Transcript Basic Concepts

Physical Layer
Propagation:
UTP and Optical Fiber
Chapter 3
Updated by Alan Holbrook July 2008
Panko’s
Business Data Networks and Telecommunications, 6th edition
Copyright 2007 Prentice-Hall
May only be used by adopters of the book
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
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
5
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…
6
Binary-Encoded Data
• Some data are inherently binary
– 48-bit Ethernet addresses
– 32-bit IP addresses
– Need no further encoding
7
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”
8
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
9
3-2: Arithmetic with Binary Numbers
Binary Arithmetic for Binary Numbers
Basic Rules
0
+0
=0
0
+1
=1
1
+0
=1
1
+1
=10
1
+1
+1
=11
3-10
3-4: ASCII
• Purpose
– To represent text (A, a, 3, $, etc.) as binary data for
transmission
• ASCII
– Traditional code to represent text data in binary
– Seven bits per character
– 27 (128) characters possible
– Sufficient for all keyboard characters (including shifted
values)
3-11
3-4: ASCII
• ASCII
– Sufficient for all keyboard characters
Category
Meaning
ASCII
Capital letters
A
1000001
Lower-case letters
a
1100001
Digits
3
0110011
Punctuation
.
0101110
@
1000000
Special characters
Space
0100000
Printing control
Carriage Return
0001101
Printing control
Line feed
0001010
3-12
3-4: ASCII
• Each ASCII Character is Sent in a Byte
– 8th Bit in Data Bytes Normally Is Not Used
Data Byte
1
0
1
0
0
ASCII Code
for Character
1
1
1
Unused.
Value does
not matter
3-13
3-4: ASCII
• To send “Hello world!” (without the quotes), how
many bytes will you have to transmit?
3-14
3-5: Graphics Image and Conversion to Binary
2
Example 2:
Screen Resolution:
1000 x 500, so
500,000 pixels per screen
If 24 bits/pixel, then
500,000 pixels/screen x
24 bits/pixel =
12,000,000 bits/screen
or
1,500,000 bytes/screen
Example 1:
8 bits per base color gives
256 levels per base color (28).
Three base colors gives 2563
or over 16 million colors
3-15
The Physical Layer
• The physical layer includes network hardware and
circuits.
• Network circuits include physical media (e.g., cables) and
special purposes devices (e.g., routers and hubs).
Networks are made of both physical and logical circuits.
– Physical circuits connect devices & include actual wires.
– Logical circuits refer to the transmission characteristics of the
circuit, such as a T-1 connection.
• Sometimes the physical and logical circuits are the same,
but they can be different. For example, in multiplexing,
one wire carries several logical circuits.
Analog and Digital Data
• Another fundamental physical layer distinction is
between digital and analog forms of data.
• Sounds waves, which vary continuously over
time are analog data.
• Computers produce digital data that is in binary
form, that is, it is represented as a series of ones
and zeros.
Advantages of Digital Transmission
• Digital transmission:
– produces fewer errors than analog transmission. Because the
transmitted data is binary (1s and 0s), it is easier to detect and
correct errors.
– permits higher transmission rates. Optical fiber, for example, is
designed for digital transmission.
– is more efficient. It’s possible to send more data through a given
circuit using digital rather than analog transmission.
– is more secure since it is easier to encrypt.
• Integrating voice, video and data on the same circuit is
also far simpler with digital transmission since signals
made up of digital data are easier to combine.
18
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.
19
Serial transmission
Digital Transmission
• Digital signals are sent as a series of “square waves” of
either positive or negative voltage. Voltages vary
between +3/-3 and +24/-24 depending on the circuit.
• Each digital transmission standard defines what voltage
levels correspond to a bit value of 0 or 1.
• Unipolar signal voltages either vary between 0 and a
positive value or between 0 and some negative value.
Digital Transmission (cont.)
• With bipolar signals, signals are sent using
both positive and negative voltages.
• A second digital transmission factor, called
return to zero (RZ) means the signal returns
to the 0 voltage level after sending a bit. In
non return to zero (NRZ), the signals
maintains its voltage at the end of a bit.
• Ethernet uses Manchester encoding in
which the bit value is defined by a mid-bit
transition. A high to low voltage transition is
a binary 0 and a low-high mid-bit transition
defines a binary 1.
