Transcript Slides: Physical Layer

Physical Layer
N Amanquah
Metric Units
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10-3
Milli
Micro 10-6
Nano 10-9
Pico 10-12
Femto 10-15
1kb= 1000bits
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Kilo 10-3
Mega 10-6
Giga 10-9
Tera 10-12
Peta 10-15
Bytes
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1Byte = 8bits
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1KB (Kilobytes) (not 1kb)
=210 = 1024 bytes =8*1024bits
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1MB = 220
1GB = 230
1TB = 240
1PB = 250
Transmission
Media
Connectors
Transmission
Media
Media
Hubs and repeaters
Transmission
Media
Repeaters
Signal Regeneration
Clean up
Amplify
Distance Extension
Hubs
Repeater functionality, plus...
Concentration Point
Signal Distribution Device
Management Functions
Transmission
Media
Hardware Stuff
MAGNETIC MEDIA:
 Sometimes it's cheaper and faster to drive a box of tapes in a car
TWISTED PAIR:
 Simply two wires twisted together - the twisting cuts down on electrical
interference.
 Heavily used in the phone system - the typical office has four pairs for
phones, etc.
 Category 3 and 5 - with 5 having more twists and better insulation.
BASEBAND COAXIAL CABLE:
 Used for digital transmissions (called baseband.)
 Good noise immunity.
 Data rates as high as 1 Gbps for short distances.
 Now being replaced by fiber.
Transmission
Media
BROADBAND COAXIAL CABLE:
 Used for analog transmissions (called broadband.)
 Can run 300 MHz for long distances.
 Interfaces must convert digital signals to analog and vice versa.
 Designed for long distances - can use amplifiers.
FIBER OPTICS:
 Transmission of light through fiber - properties include total internal
reflection and attenuation of particular frequencies.
 Fiber Optic Networks - can be used for LANs and long-haul.
Fiber
• Total internal reflection
• Light source eg LED, optocoupler, photon
detector
• Multimode
– multiple rays at different incident angles
• Single mode
– Fiber diameter a few wavelengths of light
– Behaves like waveguide
– Higher transmission rates eg 50Gbps for 100km w/o
amplification
– For longer distances, more expensive
Fiber – size of cable
• Wavelength stated in microns.
– 1micron =10-6m
– Multimode fiber core is about 50microns
– Single mode fiber core: 8-10 microns
• Note: visible light is of order 0.4-0.7microns
(400-700nm)
• Construction:
– Glass core surrounded by glass cladding of lower
refractive index (to reduce light loss & Totl int refl)
Connection & Light source
• Connectors
• Mechanical Splicing
• Fuse/melt together
Light Source
• LED
• Semiconductor Laser
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– Higher data rate
– Multimode or single
mode use
– Long distance
– Short life
– Expensive
– Significant temp
sensitivity
Low data rate
Multimode use
Short distance
Long life
Low cost
Minor temperature
sensitivity
Reception: use of Photo detector
Fiber networks
• Can be ring or start
– Passive:
• Has transceiver (for host)
• Tap is passive, very reliable even if broken
transceiver
• Light is lost at each T junction
– Active
• Signal regenerated
• Light  electrical  light conversion
• Less reliable, but allows greater distance bt hosts
Hardware Stuff
Transmission
Media
Comparison of Fiber Optics and Copper Wire
Bandwidth
Distance between repeaters
Interference
Physical
Flow
Fiber
Copper
Higher
50 Km
Low
Smaller/Lighter
Uni-directional
Lower
5 Km
High
Unidirectional Fiber: two fibers or 2 freq bands on one fiber
Can replace copper with fiber – sell on copper!
Bi-directional
Fiber (vs Copper)
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Not easily tapped
Requires skilled installers
Can be damaged if bent too much
More expensive
Not affected by power surges or em
radiation or induced electricity (eg factory
w motors)
Wireless Transmission
Scenarios
• Use on the move
• Where cable is impractical
– Difficult terrain,
– historical building,
– factory floor/other large floor space with traffic
• Future:
– Fiber & Wireless
Speed of wireless comms
speed=frequency  wavelength
 f c
m/s=cycles/s  m/cycles
Hz(hertz)
speed of light (in vacuum)=
3  10 8 m / s
2. Physical Layer
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
Electromagnetic Waves
one cycle
Transmit data by modulation.
