Download Basic Concepts

Physical Layer
Propagation:
UTP and Optical Fiber
Chapter 3
Updated January 2009
XU Zhengchuan
Fudan University
Orientation
• Chapter 2
– Data link, internet, transport, and application layers
– Characterized by message exchanges
• Chapter 3
– Physical layer (Layer 1)
– There are no messages—bits are sent individually
– Concerned with transmission media, plugs, signaling
methods, propagation effects
– Chapter 3: Signaling, UTP, optical fiber, and topologies
– Wireless transmission is covered in Chapter 5
2
Figure 3-1: Signal and Propagation
Received Signal
(Attenuated &
Distorted)
Transmitted
Signal
Propagation
Transmission Medium
Sender
Receiver
A signal is a disturbance in the media that propagates (travels)
down the transmission medium to the receiver
If propagation effects are too large, the receiver will not be able to
read the received signal
3
• Test Your Understanding
• P 141
4
Data
Representation
Binary-Encoded Data
• Computers store and process data in binary
representations
– Binary means “two”
– There are only ones and zeros
– Called bits
1101010110001110101100111
6
Binary-Encoded Data
• Non-Binary Data Must be Encoded into Binary
– Text
– Integers (whole numbers)
– Decimal numbers
– Alternatives (North, South, East, or West, etc.)
– Graphics
– Human voice
– etc.
Hello
11011001…
7
Binary-Encoded Data
• Some data are inherently binary
– 48-bit Ethernet addresses
– 32-bit IP addresses
– Need no further encoding
8
Figure 3-2: Arithmetic with Binary
Numbers
Binary Arithmetic for Whole Numbers (Integers)
(Counting Begins with 0, not 1)
Integer
0
1
2
3
4
5
6
7
8
Binary
0
1
10
11
100
101
110
111
1000
“There are 10 kinds of people—
those who understand binary and those who don’t”
9
Figure 3-2: Arithmetic with Binary
Numbers, Continued
Binary Arithmetic for Binary Numbers
Basic Rules
0
+0
=0
0
+1
=1
1
+0
=1
1
+1
=10
1
+1
+1
=11
10
Figure 3-2: Arithmetic with Binary
Numbers, Continued
Examples
Binary
1000
+1
=1001
+1
=1010
+1
=1011
+1
=1100
Decimal
8
+1
=9
+1
=10
+1
=11
+1
=12
11
Figure 3-3: Binary Encoding for
Alternatives
Encoding Alternatives
(Product number, region, gender, etc.)
(N bits can represent 2N Alternatives)
Number of Bits
In Field (N)
1
2
3
4
8
16
…
Number of Alternatives
That Can be Encoded
with N bits
2 (21)
4 (22)
8 (23)
16 (24)
256 (28)
65,536 (216)
…
Each added bit doubles the number of alternatives that can be represented
12
Figure 3-3: Binary Encoding for
Alternatives
Bits
Alternatives
Examples
1
21=2
Male = 0, Female = 1
2
22=4
Spring = 00, Summer = 01,
Autumn = 10, Winter = 11
8
28=256
Keyboard characters for U.S.
keyboards. Space=00000000, etc.
ASCII code actually uses 7 bits
13
Powers of 2
Bits
Alternatives
1
2
2
4
Start with one you know
and double or halve until
you have what you need
3
8
4
16
5
32
E.g., if you know 8 is 256,
10 must be 4 times as
large or 1,024.
6
64
7
128
8
256
Memorize for 1, 4, 8, and
16 bits
10
1,024
16
65,536
Each additional bit
doubles the number of
possibilities
14
Figure 3-3: Binary Encoding for
Alternatives
• Quiz
– How many flavors of ice cream can you represent in half
a byte of storage?
– How many bits do you need to represent 64 flavors of ice
cream?
– How many bits do you need to represent 6 sales districts?
15
Figure 3-4: ASCII and Extended ASCII
• ASCII Code to Represent Text
– ASCII is the traditional binary code to represent text data
– Seven bits per character
• 27 (128) characters possible
– Sufficient for all keyboard characters (including shifted
values)
• Capital letters (A is 1000001)
• Lowercase letters (a is 1100001)
– Each character is stored in a byte
• The 8th bit in a byte normally is not used
16
Figure 3-4: ASCII and Extended ASCII,
Continued
• Extended ASCII
– Used on PCs
– Uses a full 8 bits per character
– 28 (256) characters possible
– Extra characters can represent formatting in word
processing, etc.
