Introduction to Computer Networks · Introduction to Computer Networks Tian Song (嵩天), Ph.D.,...
Transcript of Introduction to Computer Networks · Introduction to Computer Networks Tian Song (嵩天), Ph.D.,...
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Introduction to Computer Networks
Tian Song (嵩天), Ph.D., Assoc. [email protected]
Satellite, Multiplexing and PSTN
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian3
Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian4
Tech Networks in the World
1. Satellite Network
2. Public Switched Telephone Network
3. Mobile Telephone Network
4. Cable TV Network
5. Internet
Here, we observe them in physical layer
三网融合
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian5
Tech Networks in the World
![Page 6: Introduction to Computer Networks · Introduction to Computer Networks Tian Song (嵩天), Ph.D., Assoc. Prof.songtian@bit.edu.cn Satellite, Multiplexing and PSTN](https://reader034.fdocuments.in/reader034/viewer/2022042103/5e804894019ea6566d77f21b/html5/thumbnails/6.jpg)
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian6
Satellite Network
Essence:You direct a signal to something way up in the sky, and
that reflects it back to earth.
Reflection = receipt + amplification + sending back
Question:
Where to put the satellites?
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian7
Satellite Network
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Satellite Network• GEO, Geostationary Earth Orbit
35 800 km high
40 transponder, 36 MHz bandwidth each
250 – 300 ms delay from one end to another
multicasting
distance independent
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Satellite Network•MEO, Medium Earth Orbit
GPS, Compass, GLONASS, Galileo
6 hours a circle
low power needed for a transmission
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Satellite Network
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Satellite Network• LEO, Low Earth OrbitIridium Project (铱星计划), Motorola, 1990‐1998
750 km, 6 chain, 1.5 hrs
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian12
Satellite Network• LEO, Low Earth Orbit
(a) Relaying in space. (Iridium) (b) Relaying on the ground.(Globalstar)
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian13
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Where (not) to use Satellite• BandwidthFiber wins, but access is an issue.
• Mobility and remote locations
Satellite wins
• Broadcasting
Satellite wins, as it comes for free
• Fast and Reliable
Fiber wins. Satellite are bad due to altitude and rain
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian15
Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian16
Modulation and Multiplexing• Baseband Transmission0 ~ f
• Passband Transmission
fb + 0 ~ fb + f
• Multiplexing
Sharing a channel, how to share?
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian17
Baseband Transmission• Question
In what format a 0 or 1 is transferred?
How to represent a 0 or 1 in transmission?
How can we encode signals when we
can effectively use a single frequency?
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Baseband Transmission• NRZ Non‐Return‐to‐Zero
Issue:
How to handle continuous 0 and 1?
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Baseband Transmission
NRZ, Manchester, NRZI, who wins?
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Passband Transmission• MotivationBaseband signaling is unsuitable to be
transmitted along a narrow band channel
due to
attenuation, delay distortion, and noise.
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Passband Transmission• Modulation Techniques
Asin (ωt + φ) A: amplitude, ω : frequency, φ : phase
• Amplitude Shift Keying, ASK
• Frequency Shift Keying, FSK
• Phase Shift Keying, PSK
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ASK & FSKASK & PSK
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Passband Transmission• Another Question:
How to increase the transmission rate, with low baud rate?
2Blog2V
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian24
Passband Transmission
E.g. 32 points, each baud encodes ____ bits. At 2400
baud, the date rate is ______ bps.
(a) QPSK (b) QAM‐16 (c) QAM‐64Q: Quadrature
正交调相 正交调幅(16点) 正交调幅(64点)
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School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian25
Multiplexing• Question:
1) How to share a transmission line?
2) The bandwidth of a channel can be
huge, wouldn’t it be possible to divide
the channel into sub‐channels
It is a common question to many domains.
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Multiplexing• FDM: Frequency Division Multiplexing
OFDM: Orthogonal FDM, 正交频分复用
• TDM: Time Division Multiplexing
• CDM: Code Division MultiplexingCDMA: CDM Address, 码分多址
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Multiplexing• FDM: Frequency Division Multiplexing
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Multiplexing• OFDM
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Multiplexing• WDM: wavelength, (FDM for Fiber)
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Multiplexing• TDM: Time Division Multiplexing
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Multiplexing• CDM: Code Division Multiplexing
Not for today, next time.
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Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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PSTN• FactPSTN is earlier than Internet, in 1878.
本地回路
中继线
交换局
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PSTN•1 Local loop (modem, ADSL, Fiber)
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PSTN• 1 Local loop – voice‐grade telephone line
Bandwidth is 3000 Hz, the baud‐rate is 6000.
Question:
How to encode bits in a baud as much as possible?
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PSTN• Solution
V.32, 32 points, 2400 baud/s, 4 bits with 1 bit check
the data transfer rate is ________
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PSTN•1 Local loop – ADSL
Why the bandwidth is 3000 Hz?
Because of the switch at the End office…
Solution:
How about changing a switch without filter?
Now, the B is about 1M Hz.
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PSTN
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PSTN•1 Local loop – Fiber
Another excellent link media
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PSTN•2 Trunks – How to multiplexing? T1 Carrier
[(7+1) x 24 + 1] bit x 1/125 μs x 106 = 1.544 MbpsPCM
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PSTN• 2 Trunks – T1, T2, …, Carrier
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PSTN• 2 SONET– How to make different carriers work together?
810 B x 1/125 μs x 106 = 51.84 Mbps
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PSTN• 2 SONET
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PSTN•3 Switching• Switching:A measure provided by telecom subnet to determine how to send message from source to destination.
• Circuit switching: Make a true physical connection from sender to receiver.
• Packet switching: store‐and‐forward,(1) Split any data (i.e. message) into small packets, (2) route those packets separately from sender to receiver, and (3) assemble them again.
• Message switching: store‐and‐forward, a message is completely received at a router, stored, and then put into an outgoing queue for further routing.
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PSTN•3 Switching
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Conclusion
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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