Some Definitions
Broadband
• According to International Telecommunications Union (ITU), defined as
transmission speed higher than 1.5 Mb/s.
• Any connection fast enough to support interactive multimedia.
• Any communications method that multiplexes a number of individual channels
onto a single, high-speed channel.
Wireless Communication
• Data is transmitted over the air, modulated onto a carrier signal
(e.g., FDMA, CDMA)
Wireline Communication
• Network connection is transmitted through physical media (copper or optical
fiber).
• Data is usually sent unmodulated.
• Multiple channels are aggregated via time-division multiplexing.
EECS 270C Week 1
Prof. M. Green
1
Digital Telephony Example
111
110
Analog signal:
101
100
011
010
001
000
Ts
1
0
0
1
0
1
1
1
0
Digitized signal:
(b = 3)
Bit rate is b/Ts
EECS 270C Week 1
Ts
Prof. M. Green
2
1
0
0
1
0
1
1
1
0
For digital telephony:
Voice quality requires ~4 kHz bandwidth
Ts = 125 µs (fs = 8 kHz)
b=8
b bits in Ts
8 kHz X 8 bits (bit rate 64 kb/s) gives “DS0” signal.
User-to-network interface:
24 X DS0
Framing bit
MUX
DS1 channel
DS1 bits in each TS: 24 X 8 + 1 = 193
DS1 bit rate: 193 / 125 µs = 1.544 Mb/s
DS3 channel
DS3 bits in each TS: 28 X 193 + 188 = 5592
DS3 bit rate: 5592 / 125 µs = 44.736 Mb/s
28 X DS1
188 Framing bits
MUX
“T-carrier” system: T1 line carries a DS1 signal
T3 line carries a DS3 signal
EECS 270C Week 1
Prof. M. Green
3
Ethernet
• Invented in 1973 at Xerox PARC
• IEEE 802.3 standard (10 Mb/s) created in 1985
• Used to create Local-Area Networks (LANs)
IEEE ethernet identifiers:
10 BASE 5 -- (10 Mb/s, baseband transmission, 500m max. cable length)
1000 BASE T -- (1 Gb/s, baseband transmission, twisted-pair)
Gigabit/10 Gigabit Ethernet (IEEE Standard 802.3):
1 Gb/s links can be transmitted over twisted-pair copper
10 Gb/s links can be transmitted over copper (short lengths) or fiber.
EECS 270C Week 1
Prof. M. Green
4
Networking
Wide-Area Network (WAN):
multiple LANs connected over a wide geographical area -- made possible
by very high-speed optical fibers
Metropolitan-Area Network (MAN):
Network connection within a metropolitan area
Storage-Area Network (SAN):
Uses networking techniques to manage very large amounts of data
EECS 270C Week 1
Prof. M. Green
5
Synchronization Methods
TX
Ref.
clock
RX
CMU
only data, not clock, transmitted
Plesiochronous Digital Hierarchy:
• Different parts of network operate at frequencies that are very close (~50 ppm), but
not identical.
• Such systems require additional functions to compensate for the mismatch by
repeating or adding bits.
• Reference clocks generated locally (usually with crystal oscillator).
• Used in Ethernet protocol.
Synchronous Digital Hierarchy:
• All parts of network operate at identical frequencies, accomplished by synchronizing
all Reference clocks to the “Stratum” global system of atomic clocks.
• Additional functions not required, but jitter requirement is very rigorous.
• Used in SONET/SDH protocol.
EECS 270C Week 1
Prof. M. Green
6
Other Protocols for High-Speed Networks
Synchronous Optical Network (SONET*):
• Provides a protocol (standardized by ANSI) for long-haul (> 50km) WAN
transmission over optical fiber
Optical carrier (OC)
level
*Also
Native bit rate
OC-1
51.84 Mb/s
OC-3
155.52 Mb/s
OC-12
622.08 Mb/s
OC-48
2.48832 Gb/s
OC-192
9.95328 Gb/s
OC-768
39.81312 Gb/s
OC-1536
79.62612 Gb/s
OC-3072
159.25224 Gb/s
known internationally as Synchronous Digital Hierarchy (SDH).
EECS 270C Week 1
Prof. M. Green
7
SONET Ring
OC-3
Add/Drop
MUX
OC-48
Add/Drop
MUX
OC-12
OC-12
Add/Drop
MUX
standby ring
OC-48
working ring
OC-3
OC-48
OC-3
OC-3
OC-48
Add/Drop
MUX
OC-12
OC-12
• Fiber rings can easily be deployed
• If any one link fails or is down for maintenance, data can still be
transmitted.
EECS 270C Week 1
Prof. M. Green
8
Fibre Channel:
• Often used for Storage Area Networks (SAN); allows fast transmission of
large amounts of data across many different servers.
• Serial bit rates of 1.0625 2.125, 4.25, 8.5 Gb/s
EECS 270C Week 1
Prof. M. Green
9
Some SAN Terminology
JBOD: Just a Bunch Of Disks
Refers to a set of hard disks that are not configured
together.
RAID: Redundant Array of Independent (or Inexpensive?) Disks
Multiple disk drives that are combined for fault tolerance
and performance. Looks like a single disk to the rest of
the system. If one disk fails, the system will continue
working properly.
EECS 270C Week 1
Prof. M. Green
10
Passive Optical Network (PON)
• Used to replace electronic transmission in “last mile”
• Facilitates “Fiber-to-the-home (FTTH)” or “Fiber-to-the-premises (FTTP)”
GPON protocol:
• 2.5 Gb/s upstream;1.25
Gb/s downstream
• TDMA “Burst-mode”
operation: Switching among
fibers requires fast locking at
receiver (within ~30 UI).
EECS 270C Week 1
Prof. M. Green
11
Open Systems International (OSI) Networking Protocol
of interest to
IC designers
http://http://en.wikipedia.org/wiki/OSI_model
EECS 270C Week 1
Prof. M. Green
12
Characteristics of Broadband Signals & Circuits
Primarily digital (i.e., bilevel) operation but high bit rate (multi-Gb/s)
dictates analog behavior & design techniques.
• Standard analog circuit applications:
 Continuous-time operation
 Precision required in signal domain
(i.e., voltage or current)
 Dynamic range determined by noise
& distortion
V
• Broadband communication circuits:
 Discrete-time (clocked) operation
 Precision required in time domain
(low jitter)
 Bilevel signals processed
V
V
t0
VH
Vt
VL
t
EECS 270C Week 1
Prof. M. Green
t
t
13
Binary Data Representations (time domain)
Non-return-to-zero (NRZ) format (most common):
Tb
“unit interval” (UI)
Return-to-zero (RZ) format:
1
Tb
2
1
0
1
1
0
1

