n l pointtopoint communication network PSTN Public Switched

n 데이터 통신망 l 형태 - 점대점(point-to-point) - 통신망(communication network) - 전화망(PSTN: Public Switched Telephone Network) - LAN(Local Area Network) - WAN(Wide Area Network) 2

l 교환 기술 - 회선 교환 (Circuit Switching) - 메시지 교환 (Message Switching) - 패킷 교환 (Packet Switching) 23

The Network Core n n mesh of interconnected routers the fundamental question: how is data transferred through net? l circuit switching: dedicated circuit per call: telephone net l packet-switching: data sent thru net in discrete “chunks” 32

Circuit Switching End-end resources reserved for “call” n link bandwidth, switch capacity n dedicated resources: no sharing n circuit-like (guaranteed) performance n call setup required 33

Circuit Switching network resources (e. g. , bandwidth) divided into “pieces” n pieces allocated to calls n resource piece idle if not used by owning call (no sharing) n dividing link bandwidth into “pieces” l frequency division l time division 34

Packet Switching each end-end data stream divided into packets n user A, B packets share network resources n each packet uses full link bandwidth n resources used as needed, Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention: n aggregate resource demand can exceed amount available n congestion: packets queue, wait for link use n store and forward: packets move one hop at a time l transmit over link l wait turn at next link 35

Packet Switching 10 Mbs Ethernet A B statistical multiplexing C 1. 5 Mbs queue of packets waiting for output link D 45 Mbs E 36

Packet Switching Packet-switching: store and forward behavior Message size: 7. 5 Mbits Packet size: 1. 5 Kbits Link trans. Rate: 1. 5 Mbps Assume: no congestion, no propagation delay 37

Packet switching versus circuit switching Is packet switching a “slam dunk n winner? ” Great for bursty data resource sharing l no call setup Excessive congestion: packet delay and loss l protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? l bandwidth guarantees needed for audio/video apps still an unsolved problem l n n 38

Packet-switched networks: routing n Goal: move packets among routers from source to destination n datagram network: l destination address determines next hop routes may change during session l analogy: driving, asking directions virtual circuit network: l each packet carries tag (virtual circuit ID), tag determines next hop l fixed path determined at call setup time, remains fixed thru call l routers maintain per-call state l n 39

Delay in packet-switched networks packets experience delay on end-to-end path n four sources of delay at each hop transmission A n n nodal processing: l check bit errors l determine output link queueing l time waiting at output link for transmission l depends on congestion level of router propagation B nodal processing queueing 40

Delay in packet-switched networks Propagation delay: n d = length of physical link n s = propagation speed in medium (~2 x 108 m/sec) n propagation delay = d/s Transmission delay: n R=link bandwidth (bps) n L=packet length (bits) n time to send bits into link = L/R transmission A Note: s and R are very different quantitites! propagation B nodal processing queueing 41

n Hamming Code의 예(송신측) 49

n Hamming Code의 예(수신측) 50