Digital transmission types
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.
24
Figure 3-8: Binary Signaling in 232 Serial Ports
In a clock cycle,
15 Volts
Clock Cycle
-2 to -15 volts is a zero
0
3 Volts
3 to 15 volts represents a one
0
0
0 Volts
-3 Volts
1
-15 Volts
1
This type of
signaling is used in
232 serial ports.
25
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
26
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
27
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
28
3-11: Multistate Digital Signaling
Box
• Concepts
– Bit rate: Number of bits sent per second
– Baud rate: Number of clock cycles per second
• If 1,000 clock cycles per second, 1 kbaud
• If each clock cycle is 1/1,000 second = 1,000 clock
cycles/second = 1 kbaud
3-29
Transmission Media
Communications Media
• Medium: the physical matter that carries the
transmission. Two basic categories of media:
• With Guided media the transmission flows
along a physical guide. The three main types
of guided media: twisted pair wiring, coaxial
cable and optical fiber cable.
• With Wireless media there is no wave guide
and the transmission just flows through the
air (or space). The main forms of wireless
communications are radio, infrared,
microwave, and satellite communications.
Guided Media: Twisted Pair Wires
• Twisted pair wire cables are commonly used for
telephones and local area networks.
• Twisting two wires together reduces electromagnetic
interference.
• TP cables have a number of pairs of wires.
– Telephone lines have two pairs (4 wires, usually only one
pair is used by the telephone)
– LAN cables have 4 pairs (8 wires)
• Shielded twisted pair also exists, but is more
expensive.
• TP cables are also used in telephone trunk lines and
can have up to several thousand pairs.
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
34
RJ-45 Jacks and Connectors
RJ-45
Jack
RJ-45
Jack
RJ-45
Jack
RJ-45 Connectors
35
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
36
Coax Cable
Guided Media: Coaxial Cable
• Formerly common on LANs, but now disappearing
(but still used on other comm. equipment, e.g.,
CATV).
• More expensive than twisted pair, but coax is
shielded, so it’s less prone to interference than
twisted pair.
• Coaxial Cable Structure
– Inner conductor
– Insulator
– Wire mesh ground
– Outer protective jacket or shell
38
Figure 3-5 Coaxial Cable
39
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
41
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
42
Figure 3-20: Optical Fiber Transceiver and Strand
Strand
3.
Cladding
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
43
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
44
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)
45
Figure 3-23: Frequency and Wavelengths
2.
Wavelength
Distance between comparable points in successive cycles
(Measured in nanometers for light)
Wave
1.
Amplitude
Power,
Voltage,
etc.
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).
46
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
47
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 with 850 nm signaling cannot span the
kilometer distances needed by carriers, but can span the
200-300 meters needed in LAN fiber cords
48
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. If it is too large, signals will be unreadable.
Modal distortion is the main limitation on distance in multimode fiber.
49
Figure 3-24: Carrier Fiber and LAN
Fiber
• LAN Fiber
– All multimode fiber today is graded-index multimode fiber
• The index of refraction decreases from the center of
the core to the core’s outer edge.
Lower
Higher
Incidence of
Refraction
50
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
• Modal dispersion between the modes is reduced
Mode 2 (Slowed)
Mode 1 (Speeded Up Near Edge of Core)
Lower
Modal
Dispersion
51
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
52
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
53
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)
54
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
55
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
56
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
57
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
58
Wireless Media
59
Wireless Media
• Wireless media signals are becoming popular
for LAN use. The main forms are:
– Radio: wireless transmission of electrical waves.
Includes AM and FM radio bands. Microwave is
also a form of radio transmission.
– Infrared: “invisible” light waves whose frequency
is below that of red light. Requires line of sight and
are generally subject to interference from heavy
rain. Used in remote control units (e.g., TV).
– Microwave: high frequency form of radio with
extremely short wavelength (1 cm to 1 m). Often
used for long distance, terrestrial transmissions
and cellular telephones. Requires line-of-sight.
