Computer Network Review

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

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）

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