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

1
Physical Layer II Framing, SONET, SDH, etc.

• CS 4251 Computer Networking IINick
  FeamsterSpring 2008

2
From Signals to Packets
3
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?

4
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.

5
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.

6
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

7
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

8
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

9
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.

10
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

11
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

12
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

13
Digital Data, Analog Signals

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

14
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

15
(No Transcript)
16
Amplitude-Shift Keying

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

17
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

18
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

19
Phase-Shift Keying (PSK)

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

20
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?

21
Time Division Multiplexing

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

Frequency
Frequency
22
Plesiosynchronous Digital Hierarchy (PDH)

• Infrastructure based on phone network
• Spoken word not intelligeible 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)

23
Points in the Hierarchy TDM
Level
Data Rate
DS0 64
DS1 1,544
DS3 44,736
24
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

25
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

26
Amplitude Carrier Modulation
Amplitude
Amplitude
Signal
Carrier Frequency
Modulated Carrier
27
Frequency Division MultiplexingMultiple Channels
Determines Bandwidth of Link
Amplitude
Determines Bandwidth of Channel
Different Carrier Frequencies
28
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
29
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
30
Wireless Technologies

• Great technology no wires to install, convenient
  mobility, ..
• High attenuation limits distances.
• High noise interference from other transmitters.
• Use MAC and other rules to limit interference
• Aggressive encoding techniques to make signal
  less sensitive to noise
• Other effects multipath fading, security, ..
• Ether has limited bandwidth.
• Try to maximize its use
• Government oversight to control use

31
Gigabit EthernetPhysical Layer Comparison
Medium Transmit/receive Distance Comment Cop
per 1000BASE-CX 25 m machine room
use Twisted pair 1000BASE-T 100 m not
yet defined cost? Goal4 pairs of
UTP5 MM fiber 62 mm 1000BASE-SX 260 m
1000BASE-LX 500 m MM fiber 50 mm
1000BASE-SX 525 m 1000BASE-LX 550 m SM
fiber 1000BASE-LX 5000 m Twisted pair
100BASE-T 100 m 2p of UTP5/2-4p of UTP3 MM
fiber 100BASE-SX 2000m
32
Regeneration and Amplification

• At end of span, either regenerate electronically
  or amplify
• Electronic repeaters are potentially slow, but
  can eliminate noise
• Amplification over long distances made practical
  by erbium doped fiber amplifiers offering up to
  40 dB gain, linear response over a broad
  spectrum. Ex 10 Gbps at 500 km.

pump laser
source