Satellite Communications
Satellite & Microwave
Communications
• Common Applications - Radio Relay - data, voice,
video
• Microwave
– Characteristics
– +2 GHz
– Directional
– Line of Sight
–…
Satellite Communications
• RF Bands
Band Name
HF-band
VHF-band
P-band
UHF-band
L-band
FCC's digital radio
S-band
C-band
X-band
Ku-band (Europe)
Ku-band (America)
Ka-band
Frequency Range
1.8-30 MHz
50-146 MHz
0.230-1.000 GHz
0.430-1.300 GHz
1.530-2.700 GHz
2.310-2.360 GHz
2.700-3.500 GHz
Downlink: 3.700-4.200 GHz
Uplink: 5.925-6.425 GHz
Downlink: 7.250-7.745 GHz
Uplink: 7.900-8.395 GHz
Downlink: FSS: 10.700-11.700 GHz
DBS: 11.700-12.50 0 GHz
Telecom: 12.500-12.750 GHz
Uplink: FSS and Telecom: 14.000-14.800 GHz;
DBS: 17.300-18.100 GHz
Downlink: FSS: 11.700-12.200 GHz
DBS: 12.200-12.700 GHz
Uplink: FSS: 14.000-14.500 GHz
DBS: 17.300-17.800 GHz
Roughly 18-31 GHz
Satellite Communications
• Orbits - GEO – 35,786 kilometers
22,241 statute miles
– 6,900 mph
– 7,000 circular footprint
– Spacing >= 2o degrees,
or >= 9o (broadcast)
– 1 revolution per day
– Prop delay = 22,300 miles/186,000 miles/sec = 0.1198 sec
0.1199 sec x 2 = 0.2398 seconds (one way delay)
– Freq. • C Band 4GHz-6-GHz - Interference from Microwave
• Ku 11GHz - 12 GHz - atmospheric attenuation
• Ka 14GHz - atmospheric attenuation
– Higher Frequencies used in uplink, lower loss for lower bands
used in downlink
Satellite Communications
• Orbits - MEO • ~6,000 miles
• 5,000-6,000 mile circular footprint
• 5 revolutions per day
• Prop delay = 6,000 miles/186,000 miles/sec =
0.0322 sec
0.0322 sec x 2 = 0.0644seconds (one way delay)
• Freq. – Ranges from 300 MHz - 2200 MHz
– C, S, K Band
Satellite Communications
• Orbits - LEO• >1,000 miles
• 1,000 - 3,500 mile circular footprint
• ~12 revolutions per day
• Prop delay = 1,000 miles/186,000 miles/sec =
0.0054 sec
0.0054 sec x 2 = 0. 0108 seconds (one way delay)
• Freq. – Ranges from 300 MHz - 2200 MHz
– C, S, K Band
Satellite Communications
• Orbit Tracks
• http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html
Satellite Communications
• Power and Footprint
– Low Power - 10’s to 100 watts
– Free Space Loss ~200 dB for GEOs
– Very low power at receiver
– Restricting radiated energy to specific area allow
power to be concentrated
– Antenna design - Gain
• Effective/Equivalent Isotropic Radiated Power : It is the ouptut
power at the transmitter terminal, minus feeder and mismatch
losses, plus average antenna gain relative to an isotropic
radiator in the horizontal direction in dBW
– Spot Beam
Satellite Communications
• http://www.intelsat.com/satellites/covmaps/[email protected]#
• http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html
Satellite Communications
• Transponder
– Satellites have some number of transponders
– receives a signal, amplifies it, and retransmits it
(typically at 8.5 to 60 watts
– new direct broadcast satellites use up to 120 watts
so that very small receiving antennas can be
used)
– Transponders typically have a bandwidth of 36 to
72 MHz each (though newer satellites have up to
108-MHz transponder bandwidths).
•
http://www.oreilly.com/reference/dictionary/terms/S/Satellite.htm
Satellite Communications
• Transponder
– NTSC standard analog television video (with
audio) signal requires 24 to 36 MHz of
transponder bandwidth
– Each transponder typically carries one, two, or
three television signals (two for a 54-MHz
transponder, three for a 72-MHz transponder).
– Video signal digitization and compression
schemes allow up to eight television signals to
share the bandwidth required by a single
uncompressed video signal.
