Download Link Layer

Physical Layer II:
Framing, SONET, SDH, etc.
CS 4251: Computer Networking II
Nick Feamster
Spring 2008
From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream
Packets
0 0 1 0 1 1 1 0 0 0 1
0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Body
Packet
Transmission
Sender
Header/Body
Header/Body
Receiver
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)
0
1
0
0
0
1
1
0
1
.85
V
0
-.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
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)
0
1
0
0
0
1
1
0
1
.85
V
0
-.85
• 1 -> make transition; 0 -> signal stays the
same
• Solves the problem for long sequences of 1’s,
but not for 0’s.
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
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
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

 A cos2f ct 
s t   
0


binary 1
binary 0
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

 A cos2f1t 
s t   

 A cos2f 2t 
•
•
•
•
binary 1
binary 0
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
si t   A cos 2f i t
•
•
•
•
•
1 i  M
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
Phase-Shift Keying (PSK)
• Two-level PSK (BPSK)
– Uses two phases to represent binary digits

binary 1
 A cos2f ct 
s t   
binary 0

 A cos2f ct   

 A cos2f ct 


 A cos2f ct 
binary 1
binary 0
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 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”)
Points in the Hierarchy: TDM
Level
DS0
Data Rate
64
DS1
1,544
DS3
44,736
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
Amplitude
Amplitude Carrier Modulation
Signal
Carrier
Frequency
Modulated
Carrier
Frequency Division Multiplexing:
Multiple Channels
Amplitude
Determines Bandwidth of Link
Determines
Bandwidth
of Channel
Different Carrier
Frequencies
• With frequency-division
multiplexing different users
use different parts of the
frequency spectrum.
Frequency
Frequency vs. Time-division
Multiplexing
Frequency
Bands
– 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
Slot
Time
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
Optical
Splitter
Frequency
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
Gigabit Ethernet:
Physical Layer Comparison
Medium
Transmit/receive
Distance
Copper
Twisted pair
1000BASE-CX
1000BASE-T
25 m
100 m
MM fiber 62 m
SM fiber
1000BASE-SX
1000BASE-LX
1000BASE-SX
1000BASE-LX
1000BASE-LX
260 m
500 m
525 m
550 m
5000 m
Twisted pair
100BASE-T
100 m
MM fiber
100BASE-SX
2000m
MM fiber 50 m
Comment
machine room use
not yet defined; cost?
Goal:4 pairs of UTP5
2p of UTP5/2-4p of UTP3
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