniques to link the two
  buildings.

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Possible solution
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Business Multiplexing In Action (continued)

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Business Multiplexing In Action (continued)

• Possible solutions
• Connect each terminal to the mainframe computer
  using separate point-to-point lines
• Connect all the terminals to the mainframe
  computer using one multipoint line
• Connect all the terminal outputs and use
  microwave transmissions to send the data to the
  mainframe
• Collect all the terminal outputs using
  multiplexing and send the data to the mainframe
  computer using a conducted line