introduction to computer networks · introduction to computer networks tian song (嵩天), ph.d.,...
TRANSCRIPT
Introduction to Computer Networks
Tian Song (嵩天), Ph.D., Assoc. [email protected]
Introduction to Computer Networks
Tian Song (嵩天), Ph.D., Assoc. [email protected]
Satellite, Multiplexing and PSTN
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian3
Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian5
Tech Networks in the World
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?
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian7
Satellite Network
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian8
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
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian9
Satellite Network•MEO, Medium Earth Orbit
GPS, Compass, GLONASS, Galileo
6 hours a circle
low power needed for a transmission
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian10
Satellite Network
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian11
Satellite Network• LEO, Low Earth OrbitIridium Project (铱星计划), Motorola, 1990‐1998
750 km, 6 chain, 1.5 hrs
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)
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian13
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian14
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
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian15
Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
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?
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?
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian18
Baseband Transmission• NRZ Non‐Return‐to‐Zero
Issue:
How to handle continuous 0 and 1?
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian19
Baseband Transmission
NRZ, Manchester, NRZI, who wins?
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian20
Passband Transmission• MotivationBaseband signaling is unsuitable to be
transmitted along a narrow band channel
due to
attenuation, delay distortion, and noise.
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian21
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
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian22
ASK & FSKASK & PSK
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian23
Passband Transmission• Another Question:
How to increase the transmission rate, with low baud rate?
2Blog2V
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)
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.
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian26
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
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian27
Multiplexing• FDM: Frequency Division Multiplexing
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian28
Multiplexing• OFDM
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian29
Multiplexing• WDM: wavelength, (FDM for Fiber)
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian30
Multiplexing• TDM: Time Division Multiplexing
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian31
Multiplexing• CDM: Code Division Multiplexing
Not for today, next time.
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian32
Outline
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian33
PSTN• FactPSTN is earlier than Internet, in 1878.
本地回路
中继线
交换局
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian34
PSTN•1 Local loop (modem, ADSL, Fiber)
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian35
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?
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian36
PSTN• Solution
V.32, 32 points, 2400 baud/s, 4 bits with 1 bit check
the data transfer rate is ________
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian37
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.
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian38
PSTN
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian39
PSTN•1 Local loop – Fiber
Another excellent link media
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian40
PSTN•2 Trunks – How to multiplexing? T1 Carrier
[(7+1) x 24 + 1] bit x 1/125 μs x 106 = 1.544 MbpsPCM
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian41
PSTN• 2 Trunks – T1, T2, …, Carrier
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian42
PSTN• 2 SONET– How to make different carriers work together?
810 B x 1/125 μs x 106 = 51.84 Mbps
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian43
PSTN• 2 SONET
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian44
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.
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian45
PSTN•3 Switching
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian46
Conclusion
• Satellite Communication
• Modulation and Multiplexing
• Public Switched Tele Network
School of Computer Science and Technology, BIT, http://cs.bit.edu.cn/~songtian47
End
This page is intended blank.