컴퓨터 네트워크 ch1 (3)

1.3 The Network Core

• circuit switching (telecommunication networks)
  • dedicated circuit per call (자원[가상의 선로]를 점유하고 있으므로, 정보를 전달하지 않아도 돈을 지불해야함)
  • The sender can transfer the data to the receiver at the guaranteed rate.
  • 전용도로 - 가상의 선로로 쭉 보냄

 

• packet- switching (Internet)-라우터
  • data is sent through networks in packets
    • forward packets from one router to the next, across links on path from source to destination (sotre-and-forward 방식, packet을 보내면 router로 다 전달받음)
    • each packet transmitted at full link capacity
    • The internet makes its best efforts to deliver packets in a timely manner, but it does not make any guarantees.

 

Taxonomy of Switched Networks

 

Host A    router1    router2    router3     Host B

통신선로를 1M라고 했을 때,

[Circuit Switched] : 1M를 64k로 나눠서 할당받은 내에서만 데이터 전송

[Packet Switched] : 최대 1M의 rate에서는 1 → 2로 전달됨 (full link capacity) 

→ 사용자가 많아서 1M보다 보내야 하는 양이 많으면 버퍼에 저장해뒀다가, 버퍼도 차 버리면 그냥 버림

 

Circuit Switching

End-end resources reserved for "call"

• link bandwidth, switch capacity
• dedicated resources : no sharing
  • circuit-like (guaranteed) performance
• call setup required
• Circuit switching involves three phases :

   

• circuit establishment
  • Before any data is transmitted, an end-to-end circuit must be established, i.e. network resources on a path between end devices must be reserved

   

• Data transfer
• • After some period of data transfer, the connection should be terminated and dedicated resources are releasedCircuit release (disconnect)
  • [Propagation delay] : Transmission에서 전기적 속도는 매우 빠르지만, 매체 속에서 느려지므로 거리가 길어지면 어쩔 수 없이 delay가 생긴다

 

Multiplexing in Circuit-switched Networks

• Each link can be shared among 'N' circuits
  • Each circuit gets a fraction of 1/N of the link's bandwidth
• Multiplexing
  • Set of techniques that allows simultaneous transmission of multiple signals across a single data link
  • link는 같지만 slot을 다르게 하여 multiplexing 가능
• Two basic multiplexing methods
• Frequency division multiplexing (FDM)
  • Assign a non-overlapped frequency bandwidth to each channel through a modulation
• Time division multiplexing (TDM)

 

Multiplexing : FDM and TDM

 

Multiplexing : FDM

Bandpass filter ; demodulator

 

Multiplexing : TDM

• TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one
  • Regularly assign time slots to each channel

 

Circuit Switching

• Advantages
  • Guaranteed qualith of services (QoS)
  • Delay at nodes(=switch) is negligible
• Disadvantages
  • Inefficient use of resources ; 데이터 전송을 안해도 자원을 점유하고 있기 때문. 특히 traffic이 bursty할 때. 그러나 constant rate로 data가 전송되면 비효율적이지 않다
  • Circuit establishment delay

 

Numerical example

How long does it take to send a file of 640,000bits from host A to host B over a circuit-switched network?

• All links are 1.536Mbps
• Each link uses TDM with 24 slots
• 500msec to establish end-to end circuit

• 한 슬롯 당 $1.536\times10^6bps/24$   $ [= R] $ 의 속도로 데이터 보낼 수 있음
• 그러면 총 데이터는 $640\times10^3/R$의 속도로 보내짐
• propagation delay 무시
• setup $500msec$ 더하면

$500msec + 640\times10^3/R$

 

 

 

Packet switching

• Each end-end data stream is divided into packets (header + payload)
• Store the packet, and then forward the packet to the next node (store-and-forward)
  • 일단 라우터에서 packet을 받아 저장, 목적지 주소 분석 후 다음 라우터로
• User A, B packets share network resources
• Each packet uses full link bandwidth
• Resource는 필요할 때만 쓰이는 On demand방식
  • No dedicated allocation
  • Congestion may occur
    • aggregate demand가 available resource보다 많아지면 congestion 발생
    • queue에 잉여 정보 저장 (시간 지체)

 

 

