Transcript Chapter 4 - William Stallings, Data and Computer

FIT 1005 Networks & Data Communications
Chapter 4 –Transmission Media
Physical Layer
lecture slides can also be found at: http://users.monash.edu.au/~amkhan/fit1005
www.infotech.monash.edu
Reminders
•
•
•
•
The Assignment-1 has been released.
Due Date: The submission is due on Fri 29th August 4 PM (week 5).
Marks: This assignment is worth 15% of all marks for this Unit.
Submission:
– Answer all the Five Questions which is worth 40 marks.
– submission date and time, with the appropriate cover sheet correctly
filled out and attached.
– The hard copy assignment that require written submission must be
delivered to the labeled mailbox at Building-H 6th Floor Front Office.
– In addition, a soft copy assignments must be submitted as PDF
documents on Moodle.
www.infotech.monash.edu
2
Today’s lecture is to
• provide an overview of media that is used for
data communications
• key factors in the selection of the media
• design factors that need to be considered in
the selection of the media
– for link communication
www.infotech.monash.edu
3
Classification (taxonomy) of the media
• guided
– wired transmission
• unguided – wireless transmission
www.infotech.monash.edu
4
On what to compare? - Properties
• characteristics and quality of communication is
determined by the
– type of the medium and
– signal strength decay (known as attenuation)
• For example,
– in unguided media – signal strength produced
by the antenna is more important
– in guided media - medium is more important
www.infotech.monash.edu
5
Key concerns in the selection of media are
• data rate
– amount of maximum number of bits that can be transmitted
from one end to the other end of the link per sec (bps)
• distance
– how long can the data be transferred without error?
– OR how much attenuation can be tolerated for error free
decoding?
– Attenuation rate is measured in decibels (dBs) =
power output
= - 10 log 10 -------------------power input
– Note that dB is a relative measure
– a high value  high attenuation (or loss of signal strength)
www.infotech.monash.edu
6
Key concerns in the selection are (cont.)
• Throughput of a link
– as distance increases, the signal strength decay’s and hence
the data cannot be decoded correctly at the receiving end
– impairments can distort or corrupt a signal
– when the data is not decoded correctly at the receiving end, it
may need to be retransmitted and this will reduce the
throughput of the link
– Throughput (of a link) = useful data correctly received per
unit of time
> Throughput will be ≤ data rate
> if the link is error free only than throughput = data rate
www.infotech.monash.edu
7
Design Factors
• Bandwidth
– higher bandwidth  higher data rate
• Transmission impairments
– eg. Attenuation can limit the distance of transmission
• Interference
– From competing signals which can alter the signal (in turn the
data)
• number of receivers attached to the medium
– mainly applicable to guided media
– e.g. more receivers attached to a link will introduce more
attenuation of the signal strength  that can lead to erroneous
interpretation of the data at the receiving end.
www.infotech.monash.edu
8
Interference
• Interference add noise to the signal and that can
affect the decoding of the data
• Measured in terms of Signal-to Noise Ratio and
expressed in dBs
Output Signal power
SNRdB = 10 log 10 ----------------------------Noise power
• Higher SNR  less noise or higher probability of
correct decoding of the data
www.infotech.monash.edu
9
Attenuation / Distortion / Noise / SNR examples
Figure-2 Distortion
Figure-1 Attenuation
Figure-4 Two cases of SNR: a high SNR and a low SNR
Figure-3 Noise
www.infotech.monash.edu
10
Common media are
• Wired/guided
–
–
–
–
twisted single pair (telephone lines)
twisted multiple pairs (UTP shielded/unshielded LAN cable)
coaxial (and its variants)
Optical
• Wireless/unguided
–
–
–
–
Radio
Microwave
Satellite
Infrared
• Each of them have their own characteristics which limit
their data rate and distance of usage
www.infotech.monash.edu
11
Electromagnetic Spectrum
www.infotech.monash.edu
12
Transmission Characteristics of Guided Media
www.infotech.monash.edu
13
Twisted Pair
Unshielded Twisted Pair for LAN’s
UTP / STP cable testers
Unshielded Twisted Pair termination
RJ-45 connectors at end points
www.infotech.monash.edu
14
Twisted Pair - Transmission Characteristics
• can carry either analog or digital signals
– analog
> needs amplifiers every 5km to 6km
– digital
> needs a repeater every 2-3km
•
•
•
•
•
limited distance
P-T-P analog signaling - limited bandwidth (1MHz)
P-T-P digital signaling - limited data rate (100MHz)
short distances, data rates of up to 10 Gbps
susceptible to interference and noise
www.infotech.monash.edu
15
Unshielded vs Shielded TP
• unshielded Twisted Pair (UTP)
– e.g. ordinary telephone wire / LAN Ethernet cabling
– No form of electromagnetic shielding and hence susceptible to
electro magnetic interferences from neighboring cables.
