Data Encoding (Chap. 5)

1
Data Encoding (Chap. 5)

• Transforming original signal just before
  transmission.
• Both analog and digital data can be encoded into
  either analog or digital signals.

2
Digital Transmission Terminology

• Data element bit.
• Signaling element encoding of data element for
  transmission.
• Unipolar signaling signaling elements have same
  polarization (all or all -).
• Polar signaling different polarization for
  different elements.

3
More Terminology

• Data rate rate in bps at which data is
  transmitted for data rate of R, bit duration
  (time to emit 1 bit) is 1/R sec.
• Modulation rate baud rate (rate at which signal
  levels change).

4
Approach 1 NRZ
But how do you know when to sample? Phase-locked-l
oop (PLL) measures the difference when
transitions occur on the wire and when they occur
on a local adjustable oscillator, and then make
adjustments accordingly. YOU MUST HAVE
TRANSISTIONS TO LOCK ON TO.
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Multilevel Binary
opposite direction
Pros No DC component. Can be used to force
transitions (to help PLL). Cons We are using 3
levels and could send ?? bits instead of 1
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Scrambling to help the PLL

• If there are not enough transitions, the PLL may
  have problems.
• So we force extra transitions when there are not
  enough.
• Approach 1 Use special coding so that long
  strings of zeros (or ones) dont occur.

7
Scrambling to help the PLL

• Approach 2 Use multilevel binary and set
  illegal transitions to long strings of zeros.
• Here, if an octet of zeros occurs, send a special
  illegal sequence.
• The receiver must be able to interpret this
  special sequence.

used in long-distance transmission
8
Biphase Differential Manchester(Self-Clocking)
A transition always occurs in the middle of the
period. A zero is represented by a transition
occurring at the beginning of the period. A one
is represented by no transition at the beginning
of the period.
0
0
1
1
always a transition in the middle
Used in CD players and Ethernet
9
Methods to Encode Digital Signals

• NRZ
• Multilevel binary
• Manchester
• Issues
• DC?
• Self Clocking?
• How big is the spectrum?

10
Sending Digital Signals over Analog (e.g. Modem)

• Amplitude shift keying (ASK) (Amplitude
  Modulation)
• Frequency shift keying (FSK) (Frequency
  modulation)
• Phase shift keying (PK) (Phase Modulation)
• Modems use phase and amplitude.

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Modulation Techniques
ASK
FSK
PSK
12
Fixed mistakes on last slide?
13
Phase-shift Keying

• Quadrature phase-shift keying (QPSK) - send 2
  bits.

90
0
180
270
14
QAM - Quadrature Amplitude Modulation
constellation diagrams
90
90
0
180
0
180
270
270
QAM-16 (16 levels, how many bits)
QAM - 64
15
V32
128 bits 6 data and 1 parity (error correction)
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How fast is V32?
The phone system transmits over the 300 to 3400
Hz band.
So what data rate can we use? How fast can we
send symbols?
So 2400 6 14400 bps
What is the baud rate?
V.34 2400 baud - with 12 data bits/symbol V.34
bis 2400 baud with 14 data bits/symbol Thats
the fastest there is!
To get 56K you send at 4000 baud (if the phone
system can handle it)
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Digital Subscriber Lines (DSL)

• ADSL A for asymmetric, faster down load speed
  than up.
• The 56kbps or 33kbps modem speed is due to a
  filter installed at the end office.
• If this filter is removed, then the full spectrum
  of the twisted pair is available (which is?)
• But, if you are far from the office, then you
  cant get a very high data rate because?
• The DSL standard goes up to 8 Mbps down and 1
  Mbps up.

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DSL
A total of 256 4kHz channels
Upstream
downstream
empty
25kHz (channel 6)
channel 632
Voice POTS (stands for ?) (channel 0)
250 parallel channels Each data channel uses QAM
16 (with 1 parity bit).
The quality of each channel is monitored and
adjusted. So channels may transmit at different
speeds
What is the maximum data rate?
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ADSL configuration
voice switch
telephone line
splitter
splitter
ADSL Modem
DSLAM digital subscriber line access multiplexer
TO ISP
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Cable - History

• Starting in the late 1940s, mountain people put a
  big antenna on a hill top and ran a cable to them
  and their friends.
• 1970, 1000s of independent cable systems existed.
• 1974 Time Inc started HBO

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Early CableCommunity Antenna Television
Head end
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Internet over Cable
A group of homes share the cable. (As oppose to
DSL.) The number of homes per group is around
500-2000.
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Cable Spectrum
108MHz
Upstream 5 42 MHZ
750MHz
550MHz
TV
FM
TV
HBO
shopping
Downstream data
54 MHz
88MHz
Each TV channel gets its own frequency. This is
called frequency multiplexing
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Multiplexing
It cost the same to install a high bandwidth
connection as it does to install a low bandwidth
one. So the approach is to install a few high
bandwidth connections and make it seem like many
low bandwidth connections.
One high bandwidth link is shared by many. But
how?
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Frequency Division Multiplexing

