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

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

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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 Encoding
1 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

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