MetadataDec 27, 2024
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Mobile Computing
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MC Key Notes

A short keynote review of Course at school

Lec1 ​

Mobility ​

level:

  • Macro
  • Micro
  • Ad-hoc

definition

  • User Mobility
  • Device Portability

Mobile Computing ​

Mobile Computing is a technology that allows transmission of multimedia data via any wireless device without having to be connected to a fixed physical link.

  • Mobile communication
  • Mobile hardware
  • Mobile software

Mobile Communication ​

The mobile communication is the technology to ensure that seamless and reliable communication goes on.

  • protocols (including data format),
  • services,
  • bandwidth, and
  • portals necessary to facilitate and support the services

Lec2 ​

IP ​

  • In IP networks, routing is based on stationary IP addresses, similar to how a postal letter is delivered to the fixed address on the envelope.
  • A device on a network is reachable through normal IP routing by the IP address it is assigned on the network.

MIP ​

Mobile IP is an open standard which Allow users to keep the same IP address, stay connected, and maintain ongoing applications while roaming between IP networks.

Mobile IP is scalable for the Internet because it is based on IP—any media that can support IP can support Mobile IP.

Work ​

  • Agent Discovery
  • Registration
  • Tunneling

Motivation MIP ​

  • Routing
  • Specific routes to end-systems
  • Changing the IP-address

Components ​

  • Mobile Node
  • Home Agent
  • Foreign Agent

Care of Address, COA(临时地址)

MIP components and relationships ​

Lec3 ​

Motivation ​

Transport protocols typically designed for

  • Fixed end-systems
  • Fixed, wired networks

packet loss not due to congestion, retransmit, continue with current congestion window

Indirect TCP ​

splits TCP connection into two connections

Loss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crash

Scoop TCP ​

Buffer

end-to-end semantics

  • Snooping TCP does not isolate the wireless link as good as I-TCP
  • Snooping might be tough if packets are encrypted

M-TCP ​

splits TCP connection, chokes sender via window size

Maintains end-to-end semantics

bad: software modifitcation

Window Size=0 , Standard Host->stop

Fast Retransmit/Recovery ​

  • Change of foreign agent often results in packet loss
  • not Slow Start -> congestion control
  • Duplicate ACK -> Fast Retransmit/Recovery
  • Congestion Window Size not change
  • higher performance

Lec3 ​

Free Space Loss ​

Receiving power ∝ 1/d² (d = distance between sender and receiver)

Signal propagation ​

  • Fading (frequency dependent)
  • Shadowing
  • Reflection at large obstacles
  • Refraction depending on the density of a medium
  • Scattering at small obstacles (size in the order of the wavelength)
  • Diffraction at edge

Multipath propagation ​

  • Time Dispersion -> Inter Symbol Interference
  • reaches a receiver directly and Phase Shift

Shannon's Theorem(*) ​

SNR=10⋅log_10(S/N)

C=B⋅log_2(1+S/N)

Lec4 ​

Cellular network comapre Computer network ​

  • Cellular network is an underlying technology for mobile phones, personal communication systems, wireless networking, etc.
  • The technology is developed for mobile radio telephone to replace high power transmitter/receiver systems.
  • Cellular networks use lower power, shorter range and more transmitters for data transmission.

Challenges for Cellular ​

  • Frequency allocation
    • Licensed
    • Many providers
  • Multiple Access
    • Many users
    • Wide area of coverage
    • Traffic management
  • Location management
    • High mobility (in cars, trains)
    • Multiple suppliers
    • Handoff management, roaming
  • General principles
  • Handled differently by different generations

Shape ​

  • Square Cell -> Simplifies choosing and switching to new antenna
  • Hexagon Cell -> Provides equidistant antennas & Distance from center to vertex equals length of side

Frequency Reuse ​

4, 7,12, 21

Min cluster size>= 4 & (a+b)^2-ab

每个单元的信道数量N=总信道数T/集群大小K

系统容量C=集群复制次数M×T

  • frequency reuse -> capacity -> num of user support

Cluster Size ​

d/r=sqrt(3k)

Lec5 ​

  • hand off -> strict req -> RealTime   与computer network 对比

  • CSMA/CD vs CA

  • wifi opreration 画图     - infra     - ad-hoc

  • AP channel reuse

Lec6 ​

Chararistic ​

Operates in the 2.4 GHz band - Packet switched • 1 mWatt - as opposed to 500 mW cellphone • Low cost, 10m to 100m range • Uses Frequency Hop (FH) spread spectrum, which divides the frequency band into a number of hop channels. During connection, devices hop from one channel to another 1600 times per second • Data transfer rate 1-2 megabits/second (GPRS is ~50kbits/s) • Supports up to 8 devices in a piconet (= two or more Bluetooth units sharing a channel). • Built-in security • Non line-of-sight transmission through walls and briefcases • Easy integration of TCP/IP for networking.