• FDM
• Useful bandwidth of medium exceeds required
  bandwidth of channel
• Each signal is modulated to a different carrier
  frequency
• Carrier frequencies separated so signals do not
  overlap (guard bands)
• e.g. broadcast radio
• Channel allocated even if no data
• ADSL uses FDM

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FDM
Channel 1
Channel 2
Channel n
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Analog Carrier Systems

• ATT (USA)
• Hierarchy of FDM schemes
• Group
• 12 voice channels (4kHz each) 48kHz
• Range 60kHz to 108kHz
• Supergroup
• 60 channel
• FDM of 5 group signals on carriers between 420kHz
  and 612 kHz
• Mastergroup
• 10 supergroups

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Back to Cable

• Why put downstream data at highend of spectrum?
• When they made the system they put in amplifiers
  that could work at these high frequencies (why?)
• So the downstream was all set to go. But upstream
  amplifiers had to be installed.
• Downstream data also uses FDM 6MHz or 8MHz
  channel width with QAM-64 (what is the data
  rate?)
• But with overhead you only get 27Mbps.
• The upstream also uses FDM, but with QPSK (2 bits
  per symbol).

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Cable modems

• On modem boot-up, the headend tells the modem
  which channel to use for upstream and downstream
  transmission.
• Many users may share the same channel.
• To share the same channel, cable uses time
  division multiplexing (TDM).

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Time Division Multiplexing (TDM)
channel 1
channel 2
channel n
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TDM

• Data rate of medium exceeds data rate of digital
  signal to be transmitted
• Multiple digital signals interleaved in time
• May be at bit level of blocks
• Time slots preassigned to sources and fixed
• Time slots allocated even if no data
• Time slots do not have to be evenly distributed
  amongst sources

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TDM vs FDM
FDM
TDM
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TDM

• Must provide synchronizing mechanism
• Added digit framing
• One control bit added to each TDM frame
• Looks like another channel - control channel
• Identifiable bit pattern used on control channel
• e.g. alternating 01010101unlikely on a data
  channel
• Can compare incoming bit patterns on each channel
  with sync pattern

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Back to Cable

• Downstream is easy. The headend just transmits
  packets as it wants. Each packet has a label, so
  the modem can detect that the packet belongs to
  it.
• Upstream is hard.
• The upstream channel is shared.
• TDM is used, but still each TDM slot is shared by
  many users.
• What happens if two users try to send upstream
  data on the same channel?

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Cable Upstream Media Access (link layer)

• Time is divided into minislot. It is possible to
  transmit 8 bytes in one minislot.
• The modem asks the headend if it can transmit a
  packet.
• The headend responds with an acknowledgment and
  tells the modem which minislots it can use.
• Problem How can the modem sent the request
  without permission to use minislots?
• Solution on boot-up, the headend tells the modem
  which minislots it can use for requesting
  minislots and the headend never allocates these
  minislots for upstream data.
• Problem These special control minislots are
  shared by many users (why?), so what happens if
  two users make a request at the same time?
• Solution If two users transmit at the same time,
  the signal cannot be understood by the headend
  and is ignored. Thus, no acknowledgement is made.
  The competing modems then wait a random amount of
  time and try again. It if fails again, then they
  wait a random amount of time again, but the
  maximum time they might wait is doubled.

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Cable MAC (media access control )
headend
modem
minislot

modem sends a request for upstream minislots
Headend gets request. Thinks about it. And sends
acknowledgement with which minislots to use.
modem sends data
data does not overlap with request minislot
upstream request for bandwidth minislots
time
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Cable MAC Contention
Two modems send request for bandwidth at the same
time.
The headend can figure out what was transmitted
and ignores it
The modems wait for the acknowledgement that will
never arrive. The amount of time they wait is
random.
This time the red modem gets through.
The blue modem tries again, but the green modem
also sends a request.
Now the blue doubles the maximum random amount of
time it waits before sending another request.
Green gets through
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Cable vs. DSL

• Cable could give higher bandwidth, but it might
  give less. It depends on the number of users.
• If there are too many users in a group, the cable
  operator has to put in a fiber and headend. That
  cost money, so they try not to do it.
• DSL can promise 1Mbps down and 256kbps up, and
  you will likely get it.
• Note that sharing bandwidth is much more
  efficient than assigning each user a fixed chunk.
• The telephone system is very reliable. When was
  the last time you picked up the phone and there
  was no dial tone (major earthquake or huge
  storm). Cable will go down when the power goes
  out.

39
Mobile Phone