– Cheapest and hence widely used in local area networks!
– easy to handle and install
• shielded Twisted Pair (STP)
– metal braid or sheathing that reduces interference
– more expensive
– harder to handle (thick, heavy)
• in a variety of categories - see EIA-568
– Electronic Industries Alliance(EIA)
– Telecommunications Industry Association
–
http://en.wikipedia.org/wiki/TIA/EIA-568
www.infotech.monash.edu
16
UTP Categories
Category 3
Class C
Category 5
Class D
Category 5E
Category 6
Class E
Category 7
Class F
Bandwidth
16 MHz
100 MHz
100 MHz
200 MHz
600 MHz
Cable Type
UTP
UTP/FTP
UTP/FTP
UTP/FTP
STP
Link Cost
(Cat 5 =1)
0.7
1
1.2
1.5
2.2
UTP, FTP and SFTP are terms used in specification of Ethernet cables
Refers to Ethernet cable shielded or not
UTP; Unshielded twisted pair
FTP; Aluminum-Mylar Foil Shield twisted pair
STP; inner Aluminum-Mylar Foil Shield, covered by an outer tinned copper braid shield
Pro-Plex PCCAT5P and PCCAT5EP have maximum shielding;
www.infotech.monash.edu
17
Attenuation comparison of Shielded and
Unshielded Twisted Pair
Attenuation (dB per 100 m)
Frequency
(MHz)
Category 3
UTP
Category 5
UTP
1
2.6
4
Near-end Crosstalk (dB)
150-ohm STP
Category 3
UTP
Category 5
UTP
150-ohm STP
2.0
1.1
41
62
58
5.6
4.1
2.2
32
53
58
16
13.1
8.2
4.4
23
44
50.4
25
—
10.4
6.2
—
41
47.5
100
—
22.0
12.3
—
32
38.5
300
—
—
21.4
—
—
31.3
www.infotech.monash.edu
18
Near End Crosstalk (Next) &
Far end crosstalk (FEXT)
• Near End Crosstalk (NEXT):
– is interference between two pairs of a cable measured at
the same end of the cable as the transmitter
• Far end crosstalk (FEXT):
– is interference between two pairs of a cable measured at
the other end of the cable from the transmitter
www.infotech.monash.edu
19
Coaxial Cable
www.infotech.monash.edu
20
Coaxial Cable - Transmission Characteristics
• superior frequency characteristics to TP
• performance limited by attenuation & noise
For long distance transmissions:• of analog signals
– Amplifiers needed every few km
– closer if higher frequency
– up to 500MHz
• of digital signals
– Require repeaters every 1km
– closer repeater spacing for higher data rates
www.infotech.monash.edu
21
Optical Fiber
• Fiber optic can be defined as: A cylindrical column of
glass or plastic surrounded by an opaque outer jacket
• Typically two layers ( also called columns) – the inner
one is called the core which has higher refraction index
– higher refraction index  can reflect more light
– data rates of hundreds of Gbps
www.infotech.monash.edu
22
Optical Fiber
www.infotech.monash.edu
23
Optical Fiber - Benefits
• greater capacity
– data rates of hundreds of Gbps
• smaller size & weight
• lower attenuation due to higher refraction index
• electromagnetic isolation because of light signals
• greater repeater spacing
– 10’s of km at least
www.infotech.monash.edu
24
Optical Fiber - Transmission Characteristics
• uses total internal reflection to transmit light
– effectively acts as waveguide for 1014 to 1015 Hz
(infrared and visible spectrum of frequency)
• can use several different light sources
– Light Emitting Diode (LED)
> cheaper, wider operating temp range, lasts longer
– Injection Laser Diode (ILD)
> more efficient, has greater data rate
www.infotech.monash.edu
25
Frequency Utilization for Fiber Applications
Wavelength (in
vacuum) range (nm)
820 to 900
1280 to 1350
1528 to 1561
1561 to 1620