• Converters
– Text-to-ASCII and Text-to-Extended ASCII
Converters are Readily Available on the Internet
17
Figure 3-5: Binary Coding for
Graphics Image
• Pixels
– 1. Screen is divided
into small squares
called pixels (picture
elements)
– 2. Each pixel has three
dots—red, green, and
blue. Sometimes a
black dot too
3. JPEG stores one
byte per color
(24 bits total)
This gives 256
intensity levels for
each color or
16.8 million colors
overall (2563)
18
• Test Your Understanding
• P 146
• 3c
19
Signaling
Figure 3-6: Data Encoding and Signaling
Data
“Now is the …”
Male or Female
Graphics
Human Voice
1.
First, data must be
converted to binary, as
we have just seen
Binary
Encoding
BinaryEncoded
Data
1101010
Signaling
2.
Second, bits must be covered
Into signals (voltage changes, etc.).
Voltage change, etc.
21
Figure 3-7: On/Off Binary Signaling
Clock
Cycle
Light
Source
Off=
0
On=
1
On=
1
Off=
0
On=
1
Off=
0
On=
1
Optical Fiber
During each clock cycle, light is turned on for a one or off for a zero.
22
Figure 3-8: Binary Signaling in 232 Serial Ports
In a clock cycle,
15 Volts
Clock Cycle
0
3 Volts
0
3 to 15 volts represents a
zero
-3 to -15 volts is a ONE
0
0 Volts
-3 Volts
1
-15 Volts
1
This type of
signaling is used in
232 serial ports.
23
Figure 3-9: Relative Immunity to Errors in
Binary Signaling
15 Volts
0
Transmitted
Signal
(12 Volts)
Received
Signal
(6 volts)
3 Volts
0 Volts
-3 Volts
1
Despite a 50% drop in voltage,
the receiver will still know
that the signal is a zero
-15 Volts
24
Binary and Binary Signaling
• In binary signaling, there are two states
– This can represent a single bit per clock cycle.
• In digital signaling, there are a few bits per clock cycle—2, 4,
8, 16, 32, …
• With more states, several bits to be sent per clock cycle
• Note that all binary transmission (2 states) is digital (few
states)
• But not all digital
transmission is
binary
11
11
10
01
00
10
01
Clock
Cycle
01
00
25
• Test Your Understanding
• P 149
• 4 a, b, c
26
Figure 3-10: 4-State Digital Signaling
Box
Clock Cycle
11
11
10
01
00
Client PC
10
01
01
00
Server
Digital signaling has a FEW possible states per clock cycle (4 in this slide)
This allows it to send multiple bits per clock cycle
This increases the bit transmission rate per clock cycle
It reduces error resistance because differences between states are smaller
27
Quiz
Box
• Which Is Binary? Which Is Digital?
2.
Number
of
Fingers
3.
On/Off Switch
1.
Calendar
4.
Day of the Week
5.
Gender
Male or Female
28
Figure 3-10: 4-State Digital Signaling, Continued
Box
• Equation 3-1:
Bit rate = Baud rate * Bits sent per clock cycle
– Baud rate is the number of clock cycles per second
• If the clock cycle is 1/1000 of a second, the baud rate
is 1,000 baud
– Bit rate is then the number of clock cycles per second
times the number of bits sent per clock cycle
• If the three bits are sent per clock cycle, the bit rate is
3,000 bps or 3 kbps
29
Figure 3-10: 4-State Digital Signaling, Continued
• Equation 3-2: States =
2Bits
– Bits is the number of bits to
be sent per clock cycle
– States is the number of
states needed to send that
many bits
• Doubling the number of
states transmits one more
bit per clock cycle.
Box
Bits to be Number of
sent per
states
clock cycle
required
1
2
2
4
3
8
4
16
30
Figure 3-10: 4-State Digital Signaling,
Continued
Box
• Example:
– The clock cycle is 1/100,000 second
• The baud rate is 100 kbaud (not kbauds)
– You want a bit rate of 500,000 bps
• Solution:
– You have to send 5 bits per clock cycle (baud)
– This will require 32 states
• States = 2bits
• States = 25
• States = 32
31
Figure 3-10: 4-State Digital Signaling,
Continued
Box
• Example:
– Suppose there a system has 8 states
– Suppose that the clock cycle is 1/10,000 second
– How fast can the system transmit?
