Physical Layer II: Framing, SONET, SDH, etc.

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Physical Layer II Framing, SONET, SDH, etc.

• CS 4251 Computer Networking IINick
  FeamsterSpring 2008

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From Signals to Packets
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Analog versus Digital Encoding

• Digital transmissions.
• Interpret the signal as a series of 1s and 0s
• E.g. data transmission over the Internet
• Analog transmission
• Do not interpret the contents
• E.g broadcast radio
• Why digital transmission?

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Why Do We Need Encoding?

• Meet certain electrical constraints.
• Receiver needs enough transitions to keep track
  of the transmit clock
• Avoid receiver saturation
• Create control symbols, besides regular data
  symbols.
• E.g. start or end of frame, escape, ...
• Error detection or error corrections.
• Some codes are illegal so receiver can detect
  certain classes of errors
• Minor errors can be corrected by having multiple
  adjacent signals mapped to the same data symbol
• Encoding can be very complex, e.g. wireless.

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Encoding

• Use two discrete signals, high and low, to encode
  0 and 1.
• Transmission is synchronous, i.e., a clock is
  used to sample the signal.
• In general, the duration of one bit is equal to
  one or two clock ticks
• Receivers clock must be synchronized with the
  senders clock
• Encoding can be done one bit at a time or in
  blocks of, e.g., 4 or 8 bits.

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Nonreturn to Zero (NRZ)

• Level A positive constant voltage represents one
  binary value, and a negative contant voltage
  represents the other
• Disadvantages
• In the presence of noise, may be difficult to
  distinguish binary values
• Synchronization may be an issue

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Non-Return to Zero (NRZ)
0
0
0
1
1
0
1
0
1
.85
V
0
-.85

• 1 -gt high signal 0 -gt low signal
• Long sequences of 1s or 0s can cause problems
• Sensitive to clock skew, i.e. hard to recover
  clock
• Difficult to interpret 0s and 1s

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Improvement Differential Encoding

• Example Nonreturn to Zero Inverted
• Zero No transition at the beginning of an
  interval
• One Transition at the beginning of an interval
• Advantage
• Since bits are represented by transitions, may be
  more resistant to noise
• Disadvantage
• Clocking still requires time synchronization

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Non-Return to Zero Inverted (NRZI)
0
0
0
1
1
0
1
0
1
.85
V
0
-.85

• 1 -gt make transition 0 -gt signal stays the same
• Solves the problem for long sequences of 1s, but
  not for 0s.

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Biphase Encoding

• Transition in the middle of the bit period
• Transition serves two purposes
• Clocking mechanism
• Data
• Example Manchester encoding
• One represented as low to high transition
• Zero represented as high to low transition

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Aspects of Biphase Encoding

• Advantages
• Synchronization Receiver can synchronize on the
  predictable transition in each bit-time
• No DC component
• Easier error detection
• Disadvantage
• As many as two transitions per bit-time
• Modulation rate is twice that of other schemes
• Requires additional bandwidth

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Ethernet Manchester Encoding
0
1
1
0
.85
V
0
-.85
.1?s

• Positive transition for 0, negative for 1
• Transition every cycle communicates clock (but
  need 2 transition times per bit)
• DC balance has good electrical properties

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Digital Data, Analog Signals

• Example Transmitting digital data over the
  public telephone network
• Amplitude Shift Keying
• Frequency Shift Keying
• Phase Shift Keying

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Amplitude-Shift Keying

• One binary digit represented by presence of
  carrier, at constant amplitude
• Other binary digit represented by absence of
  carrier where the carrier signal is
  Acos(2pfc

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Amplitude-Shift Keying

• Used to transmit digital data over optical fiber
• Susceptible to sudden gain changes
• Inefficient modulation technique for data

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Binary Frequency-Shift Keying (BFSK)

• Two binary digits represented by two different
  frequencies near the carrier frequency
• f1 and f2 are offset from carrier frequency fc by
  equal but opposite amounts

• Less susceptible to error than ASK
• On voice-grade lines, used up to 1200bps
• Used for high-frequency (3 to 30 MHz) radio
  transmission
• Can be used at higher frequencies on LANs
  w/coaxial cable

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Multiple Frequency-Shift Keying

• More than two frequencies are used
• More bandwidth efficient but more susceptible to
  error
• f i f c (2i 1 M)f d
• f c the carrier frequency
• f d the difference frequency
• M number of different signal elements 2 L
• L number of bits per signal element

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Phase-Shift Keying (PSK)

• Two-level PSK (BPSK)
• Uses two phases to represent binary digits

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Modulation Supporting Multiple Channels

• Multiple channels can coexist if they transmit at
  a different frequency, or at a different time, or
  in a different part of the space.
• Space can be limited using wires or using
  transmit power of wireless transmitters.
• Frequency multiplexing means that different users
  use a different part of the spectrum.
• Controlling time is a datalink protocol issue.
• Media Access Control (MAC) who gets to send when?

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Time Division Multiplexing

• Users use the wire at different points in time.
• Aggregate bandwidth also requires more spectrum.

Frequency
Frequency
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Plesiosynchronous Digital Hierarchy (PDH)

• Infrastructure based on phone network
• Spoken word not intelligible above 3400 Hz
• Nyquist 8000 samples per second
• 256 quantization levels (8 bits)
• Hence, each voice call is 64Kbps data stream
• Almost synchronous Individual streams are
  clocked at slightly different rates
• Stuff bits at the beginning of each frame allow
  for clock alignment (more complicated schemes
  called B8ZS, HDB3)

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Points in the Hierarchy TDM
Level
Data Rate
DS0 64
DS1 1,544
DS3 44,736
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TDM Moving up the Hierarchy

• Additional bits are stuffed into frames to allow
  for clock alignment at the start of every frame
• In North America, a DS0 data link is provisioned
  at 56 Kbps. Elsewhere, it is 64 Kbps.
• Circuits can be provided in composite bundles

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Synchronous Digital Hierarchy (SDH)

• Tightly synchronized clocks remove the need for
  any complicated demultiplexing
• Typically allows for higher data rates
• OC3 155.52 Mbps
• OC12 622.08 Mbps

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Baseband versus Carrier Modulation

• Baseband modulation send the bare signal.
• Carrier modulation use the signal to modulate a
  higher frequency signal (carrier).
• Can be viewed as the product of the two signals
• Corresponds to a shift in the frequency domain
• Same idea applies to frequency and phase
  modulation.
• E.g. change frequency of the carrier instead of
  its amplitude

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Amplitude Carrier Modulation
Amplitude
Amplitude
Signal
Carrier Frequency
Modulated Carrier
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Frequency Division MultiplexingMultiple Channels
Determines Bandwidth of Link
Amplitude
Determines Bandwidth of Channel
Different Carrier Frequencies
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Frequency vs. Time-division Multiplexing

• With frequency-division multiplexing different
  users use different parts of the frequency
  spectrum.
• I.e. each user can send all the time at reduced
  rate
• Example roommates
• With time-division multiplexing different users
  send at different times.
• I.e. each user can sent at full speed some of the
  time
• Example a time-share condo
• The two solutions can be combined

Frequency
Frequency Bands
Slot
Frame
Time
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Wavelength-Division Multiplexing

• Send multiple wavelengths through the same fiber.
• Multiplex and demultiplex the optical signal on
  the fiber
• Each wavelength represents an optical carrier
  that can carry a separate signal.
• E.g., 16 colors of 2.4 Gbit/second
• Like radio, but optical and much faster

Optical Splitter
Frequency