1. Physical Layer II: Framing, SONET, SDH, etc.
CS 4251: Computer Networking II Nick Feamster Spring 2008

2. From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream
Packets
Header/Body
Receiver
Sender
Packet
Transmission

3. Analog versus Digital Encoding
Digital transmissions.
Interpret the signal as a series of 1’s and 0’s
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
Receiver’s clock must be synchronized with the sender’s 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)1 1 1 1
.85 V
-.85
1 -> high signal; 0 -> low signal
Long sequences of 1’s or 0’s can cause problems:
Sensitive to clock skew, i.e. hard to recover clock
Difficult to interpret 0’s and 1’s

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)1 1 1 1
.85 V
-.85
1 -> make transition; 0 -> signal stays the same
Solves the problem for long sequences of 1’s, but not for 0’s.

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 Encoding1 1
.85 V
-.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(2πfc

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 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”)

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

24. 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

25. Synchronous Digital Hierarchy (SDH)
Tightly synchronized clocks remove the need for any complicated demultiplexing
Typically allows for higher data rates
OC3: Mbps
OC12: Mbps …

26. 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

27. Amplitude Carrier Modulation
Signal
Carrier
Frequency
Modulated
Carrier

28. Frequency Division Multiplexing: Multiple Channels
Determines Bandwidth of Link
Amplitude
Determines
Bandwidth
of Channel
Different Carrier
Frequencies

29. 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

30. 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