Higher bandwidth =higher data
rate
2. Physical Layer
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
Characteristics of Radio Txs
• Capacity/data rate is proportional to
bandwidth (Shannon)
• Omni directional
• Penetrates buildings
• Travel long distance
• Interference is a challenge
• Prx falls of at 1/r2
Behavior is frequency dependent….
VLF, LF, MF
waves follow the curvature of the earth.
VLF, LF, MF (ground wave) 1000km..
Travel long distance, but low bandwidth
Penetrate buildings easily
AM is MF
HF, VHF
At height 100 to 500km
In the HF they bounce off the ionosphere.
Microwave
• >100 MHz, Very high Frequencies eg
microwave
• Behaves like light- reflection, use of parabolic
antennas to focus, straight lines
• LOS often required
• Absorbed by rain
• Affected by earths curvature. Repeaters required.
Microwave II
Disadvantages:
•do not pass through buildings well
•multipath fading problem (the delayed waves cancel the signal)
•absorption by rain above 8 GHz
•severe shortage of spectrum
Advantages:
•no right way is needed (compared to wired media)
•relatively inexpensive
•simple to install
Regulation & transmit strategy
• Government control bc of distance
covered & interference
– FDMA: Frequency Division Multiple Access
– TDMA: Time Division Multiple Access
– CDMA: Code Division Multiple Access
(using spread spectrum technique)
ISM
ISM (Industrial/Scientific/Medical) Band
NO licensing. One worldwide band 2.400-2.484 GHz.
Used for
cordless telephones, garage door openers, wireless hi-fi speakers,
security gates, etc.
Infra red
For short-range communication.
Eg remote controls, to printer
directional, cheap,
do not pass through solid objects.
Minimal interference.
Harder to intercept –LOS
Laser/Light wave
Affected by fog or rain
2. Physical Layer
2.8 Communication Satellites
Contain several transponders.
Properties:
1. Longer delay
2. Broadcast in nature
3. Bad security
4. Deployment is fast
5. Amplifies & re-Tx
downlink channel
uplink channel
Geosynchronous Satellites
• GEO
• 36,000km, T=24hrs
Commercial bands for satellites
Sat Comms
VSATs
Footprint
Large antenna Vs longer delay in
return for having cheaper end-user
stations. (use a hub)
Other satellites
• MEO eg 24 GPS sats
• LEO
• Lower EO sats have smaller foot print.
• Need to be tracked.
2. Physical Layer
2.8 Communication Satellites
2.8. Satellites versus Fiber
Niche for satellites
1. Bypass local loop
2. Mobile communications eg roving journalists
3. Broadcasting
4. Hostile terrain or a poorly developed terrestrial infrastructure
5. Obtaining the right of way for laying fiber is difficult
6. Rapid deployment
7. Transmission cost independent of distance
Paging Systems
Cellular Radio
Pagers/
Beepers
(one way)
Mobile phones
(two ways)
Cordless telephony
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Standard
CT-1 (analog)
CT-2 (digital)
third generation, CT-3 or DECT (digital
European CT), -supported roaming over
base stations.
Mobile phones
• Analog voice
– AMPS – advanced Mobile phone systems
• Digital Voice
– D-AMPS (digital AMPS,
– GSM: global system for mobile comms
– CDMA
• Digital voice & data
– 2.xG eg 2.1, 2.5, 3 , 4G (more later)
Physical Layer
Cellular Radio
Cell structure and
frequency reuse
Handoff: hard vs soft
Microcells to increase
frequency reuse and
cheaper handset
Physical Layer
Cellular Radio
2.7.3 Analog Cellular
Telephones
AMPS (Advanced Mobile Phone System)
The AMPS system uses 832 full-duplex channels, each
consisting of a pair of simplex channels. There are 832 simplex
transmission channels from 824 to 849 MHz and 832 simplex
receive channels from 869 to 894 MHz. Each of these simplex
channels is 30 kHz wide. Thus AMPS uses FDM to separate the
channels.