Frequency
Range (THz)
366 to 333
234 to 222
196 to 192
192 to 185
Band
Label Fiber Type
Application
Multimode
LAN
S
Single mode Various
C
Single mode
WDM
L
Single mode
WDM
www.infotech.monash.edu
26
Attenuation in Guided Media
www.infotech.monash.edu
27
Wireless (Unguided) Transmission Frequencies
• Microwave
– Operates in the frequency range of 2GHz to 40GHz
– highly directional
– point to point
• Satellite
– operates in the frequency range of 30MHz to 1GHz
– Need different frequencies for up and down links for
continuous transmission and to avoid interference
because the same medium is shared
• Broadcast radio
– omnidirectional
– Operates in the frequency range of 30 MHz to 1 GHz
www.infotech.monash.edu
28
Wireless (Unguided) Transmission Frequencies
infrared
•Range - Infrared communication - short distances
– Very local - (no more than 5 meters)
– Operates in the frequency range of 300 GHz to 400 THz
•Unlike Wi-Fi and Bluetooth technologies, infrared network signals
cannot penetrate walls or other obstructions and work only in the
direct "line of sight."
•Performance - Infrared technology used in local networks exists in
three different forms:
– IrDA-SIR (slow speed) infrared supporting data rates up to 115 Kbps
– IrDA-MIR (medium speed) infrared supporting data rates up to 1.15
Mbps
– IrDA-FIR (fast speed) infrared supporting data rates up to 4 Mbps
www.infotech.monash.edu
29
Satellite Point to Point Link
In satellite telecommunication:
Uplink
– a downlink is the link from a
frequency Uf
satellite down to one or more
ground stations or receivers, and
– an uplink is the link from a ground
station up to a satellite.
Down link
frequency Ud
The following table shows the main frequency bands used for satellite links.
Frequency Band
Downlink
Uplink
C
3,700 - 4,200 MHz
5,925 - 6,425 MHz
Ku
11.7 - 12.2 GHz
14.0 - 14.5 GHz
Ka
17.7 - 21.2 GHz
27.5 - 31.0 GHz
www.infotech.monash.edu
30
Satellite Broadcast
www.infotech.monash.edu
31
Broadcast Radio
• Omnidirectional
• Radio frequency range is 3kHz to 300GHz
• use broadcast radio, 30MHz - 1GHz, for:
– FM radio
– UHF and VHF television
• still need line of sight
• suffers from multipath interference
– reflections from land, water, other objects
www.infotech.monash.edu
32
Infrared
•
•
•
•
•
Modulate non-coherent infrared light
line of sight (or reflection)
Blocked by walls
no licenses required
typical uses example
– TV remote control
– IRD port
www.infotech.monash.edu
33
Multipath Interference
www.infotech.monash.edu
34
Antennas
•
electrical conductor are used to radiate or collect electromagnetic energy
• The two functions of an antenna are:
– transmission and reception
1. transmission antenna
Radio frequency
electrical energy
Is converted by
Antenna
electromagnetic
energy
radiated into
surrounding
environment
2. reception antenna
electromagnetic
energy impinging
on Antenna
Is collected
by Antenna
Converts to Radio
frequency
electrical energy
And Feeds to
the Receiver
Receiver
– Usually same antenna are often used for both purposes Tx and Rx.
www.infotech.monash.edu
35
Radiation Pattern
• Antennas radiate power in all directions
• performance degrades when radiating in all
directions
– as seen in a radiation pattern
• an isotropic antenna is a (theoretical) point in
space
– radiates in all directions equally
– with a spherical radiation pattern
www.infotech.monash.edu
36