学习笔记：高级信息网络Ch5.Communication Media
(来源：notes of course Advanced IT Network.Ch5.Communication Media )
Four kind of media
- Optical fiber
- Coaxial cable
- Twisted pair
- Radio
Optical fiber
-based on Snell’s law（折射定律）,different n–>different velocity
2 kind
- step index fiber ( above )
- graded index fiber
Multi-mode propagation
-because the total distance of light traveling varies, the light wave’s speed will vary
Multimode dispersion
-because of the multimode propagation, the time for light to reach destination vary. In the time graph, the pulses will get merged.
-this limit the maximum bit rate.
-graded index fiber suffer less , but nevertheless, multimode fiber are restricted to short distance applications.
Monomode fiber
-no reflection—–> one propagate mode
-diameter decrease to 10 times or less.
Wave domain multiplexing
-different color lights carry diffent data
Coaxial cable
-carry less bandwidth than fiber
-moisture matters!
-the coaxial conductor mainly for creating electromagnetic protection
Twisted pairs
-mainly used one for carrying data for short distance.
-the sum voltage of two pairs is closed to ground potencia.
magnetical induction
-when a magnetic induction B is put on the twisted pairs, there will conduct a voltage e
-the voltage induced in successive twists will compensate each other.
-电磁感应，周围的环路可以进行补偿，但是尾部的环路不会补偿
capacity couping
-double twisted pairs would constitute a capacitor
-to break this effect—-> don’t twist pair with same step.
-两对TP会组成电容器
Radio(wireless media)
restrictions:
- limited available bandwidth
- uncontrolled sources of noise
A useful formula to describe the power of radio:
$$P_{r}=\frac{S_{a}P_{t}}{4\pi r^2}$$
Microwave point to point link:
**-**highly directive antennas
-cost effective
-high transmission capacity
-severely influenced by weather
Satellite communications:
**-**high delay
-better receiverc device is needed
-low orbit satellite: not profitable,only use to relay between airplane and earth.
Mobile radio network:
Principle: reuse of the same frequencies in different, non overlapping location
-ideal situation , 3 sets of frequencies are enough, but in reality use 7 sets.
-could severely be change by the complexing modeling( eg. moutain area )
GSM station authentication(认证方式):
- SIM card is used with the inside K (key), MS sends a imsi to VLR （请求连接）
- VLR sends imsi to HLR
- HLR sends rand (random number sequence) and the sres(answer to rand),Kc( another key)
- MS could use K to compute the rand to sres
- MS sends the sres , if same, then connection is built
- latter exchange message use the new key Kc
（确认身份的过程比较有趣，有点像哈利波特7下， 被追杀确认哈利身份的过程，约定好钥匙，由中间人传达问题和答案，匹配的话就建立连接。）
1G：
-analog radio technology, is designed to carry voice and a short data message(SMS)
2G:
-two channel is separated by 45MHz, one for transmitting, one for receiving
-to ensure the low attenuation when moving, frequency changes every 557miu-m(timeslot)
-the receiving frequencies sequence follows the sending sequency with a delay of 2 timeslot
-handover feature is available
3G:
theory:
add pseudo sequence
-–> data rate rises
-–> according to the Shannon formula:
$$DataRate=Bandwidth\times\log_{2}{S/N}$$ —> B rises!
-–> true Datarate is constantial
-–> according to the Shannon again
-–> S/N decrease!
Because the pseudo sequence is added, there is another amazing feature showing up.
-radio could reflet from the building, which causes the interference.
-because of the pseudo numbers, 3G only read the message decrypted. which means the reflected signal maybe decrypted twice and to strengthen the whole signal.
4G:
**-**there are n flows where each is R bits per second in frequency domain
-because of the n times flow, the succssive symbols also is n times. then the interference safety margin can be reduced by a factor of n
(有点像并联电路，一起出发，外界的影响就减小了n倍，徒手摆一把和一根筷子的原理）
5G:
（感觉没讲什么东西，国外5G不发达。。。）
MIMO: use antenna arrays that can focus the radio wave in narrow beams