[강의정리] 데이터통신과네트워크 Computer Network and the Internet

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What’s the Internet

“nuts and bolts” view

billions of connected computing devices:

• hosts = end systems
• running networks apps

communication links

• fiber, copper, radio, stellite
• transmission rate: bandwidth

packet switches: forward packets (chunks of data)

• routers and switches

Internet: “network of networks”

protocols control sending, receiving of message

• e.g., TCP, IP, HTTP, …

Internet standards

• Internet Engineering Task Force (IETF) 에서 Request for comment (RFC)라는 표준 문서들을 개발함

A service view

infrasturcture that provides services to applications:

• Web, VoIP, email, games, e-commerce, social nets, …

Socket interface

• allows sending and receiving app programs to “connect” to Internet
• provides service options, analogous to postal service

What’s a protocol?

network protocols:

• machine rather than humans
• three way handshaking

protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt

Network edge

A closer look at network structrue

Network edge (종단 시스템)

• hosts: clients and servers

Access networks

• Home, Enterprise, Mobile, …

  Physical media

• wired, wireless communication links

Access networks and pysical media

How to connect end systems to end router?

• residential(Home) access networks
• institutional access networks
• mobile access networks

keep in mind:

• bandwidth (bits per second) of access network? (속도)
• shared or dedicated? (나만쓰는가?)

Home Access

Home Access: DSL

Digital Subscriber Line (DSL)

• 전화선을 이용한 통신

Home Access: Cable Internet Access

HFC: hybrid fiber coax (동축 케이블)

Coaxial and fiber cables attach homes to ISP router

• fiber와 Coaxial 동시에 사용

Home Access: FTTH

Fiber to the home

• fiber만 사용

Home Access: A typical home network

Enterprise access

Enterprise access: Ethernet

Physical media

Physical media: Twisted-Pair Copper Wire

• Unshielded twisted pair (UTP)
• Shielded twisted pair (STP)

Pysical media: Radio

• no physical “wire”

Network core

• Mesh of interconnected routers

Packet-switching

• Message는 application이 원하는 데이터를 포함
• Message를 잘게 쪼갠게 packets

Packet들이 Communication link로 이동할때 transmission rate기반으로 이동

• 패킷의 크기는 L bits, 전송 속도는 R bits/sec
  

Packet-switching: Store-and-Forward

• 저장하고 전달

Ex) L = 7.5 Mbits, R = 1.5 Mbps, one-hop transmisson delay = 5 s

• End to End Delay = 2L/R

• 3개의 패킷을 보내는 시간

  • L/R시간, 첫번째 패킷이 라우터에 도착
  • 2L/R시간, 첫번째 패킷이 목적지 도착, 두번째 패킷 라우터 도착
  • 3L/R시간, 두번째 패킷이 목적지 도착, 세번째 패킷 라우터 도착
  • 4L/R시간, 세번째 패킷이 목적지 도착

• 하나의 패킷을 N개의 링크가 있는 곳으로 보내는 End to End 딜레이 (N links (eash of rate R), N-1 Routers)

  • D_end-to-end = N X (L/R)

Queuing Delays and Packet Loss

Queuing and Loss:

• 도착하는 패킷보다 보내는 패킷이 느릴경우
• 딜레이 + 공간 꽉차면 Loss까지 생김
  

Forwarding and Routing

• Forwarding: input에서 output으로 움직이는것
• Routing: 어디로 갈지 정해주는것

Circuit switching

• 데이터 회선이 정해지고 회선단위로 데이터를 주고받음

frequency-division multiplexing (FDM) - 주파수분할

time-division multiplexing (FDM) - 시분할

Silend Period -> 네트워크 자원낭비로 이어짐

Packet Switching vs. Circuit switching

• Packet switching allows more user to use network

  • 1 Mb/s link
  • each user:
    • 100 kb/s when “active”
    • the probability that a specific user is active is 0.1

• circuit switching:

  • support only simultaneous 10 users (1 Mbs/ 100 Kbps)

• Packet switching:

  • With 35 users, probability > 10 active at same time is less than 0.0004
  • 35C11(0.9)24(0.1)11 < 0.0004

Packet switching이 좋지만, packet delay나 loss가 발생할 수 있음

• reliable한 데이터 전송이 필요

Network of networks

• End systems은 access ISPs (Internet Service Providers)

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Delay, Loss, Throughput in Networks

Four sources of packet delay

• Total nodal delay (d_nodal)

  • nodal processing delay (d_proc),
  • queuing delay (d_queue),
  • transmission delay (d_trans),
  • propagation delay (d_prop)

  

d_proc: nodal processing

• check bit errors
• determine output link
• typically < msec

d_queue: queueing delay

• 라우터 개수에 의존
• 가장 중요

d_trans: transmission delay

• L: packet length (bits)
• R: link bandwidth (bps)
• d_trans = L/R

d_prop: propagation delay

• d: length of physical link
• s: propagation speed (통신매체에 따라 다름)
• d_prop = d/s

Transmission delay vs. Propagation delay

Caravan analogy

• cars “propagate” at 100 km/hr

• toll booth takes 12sec to service car (bit transmission time)

• 2번째 톨게이트까지 가는 시간?

  • 첫번째 톨게이트 12*10 = 120s
  • 100km 가는 시간 = 1hr
  • A: 62 min

Queueing delay

• R: link bandwidth (bits / sec)
• L: packet length (bits)
• a: average packet arrival rate (packets/sec)
• La/R: traffic intersity
  • La/R이 0에 가까우면: 딜레이가 거의 없음
  • La/R이 1에 가까우면: queueing delay가 커짐
  • La/R이 1보다 크면: 거의 무한대의 딜레이

Packet loss

• 버퍼가 가득 차면 loss 됨

End-to-End Delay

• N-1개의 라우터가 있을때
• End-to-End Delay = N x (d_proc+d_trans+d_prop)

Throughtput(처리량)

Networks under attack: Security

Network security