• Solution:
– With four states, 3 information bits can be sent per clock
cycle (8=2X) [Equation 3-2] X=3
– With a clock cycle of 1/10,000, baud rate is 10,000 baud
– The bit rate will be 30 kbps (3 bits/clock cycle times
10,000 clock cycles per second). [Equation 3-1]
32
• Test Your Understanding
• P 151
33
UTP Propagation
Unshielded Twisted Pair wiring
Figure 3-12: 4-Pair UTP Cord with RJ45
Connector
3.
RJ-45
Connector
1.
UTP Cord
Industry Standard Pen
2.
8 Wires
Organized
as 4
Twisted
Pairs
UTP Cord
35
RJ-45 Jacks and Connectors
RJ-45
Jack
RJ-45
Jack
RJ-45
Jack
RJ-45 Connectors
36
Figure 3-11: Unshielded Twisted Pair
(UTP) Wiring, Continued
• UTP Characteristics
– Inexpensive and to purchase and install
– Dominates media for access links between computers
and the nearest switch
37
• Test Your Understanding
• P 154
38
Figure 3-13: Attenuation and Noise
Power
1.
Signal
Signals in UTP attenuate with
propagation distance.
If attenuation is too great, the
signal will not be readable by the
receiver.
Distance
39
Figure 3-14: Decibels
• Attenuation is Sometimes Expressed in
Decibels (dB)
• The equation for decibels is
– dB = 10 log10(P2/P1)
– Where P1 is the initial power and P2 is the final
power after transmission
– If P2 is smaller than P1, then the answer will be
negative
40
Figure 3-14: Decibels, Continued
• Example
– Over a transmission link, power drops to 37% of its
original value
– P2/P1 = 37/100 = .37 (37%/100%)
– LOG10(0.37) = -0.4318
– 10*LOG10(0.37) = -4.3 dB (negative, reflecting
power reduction through attenuation)
– In calculations, the Excel LOG10 function can be
used
41
Figure 3-14: Decibels, Continued
• There are two useful approximations
• 3 dB loss is a reduction to very nearly 1/2 the
original power
– 6 dB loss is a decrease to 1/4 the original power
– 9 dB loss is a decrease to 1/8 the original power
–…
• 10 dB loss is a reduction to very nearly 1/10 the
original power
– 20 dB loss is a decrease to 1/100 the original power
–…
42
Figure 3-13: Attenuation and Noise, Continued
Power
Signal
Signalto-Noise
Ratio (SNR)
Noise Spike
Error
Noise Floor
Noise
Distance
Noise is random unwanted energy within the wire
Its average is called the noise floor (噪声基底)
Random noise spikes (噪声毛刺) cause errors
-A high signal-to-noise ratio reduces noise error problems
As a signal attenuates with distance, damaging noise spikes
become more common
43
Limiting UTP Cord Length
• Limit UTP cord length to 100 meters
– Limits attenuation to being a negligible problem
– Limits noise problems being a negligible problem
– Note that limiting cord lengths limits BOTH noise and
attenuation problems
100 Meters Maximum
Cord Length
44
• Test Your Understanding
• P 157-158
45
Figure 3-11: Unshielded Twisted Pair
(UTP) Wiring, Continued
• Electromagnetic Interference (EMI) (Fig. 3-15)
– Electromagnetic interference (电磁干扰) is
electromagnetic energy from outside sources that
adds to the signal
• From fluorescent lights, electrical motors,
microwave ovens, etc.
– The problem is that UTP cords are like long radio
antennas.
• They pick up EMI energy nicely
• When they carry signals, they also send EMI
energy out from themselves
46
Figure 3-15: Electromagnetic
Interference (EMI) and Twisting
Electromagnetic
Interference (EMI)
Twisted
Wire
Interference on the Two Halves of a Twist Cancels Out
47
Figure 3-16: Crosstalk Interference and Terminal
Crosstalk Interference (交互干扰)
Untwisted
at Ends
Signal
Crosstalk Interference
Terminal Crosstalk
Interference
Terminal crosstalk interference
Normally is the biggest EMI problem for UTP
48
Figure 3-11: Unshielded Twisted Pair
(UTP) Wiring, Continued
• Electromagnetic Interference (EMI) (Fig. 3-15)
– Terminal crosstalk interference dominates
interference in UTP
– Terminal crosstalk interference is limited to an
acceptable level by not untwisting wires more than a
half inch (1.25 cm) at each end of the cord to fit into
the RJ-45 connector
– This reduces terminal crosstalk interference
to a negligible level.
