Computer Network Review_南京大学陈天-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_40891899/article/details/107029384

本文基于南京大学陈天老师的课程，涵盖了交换网络的电路网络、分组网络和混合虚拟电路，网络评估的延迟、带宽和丢包率，网络构成如DSL、以太网和卫星通信，OSI七层模型，数据链路层的帧封装、错误检测与纠正，以及无线网络面临的挑战和解决方案。

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This blog is based on the course of Teacher Chen Tian of NJU

1、Switched networks

1） Circuit network：

Time division multiplexing
Frequency division multiplexing

Pros: Predicted performance，simple or fast switching
Cons：delay 、inefficient and complexity of circuit setup or teardown， Switch fails -> circuit fails

2）Packet network

Pros：efficient use of network resource， simple to implement，robust->“can route trouble”
Cons：unpredictable performance， require buffer management and congestion control

3）Hybrid：Virtual circuit

2、How do we evaluate a network？

Delay：Transmission 、Propagation、Queue、Processing
BDP = Bandwidth x Propagation-Delay
Little’s Law : L = A x W
Loss
Throughput

3、What is a network made of？

DSL
cable
Ethernet
Fiver to the Home
Optical Carrier
Satellite
Cellular

4、OSI layers

Application
Presentation
Session
Transport
Network
Data Link
Physical

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Layers Pros and Cons：
Pros： Reduce Complexity， improve flexibility
Cons：Higher overheads， Cross-layer information often useful

Layering is a good way to organize networks
Unified Internet layer decouples applications from networks
E2E argument encourages us to keep IP simple

5、Data Link layer

Framing：Encapsulates network layer data
Link access：Medium access control (MAC) protocol defines when to transmit frames
Reliable delivery：Primarily for mediums with high error rates (e.g., wireless)
Error detection and correction

Point to point Vs. Broadcast

PtP：dedicated pairwise communication
Broadcast：shared wire or medium

Thress classes of techniques in a shared broadcast channel：

Channel partitioning: divide channel into pieces（TDMA、FDMA、CDMA）
Taking turns: scheme for deciding who transmits
Random access: allow collisions, and then recover（ALOHA、Slotted ALOHA、CSMA、CSMA/CD、CSMA/CA-Wireless）

ALOHA: ACK
Slotted ALOHA: central clock
CSMA:listen before transmit

Three key ideas of random access：

carrier sense
collision detection
randomness：waiting for a random time before try again（Binary exponential back-off）

6、Ethernet

MAC（Medium Access Control）
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Spanning tree protocol——eliminate looping

Data link layer transfers data between adjacent nodes or nodes connected to the same switch
Ethernet evolved from a broadcast medium to switched

ARP：Address resolution Protocol

Every host maintains an ARP table
Map dest. IP address to dest. MAC address

DHCP：Dynamic Host Configuration Protocol
A host uses DHCP to discover

Its own IP address
Its netmask
IP address(es) for its local DNS name server(s)
IP address(es) for its first-hop “default” router(s)

7、Wireless

Wireless networking introduces more challenges than wired networks
Interference, attenuation, multipath, hidden terminals, etc.
CSMA/CD doesn’t work because collision detection is difficult
Instead, CSMA/CA is used that avoid collisions by reserving the channel a priori