• Higher bandwidth RZ signals require faster circuitry than NRZ, but are more
easily synchronized due to more transitions.
EECS 270C Week 1
Prof. M. Green
14
Some Definitions (1)
Transition Density is the ratio of transitions to the number of unit intervals in
a data stream. A high transition density is desirable in a communication
system.
6 transitions/12 clock cycles  transition density = 0.5
Equivalent to density of 0011 repeating pattern
EECS 270C Week 1
Prof. M. Green
15
Some Definitions (2)
Run Length is the maximum of consecutive 0’s or 1’s that occur in a data
stream. A maximum run length is often specified in a communication
system to avoid long periods where no transitions are present.
(Also known as Consecutive Identical Digit – CID)
Run length = 10 bits
EECS 270C Week 1
Prof. M. Green
16
Some Definitions (3)
Pseudo-Random Bit Sequence (PRBS) is a repeating pattern that has
properties similar to random sequences.
• Parameterized by n, number of DFFs in generator.
• Gives almost equal number of 1’s & 0’s
• Sequence length = 2n-1; max. run length = n
D1
Q1
Q2
Q3
CK
D1  Q1 Q3
23-1 PRBS
EECS 270C Week 1
Prof. M. Green
Q1
Q2
Q3
D1
1
0
0
1
1
1
0
1
1
1
1
0
0
1
1
1
1
0
1
0
0
1
0
0
0
0
1
1
1
0
0
1
..
.
17
Definitions of Common PRBS Signals
Sequence
Sequence
Length
Run
Length
Feedback
(defined by ITU)
27-1
127
7 D 1 = Q1  Q3
29-1
511
9 D 1 = Q5  Q9
211-1
2 047
11 D1 = Q9  Q11
215-1
32 767
15 D1 = Q14  Q15
220-1
1 048 575
20 D1 = Q3  Q20
223-1
8 388 607
23 D1 = Q18  Q23
231-1 2 147 483 647
31 D1 = Q28  Q31
Bit error-rate testing (BERT) equipment is programmed to recognize these patterns.
EECS 270C Week 1
Prof. M. Green
18
Decimation Properties of PRBS
23-1 PRBS:
PRBS demuxed into 2 parallel channels
Resulting bit sequences are both also 23-1 PRBS!
EECS 270C Week 1
Prof. M. Green
19
Typical broadband data waveform:
Length of single bit = 1 Unit Interval (1 UI)
Eye diagram
An eye diagram maps a random bit sequence
to a regular structure that can be used to
analyze jitter.
EECS 270C Week 1
Prof. M. Green
20
Close-up of measured eye
diagram:
trise = tfall
voltage swing
1 UI
(Unit Interval)
Zero-crossing width
indicates jitter.
Zero crossings
EECS 270C Week 1
Prof. M. Green
21
Types of Jitter (1)
Random Jitter (RJ):
• Originates from external and internal random noise sources
• Stochastic in nature (probability-based)
• Measured in rms units
• Observed as Gaussian histogram around zero-crossing
• Grows without bound over time
Histogram measurement at zero crossing
exhibiting Gaussian probability distribution
EECS 270C Week 1
Prof. M. Green
22
Types of Jitter (2)
Deterministic Jitter (DJ):
• Originates from circuit non-idealities (e.g., finite bandwidth, offset, etc.)
• Amount of DJ at any given transition is predictable
• Measured in peak-to-peak units
• Bounded and observed in various eye diagram “signatures”
• Different types of DJ:
a) Intersymbol interference (ISI)
b) Duty-cycle distortion (DCD)
c) Periodic jitter (PJ)
EECS 270C Week 1
Prof. M. Green
23
a) Intersymbol interference (ISI)
Consider a 1 UI output pulse applied to a buffer:
1UI
< 1UI