Satellite Communications
• VSAT
Satellite Communications
• http://www.gilat.com/Technology_SatelliteBas
ics.asp
• http://www.tbssatellite.com/tse/online/mis_telecom_geo.htm
l
• http://www.ssloral.com/products/satint.html
Media Selection depends on many
factors including:
• Type of network
• Cost
• Transmission distance
• Security
• Error rates
• Transmission speeds
IMPAIRMENTS TO TRANSMISSION
Transmission Media Data Rate
• Data rate is effected by
– Bandwidth
– Transmission impairments
– Interference
– Number of nodes (guided medium only)
Transmission Impairment/Limitations
• Guided medium impairments
– Noise
•
•
•
•
Thermal (white) noise
Intermodulation noise
Crosstalk
Impulse noise
– Attenuation - signal amplitude reduced
• rcvr signal strength, signal/noise,greater at higher freq....
– Delay distortion - different freq..... propagate at
different rate. Highest near center freq.…
– Phase Jitter, Echo, Dropouts
Transmission Impairment/Limitations
• Unguided medium impairments
– Free-space loss - signal disperses
– Atmospheric absorption - water vapor, oxygen
– Multipath - reflections
– Refraction - signals bend through atmosphere
– Thermal Noise - thermal activity
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
79
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
80
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
81
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
–…
82
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
Random noise spikes cause errors
-A high signal-to-noise ratio reduces noise error problems
As a signal attenuates with distance, damaging noise spikes
become more common
83
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
84
Figure 3-11: Unshielded Twisted Pair
(UTP) Wiring, Continued
• Electromagnetic Interference (EMI) (Fig. 3-15)
– Electromagnetic interference 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
85
Effect of Noise
Amplifier - Repeater
Figure 3-16: Crosstalk Interference and Terminal
Crosstalk Interference
Untwisted
at Ends
Signal
Crosstalk Interference
Terminal Crosstalk
Interference
Terminal crosstalk interference
Normally is the biggest EMI problem for UTP
88
Figure 3-16: Crosstalk Interference
and Terminal Crosstalk Interference,
Continued
• EMI is any interference
– Signals in adjacent pairs interfere with one another
(crosstalk interference). This is a specific type of EMI
• Crosstalk interference is worst at the ends, where the
wires are untwisted. This is terminal crosstalk
interference—a specific type of crosstalk EMI
EMI
Crosstalk Interference
Terminal Crosstalk
Interference
89
Figure 3-11: Unshielded Twisted Pair
(UTP) Wiring, Continued
• Electromagnetic Interference (EMI) (Fig. 3-15)
– NEXT
– 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
1.25 cm
or 0.5 inches
to a negligible level.
90
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
91
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.
92
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
93
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
94
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
95
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
96
Figure 3-19: Wire Quality Standards
Category Technology Maximum Speed
1
2
3
4
5
5e
6
6
6A
7
UTP
UTP
UTP
UTP
UTP
UTP
UTP
UTP
UTP
STP1
Never defined
Never defined
10 Mbps
10 Mbps
1 Gbps
1 Gbps
1 Gbps
10 Gbps
10 Gbps
10 Gbps+
Maximum Ethernet
Distance at this
Speed
Not Applicable
Not Applicable
100 meters
100 meters
100 meters
100 meters
100 meters
55 meters
100 meters
100 meters
Category numbers indicate wire quality
3-97
Optical Fiber
Transmission
Light through Glass
Spans Longer Distances than UTP
3-20: Optical Fiber Transceiver and Strand
An optical fiber strand has a thin glass core
This core is 8.3, 50, or 62.5 microns in diameter
This glass core is surrounded by a tubular glass cladding
The outer diameter of the cladding is 125 microns,
regardless of the core’s diameter
The transceiver injects laser light into the core
3-99
3-20: Optical Fiber Transceiver and Strand
When a light wave ray hits the core/cladding boundary,
there is perfect internal reflection. There is no signal loss
3-100
3-21: Roles of UTP and Optical Fiber in LANs
3-101
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
3-102
3-22: Full-Duplex Optical Fiber Cord with SC and
ST Connectors
SC Connector
(push and click)
ST Connector
(bayonet connectors:
push and click)
In contrast to UTP, which always uses RJ-45 connectors,
there are several optical fiber connector types
SC and ST are the most popular
3-103
3-23: Frequency and Wavelength
Wave
Light travels in waves
The amplitude is the intensity of the wave
In sound waves, amplitude is loudness
Amplitude is a measure of power
3-104
3-23: Frequency and Wavelength
Wave
Wavelength is the physical distance
between comparable points on adjacent cycles
(peak-to-peak, trough-to-trough, start-to-start, etc.)