Connection States ​

8个, 4 层   -> 节能

Active=Transmit

Bluetooth Architecture ​

  • master+ n slave(max 7 active)
  • piconet基本单位

Scatternet（散射网）:

一个设备只能是一个 Piconet 的 Master，但可以是多个 Piconet 的 Slave

当多个 piconet 在同一地理区域中时，它们之间可能会产生干扰。 为了解决这一问题，蓝牙使用了 跳频扩频（FH-SS）技术

TDMA

Security ​

  • Encryption Mode 1—Nothing is encrypted
  • Encryption Mode 2—Traffic from master to one slave is encrypted, but traffic from master to multiple slaves is not
  • Encryption Mode 3—All traffic is encrypted

Authentication: challenge-response strategy

Applications ​

Smart home Wearable devices BLE broadcast

zigbee vs bluetooth ​

  • Focus on low data rates/low power/moderate range/low complexity devices for WPAN sensor networks
  • Very Low power consumption
  • Full handshake protocol for transfer reliability

Lec7 ​

WSN(Wireless Sensor Network) 无线传感器网络 ​

  • Sensors exmple
  • sensing process(*)   - sensing   - signal processing   - actuation

MANET(Mobile Ad Hoc Network) ​

Lec7 ​

MANET

Dynamic Source Routing

源路由（Source Routing）：每个数据包的头部包含从源节点到目标节点的完整路由信息。中间节点根据包头中的路由信息转发数据，而不需要存储额外的路由表。

Routing ​

  • discovery: RREQ 每个中间节点在转发RREQ时，会在消息中附加自己的标识符
  • reply: RREP -> 逆转RREQ中记录的路径返回
  • estab: 缓存该路径,建立源路由

Power-Aware Routing

Lec8 ​

Viruses  VS trojan horse ​

A software virus is a parasitic program written intentionally to alterthe way your computer operates without your permission or knowledge

Trojan horses are programs that hidden in other programs. These programs may be to damage files, to place a virus on your computer or to allow a hacker to gain access to your machine.

level of damage via message passing ​

  • 中断（Interrupt）：
    • 描述：攻击者通过阻止消息到达接收者 R 来实施干扰。
    • 影响：破坏了消息的可用性（Availability）。
  • 拦截（Intercept）：
    • 描述：攻击者能够查看或窃听消息内容。
    • 影响：威胁了消息的保密性（Secrecy）。
  • 修改（Modify）：
    • 描述：攻击者截获消息并对其进行更改。
    • 影响：破坏了消息的完整性（Integrity）。
  • 伪造（Fabricate）：
    • 描述：攻击者创建一条看似来自发送者 S 的消息，并将其发送给接收者 R。
    • 影响：同样破坏了消息的完整性（Integrity）。

Public-key Cryptography ​

Secret-key cryptography ​

Substitution ​

  • Monoalphabetic Substitution -> 频率分析
  • Polyalphabetic Substitution: 使用多个替代表，每个表适用于明文的不同部分,使密文字母的频率分布变得更均匀，从而抵御频率分析攻击 Vigenere Tableaux /Use 26 Characters as a key -> Kasiski method破解(如果一个序列的重复间隔是某些整数的公因数，可能表明关键字的长度是这些公因数之一)

IC ​

IC=$ ∑_{i=1}^{26} \frac{f(i)f(i-1)}{N(N−1)}$

​

Transposition vs subsitution ​

只改变字符的排列顺序

不改变字符的频率分布，因此容易被统计方法破解。 如果密钥被泄露，攻击者可以轻松解密。

MH Knapsack ​

超级递增序列

A,M互质转换序列序列-> 密钥

二进制加权密钥求和

使用逆元反推

RSA ​

digital ​

The digital signature is an un-forgeable piece of data as inserting that a named person wrote or otherwise agreed tothe document to which the signature is attached.

  • Non-repudiation
  • Integrity
  • Authenticity

发送方使用自己的 私钥 对消息进行哈希操作后加密，生成数字签名。
接收方使用发送方的 公钥 解密数字签名，得到哈希值。 然后将消息重新计算哈希值，与解密得到的哈希值比较：

Lec9 ​

  • 什么是QT,优点
  • QT signal & slots event driven control