  UI
  UI
  UI

If rise/fall time << 1 UI, then the output pulse is attenuated
and the pulse width decreases.
EECS 270C Week 1
Prof. M. Green
24
ISI (cont.)
Consider 2 different bit sequences:
0
0
1
1
0
1
Steady-state not reached
at end of 2nd bit
t = ISI
2 output sequences
superimposed
ISI is characterized by a double edge
in the eye diagram.
EECS 270C Week 1
Prof. M. Green
25
Effect of ISI on measured
eye diagram:
Double-edge (DJ) combined with RJ
EECS 270C Week 1
Prof. M. Green
26
b) Duty cycle distortion (DCD)
• Occurs when rising and falling edges exhibit different delays
• Caused by circuit mismatches
Nominal data sequence
Tb
2Tb
Data sequence with late falling edges
& early rising edges due to threshold shift
t = DCD
Eye diagram with DCD
Crossing offset from
nominal threshold
EECS 270C Week 1
Prof. M. Green
27
c) Periodic Jitter (PJ)
Timing variation caused by periodic sources unrelated to the data pattern.
Can be correlated or uncorrelated with data rate.
Clock source with
duty cycle ≠50%
t1
PJ  t1  t0
t0
Synchronized data
exhibiting correlated PJ

Uncorrelated jitter (e.g., sub-rate PJ due to supply ripple) affects the
eye diagram in a similar way as RJ.
EECS 270C Week 1
Prof. M. Green
28
Binary Data Representations in Frequency Domain (1)
A random data signal x(t) can be represented as:
x(t) 
b  pt  kT 
k
b
k
where bk  1,1 is the bit sequence and p(t) is a unit-interval pulse::
P(f)


p(t)
1 
0

P f Tb 
Tb
t
sin(  fTb )
fTb
1
Tb
2
Tb
3
Tb
f
If there is equal
 probability of low or high logic levels (i.e., dc level is 0),
the power spectral density of x(t) is given by:


1
Sx f 
Pf
Tb
EECS 270C Week 1
2

sin  fT
b
Tb  
  fTb


2





Prof. M. Green
29
Binary Data Representations in Frequency Domain (2)

Sx f


1
Sx f 
Pf
Tb
2

sin  fT
b
Tb  
  fTb


2
 


1
Tb


Example: 10 Gb/s data signals
Sx f
2
Tb

3
Tb
f

random data

repeating 0101
f (GHz)
5
10
15
20
25
30
100 ps
EECS 270C Week 1
Prof. M. Green
30
Transmission over Copper
Ideal transmission line:
l
l
For l, c  0, transmission line behaves
like a constant delay.
c
c
Lossy transmission line:
rs
rs
l
l
c
EECS 270C Week 1
gp
c
gp
Series loss rs and shunt loss gp cause
attenuation and reduce bandwidth.
Prof. M. Green
31

l
rs
l
rs
c
gp
c
gp
At high frequencies, skin effect causes rs to increase with frequency:
H( )  e L