Wavelengths are measured in meters
Light is measured in wavelength
So optical fiber transmission is specified by wavelength
3-105
3-20 Optical Fiber Strand
In optical fiber transmission, light is expressed in nanometers.
The transceiver transmits at 850 nm, 1,310 nm, or 1,550 nm
Shorter-wavelength (850 nm) transceivers are less expensive
Longer-wavelength (1,310 or 1,550 nm) light travels farther for a given speed
For LAN fiber, 850 nm provides sufficient distance and dominates
3-106
3-23: Frequency and Wavelength
Wave
Waves can also be measured in frequency
The frequency is the number of complete cycles per second
Hertz (Hz) is the term for cycles per second
Radio transmission is measured in frequency
Radio transmission usually takes place in the MHz or GHz range 3-107
3-25: Multimode Fiber and Single-Mode Fiber
Multimode fiber has a thick core (50 or 62.5 microns in diameter)
Light can only enter the core at certain angles, called modes
Modes traveling straight through arrive faster than
modes that bounce against the cladding several times
3-108
3-25: Multimode Fiber and Single-Mode Fiber
Modal dispersion is the difference in time it takes modes to propagate
If modal dispersion is too large, light from
adjacent clock cycles will overlap, producing errors
Modal dispersion is the limiting distance factor for multimode fiber
3-109
3-25: Multimode Fiber and Single-Mode Fiber
Modal dispersion can be reduced by having a graded index of
refraction in the core—decreasing from the center to the cladding.
All multimode fiber is graded index multimode fiber today.
Modal dispersion is also reduced by better-quality multimode fiber.
Modal bandwidth (measured as MHz-km)
is the measure of multimode fiber quality.
(In UTP, quality is expressed by Category number)
3-110
3-26: Wavelength, Core Diameters, Modal
Bandwidth, and Maximum Propagation Distance
for Ethernet 1000BASE-SX
Wavelength
Core Diameter
Modal Bandwidth
850 nm
62.5 microns
160 MHz.km
Maximum
Propagation
Distance
220 m
850 nm
62.5 microns
200 MHz.km
275 m
850 nm
50 microns
500 MHz.km
550 m
With 850 nm light, distance can be increased by
using a smaller core diameter or
using better-quality fiber with higher modal bandwidth
3-111
3-25: Multimode Fiber and SingleMode Fiber
Single-mode fiber has a core diameter that is so small
(8.3 microns) that only one mode can propagate.
Consequently, there is no modal dispersion.
Single mode fiber transmission distance is limited
only by absorptive attenuation, which is extremely low.
Consequently, single-mode fiber can carry signals for kilometers.
However, single-mode fiber is more expensive than multimode.
It is rarely used in LANs
It is almost always used in carrier transmission lines
3-112
3-24: LAN Fiber Versus Carrier WAN Fiber
Required Distance
Span
Transceiver
Wavelength
Type of Fiber
Core Diameter
Primary Distance
Limitation
Quality Metric
LAN Fiber
Carrier WAN Fiber
200 m to 300 m
1 to 40 kilometers
850 nm
1,310 nm (and
sometimes 1,550 nm)
Multimode (thick core) Single mode (thin
core)
50 microns or 62.5
8.3 microns
microns
Modal dispersion
Absorptive attenuation
Modal bandwidth
(MHz.km)
NA
LAN distance requirements are so short (200-300 m)
that multimode fiber and 850 nm light are sufficient.
Multimode fiber quality (modal bandwidth), however, is important. 3-113
3-24: LAN Fiber Versus Carrier WAN Fiber
Required Distance
Span
Transceiver
Wavelength
Type of Fiber
Core Diameter
Primary Distance
Limitation
Quality Metric
LAN Fiber
Carrier WAN Fiber
200 m to 300 m
1 to 40 kilometers
850 nm
1,310 nm (and
sometimes 1,550 nm)
Multimode (thick core) Single mode (thin
core)
50 microns or 62.5
8.3 microns
microns
Modal dispersion
Absorptive attenuation
Modal bandwidth
(MHz.km)
NA
Carrier distances are so long (1 to 40 km) that carrier fiber
is single-mode fiber, and wavelengths are long (1,310 or 1,550 nm).