1.25 cm or 0.5 inches
49
UTP Limitations
• Limit cords to 100 meters
– Limits BOTH noise AND attenuation problems to an
acceptable level
• Do not untwist wires more than 1.25 cm (a half inch)
when placing them in RJ-45 connectors
– Limits terminal crosstalk interference to an acceptable
level
• Neither completely eliminates the problems but
they usually reduce the problems to negligible
levels
50
• Test Your Understanding
• P 160
51
Figure 3-17: Serial Versus Parallel
Transmission
One Clock Cycle
1.
Serial
1 bit
Transmission
(1 bit per clock cycle)
2.
Parallel
Transmission
(1 bit per clock cycle
per wire pair)
4 bits per clock cycle
on 4 pairs
1 bit
1 bit
1 bit
1 bit
Parallel transmission increases speed.
But it is only workable over short distances.
Parallel is not 4. It is more than one.
52
• Test Your Understanding
• P 161
53
Figure 3-18: Wire Quality Standards
• Wiring Quality Standards
– Rated by Category (Cat) Numbers
• Category Standards are Set by ANSI/TIA/EIA and
ISO/IEC
– In the United States, the TIA/EIA/ANSI-568 governs UTP
and optical fiber standards
– In Europe and many other parts of the world, the
standard is ISO/IEC 11801
– The two sets of standards are close but not identical
54
Figure 3-18: Wire Quality Standards
• UTP Categories 3 and 4
– Early data wiring, which could only handle Ethernet
speeds up to 10 Mbps
• UTP Categories 5 and 5e
– Most wiring installed today is Category 5e (enhanced)
– Cat 5e and Cat 5 can handle Ethernet up to 1 Gbps
– Most wiring sold today is Cat 5e
55
Figure 3-18: Wire Quality Standards
• UTP Category 6
Errors
– Relatively new
– No better than Cat 5 or Cat 5e at 1 Gbps
– Developed for higher Ethernet speeds of 10 Gbps
• But can only span 55 meters at that speed
• Book says cannot be used. This is an error.
• Category 6A (Augmented)
– Able to carry Ethernet signals at 10 Gbps up 100 meters
– The book said 55 meters, but this is an error
56
Figure 3-18: Wire Quality Standards
• Category 7 STP
– Shielded twisted pair (STP) rather
than unshielded twisted pair (UTP)
• Metal foil shield around each pair to reduce crosstalk
interference
• Metal mesh around all four pairs to reduce crosstalk
from other cords
– STP is expensive and awkward to lay
– Can 10 Gbps Ethernet to 100 meters
57
• Test Your Understanding
• P 164
58
Optical Fiber
Transmission
Light through Glass
Better than UTP:
More Easily Spans Longer Distances at High Speeds
Figure 3-19: UTP in Access Lines and
Optical Fiber in Trunk Lines
1.
Workgroup
Switches Link
Computers to
the Network
Workgroup
Switch
UTP
Access
Line
2.
UTP dominates access lines
between stations and
their workgroup switches
UTP
Access
Line
UTP
Access
Line
60
Figure 3-19: UTP in Access Lines and
Optical Fiber in Trunk Lines, Continued
1.
Core switches
connect
other switches
Fiber
Trunk
Fiber Trunk
Fiber Trunk
Core Switch
Core Switch
Core
Fiber
Trunk
Core Switch
Fiber Trunk
2.
Fiber dominates trunk lines
between switches
61
• Test Your Understanding
• P 165
• 14-15
62
Figure 3-20: Optical Fiber Transceiver and Strand
Strand（股）
3.
Cladding （镀层）
125 micron diameter
Transceiver
1.
(Transmitter/Receiver)
Light Source
5.
850 nm,
Perfect internal reflection at
1,310 nm,
core/cladding boundary;
and 1,550 nm
No signal loss, so low attenuation
2.
Core
8.3, 50
or 62.5
micron
diameter
4.