And dielectric loss causes gp to increase with frequency:
H()  eL

L = transmission line length;  are constants



For H( )  exp L     



10logH( )  4.34L    
This results in a very steep drop in a log-log scale …

EECS 270C Week 1

Prof. M. Green
32

Effect of High-Frequency Loss in Copper Cable
|H(f)| (dB)
108
109
1010
1011
f (Hz)
EECS 270C Week 1
Prof. M. Green
33
Coaxial cable
grounded shield
inner conductor (signal)
Purpose of outer conductor:
• Shields region inside from external electromagnetic fields
• Provides return path
Typical loss @ 100 MHz: 9 dB/foot
“ @ 1 GHz: 22 dB/foot
EECS 270C Week 1
Prof. M. Green
34
Twisted Pair
+
_
•
•
•
•
Signal sent differentially.
Twisting gives each line nearly equal exposure to outside interference.
Lighter and less expensive the shielded cable.
Quality specified in # twists/foot
Cat 3 unshielded twisted pair (UTP):
Cat 5e UTP:
Cat 6 UTP:
EECS 270C Week 1
Prof. M. Green
< 16 MHz
< 100 MHz
< 250 MHz
35
Backplane
A circuit board that allows connection of several connectors together, forming
a bus. For high-speed signals, the metal traces are considered to be
microstrip lines.
http://en.wikipedia.org/wiki/Industry_Standard_Architecture
EECS 270C Week 1
Prof. M. Green
36
Transmission over Optical Fiber
Snell’s Law of Refraction:
n1
sin  1 n2 v 1


sin  2 n1 v 2

n2
n1
reflected ray
reflected ray
n2
refracted ray
refracted ray
1
1
incident
ray


1
1
2
incident ray


n2  n1

n2  n1

EECS 270C Week 1
2
Prof. M. Green

37
Total Internal Reflection
n1
reflected ray
n2
refracted ray
1
2
Let 2 = /2:
Then sin 1 
n 
 c  sin  2 
n1 
1
1

For 1 > c, light ray is completely reflected.

incident ray

n2
n1
n2  n1
 Total internal reflection

EECS 270C Week 1
Prof. M. Green
38
Optical Fiber Transmission
ncladding ncore ncladding
n1 n2
reflected ray
refracted ray
1
2
1

ncladding  ncore
Total internal reflection keeps all
optical energy within the core,
even if the fiber bends.

incident ray

n  n1
core2
cladding

EECS 270C Week 1
Prof. M. Green
39
Advantages of Optical Fibers
over Copper Cable
• Very high bandwidth (bandwidth of optical transmission network
determined primarily by electronics)
• Low loss
• Interference Immunity (no antenna-like behavior)
• Lower maintenance costs (no corrosion, squirrels don’t like the
taste)
• Small & light:
1000 feet of copper weighs approx. 300 lb.
1000 feet of fiber weighs approx. 10 lb.
• Different light wavelengths can be multiplexed onto a single
fiber via Dense Wavelength Division Multiplexing (DWM).
• 10Gb/s & 40 Gb/s transmission networks are state-of-the art.
EECS 270C Week 1
Prof. M. Green
40
Commonly-used wavelengths
Fiber Loss vs. Wavelength
850nm
(LED)
EECS 270C Week 1
1310nm 1550nm
Prof. M. Green
41
Types of Optical Fiber
Diameter  125 µm
inexpensive; used for shorter
distances; dispersion causes
jitter.
Diameter = 2~8 µm
Expensive; used for long
distances
Optical dispersion
compensation; non-uniform n1
EECS 270C Week 1
Prof. M. Green
42
40Gbps NRZ signal
Optical Signals
25ps
Dr
-40
Laser
source
40 80 f (GHz)
λ = 1550nm
f = 193 THz
Modulator
193THz
f
40GHz
λ=v/f
1550
λ (nm)
0.32
EECS 270C Week 1
Prof. M. Green
43
Chromatic Dispersion (1)
• Chromatic dispersion is due to the fact that different wavelength
travel at different speeds.
EECS 270C Week 1
Prof. M. Green
44
Chromatic Dispersion (2)
• CD is measured in ps/nm.
CD 
• CD is proportional to fiber length:


CD  17ps/nm/m L
d
d
Relative
group
Delay,


(ps)
λ (nm)
EECS 270C Week 1
Prof. M. Green
45
Chromatic Dispersion at Different Data Rates
10Gb/s
CD=0
CD=600ps/nm
CD=1600ps/nm
CD=2200ps/nm
CD=40ps/nm
CD=100ps/nm
CD=140ps/nm
40Gb/s
CD=0
EECS 270C Week 1
Prof. M. Green
46
Polarization Mode Dispersion
• PMD is due to the fact that light travels at different speed across
the two orthogonal polarization states.
• Output contains two delayed images of the input pulse.
EECS 270C Week 1
Prof. M. Green
47
Eye Diagrams due to PMD
DGD=0
(BER<1e-15)
DGD=20ps
(BER=2e-4)
EECS 270C Week 1
DGD=10ps
(BER<1e-15)
DGD=15ps
(BER=2e-11)
DGD=25ps
(BER=2e-2)
DGD=30ps
(BER=2e-2)
Prof. M. Green
48