This is very expensive.
3-114
Radio Propagation
Radio Propagation
Radio signals also propagate as waves.
As noted earlier, radio waves are measured in hertz (Hz),
which is a measure of frequency.
Radio usually operates in the MHz and GHz range.
3-116
3-27: Omnidirectional and Dish Antennas
3-117
3-28: Wireless Propagation Problems
UTP and optical fiber propagation are fairly predictable.
However, radio suffers from many propagation effects.
This makes radio transmission difficult to manage.
We will look at these problems one at a time.
3-118
3-28: Wireless Propagation Problems
The first propagation problem is electromagnetic
interference (EMI) from nearby radio sources
This includes other wireless devices
It can include microwave ovens an other devices
3-119
3-28: Wireless Propagation Problems
Another problem is inverse square law attenuation.
As a signal propagates, its energy spreads out over the
Surface of an ever-expanding sphere.
3-120
3-28: Wireless Propagation Problems
• An Example of Inverse Square Law Attenuation
–
–
–
–
–
P1 = Power at Point A.
P2 = Power at Point B (which is farther from A).
r1 = Distance to Point A.
r2 = Distance 2Point B (which is farther from A).
P2 = P1 * (r1/r2)2
– If the power is 400 mW (milliwatts) at 100 meters
– What is the power at 200 meters?
– P2 = 400 mW * (100/200)2
– P2 = 400 mW * (1/2)2 = 400 mW * 1/4 = 100 mW
3-121
3-28: Wireless Propagation Problems
• Another Example of Inverse Square Law
Attenuation
–
–
–
–
–
P1 = Power at Point A.
P2 = Power at Point B (which is farther from A).
r1 = Distance to Point A.
r2 = Distance to Point B (which is farther from A).
P2 = P1 * (r1/r2)2
– If the power is 900 mW (milliwatts) at 10 meters
– What is the power at 30 meters?
3-122
3-28: Wireless Propagation Problems
Confusingly, wireless propagation suffers from two forms
of attenuation. We have just seen inverse square law attenuation.
There is also absorptive attenuation, which is attenuation
because power is absorbed by water molecules along the way.
Absorptive attenuation increases with frequency.
3-123
3-28: Wireless Propagation Problems
When radio waves hit thick objects, they may not be able to penetrate.
This creates shadow zones, which are also called dead spots.
Shadow zones get worse as frequency increases.
3-124
3-28: Wireless Propagation Problems
Multipath interference is the oddest propagation problem for radio.
It is also the most important at wireless LAN frequencies.
Sometimes, a reflected signal arrives just slightly after
the direct signal. The direct and reflected signals will
add together. If one signal is at its peak and the
other is at its trough, then they may partially
or completely cancel out.
3-125
Topology
Network topology is the physical
arrangement of a network’s
computers,
switches, routers, and transmission
lines
It is a physical layer concept
3-29: Major Topologies
The simplest topology is the point-to-point topology
3-127
3-29: Major Topologies
Ethernet uses a star topology
Note that the switch does not have to be in the middle of the star
3-128
3-29: Major Topologies
Larger Ethernet LANs use an extended star topology
This is better called a hierarchical topology
3-129
3-29: Major Topologies
In a mesh topology, there are many connections
between switches or routers
Consequently, there are many alternative routes between hosts
3-130
3-29: Major Topologies
In the ring topology, messages travel around a loop
3-131
3-29: Major Topologies
The bus topology uses broadcasting.
The message receives each host at almost the same time.
All wireless transmission uses a bus topology.
3-132
Topics Covered
Topics Covered
• Binary Data Representation
– Must first convert data into bits
– For instance, keyboard characters are represented with
ASCII
• Signaling
– Then, bit streams must be converted into signals
– Binary versus digital signaling
3-134
134
Topics Covered
• UTP wiring
– Limit cords to 100 meters to make both noise and
attenuation negligible problems
– Limit the untwisting of wires at the ends to 1.25 cm (a
half inch) to reduce terminal crosstalk interference to a
negligible problem
– Category number specifies UTP wiring quality
– Serial versus parallel transmission
3-135