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Physical Layer II: Framing, SONET, SDH, etc.
CS 4251: Computer Networking IINick FeamsterSpring 2008
From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream 0 0 1 0 1 1 1 0 0 0 1
Packets0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Body Header/Body Header/Body
ReceiverSenderPacket
Transmission
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?
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.
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.
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
Non-Return to Zero (NRZ)
• 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
V 0
.85
-.85
0 0 0 11 0 1 0 1
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
Non-Return to Zero Inverted (NRZI)
• 1 -> make transition; 0 -> signal stays the same
• Solves the problem for long sequences of 1’s, but not for 0’s.
V 0
.85
-.85
0 0 0 11 0 1 0 1
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
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
Ethernet Manchester Encoding
• 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
V 0
.85
-.85
0 1 1 0
.1s
Digital Data, Analog Signals
• Example: Transmitting digital data over the public telephone network
• Amplitude Shift Keying• Frequency Shift Keying• Phase Shift Keying
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
ts tfA c2cos0
1binary 0binary
Amplitude-Shift Keying
• Used to transmit digital data over optical fiber
• Susceptible to sudden gain changes
• Inefficient modulation technique for data
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
ts tfA 12cos tfA 22cos
1binary 0binary
• 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
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
tfAts ii 2cos Mi 1
Phase-Shift Keying (PSK)• Two-level PSK (BPSK)
– Uses two phases to represent binary digits
ts tfA c2cos tfA c2cos
1binary 0binary
tfA c2cos
tfA c2cos1binary 0binary
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?
Time Division Multiplexing
• Users use the wire at different points in time.• Aggregate bandwidth also requires more spectrum.
Frequency
Frequency
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”)
Points in the Hierarchy: TDM
DS0 64
DS1 1,544
DS3 44,736
Level Data Rate
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
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– …
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
Amplitude Carrier ModulationA
mpl
itude
Signal CarrierFrequency
Am
plitu
de
ModulatedCarrier
Frequency Division Multiplexing:Multiple Channels
Am
plitu
de
Different CarrierFrequencies
DeterminesBandwidthof Channel
Determines Bandwidth of Link
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
Fre
quen
cy
Time
FrequencyBands
Slot Frame
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
OpticalSplitter
Frequency