Statistical Multiplexing

• 데이터를 보냈다 안보냈다 함 - 어느 순간은 traffic이 많고 그래도 평균적으로 같이 link 사용한다.
• 그래서 스케쥴링하여 공평하게! 내가 안쓰면 남이 쓸 수 있도록 하여 bursty traffic에 더 효율적이게!
• Schedule link on demand basis rather than predetermined basis of resources
• Link capacity is shared on packet-by-packet basis only among those users who have packets that need to be transmitted over the link
• Buffer packets that are contending for the link
• Congestion may occur (queueing and loss)

 

 

Two key network-core functions

routing : 경로 정하기 - forward table이 생김

forwarding : forwarding table에 따라, 받은 packet 보내기

 

 

Packet switching vs circuit switching

Example

• N users share 1Mb/s link
• each user :
  • 100kb/s when "active"
  • active 10% of time
• Q : How many users can be supported with CS and PS?
• Assumption : The probability that more than 10 users are active should be less than 0.0004
• Circuit-switching : $1Mbps/100kbps = 10$ simultaneous users
• packet switching :
  • 10명까지는 다 더해도 1M이므로, 10명은 일단 수용 가능 
  • 11명부터 N명까지, i명이 동시에 packet을 사용할 확률은
    • $\sum_{i=11}^{N} {}_N \mathrm{C}_i (0.1)^{i}(0.9)^{N-i}<0.0004$
• Packet switching allows more users to use network!

 

Packet Switching

Advantages

• Efficinet for bursty data
• Resource sharing
• Simpler, no call setup

Disadvantages

• No guarantee QoS : variable packet delay and loss
• protocols for reliable data transfer, congestion control, throughput도 보장되지 않음

Q : How to guarantee QoS?

• QoS needed for audio/video apps
• still an unsolved problem
• congestion을 줄여야 함 - 어떻게 detect하고 줄이느냐가 핵심!

 

Packet switching (store-and-forward)

• Packet size = L bits, transmission rate = R bps
• If queueing delay and propagation delay are ignored,
  • Total delay (latency) = 3L/R

cf) 한 패킷을 보내는 데 필요한 Transmission delay = L/R

     propagation delay는 한 비트 당!

 

 

 

Datagram Networks

• packet switching이라고 하면 보통 datagram networks를 말한다
• Analogy : postal service
• No connection setup phase
• Stateless switch
  • 스위치가 꺼졌다 켜져도 정보전달은 된다? stateless!
  • setup 때 테이블을 만들기 때문에 setup과정은 필요 없다
• Each packet is forwarded independently
  
  • Based on the routing table
  • packet의 목적지가 같다고 같은 route를 통해서 가는 것은 아니다!!!
• Each packet has the complete destination address
  • Addres is a globally unique identifier
  • 주소는 32bit이고, 관리하는 단체가 산하기관에 IP를 나눠준다
• Sometimes called connectionless model

Reordering 필요

 

 

Virtual Circuit Networks

• Establish a virtual connection from the sender to the destination before data transfer
  • 여기서 테이블 생성
  • A single route, so-called virtual circuit, is chosen between the sender and the destination
  • Signaling protocol is used for connection setup
• Each switch maintains connection state 
  • VC entry $\leq$ in_interface, in_VCI, out_interface, outVCI
  • Generally VCI[Virtual Circuit Identifier] is not a globally significant identifier
  • VCI (Virtual Circuit Identifier) : 통신하고자 하는 상대방까지 어떻게 갈지 알려줌! (globally unique X)
• Connection setup provides an opportunity to reserve resources
  • However, a virtual circuit does not have deicated resource
• VC types
  • PVC : permanent VC - 사람이 테이블 만든다
  • SVC : Switched VC - 쉽지 X

 

Virtual Circuit Networks

incoming, outcoing VCI가 자주 바뀌는 건 packet에게 좋지 않다. (bit가 많이 필요할 것이므로)

unique한 Identifier를 만들지 않게 하기 위함이다.

 

 

Advantages

• Small look-up table
  • 그 당시에 사용하는 애들의 정보를 가짐 
  • Fast packet switching (그러나 요즘, 테이블이 커져도 하드웨어적으로 search해 빨리 찾음)
• Possibility to support QoS
  • Resource can be allocated during call setup
• In-Order packet delivery

Disadvantages

• Need call setup procedure
• Each switch should maintain state-informaiton
  • susceptibility to failure

 

 

Internet structure

• End systems connect to Internet via access ISPs (Internet Service Providers)
  • Residential, company and university ISPs
• Access ISPs in turn must be interconnected
  • So that any two hosts can send packets to each other
• Resulting network of networks is very complex
  • Evolution was driven by economics and national policies