Light
Ray
63
Figure 3-22: Two-Strand Full-Duplex Optical Fiber
Cord with SC and ST Connectors
Cord
Two
Strands
A fiber cord has
two-fiber strands
for full-duplex (twoway) transmission
SC Connectors
ST Connectors
64
Figure 3-22: Pen and Full-Duplex Optical Fiber
Cord with SC and ST Connectors
SC Connectors
(Push in and Snap)
ST Connectors
(Bayonet: Push in and Twist)
65
Figure 3-23: Frequency and Wavelengths
2.
Wavelength
Distance between comparable points in successive cycles
(Measured in nanometers for light)
1.
Amplitude
Power,
Voltage,
etc.
Wave
Amplitude
1 Second
3.
Frequency is the number of cycles per second.
1 Hz = 1 cycle per second
In this case, there are two cycles in 1 second,
so frequency is two hertz (2 Hz).
66
Light Wavelengths
• Light signals are measured by wavelength
• Light wavelengths measured in nanometers (nm)
• There are three fiber wavelength “windows” with
good propagation characteristics
– 850 nm
– 1310 nm
– 1550 nm
• Shorter wavelength allows cheaper transceivers
• Longer-wavelength light travels farther
67
• Test Your Understanding
• P 169
• 16
68
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– Uses multimode fiber, which has a “thick” core diameter
of 50 or 62.5 microns
• Less expensive than single-mode fiber (later)
• 62.5 micron fiber is more common in the US but does
not carry signals as far as 50 micron fiber
– Also uses inexpensive 850 nm transceivers
– Multimode fiber （多模光纤）with 850 nm signaling
cannot span the kilometer distances needed by carriers,
but can span the 200-300 meters needed in LAN fiber
cords
69
Figure 3-24: Multimode and Single-Mode
Optical Fiber
Mode 2
Light
Source
(Usually
Laser)
Core
Multimode Fiber
Mode 1
Arrives
Later
In thicker fiber, light only travels in one of several allowed modes.
Different modes travel different distances and arrive at different times
(See that Mode 1 light takes longer to arrive than Mode 2 light.)
If distance is too long, modes from successive light pulses will overlap.
This is modal distortion（模态散射）. If it is too large, signals will be
unreadable.
Modal distortion is the main limitation on distance in multimode fiber. 70
• Test Your Understanding
• P 172
• 17
71
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– All multimode fiber today is graded-index multimode fiber
• The index of refraction （折射级数）decreases from
the center of the core to the core’s outer edge.
Lower
Higher
Incidence of
Refraction
72
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– Graded-index multimode fiber
• Light speed increases when the index decreases
• The central mode (Mode 2) is slowed
• High-angle modes (Mode 1) are speeded up
Mode 2 (Slowed)
Mode 1 (Speeded Up Near Edge of Core)
Lower
Modal
Dispersion
73
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– UTP quality is measured by category number.
– Multimode Fiber Quality
• Measured as modal bandwidth (MHz.km or MHz-km)
• More modal bandwidth is better
• Increases the speed–distance product
– With greater mobile bandwidth, can go faster,
farther, or some combination of the two
74
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– Example: 1000BASE-SX Ethernet
• Uses inexpensive 850 nm light
• With 62.5 micron fiber and 160 MHz-km modal
bandwidth, maximum distance is 220 m
• With 62.5 micron fiber and 200 MHz-km bandwidth,
maximum distance is 275 m
• Some vendors with higher-than-standard modal
bandwidth can carry traffic farther
75
Figure 3-24: Carrier Fiber and LAN
Fiber
• LANs and WAN carriers use different types of fiber
• Carrier Fiber
– Carrier fiber must span long distances
– This requires expensive long-wavelength laser light
sources (1,310 and 1,550 nm)
– It also requires expensive “single-mode” fiber with a very
narrow core (8.3 microns)
76
Figure 3-24: Multimode and Single-Mode
Optical Fiber , Continued
Single Mode
Light
Source
Cladding
Core
Single-Mode Fiber
Light enters only at certain angles called modes
Single-mode fiber cores are so thin that only one mode can
propagate—the one traveling straight through
No modal dispersion (discussed earlier), so can span long distances
without this distortion
Expensive but necessary in WANs
77
• Test Your Understanding
• P 174
• 18
78
Figure 3-24: Carrier Fiber and LAN
Fiber
• Carrier Fiber
– Main propagation effect for single-mode fiber is
attenuation, which is very low
• For 850 nm light, attenuation is around 2.5 dB/km
• At 1,310 nm, attenuation is lower—about 0.8 dB/km
• At 1,550 nm, attenuation falls even lower—about
0.2 dB/km
– Longer wavelengths carry farther but cost more
– Carrier fiber uses wavelengths of 1,310 or 1,550 nm
79
Figure 3-24: Carrier Fiber and LAN
Fiber
• Noise and Electromagnetic Interference (EMI) Are
Not Problems for Either LAN or Carrier Fiber
– Noise from moving electrons cannot interfere
with light signals
– EMI would have to be light signals
• Wrapping the cladding in an opaque covering
prevents light from coming in
80
Figure 3-24: Carrier Fiber and LAN Fiber
Cost
Fiber Type
Corporate LAN
Multimode Fiber
Only 200-300
meters
Much Lower ($)
Multimode ($)
Wavelength
Usually 850 nm ($)
Needed Distance
Carrier (WAN)
Single-Mode Fiber
Many kilometers
Very high ($$$$)
Single-mode ($$$$)
Typical Core
Usually 1,310 or
1,550 nm ($$$$)
50/62.5 microns ($) 8.3 microns ($$$)
Propagation Limit
Modal Distortion
Is Modal Bandwidth Yes
Important?