GSM
Specification Summary for GSM Cellular System
Multiple access technology
FDMA / TDMA
Duplex technique
FDD
Uplink frequency band
933 -960 MHz
(basic 900 MHz band only)
Downlink frequency band
890 - 915 MHz
(basic 900 MHz band only)
Channel spacing
200 kHz
Speech channels per RF channel
8
Channel data rate
270.833 kbps
GSM
• Identification of phones: IMSI, IMEI,
• Phone scans preprogrammed channels
when turned on
• Preprogrammed Channels for different
purposes eg paging
Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Security Issues
Message easily tapped
Use stolen telephone number for calls
Damages to antennas and base stations
Next generation
• GPRS dedicated time slots for data (overlay’s
voice)
• EDGE- 2.5 (more bits per baud), a number of
different error correction
• 3G :
– UMTS (= W-CDMA) (European std). Can hand over
to GSM, but not backwd compatible
– CDMA 2000 (backward compat w IS-95)
• 4G:
– QoS, ubiquitous, high bandwidth, integration w wired
networks etc
– LTE
Mobile vs wireless computing: use of wireless
Wireless transmission bypasses a great amount of infrastructure. Can leap ahead
in technology without running miles of physical media.
Cell Phones:
Wireless Computing:
STRUCTURE OF THE
PHONE SYSTEM
The Telephone
System
The use of analog and digital signals has pros and cons:
Signals
Attenuation/Noise
Amplification/Regeneration
Information Loss
Analog
Digital
Originally
Low
Hard
Some
Increasingly
High
Easy
Little
The Telephone
System
The Local Loop
This is the connection from the local switching station to your house.
ultimately what controls the transmission speed to your house.
This is
Transmission Impairments:
 Attenuation - the loss of energy as the signal propagates.
 Delay Distortion - different frequencies travel at different speeds so
the wave form spreads out.
 Noise - unwanted energy that combines with the signal - difficult to tell
the signal from the noise.
Modems
The Telephone
System
Modem: converts digital data to /from an analog signal for transmission.
Because attenuation is frequency dependent, modems use a sine wave carrier of a
particular frequency
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Amplitude modulation: Two
different amplitudes
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Frequency modulation: Two
(or more) different frequencies,
close to the carrier frequency.

Phase modulation: The phase
of the sine wave is changed by
some fixed amount.
Binary Signal
Modems
Modems use compression and error correction to increase the effective bits per
second.
Find out modem pins’s meaning (electrical standards)
The Telephone
System
Better use of the medium:
Frequency Division Multiplexing:
Divide among the logical channels
TRUNKS AND MULTIPLEXING:
The Telephone
System
TRUNKS AND MULTIPLEXING:
Wavelength Division Multiplexing: The same as FDM, but applied to fibers.
Time Division Multiplexing: users take turns, each one having exclusive use of
the medium in a round robin fashion.
TRUNKS AND MULTIPLEXING:
T1 & E1
4 KHZ Analog/Voice 
8,000 samples/sec ( sample every 125 usecond ).
T1 is the combination of 24 of these voice channels.
slide.
See Figure on previous
24 X 8 + 1 Framing Bit = 193 bits/125 usec --> 1.544 Mbps. When T1 is being
used for digital data, the 24th channel is converted for use as synchronization.
T2 combines 4 X T1;
T3 combines 6 X T2;
T4 combines 7 X T3.
T1 and E1
A dedicated phone connection supporting data rates of 1.544Mbits per second. A
T-1 line =24 individual channels, each is 64Kbits per second.
E1 (European format for digital transmission)
E1 carries signals at 2 Mbps (32 channels at 64Kbps, with 2 channels reserved for
signaling and controlling)
Core Network
SONET
(Synchronous Optical NETwork).
Sonet is TDM – both sender & receiver use same master clock.
transmitted SYNCHRONOUSLY.
Data is
This basic channel is called STS-1. (Synchronous Transport Signal-1) Multiple
channels can be multiplexed to get higher bandwidth.
The Telephone
System
SWITCHING
Circuit Switching: A connection (electrical, optical, radio) is established from the
caller phone to the callee phone. This happens BEFORE any data is sent.
Message Switching: store and forward whole messages.
may tie up routers for long periods of time
Packet Switching: Divides the message up into blocks (packets).
packets use the transmission lines for short time periods
The Telephone
System
COMPARISON OF CIRCUIT
SWITCHED AND PACKET
SWITCHED NETWORKS
What are the relative characteristics of these two technologies?
Characteristic
Dedicated "copper" path
Bandwidth Available
Potentially Wasted Bandwidth
Store-and-Forward Transmission
Each Packet Follows The Same
Route
Call Setup
When can Congestion Occur
How are $$ Charged
Circuit Switched
Yes
Fixed
Yes
No
Yes
Packet Switched
No
Dynamic
No
Yes
No
Required
At Setup Time
Per Minute
Not Needed
On every Packet
Per Packet