네트워크 이론 핵심정리 - 1

• Lan과 Wan의 차이점 3가지

LAN	WAN
Span limited area (campus, office)	Span greater geographical area
Interconnects hosts	Interconnects connecting devices
Privately owned	Created by communication companies

• Point to point WAN vs Switched WAN

  • Point to point: connect two connecting devices
  • Switched: combine point to point WANs by switches

• Internet의 정의

  • two or more networks are connected(두개 이상의 네트워크가 연결되어 있는 상태)

• 네트워크의 정의

  • Interconnection of set of devices. (기기들의 집합이 연결되어 있는 상태)

• 네트워크의 종류 2가지 비교(Circuit switched vs Packet switched)

  • Circuit switch 정의: there is one circuit(a dedicated connection) between two end system.
  • Packet switch 정의: a router has a queue and packets are delivered independently.

Circuit switched	Packet switched
Effective Only when working in full capacity	More Effective than Circuit switched net.
No delays	Packet may encounter delays
	Store packets to router(queue)

• Protocol의 정의

  • rules that sender and receiver should follow to communicate effectively.

• Protocol layering의 principle

  • Each layer should perform two opposite tasks one in each direction
    • (한 레이어가 두 반대 방향의 일을 할 수 있어야함(ex, 암호화-해독))
  • Two objects under each layer at both sites should be identical.
    • (message – message, segment – segment처럼 같은 레이어면 같은 object)

• Logical connection의 정의

  • imaginary connection between two systems at the same level of TCP/IP model

• 스위치와 라우터

  • 라우터에서 network layer가 필요한 이유: Each link may use its own data link or physical protocol.
    (라우터로 오는 protocol A를 쓰는 Link 1의 packet을 전달받아, protocol B를 쓰는 Link 2에 맞춰 packet을 전달하기 위해서)
  • 스위치는 network layer가 필요 없는 이유: 같은 link안에 있는 두 개의 connection을 연결하는 기기이기 때문에,
    같은 link면 동일한 protocol을 사용하므로 network-layer 필요 x

Switch	Router
Layer 2 in TCP/IP	Layer 3 in TCP/IP
Link 내의 connection을 담당 (같은 link -> 같은 protocol)	여러 link 사이의 connection 담당 (다른 link -> 다른 Protocol)

• Application/Transport/Network layer

  • Internet상에서 일을 한다(domain of duty), host-to-host

• Data-link/Physical layer

  • link상에서 일을 한다(domain of duty), hop-to-hop(hop: host or router)
  • Identical objects: Messages / segment or user datagram / datagram / frame / bits

• TCP/IP 5 layer : 각 layer별 하는 일

  • Application: process-to-process communication
  • Transport: give services to application layer
  • Network: Choose the best direction for each packet.
  • Datalink: take the datagram and move it across the link
  • Physical: carry individual bits

• OSI가 망한이유

  • OSI was completed when TCP/IP was fully in place
  • A lot of time and money was needed for OSI
  • Some layers in the OSI model were never fully defined.
  • When OSI did not show a high enough level of performance.

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