Attenuation
No. Only
attenuation matters
81
• Test Your Understanding
• P 174
• 19
82
Topology
Figure 3-26: Major Topologies
• Topology
– Network topology refers to the physical arrangement
of a network’s computers, switches, routers, and
transmission lines
– Topology is a physical layer concept
– Different network (and internet) standards specify
different topologies
Point-to-Point Topology
(Telephone Modem Communication, Private Lines)
84
Figure 3-26: Major Topologies, Continued
Star (Modern Ethernet)
Example:
Pat Lee’s House
in Chapter 1a
85
Figure 3-26: Major Topologies, Continued
Extended Star or Hierarchy
(Modern Ethernet)
A
C
B
X
Only one possible path
between any two computers
For computers X and Y,
the path is XBACDY
E
D
Y
Z
86
Figure 3-26: Major Topologies, Continued
Mesh (Routers, Frame Relay, ATM)
A
Path
ABD
B
C
D
Multiple alternative
paths between two
computers
Path
ACD
87
Figure 3-26: Major Topologies, Continued
Ring (SONET/SDH)
88
• Test Your Understanding
• P 176
• 20
89
Figure 3-26: Major Topologies, Continued
Bus Topology (Broadcasting)
Used in Wireless LANs
90
Topics Covered
Topics Covered
• Binary Data Encoding
• Inherently binary data (IP addresses, etc.)
• Integers (binary arithmetic)
• Alternatives (N bits can represent 2N Alternatives)
• Text (ASCII and Extended ASCII)
• Graphics (pixels, bits per pixel color)
•…
• For transmission the sender converts bits to signals
(on/off, voltage levels, etc.)
92
Topics Covered, Continued
• Digital Transmission (Box)
• A few states instead of just two states (binary)
• All binary transmission is digital transmission
• Only some digital transmission (transmission with two
states) is binary
• In the box: bit rates and baud rates
93
Topics Covered, Continued
• UTP
– 4-pair UTP cords and RJ-45 connectors and jacks
– Attenuation (often expressed in decibels) and noise
• Limit UTP cords to 100 meters
– Electromagnetic interference, crosstalk interference, and
terminal crosstalk interference
• Limit wire unwinding to 1.25 cm (a half inch) to limit
terminal crosstalk interference
– Serial versus parallel transmission
94
Topics Covered, Continued
• Optical Fiber
– On/off light pulses from transceiver
– Core and cladding; perfect internal reflection
– Dominates for trunk lines among core switches
– 2 fiber strands/fiber cord for full-duplex transmission
– SC and ST connectors are the most common
– Carriers use single-mode fiber and long wavelengths
– LANs use multimode fiber and short wavelengths
95
Topics Covered, Continued
• Multimode Optical Fiber Distance Increases
With …
– Greater Wavelength
• 850 nm < 1310 nm < 1550 nm “windows”
• But larger-wavelength transceivers cost more
– Smaller Core Diameter
• 50 microns > 62.5 microns
– Greater Modal Bandwidth (MHz.km)
• Measure of multimode fiber quality
96
Topics Covered, Continued
• Topologies
– Organization of devices and transmission links
– Physical layer concept
– Point-to-point, star, hierarchy, ring, etc.
97